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Advanced Level Organic Chemistry: Aromatic (Aryl) Amine Chemistry

Part 7. The chemistry of AROMATIC COMPOUNDS

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 7.10 The physical and chemical properties of aromatic amines e.g. phenylamine, selected derivatives including diazonium ions and dyes

Sub-index for this page

7.10.1 The structure and physical properties of aromatic amines like phenylamine

7.10.2 Methods of preparing phenylamine (in principle for other aromatic amines)

7.10.3 Relating the structure of phenylamine to electrophilic substitution reactions and orientation of products - synthesis restrictions

7.10.4 Electrophilic substitution reactions of phenylamine (and other aromatic amines)

7.10.5 Reactions of phenylamine as a nucleophile e.g. formation of amides with acid chlorides/anhydrides, but a useful intermediate is made for further electrophilic substitution

7.10.6 Preparation of the benzenediazonium ion (diazotisation) and their coupling reactions to synthesise dyes

7.10.7 The acid-base chemistry of aromatic amines - weak bases and relative strength


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

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7.10.1 The structure and physical properties of aromatic amines like phenylamine

A note of classifying amines, using aromatic (aryl) amines as examples

In aromatic (aryl) amines, the amino/amine group is directly attached to the benzene ring,

Functional group of the homologous series PRIMARY SECONDARY TERTIARY Comments
aromatic amine examples (c) doc b

phenylamine

(c) doc b

diphenylamine

(c) doc b

triphenylamine

Aromatic amine examples with 1, 2 or 3 aryl groups attached to the nitrogen of the amine functional group

Physical properties of selected aromatic amines and selected derivatives

Abbreviations used: mpt = melting point;  bpt = boiling point; sub. = sublimes, dec.= thermally decomposes

Many aromatic amines have two functional groups e.g. -Cl, -OH, -COOH, NO2 as well as -NH2.

Name of aromatic amine Structure Mpt/oC Bpt/oC Comment
phenylamine structural formula phenylamine molecular structure -6 184 Aniline, colourless liquid. Primary aromatic amine. Solubility ~3.6/100g water. It is a toxic material (carcinogen) that can be absorbed through the skin.
2-methylphenylamine structural formula 2-methylphenylamine molecular structure methyl-2-phenylamine structural formula 3-methylphenylamine molecular structure methyl-3-phenylamine structural formula 4-methylphenylamine molecular structure methyl-4-phenylamine -16 200 Three positional structural isomers.

All slightly soluble in water

Primary aromatic amines.

3-methylphenylamine -44 203
4-methylphenylamine 44 200
N-methylphenylamine structural formula N-methylphenylamine molecular structure -57 196 Insoluble in water.

Secondary aromatic-aliphatic amine

N,N-dimethylphenylamine structural formula N,N-dmethylphenylamine molecular structure 2 194 Slightly soluble in water.

Tertiary aromatic-aliphatic amine

Diphenylamine structural formula diphenylamine molecular structure 53 302 Secondary aromatic amine

Insoluble in water.

Triphenylamine structural formula triphenylamine molecular structure 126 365 Tertiary aromatic amine

Insoluble in water.

 
2-chlorophenylamine structural formula 2-chlorophenylamine molecular structure chloro-2-phenylamine structural formula 3-chlorophenylamine molecular structure chloro-3-phenylamine structural formula 4-chlorophenylamine molecular structure chloro-4-phenylamine liquid 209 Three positional structural isomers.

Primary aromatic amines.

3-chlorophenylamine liquid 236
4-chlorophenylamine 70 231
2-aminophenol structural formula 2-aminophenol molecular structure structural formula 3-aminophenol molecular structure structural formula 4-aminophenol molecular structure 174 dec.? All three isomers slightly soluble in cold water, more soluble in hot water.

Carboxylic acids with an aromatic amine group,

3-aminophenol 122 dec.?
4-aminophenol 187 284
2-nitrophenylamine structural formula 2-nitrophenylamine molecular structure 2-nitroaniline structural formula 3-nitrophenylamine molecular structure 3-nitroaniline structural formula 4-nitrophenylamine molecular structure 4-nitroaniline 71    
3-nitrophenylamine 114    
4-nitrophenylamine 146    
1,3-diaminobenzene structural formula 1,3-diaminobenzene molecular structure 63 282 Solubility 42g/100g water - relatively high due to amino groups that can hydrogen bond with water.
2-aminobenzoic acid structural formula 2-aminobenzoic acid molecular structure structural formula 3-aminobenzoic acid molecular structure structural formula 4-aminobenzoic acid molecular structure 147 200 sub. Solubility 0.57g, 0.59 and 1.1g/100g

3 isomeric molecules with both an acidic and basic group

3-aminobenzoic acid 179  
4-aminobenzoic acid 188  

Extra notes on the structure and data table on aromatic (aryl) amines

(a) A few notes on naming aromatic amines

(i) Alternative names: e.g. 2-methylphenylamine is methyl-2-phenylamine.

(ii) The old, and still widely used name for phenylamine is 'aniline' and many names of its derivatives use 'aniline' too - I'm afraid lots of older 'historic' names are still used and accepted, particularly in universities and the chemical industry e.g. acetanilide C6H5NHCOCH3, whose IUPAC systematic name is N-phenylethanamide!

(iii) Carboxylic acid and phenol are higher ranking groups, so in this case the amine group is named as a substituent group.

(iv) Aminophenols can also be named as hydroxyphenylamines e.g. 2-aminophenol is 2-hydroxyphenylamine.

(b) A comparison of  methylbenzene and phenylamine

Property (c) doc b Methylbenzene (c) doc b Phenylamine
Melting point oC -95 -6
Boiling point oC 111 184
Solubility in water insoluble slightly soluble
Hydrocarbon solvents very soluble very soluble

(i) Melting points and boiling points

Methylbenzene and phenylamine molecules have similar shapes, sizes, molecular masses and numbers of electrons, but they have quite different melting points and boiling points.

The Pauling electronegativities are: H = 2.1,  C = 2.5 and N = 3.0

Methylbenzene does not have a significantly polar bond and hence a relatively non-polar molecule.

There the intermolecular force is almost entirely due to instantaneous dipole - induced dipole forces (Van der Waals dispersive forces).

However, due to a more significant difference in electronegativity, phenylamine does have a significant polar bond in the amine group :Nδ--Hδ+, and in the liquid or solid state, you get hydrogen bonding (N-Hδ+llllδ-N-H shown in the diagram below).

Therefore for phenylamine, there will be the same instantaneous dipole - induced dipole force plus permanent dipole - permanent dipole attraction, including the directional hydrogen bonding.

The increases in intermolecular bonding forces leads to higher melting points and boiling points of phenylamine compared to benzene.

diagram of hydrogen bonding between phenylamine molecules in liquid solid aniline

(ii) Solubility in water

Carrying the polar bond arguments from (i):

The lack of a polar bond means that methylbenzene will not dissolve in water, but phenylamine will dissolve, albeit a fairly low solubility, but still significantly higher than for an aromatic hydrocarbon.

The hydrophobic methylbenzene disrupts the hydrogen bonds without compensating solute - solvent intermolecular bonding and so is virtually insoluble in water.

diagram of hydrogen bonding between phenylamine and water molecules in aqueous solutions of aniline

The reason for the difference in water solubility is due to the hydrogen bonding between phenylamine and water molecules (the hydrogen bonding N-Hδ+llllδ-O-H or O-Hδ+llllδ-N-H diagram above).

However, the hydrophobic benzene ring does limit the solvation of the phenylamine, so the solubility in water is moderate at ~3.6g/100g water = ~(10 x 3.6/93 = 0.39 mol/dm3).

Phenylamine, like most aromatic compounds is much more soluble in organic solvents like ethoxyethane ('ether'), ethanol and propanone,

(c) Don't confuse aromatic side chain aliphatic amines with 'true' aryl amines.

(c) doc b (c) doc bleft is a primary aliphatic amine with the amine group -NH2 in a side-chain off the benzene ring,

and right, the isomeric aryl amine with the -NH2 directly attached to the benzene ring.

(d)


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7.10.2 The preparation of phenylamine and other aromatic amines

Method 1. Using lithium tetrahydridoaluminate(III) as the reducing agent

LiAlH4 is a more powerful reducing agent than NaBH4 and in ether solvent readily reduces nitro–aromatics to primary aromatic amines, the simplified equation for nitrobenzene to phenylamine is ...

C6H5NO2  +  6[H]  ===>  C6H5NH2  +  2H2O

and methylnitrobenzenes would be reduced to methylphenylamine primary aromatic amines, i.e.

CH3C6H4NO2  +  6[H]  ===>  CH3C6H4NH2  +  2H2O

as will any aromatic compound with a nitro group (–NO2) attached directly to the benzene ring.

This is an example of the reduction of a polar N-O pi bond via the hydride ion (H-) acting as a nucleophile generated by the tetrahydridoaluminate(III) ion, which does not reduce the non-polar pi bond C=C you find in alkenes.

 

diagram apparatus for preparation of phenylamine aniline reflux condenser nitrobenzene tin hydrochloric acid refluxed mixture

Method 2. Using an appropriate metal and acid as a reducing agent

Reduction of nitro–aromatics with tin and concentrated hydrochloric acid

In the laboratory, reacting a nitro–aromatic with a mixture of tin and conc. hydrochloric acid by heating under reflux will reduce it to a primary aromatic amine (–NH2 directly attached to benzene ring). In industry a cheap metal like iron powder and acid are used or a direct reduction in the gas phase with hydrogen/transition metal catalyst, but here in the school laboratory e.g.

2 cm3 of nitrobenzene, 4g of tin and 10 cm3 of conc. hydrochloric acid that is slowly and carefully added to the mixture.

In the 'laboratory' preparation, the mixture may need cooling with a beaker of cold water if the reaction is too vigorous, then gentle heating with a beaker of boiled water to complete the reaction (the condenser causes avoids loss of product) when hydrogen will stop being evolved

The formation of phenylamine (aniline) from nitrobenzene can be summarised as

C6H5NO2  +  6[H]  ===>  C6H5NH2  +  2H2O

but the 'real' equations are rather more complicated, the simplest redox equation I can come up with is

2C6H5NO2(aq) + 14H+(aq) + 3Sn(s) ==> 2C6H5NH3+(aq) + 3Sn4+(aq) + 4H2O(l)

which shows the formation of the phenylammonium cation because the amine is a base and formed in an acid medium.

You should appreciate that any phenylamine formed, will immediately react with hydrochloric acid and dissolve to form the salt

C6H5NH2(l)  +  HCl(aq)   ===>  C6H5NH3+(aq)  +  Cl(aq)

and then conc. aqueous sodium hydroxide is added to free the amine (immiscible with water) from its arylammonium cation

C6H5NH3+(aq)  +  OH(aq)  ===>  C6H5NH2(l)  +  H2O(l)

 

The flask is cooled and the apparatus assembled to perform a steam distillation (diagram below).

The primary aromatic amine must be extracted by steam distillation because the reaction mixture is quite messy to deal with in any other way!

For more on the theory of steam distillation see Equilibria Part 8.5

diagram apparatus steam distillation of phenylamine aniline from reaction mixture preparation from nitrobenzene 

using a separating funnelOn addition of conc. sodium hydroxide (a strong soluble base) the amine separates out as an oily layer and the mixture is heated with the steam input via the procedure known as steam distillation. The addition of an alkali is necessary because the phenylamine base is soluble in the excess acid and must be freed to steam distil over

A mixture of the amine and water 'steam distils' into the condenser and separates into two layers in the collection flask. Steam distillation is the most efficient way of extracting the phenylamine from the reaction mixture, leaving behind all the inorganic residues. The inorganic residues are soluble, so can't be filtered of. It would prove difficult to extract the phenylamine from the reaction mixture by direct distillation OR trying to use an extracting solvent directly on the reaction mixture. Its best leave as much of the reaction residues behind before attempting the final purification procedures.

2g of salt can be added to the immiscible liquid mixture to help the two layers separate out. and reduce the solubility of phenylamine in the water layer.

The phenylamine layer is separated out with a separating funnel

Some methods use ether solvent to extract the phenylamine from the steam distilled mixture. Can't say I like this idea since a fractional distillation is done later, ether is very volatile and very flammable!

Either way, the 'damp' phenylamine or mixture with ether, can be dried with anhydrous solid sodium hydroxide or anhydrous potassium carbonate.

The drying agent is filtered off and the dried liquid fractionally distilled to obtain pure phenylamine liquid - the fraction boiling between 180 and 185oC should be reasonably pure phenylamine.

If ether isn't used, the phenylamine is distilled with an air condenser (just a tube, not Liebig style), though some texts say distil under reduced pressure because phenylamine can decompose at its boiling point.

Phenylamine distils over between 180-185oC at normal pressure, quite a high boiling point, and a water condenser can crack of very hot phenylamine vapour.

 

Method 3. Using hydrogen gas and catalyst - industrial hydrogenation of nitro group

In the chemical industry aromatic nitro–compounds are more efficiently reduced with hydrogen gas using a Ni or Cu catalyst at elevated temperatures, rather than a 'laboratory style' preparation. The resulting primary aromatic amines are very important intermediate compounds in dye and drug manufacture e.g.

C6H5NO2  +  3H2  ===>  C6H5NH2  +  2H2  (nitrobenzene  ==>  phenylamine)

CH3C6H4NO2  +  3H2 ===>  CH3C6H4NH2  +  2H2O  

(nitromethylbenzenes  ===>  aminomethylbenzenes/methylphenylamines)

CH3C6H3(NO2)2  +  6H2  ===>  CH3C6H3(NH2)2  +  4H2O

(dinitromethylbenzenes  ===>  diaminomethylbenzenes)

 

 


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7.10.3 Relating the structure of phenylamine to electrophilic substitution reactions and orientation of products - synthesis restrictions

The diagrams below give an 'impression' of an electron density 'map' of the delocalised pi electrons of the benzene ring and the non-bonding pair of electrons on the nitrogen atom.

The amine group has a plus inductive effect (+I electron shift) which increases the electron density of the ring compared to benzene, so you expect phenylamine to be more reactive than benzene, particularly to electrophilic attack at the 2, 4 an 6 substitution positions where the electron density is highest.

So, the lone pair of electrons on the nitrogen atom directly attached to the ring activates the benzene ring compared to benzene itself in terms of the potential for electrophilic substitution.

The +I effect and delocalisation of the lone pair of electrons on the nitrogen reduces their availability to act as a proton acceptor, so phenylamine and other aromatic amines are usually weaker bases than aliphatic amines.

Discussed further in 7.10.7 The acid-base chemistry of aromatic amines and their relative strength

delocalised electron system in phenylamine activating substituent 2 4 and 6 positions ortho and para directing orientation on substitution

To some extent the lone pair of electrons on the nitrogen atom interact with pi orbital electrons of the benzene ring i.e. the lone pair of the nitrogen atom become part of the delocalised system and are more or less in the same plane as the pi electrons of the benzene ring (this has been shown experimentally and I've tried to indicate this with my 'fuzzy' diagram!).

 

resonance hydbrid structures of phenylamine aniline effect on substitution postions in the benzene aromatic ring

The resonance structures of phenylamine and the three on the right correspond to the increase in density at the 2, 4 and 6 substitution positions.

There are three problems in synthesising derivatives of phenylamine:

(i) It is difficult to get just one extra substituent group into the benzene ring of phenylamine because of its activated benzene ring - see the reaction with bromine water in section 7.10.4.

(ii) Any synthesis involving an acid as a reagent, an acid catalyst or produced an acid in the reaction, will immediately cause problems because the amine group acts as a base, accepting a proton.

C6H5NH2  +  H+    C6H5NH3+

The protonated amine group deactivates the ring, inhibiting electrophilic substitution.

Therefore you cannot nitrate phenylamine directly or perform a Friedel-Crafts style synthesis.

(iii)  Phenylamine is quite easily oxidised e.g. by nitric acid to a dark mass of complex composition!

 

All three of these potential problems can be avoided by a technique known as group protection.

This is described in section 7.10.5.


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7.10.4 Electrophilic substitution reactions of phenylamine (and other aromatic amines)

Phenylamine is so reactive (argument above in 7.10.3) it rapidly reacts with bromine water at room temperature to form a precipitate of 2,4,6-tribromophenylamine, without catalyst!

A reaction similar to phenol. See 7.9 The chemical properties of phenol

diagram balanced equation for phenylamine reacting with bromine to yield 2,4,6-tribromophenylamine 2,4,6-tribromoaniline

A similar reaction happens with chlorine water to rapidly give a similar precipitate of 2,4,6-trichlorophenylamine.

As already mentioned, you can't halogenate, acylate, alkylate with Friedel-Crafts reactions  or nitrate phenylamine because of the basicity of the  amino group in any controlled way because

(i) the amine group reacts with any acid present as a product or reactant of the reaction

(ii)  The ensuing positive phenylammonium ion group strongly deactivates the benzene ring with respect o electrophilic substitution.

However, you can get round this problem with group protection i.e. see use of N-phenylethanamide in section 7.10.5 next.


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7.10.5 The reactions of phenylamine as a nucleophile e.g. formation of amides with acid chlorides/anhydrides, but a useful intermediate is made for further electrophilic substitution

When primary aromatic amines are reacted with an acid chloride (RCOCl) you do not get electrophilic substitution, even with a catalyst like AlCl3 or FeCl3.

Instead, the amine acts as a nucleophile and forms an N-substituted amide.

e.g. phenylamine reacts rapidly with at room temperature with ethanoyl chloride to yield ...

N-phenylethanamide (N-phenylacetamide, acetanilide)

C6H5NH2  +  CH3COCl  ===>  C6H5NHCOCH3  +  HCl

In industry, it is cheaper to use ethanoic anhydride employing an acid catalyst e.g.

C6H5NH2  +  (CH3CO)2O  ===>  C6H5NHCOCH3  +  CH3COOH

Both reactions are illustrated below using structural formulae.

Note the term 'N-substituted' i.e. the substitution involves replacing a hydrogen of the amino group and NOT a hydrogen in the benzene ring.

diagram balanced equation for phenylamine reacting with ethanoyl chloride ethanoic anhydride to yield N-phenylethanamide acetyl chloride acetic anhydride acetanilide

N-phenylethanamide is less reactive than phenylamine, but more than benzene and more controllable than phenylamine.

Electrophilic substitution reactivity order: C6H5NH2  >  C6H5NHCOCH3  > C6H6

 

Note that N-phenylethanamide is hydrolysed to the free amine by aqueous sodium hydroxide

N-phenylethanamide  +  sodium hydroxide  ===>  phenylamine  +  sodium ethanoate

C6H5NHCOCH3  +  NaOH  ===>  C6H5NH2  +  CH3COO-Na+

In my examples here, it is the final step to synthesise a desired derivative of phenylamine and other aryl amines.

We are now ready to look at 'group protection' synthesis!

 

Group protection in aromatic synthesis

Starting with 'previously prepared' N-phenylethanamide, I'll use four electrophilic substitution reactions to illustrate the idea of group protection - in this case protecting the amine group in phenylamine.

You protect this group by converting phenylamine to N-phenylethanamide, carry out the electrophilic substitution reaction and then obtain the desired 'orientated' product by hydrolysing the substituted N-phenylethanamide.

(1) Mono-nitration of the benzene ring to make 4-nitrophenylamine

Using a cold mixture of conc. nitric and sulfuric acid.

C6H5NHCOCH3  +  HNO3  ===>  O2NC6H4NHCOCH3  +  H2O

The structural formula equation for the formation of the majority product is shown below.

diagram equation nitration of N-phenylethanamide acetanilide to give N-(4-nitrophenyl)ethanamide hydrolysis of product to yield 4-aminophenol 4-nitroacetanilide

Comments on equation 14B:

(i) The intermediate is called N-(4-nitrophenyl)ethanamide and is hydrolysed with aqueous sodium hydroxide solution, so the full synthetic sequence from phenylamine is

phenylamine ==> N-phenylethanamide ==> N-(4-nitrophenyl)ethanamide  ==> 4-nitrophenylamine

(you can even start the synthesis sequence from benzene ===> nitrobenzene  ==> phenylamine etc.)

The product can be reduced to form 1,4-diaminobenzene and important intermediate in the production of aramid fibres and Kevlar polymers.

See section 7.12 The structure, properties and uses of polyamides

(ii) Typical orientation yields of the substitution products are: 2% in the 2 position, 8% in the 3 position and 90% in the 4 position of the benzene ring (2 + 4 substitution = 98%).

(iii) The typical % yields fit in with the activation of the benzene ring theory and favoured substitution orientation, particularly the 4 position of the benzene ring of the N-phenylethanamide.

(iv) Substitution in position 4 is also enhanced by the steric hindrance of the 2 position by the -NHCOCH3 group, so this synthesis route is very good at giving high yields of substituted products in the 4 position of the benzene ring of phenylamine e.g. to synthesise 4-nitrophenylamine.

These comments also apply to reactions (2) to (4) described next

 

(2) Mono-chlorination of the benzene ring

Using a cold mixture of phenylamine, pure ethanoic acid and chlorine bubbled in.

C6H5NHCOCH3  +  Cl2  ===>  ClC6H4NHCOCH3  +  HCl

The structural formula equation for the formation of the majority product is shown below.

diagram equation chlorination of N-phenylethanamide acetanilide to give N-(4-chlorophenyl)ethanamide 4-chloroacetanilide hydrolysis of product to yield 4-chlorophenylamine 4-chloroaniline

Comments on equation 14C:

(i) The intermediate is called N-(4-chlorophenyl)ethanamide and is hydrolysed with aqueous sodium hydroxide solution, so the full synthetic sequence from phenylamine is

phenylamine ==> N-phenylethanamide ==> N-(4-chlorophenyl)ethanamide  ==> 4-chlorophenylamine

(ii) Typical orientation yields of the substitution products are: >80% in the 4 position of the benzene ring and smaller % for the 2 and 3 positions).

(iii) The typical % yields fit in with the activation of the benzene ring theory and favoured substitution orientation, particularly the 4 position of the benzene ring of the N-phenylethanamide.

(iv) Substitution in position 4 is also enhanced by the steric hindrance of the 2 position by the -NHCOCH3 group, so this synthesis route is very good at giving high yields of substituted products in the 4 position of the benzene ring of phenylamine e.g. to synthesise 4-chlorophenylamine.

 

(3) Mono-bromination of the benzene ring

Using a cold mixture of phenylamine, pure ethanoic acid and bromine.

C6H5NHCOCH3  +  Br2  ===>  BrC6H4NHCOCH3  +  HBr

The structural formula equation for the formation of the majority product is shown below.

diagram equation bromination of N-phenylethanamide acetanilide to give N-(4-bromophenyl)ethanamide 4-bromooacetanilide hydrolysis of product to yield 4-bromophenylamine 4-bromoaniline

Comments on equation 14D:

(i) The intermediate is called N-(4-bromophenyl)ethanamide and is hydrolysed with aqueous sodium hydroxide solution, so the full synthetic sequence from phenylamine is

phenylamine ==> N-phenylethanamide ==> N-(4-bromophenyl)ethanamide  ==> 4-bromophenylamine

(ii) Typical orientation yields of the substitution products are: >80% in the 4 position of the benzene ring and smaller % for the 2 and 3 positions).

(iii) The typical % yields fit in with the activation of the benzene ring theory and favoured substitution orientation, particularly the 4 position of the benzene ring of the N-phenylethanamide.

(iv) Substitution in position 4 is also enhanced by the steric hindrance of the 2 position by the -NHCOCH3 group, so this synthesis route is very good at giving high yields of substituted products in the 4 position of the benzene ring of phenylamine e.g. to synthesise 4-bromophenylamine.

 

(4) Mono-acylation of the benzene ring

Using ethanoyl chloride (or ethanoic anhydride in industry) and Lewis acid catalyst.

C6H5NHCOCH3  +  CH3COCl  ===>  CH3COC6H4NHCOCH3  +  HCl

The structural formula equation for the formation of the majority product is shown below.

diagram equation acylation of N-phenylethanamide acetanilide to give hydrolysis of product to yield 4-aminoacetophenone

Comments on equation 14E:

(i) The intermediate is called N-(4-ethanoylphenyl)ethanamide and is hydrolysed with aqueous sodium hydroxide solution, so the full synthetic sequence from phenylamine is

phenylamine ==> N-phenylethanamide ==> N-(4-ethanoylphenyl)ethanamide  ==> 1-(4-aminophenyl)ethanone

(ii) Typical orientation yields of the substitution products are: >80% in the 4 position of the benzene ring and smaller % for the 2 and 3 positions).

(iii) The typical % yields fit in with the activation of the benzene ring theory and favoured substitution orientation, particularly the 4 position of the benzene ring of the N-phenylethanamide.

(iv) Substitution in position 4 is also enhanced by the steric hindrance of the 2 position by the -NHCOCH3 group, so this synthesis route is very good at giving high yields of substituted products in the 4 position of the benzene ring of phenylamine e.g. to synthesise 1-(4-aminophenyl)ethanone.

 


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7.10.6 Preparation of benzenediazonium ions and their coupling reactions to synthesise dyes

(a) The structure of benzenediazonium compounds

Primary aromatic amines form a diazonium salts with nitrous acid, a cation balanced by an anion e.g.

the salt benzenediazonium chloride has the formula C6H5N2+Cl-(aq)

The benzenediazonium ion from phenylamine has the structure  C6H5N+≡N:

The positive charge of any diazonium ion is on the nitrogen atom directly attached to the benzene ring.

 

(b) The preparation of a benzenediazonium salt solution

Phenylamine (or any aromatic amine) is dissolved in hydrochloric acid and (ideally) cooled to ~5oC.

A solution of sodium nitrite (NaNO2) previously cooled to ~5oC is slowly added to the amine solution.

The mixture should be kept cool at ~5oC.

At 0oC the ensuing reactions are too slow and above 10oC the diazonium ion decomposes evolving nitrogen gas.

 

(c) Diazotisation equations

The equations for the diazotisation of phenylamine or a substituted primary amine are:

C6H5NH2(aq)  +  HNO2(aq)  +  H+(aq)  ==>  C6H5N2+(aq)   +  2H2O(l)

XC6H4NH2(aq)  +  HNO2(aq)  +  H+(aq)  ==>  XC6H4N2+(aq)   +  2H2O(l)

X can be CH3, NO2, Cl, Br etc.

 

(d) Coupling reactions of diazonium compounds (ions) with phenols and aromatic amines to form dyes

As mentioned, one nitrogen on the diazonium ion carries a positive charge and since nitrogen is quite an electronegative element, the diazonium ion can act as a powerful electrophile.

So the benzenediazonium ions can substitute into a benzene ring activated by an OH group (phenol) or an NH2 group (aromatic amine).

These 'coupling reactions' produce coloured precipitates, many of which are useful dyes ('dyestuffs').

These dye compounds can form hydrogen bonds via e.g. N-Hδ+llllδ-O-H, O-Hδ+llllδ-O-H or O-Hδ+llllδ-N-H with groups on the molecules of cotton and wool fibres, which strongly bind the dye molecules to the fabric.

The coupling reactions often involve two aromatic compounds, a benzenediazonium compound and a phenol in alkaline solution or an aromatic (aryl) amine in neutral solution.

Four examples are now described.

DYE formation diagram A

A cooled sodium hydroxide solution of phenol (phenoxide ion C6H5O- formed) is slowly added to the diazotised solution of phenylamine and a yellow precipitate forms.

diazotisation phenylamine aniline benzenediazonium ion coupling reaction with phenol phenoxide ion 4-hydroxyazobenzene yellow dye

DYE diagram A

(i) The diazotisation of phenylamine to form the benzenediazonium ion

(ii) Formation of the phenoxide ion followed by its coupling with the benzenediazonium ion to give a yellow dye precipitate of 4-hydroxybenzene.

(iii) A shorthand version of (ii) omitting the formation of the phenoxide ion.

 

DYE formation diagram B

Phenylamine is slowly added to the diazotised solution of phenylamine and a yellow precipitate forms.

diazotisation phenylamine aniline benzenediazonium ion coupling reaction with phenylamine 4-aminoazobenzene Aniline yellow dye

DYE diagram B

(i) The diazotisation of phenylamine to form the benzenediazonium ion

(ii) The coupling of the benzenediazonium ion with phenylamine to give a yellow precipitate of 4-aminoazobenzene ('Aniline yellow' dye).

 

DYE formation diagram C

A cooled sodium hydroxide solution of 2-naphthol (2-naphthoxide ion formed) is slowly added to the diazotised solution of phenylamine and a red precipitate forms.

diazotisation phenylamine aniline benzenediazonium ion coupling reaction with 2-naphthol red dye precipitate

DYE diagram C

(i) The diazotisation of phenylamine to form the benzene diazonium ion

(ii) Formation of the 2-naphthoxide anion followed by its coupling with the benzenediazonium ion to give a red dye precipitate.

(iii) A shorthand version of (ii) omitting the formation of the 2-naphthoxide ion.

 

DYE formation diagram D

A cooled sodium hydroxide solution of 2-naphthol (2-naphthoxide ion formed) is slowly added to the diazotised solution of 2-nitrophenylamine and a red precipitate forms.

diazotisation 4-nitrophenylamine 4-nitroaniline 4-nitrobenzenediazonium ion coupling reaction with 2-naphthol Para red dye precipitate

DYE diagram D

(i) The diazotisation of 4-nitrophenylamine to form the 4-nitrobenzenediazonium ion

(ii) The coupling of the 4-nitrobenzenediazonium ion with alkaline 2-naphthol to give a red dye precipitate 'Para red'.

 

Origin of the colour in azo-compounds

Azo-compounds are coloured because the -N=N- grouping attached to benzene rings absorbs visible light - the colour of the dye is what isn't absorbed.

The group responsible for the 'dye' colour is called a chromophore.

The particular colour depends on the substituents present in the benzene ring.


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7.10.7 The acid-base chemistry of aromatic amines

Reminder 1. Defining an aromatic (aryl) amine

In aromatic (aryl) amines, the amino/amine group is directly attached to the benzene ring,

 

Reminder 2. Effect of the benzene ring on the 'base' strength of aromatic amines

The amine group has a plus inductive effect (+I electron shift) which increases the electron density of the ring compared to benzene ('fuzzy' diagram below). 

The +I effect and delocalisation of the lone pair of electrons on the nitrogen reduces their availability to act as a proton acceptor (N: less δ-), so phenylamine and other aromatic amines are usually weaker bases than aliphatic amines.

delocalised electron system in phenylamine activating substituent 2 4 and 6 positions ortho and para directing orientation on substitution

 

Reminder 3. Here, the term weak base refers to one that only ionises (dissociates) to a small extent in aqueous solution.

 e.g. ammonia is a weak base in water: NH3(aq)  +  H2O(l)  == ~2% ==>  NH4+(aq)  +  OH-(aq)

A strong base like sodium hydroxide ionises completely:  NaOH(s)  +  aq  == 100%  ==> Na+(aq)  +  OH-(aq)

 

The equilibrium involved when a weak base B dissolves in water:

(i) :B(aq)  +  H2O(l)    BH+(aq)  +  OH-(aq)   (for any soluble base, note the lone pair on N)

(ii) :NH3(aq)  +  H2O(l)    NH4+(aq)  +  OH-(aq)   (for ammonia)

(iii) RNH2(aq)  +  H2O(l)    RNH3+(aq)  +  OH-(aq)   (for an organic base, R = alkyl or aryl)

From equation (iii) for a weak organic base, the ionisation constant, the equilibrium constant (Kb) for equilibrium (iii), is given by the expression (water is a subsumed constant):

Kb =

  [RNH3+(aq)] [OH(aq)]

 ––––––––––––––––––  mol dm-3

       [RNH2(aq)]

Like pH, the range of Kb is so great, it is often quoted on the logarithmic scale where

 pKb = -log10(Kb/mol dm-3)   and  Kb = 10-pKb

The stronger the base, the greater the Kb and the lower the pKb value.

More on Definition of a weak base, theory and examples of Kb, pKb, Kw weak base CALCULATIONS

In the data table below I've quoted both values and a few comments what affects the value of Ka.

Neutralisation equations - illustrating why relatively insoluble aromatic amines dissolve in acids

C6H5NH2(aq)  +  HCl(aq)   ===>  C6H5NH3+(aq)  +  Cl-(aq)

C6H5NH2(aq)  +  H+(aq)   ===>  C6H5NH3+(aq)   (the phenylammonium ion/cation)

From the salt solutions you can crystallise the ionic compounds e.g. and noting the name of the cation

Hydrochloric acid yields phenylammonium chloride:  C6H5NH3+Cl-

and sulfuric acid yields phenylammonium sulfate:  (C6H5NH3+)2SO42-

Name of aromatic amine Structure pKb Kb

moldm-3

Comment

Ammonia and a primary aliphatic amine are added for comparison purposes

ammonia NH3 4.75 1.78 x 10-5  
ethylamine CH3CH2NH2 3.27 5.37 x 10-4 Most primary aliphatic amines are of similar strength.
phenylamine structural formula phenylamine molecular structure 9.38 4.16 x 10-10 Aniline, colourless liquid. Primary aromatic amine. Solubility ~3.6/100g water. It is a toxic material (carcinogen) that can be absorbed through the skin.
2-methylphenylamine structural formula 2-methylphenylamine molecular structure methyl-2-phenylamine structural formula 3-methylphenylamine molecular structure methyl-3-phenylamine structural formula 4-methylphenylamine molecular structure methyl-4-phenylamine 9.62 2.40 x 10-10 Three positional structural isomers.

All slightly soluble in water

Primary aromatic amines of similar strength to phenylamine.

3-methylphenylamine 9.33 4.68 x 10-10
4-methylphenylamine 9.00 1.00 x 10-9
2-chlorophenylamine structural formula 2-chlorophenylamine molecular structure chloro-2-phenylamine structural formula 3-chlorophenylamine molecular structure chloro-3-phenylamine structural formula 4-chlorophenylamine molecular structure chloro-4-phenylamine 11.44 1.70 x 10-12 Three positional structural isomers.

Primary aromatic amines much weaker bases than phenylamine - the electron withdrawing effect on the N lone pair of electrons.

3-chlorophenylamine 10.56 2.75 x 10-11
4-chlorophenylamine 10.07 8.51 x 10-11
2-nitrophenylamine structural formula 2-nitrophenylamine molecular structure 2-nitroaniline structural formula 3-nitrophenylamine molecular structure 3-nitroaniline structural formula 4-nitrophenylamine molecular structure 4-nitroaniline 14.28 5.25 x 10-15 Three positional structural isomers.

Primary aromatic amines very much weaker bases than phenylamine - an even greater electron withdrawing effect of the NO2 on the N lone pair of electrons.

3-nitrophenylamine 11.55 2.82 x 10-12
4-nitrophenylamine 13.02 9.55 x 10-14
         

Comments on the above data table on primary aliphatic and aromatic amines and don't forget to appreciate the significance of the relative Kb and pKb values

(a) Comparing the base strength of ammonia and the primary aliphatic amine ethylamine

ionisation of ammonia in water Kb pKb values diagram balanced equation of ionization ammonium ion hydroxide ion

Equation 20A: As you can see from the Kb value, ammonia is a weak base, with only a few % of the molecules ionising to give an alkaline solution of ammonium ions and hydroxide ions (pH <7).

Note (i) the ammonium ion is the conjugate acid of the ammonia base.

(ii) Ammonium salts of strong acids (e.g. HCl, H2SO4) in aqueous solution are acidic because when you dissolve them in water the ammonium ion (the conjugate acid) protonates water, lowering the pH.

i.e. NH4+(aq)  +  H2O(l)    NH3(aq)  +  H3O+(aq)

 

ionisation of prmary aliphatic amine ethylamine in water Kb pKb values diagram balanced equation of ionization ethylammonium ion hydroxide ion

Equation 20B: As you can see from the low Kb value, ethylamine  is a weak base, with only a few % of the molecules ionising to give an alkaline solution of ethylammonium ions and hydroxide ions.

The ethyl alkyl group has a +I effect electron shift increasing the availability of the lone pair of electrons on the nitrogen atom of the amine group to accept a proton.

Therefore ethylamine is a stronger base than ammonia i.e. Kb(ethylamine)  >  Kb(ammonia)

Note (i) the ethylammonium ion is the conjugate acid of the ethylamine base.

(ii) Ethylammonium salts of strong acids (e.g. HCl, H2SO4) in aqueous solution are acidic because when you dissolve them in water the ethylammonium ion (the conjugate acid) protonates water, lowering the pH.

i.e. CH3CH2NH3+(aq)  +  H2O(l)    CH3CH2NH2(aq)  +  H3O+(aq)

 

(b) Comparing the base strength of ammonia and two aromatic amines

ionisation of primary aromatic aryl amine phenylamine in water Kb pKb values diagram balanced equation of ionization phenylammonium ion hydroxide ion

Equation 20C: The very low Kb value for phenylamine means it is a very weak base, with only a tiny fraction of the molecules ionising to give an alkaline solution of phenylammonium ions and hydroxide ions.

The benzene ring group has a -I effect electron shift decreasing the availability of the lone pair of electrons on the nitrogen atom of the amine group to accept a proton.

Therefore phenylamine is a weaker base than ammonia i.e.

Kb(ethylamine)  >  Kb(ammonia)  >  Kb(phenylamine)

Note (i) the phenylammonium ion is the conjugate acid of the phenylamine base.

(ii) Phenylammonium salts of strong acids (e.g. HCl, H2SO4) in aqueous solution are acidic because when you dissolve them in water the phenylammonium ion (the conjugate acid) protonates water, lowering the pH.

i.e. C6H5NH3+(aq)  +  H2O(l)    C6H5NH2(aq)  +  H3O+(aq)

 

ionisation of primary aromatic aryl amine 4-nitrophenylamine in water Kb pKb values diagram balanced equation of ionization 4-nitrophenylammonium ion hydroxide ion

Equation 20D: The extremely low Kb value for 4-nitrophenylamine means it is an extremely weak base, with only a minute fraction of the molecules ionising to give an alkaline solution of phenylammonium ions and hydroxide ions.

The benzene ring group plus the very electronegative nitro group has a big -I effect electron shift decreasing the availability of the lone pair of electrons on the nitrogen atom of the amine group to accept a proton.

Therefore 2-nitrophenylamine is even a much weaker base than phenylamine i.e.

Kb(ethylamine)  >  Kb(ammonia)  >  Kb(phenylamine)  >  Kb(4-nitrophenylamine)

Note (i) the 4-nitrophenylammonium ion is the conjugate acid of the 4-nitrophenylamine base.

(ii) 4-nitrophenylammonium salts of strong acids (e.g. HCl, H2SO4) in aqueous solution are acidic because when you dissolve them in water the phenylammonium ion (the conjugate acid) protonates water, lowering the pH.

i.e. O2NC6H4NH3+(aq)  +  H2O(l)    O2NC6H4NH2(aq)  +  H3O+(aq)

 

(c) More on conjugate acids and their 'acidic' nature

Below are another series of equations illustrating the formation of the conjugate acid from the protonation of the base.

To free an organic amine (aliphatic or aromatic) you add a strong alkali like sodium hydroxide which neutralises the conjugate acid and reverses the reaction.

The proportions of free base and conjugate acid present in the equilibrium are determined by the pH.

protonation of inorganic base ammonia in water diagram balanced equation of formation of conjugate acid ammonium ion cation

ammonia   ===>  ammonium ion

 

protonation of base primary aliphatic amine ethylamine in water diagram balanced equation of formation of conjugate acid ethylammonium ion cation

ethylamine  ===> ethylammonium ion

To free ethylamine: CH3CH2NH3+(aq)  +  OH-(aq)  ===>  CH3CH2NH2(aq)  +  H2O(l)

 

protonation of base primary aromatic aryl primary amine phenylamine in water diagram balanced equation of formation of conjugate acid phenylammonium ion cation

phenylamine  ===>  phenylammonium ion

To free phenylamine: C6H5NH3+(aq)  +  OH-(aq)  ===>  C6H5NH2(aq/l)  +  H2O(l)

The phenylammonium ion is sufficiently acidic to liberate carbon dioxide from sodium hydrogencarbonate solution e.g.

C6H5NH3+(aq)  +  HCO3-(aq)  ===>  C6H5NH2(aq/l)  +  H2O(l)  +  CO2(g)

 

protonation of base primary aromatic aryl primary amine 2-nitrophenylamine in water diagram balanced equation of formation of conjugate acid 2-nitrophenylammonium ion cation

4-nitrophenylamine  ===>  4-nitrophenylammonium ion, and to free the 4-mitrophenylamine:

 O2NC6H4NH3+(aq)  +  OH-(aq)  ===>  O2NC6H4NH2(aq/l)  +  H2O(l)

Again, the 4-nitrophenylammonium ion is more than sufficient acidic to liberate carbon dioxide from sodium hydrogencarbonate solution e.g.

C6H5NH3+(aq)  +  HCO3-(aq)  ===>  C6H5NH2(aq/l)  +  H2O(l)  +  CO2(g)

 

(d) Comparing methylphenylamines and chlorophenylamines with phenylamine

structural formula 2-methylphenylamine molecular structure methyl-2-phenylamine structural formula 3-methylphenylamine molecular structure methyl-3-phenylamine structural formula 4-methylphenylamine molecular structure methyl-4-phenylamine Phenylamine has a pKb of 9.38, the methylphenylamines have pKb values of 9.00 to 9.62, so the methyl substituent doesn't to have any great effect on the strength of these amines.

structural formula 2-chlorophenylamine molecular structure chloro-2-phenylamine structural formula 3-chlorophenylamine molecular structure chloro-3-phenylamine structural formula 4-chlorophenylamine molecular structure chloro-4-phenylamine Phenylamine has a pKb of 9.38, the chlorophenylamines have pKb values of 10.07 to 11.44, so the chloro substituent does have an effect on the strength of these amines.

The minus inductive effect (-I) electron shift of the electronegative chlorine atom makes the lone pair of non-bonding electrons on the nitrogen less available for protonation compared to phenylamine.

This makes them weaker bases, though the chlorine -I effect is not as much as that cause by the nitro group (NO2) discussed above with three very electronegative atoms in the group.

 

(f) The aminophenols (or hydroxyphenylamines) structural formula 2-aminophenol molecular structure structural formula 3-aminophenol molecular structure structural formula 4-aminophenol molecular structure are amphoteric and react with acids or bases depending on which functional group is active at a particular pH e.g.

(i) In acid media: H2NC6H4OH +  H3O+ ===>  +H3NC6H4OH  + H2O  (+ on nitrogen atom)

(ii) In alkaline media: H2NC6H4OH  +  OH-  ===>  H2NC6H4O-  +  H2O   (- on oxygen atom)


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