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Advanced Organic Chemistry: Physical & chemical properties of benzoic acid & derivatives

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 7.11 The physical and chemical properties of benzoic acid and some of its derivatives

R-COOH where R is aromatic and the COOH functional group directly attached to a benzene ring

Sub-index for this page

7.11.1 The naming, structure and physical properties of selected aromatic carboxylic acids

7.11.2 The synthesis and preparation of aromatic carboxylic acids

7.11.3 The structure of aromatic acids and strength as weak carboxylic acids

7.11.4 The reactions of benzoic acid (or any aromatic acid) with alkalis and carbonates/hydrogencarbonates

7.11.5 The reduction of aromatic acids to aliphatic primary alcohols e.g. benzoic acid

7.11.6 Aromatic acid chlorides - preparation, properties and reactions

7.11.7 Examples of electrophilic substitution in the benzene ring of benzoic acid

7.11.8 Esters of aromatic carboxylic acids - preparation, properties and reactions

7.11.9 Brief notes on aromatic amides - physical and chemical properties of benzamide

7.11.10 Brief notes on aromatic nitriles - physical and chemical properties of benzonitrile

7.11.11 Brief notes on aromatic aldehydes - benzaldehyde

For basic notes e.g. GCSE level see

Carboxylic acids - molecular structure, chemistry and uses

AND Esters, chemistry and uses including perfumes, solvents


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

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7.11.1 The structure and physical properties of selected aromatic carboxylic acids

Note: quite a few of the 'old' names are still used and accepted

The name ends in ...carboxylic acid or ...oic acid, in salts and esters the suffix is ...oate

The derivative names are similar for aliphatic carboxylic acids and their derivatives e.g.

acid chlorides -COCl, are named ...oyl chloride,  amides -CONH2, are named ...amide

and esters are named in the same way e.g. alkyl/aryl ...oate

Names of aromatic acid Structure or abbreviated formula

mpt/oC

Solubility g/100g water 25oC

Comments
Benzoic acid (c) doc b  (c) doc b structural formula sodium benzoate ionic molecular structure acid 122

salt 410

acid 0.29

salt 63

 

The simplest possible aromatic acid based on benzene and its salt, sodium benzoate, which is far more soluble in water
2-hydroxybenzoic acid structural formula 2-hydroxybenzoic acid molecular structure Salicylic acid structural formula 3-hydroxybenzoic acid molecular structure structural formula 4-hydroxybenzoic acid molecular structure 158 0.22 Salicylic acid.
3-hydroxybenzoic acid 202 0.72  
4-hydroxybenzoic acid 214 0.5  
2-methylbenzoic acid structural formula 2-methylbenzoic acid molecular structure  structural formula 3-methylbenzoic acid molecular structure  structural formula 4-methylbenzoic acid molecular structure 104 ~0.12  
3-methylbenzoic acid 112 <0.1  
4-methylbenzoic acid 180 very low  
2-chlorobenzoic acid molecular structure 2-chlorobenzoic acid structural formula  molecular structure 3-chlorobenzoic acid structural formula  molecular structure 4-chlorobenzoic acid structural formula 139 ~0.2  
3-chlorobenzoic acid 155 <0.1  
4-chlorobenzoic acid 238 <0.1  
2-nitrobenzoic acid molecular structure 2-nitrobenzoic acid structural formula advanced A level organic chemistry  molecular structure 3-nitrobenzoic acid structural formula advanced A level organic chemistry  molecular structure 4-nitrobenzoic acid structural formula advanced A level organic chemistry   147 0.68  
3-nitrobenzoic acid 140 0.3  
2-nitrobenzoic acid 237 <0.1  
2-aminobenzoic acid molecular structure 2-aminobenzoic acid structural formula advanced A level organic chemistry  molecular structure 3-aminobenzoic acid structural formula advanced A level organic chemistry  molecular structure 4-aminobenzoic acid structural formula advanced A level organic chemistry 147    
3-aminobenzoic acid 173 5<0.1  
4-aminobenzoic acid 188 ~0.6  
benzene-1,2-dicarboxylic acid structural formula benzene-1,2-dicarboxylic acid molecular structure  structural formula benzene-1,3-dicarboxylic acid molecular structure  structural formula benzene-1,4-dicarboxylic acid molecular structure 206 dec. ~0.6 Very low solubility in water
benzene-1,3-dicarboxylic acid 347 0.012 ~insoluble in water
benzene-1,4-dicarboxylic acid   0.0017 ~insoluble in water
         

Extra notes on the data table of aromatic carboxylic acids

(a) Most aromatic carboxylic acids are white crystalline solids, non are liquid at room temperature.

The instantaneous dipole - induced dipole force, plus permanent dipole - permanent dipole (including hydrogen bonding), ensures the intermolecular bonding forces are too great for any of these aromatic acids to be a liquid at room temperature - all solids.

hydrogen bonding in the dimer of benzoic acid in non-aqueous solvents and crystals advanced organic chemistry revision notes doc brown

Like aliphatic carboxylic acids, aromatic acids form 'dimers' via hydrogen bonding e.g. benzoic acid above.

It is not a true dimer in the sense that the two benzoic acid molecules are covalently bonded together, but the hydrogen bonding is strong enough to hold the two molecules together, not only in the crystalline state, but when dissolved in most non-aqueous solvents (e.g. benzene, hexane, trichloromethane) i.e. solvents other than water - in which benzoic acid is mainly bonded to water molecules (see (b) below).

The 'dimerisation' effectively doubles the size of the benzoic acid molecule and roughly doubles the instantaneous dipole - induced dipole intermolecular forces between the neighbouring dimers.

A greater kinetic energy is needed to melt/vapourise the 'dimer' compared to the 'monomer', hence the higher than expected melting points for aromatic carboxylic acids.

 

(b) Aromatic carboxylic acids usually have a relatively low solubility in water, but greater than arene hydrocarbons

(c) doc bIf we compare benzoic acid (left) with 2-phenylpropane (right). Both have a similar size, shape, molecular mass and numbers of electrons (64 and 66), so they will have similar instantaneous dipole - induced dipole forces. Methylbenzene is non-polar and only weakly bonds with water and not sufficient to disrupt the hydrogen bonds of water, so is ~insoluble in water because there are no permanent dipole - permanent dipole attractions possible, only weak instantaneous dipole - induced dipole attractive forces. No hydrogen bonding possible compared to benzoic acid.

In organic solvents benzoic acid usually exists mainly as the dimer molecule e.g. you have the equilibrium.

2C6H5COOH  (C6H5COOH)2

Kdimer = [(C6H5COOH)2] / [C6H5COOH]2 = 167 (benzene)  and 78 (trichloromethane) mol-1dm3

These association equilibrium constant values (from the internet) show the benzoic acid dimer predominates in organic solvents, BUT ....

.... in aqueous solution, carboxylic acids can also hydrogen bond directly with water - which accounts for why the lower members are so soluble in water (compared to arene hydrocarbons) and benzoic acid mainly exists as the hydrated monomer in aqueous solution (diagram below).

hydrogen bonding solvation of benzoic acid between water molecules in aqueous solution OR dimer formation solid benzoic acid or dissolved in organic solvent

The hydrogen bonding solvation of benzoic acid in aqueous solution OR dimer formation liquid ethanoic acid or dissolved in an organic solvent

However, benzoic acid and other aromatic carboxylic acids have a hydrophobic C6H5- benzene ring that disrupts the hydrogen bonding between water molecules, but also a hydrophilic -COOH group.

Although water - water hydrogen bonds are disrupted (Oδ--Hδ+ǁǁǁ:Oδ-), some new carboxylic acid - water hydrogen bonds are formed e.g. (C-Oδ--Hδ+ǁǁǁ:Oδ--Hδ+) OR (H-Oδ--Hδ+ǁǁǁ:Oδ-=Cδ+) partly compensate for this, allowing limited solvation with low solubility.

(ǁǁǁ hydrogen bonds in diagram above, allowing some solvation - but low solubility)

Although benzoic acid is only slightly soluble in cold water, but is much more soluble in hot water - in fact its a good solvent to purify it by recrystallisation.

C6H5COOH(s)  +  aq    C6H5COOH(aq)

The solvation of benzoic acid when dissolving in water is endothermic, therefore increase in temperature drives the equilibrium to the right (energy absorbing direction, Le Chatelier's Rule).

The presence of a 2nd polar group in the benzene ring e.g. amino -NH2 or hydroxy -OH can increase the solubility in water, but not by a great deal despite the scope for more permanent dipole - permanent dipole (including hydrogen bonding) intermolecular attractions - see the solubilities of hydroxybenzoic acids and aminobenzoic acids.

BUT things are complicated, solubility is a complex subject e.g. two of the benzendicarboxylic acids are almost insoluble, despite the fact of having two hydrogen bonding hydrophilic groups attached to the benzene ring!

 

(c) Some aromatic acids exhibit intramolecular hydrogen bonding

e.g. 2-hydroxybenzoic acid, 2-aminobenzoic acid and 2-nitrobenzoic acid.

See 2-nitrophenol on my phenols page

 

 


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7.11.2 The synthesis and preparation of carboxylic acids

Oxidation of aromatic hydrocarbons to make aromatic acids

Acidified potassium dichromate(VI) might oxidise alkyl benzene compounds to benzoic acid, but if it does, it will be much slower than with the alkaline manganate(VII) method described below - NaOH/KMnO4 is a much stronger oxidising agent.

preparation of benzoic acid reflux diagram oxidation of methylbenzene dimethylbenzenes to aromatic carboxylic acid potassium manganate(VII)sodium hydroxide reaction conditions equations methylbenzene to benzoic acid dimethybenzene to benzenedicarboxylic acidsOverall change is represented by the equations for the conversion:

(c) doc b  == oxidation ==>  (c) doc b

C6H5CH3 + 3[O] ===> C6H5COOH + H2O

Aromatic are not easily oxidised and longish reflux times are necessary (illustrated, fig. PD2 right).

Hydrocarbons are difficult to oxidise with typical organic oxidising agents compared to compounds like alcohols.

However, aromatic hydrocarbons with an alkyl side chain can be oxidised with strong reagents such as aqueous potassium manganate(VII)/sodium hydroxide.

Whatever the length of the alkyl group on a benzene ring it gets whittled down to carbon of the carboxylic acid group e.g. propylbenzene ends up as benzoic acid, the same product obtained by oxidising the shorter methylbenzene.

The more stable aromatic benzene ring is left intact.

The overall process for producing benzoic acid from methylbenzene can be summarised ..

C6H5CH3 (l) + NaOH(aq) + 3[O] ===> C6H5COO-Na+ (aq) + 2H2O(l)

After removing the excess KMnO4/MnO2 with a reducing agent, the weak acid, benzoic acid, is freed from its sodium salt  by adding dilute hydrochloric acid.

C6H5COO-(aq) + H+(aq) ===> C6H5COOH

and you can oxidise the three isomeric dimethylbenzenes to make the three isomeric aromatic dicarboxylic acids via the same procedure:

(i) Synthesis of benzene-1,2-dicarboxylic acid from 1,2-dimethylbenzene

ea02 +  6[O]   1847 +  2H2O

(ii) benzene-1,3-dicarboxylic acid from 1,3-dimethylbenzene

(c) doc b +  6[O]  (c) doc b +  2H2O

(iii) benzene-1,4-dicarboxylic acid from 1,2-dimethylbenzene

(c) doc b +  6[O]  1847 +  2H2O

 

In the chemical industry, many of these oxidations can be done directly with oxygen at raised temperatures and suitable catalyst.

 

See also hydrolysis of nitriles, esters and amides to yield carboxylic acids

(These reactions also have a brief mention on this page too)


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7.11.3 The structure of aromatic acids and behaviour as weak carboxylic acids

general structure carboxylic acid corresponding carboxylate ion resonance hybrids delocalised O-C-O bond system advanced organic chemistry

See Section 6.4 The acidic nature of carboxylic acids for a more detailed discussion based on this diagram.

electron density picture of benzoate ion dispersion of charge planar arrangement of bonds in carboxylate group carboxy structure

The R-C-O and O-C-O bond angles are 120o - a trigonal planar sigma bond arrangement, plus the delocalised pi bond system in which two electrons reside in the pi orbitals above and below the plane of the O-C-O bonds.

The result of the possible 'resonance hybrids' is that the theoretical pi bond pair of electrons from the C=O group actually form a delocalised system across the two C-O bonds (blue arc in the diagram above).

This means both of the C-O bonds in the carboxylate ion are identical.

The diagram above gives an impression of how the delocalised pi electrons are distributed around the benzene ring and connected to the delocalised electrons of the two C-O bonds - illustrating the dispersion, to some extent, of the negative charge of the carboxylate group.

For a 'higher level' approach, the diagram of the resonance structures of the benzoate ion illustrate how the negative charge of the anion is partially further distributed around the benzene ring. This further stabilisation of the benzoate ion explains why benzoic acid is a stronger acid than phenol.

For more on the last point see chemical properties of phenol

diagram resonance structures of the bezoate ion resonance hybrid stablises stabilizes the anion Kekule style molecular ionic structure advanced oranic chemistry

 

Expressing the ionisation, both chemically and mathematically

In aqueous solution, carboxylic acids are weak acids, that is only ionised or dissociated to a small extent.

RCOOH(aq)  +  H2O(l)    H3O+(aq)  +  RCOO-(aq)

RCOOH(aq)    H+(aq)  +  RCOO-(aq)   (useful when doing pH and Ka calculations)

e.g. (c) doc b (aq)   +  H2O(l)    structural formula benzoate ion molecular ionic structure (aq)   +  H3O+(aq)

The ionisation constant (dissociation constant, acidity constant), that is the equilibrium constant (Ka) for the above equilibrium, is given by the expression:

Ka =

  [H+(aq)] [RCOO(aq)]

   mol dm-3

       [RCOOH(aq)]

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

 pKa = -log10(Ka/mol dm-3)   and  Ka = 10-pKa

The stronger the acid, the greater the Ka and the lower the pKa value.

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

See equilibrium calculations section 5c for calculations involving pH, Ka and pKa

 

Dissociation of aromatic dicarboxylic acids in aqueous solution

The ionization of an aromatic dicarboxylic acid requires two equations e.g. in 'shorthand' for benzenedicarboxylate acids

(i)  HOOC-C6H4-COOH(aq)  HOOC-C6H4-COO-(aq)  +  H+(aq)

(ii)  HOOC-CH2-COO-(aq)  -OOC-C6H4-COO-(aq)  +  H+(aq)

Although they are generally weak acids, the range of ionisation is quite wide and greatly affected by substituent groups in the 'alky' or 'aryl' hydrocarbon part of the molecule, so, like with pH, a logarithmic (base 10) scale is used ...

... for examples see the relative strengths of carboxylic acids in the data table below.

diagram of carboxylic acid what influences the strength of the acid electron shifts minus inductive -I effect plus inductive +I effects advanced organic chemistry

If the R group contains electronegative, electron cloud withdrawing atoms, the electron shift affects the δ-O-Hδ+ bond (-I induction effect) and promotes the donation of a proton to a base (left of bottom diagram).

On the other hand, other groups like alkyl groups, can have a +I inductive electron shift effect and make the proton less available to a base.

Reminders of mathematical connections:

Ka = [H+(aq)] [RCOO-(aq)] / [RCOOH(aq)]

(R = H, alkyl, aryl, the latter may be substituted with other atoms)

pKa = - log10(Ka/moldm-3)   and  Ka = 10-pKa

Like the pH scale, a base 10 logarithmic scale means each unit of pH is equal to a factor of 10 in the hydrogen ion concentration [H+(aq)] and each unit of pKa represents a factor of 10 in the value of the ionization constant (dissociation constant) in aqueous solution.

Note: For dicarboxylic acids or tricarboxylic acids, the ionisation decreases from the 1st, 2nd ionisation etc.

so that:   Ka1 > Ka2  etc.  and  pKa1 < pKa2 etc.

Aromatic acid Abbreviated formula pKa Ka/mol dm-3 Comments
Benzoic acid C6H5COOH 4.20 6.31 x 10-5 pH of 0.10 moldm-3 solution is 2.66
(CH3COOH) (CH3COOH) 4.76 (1.74 x 10-5) (Aliphatic acid for comparison)

(pH of 0.10 moldm-3 solution is 2.88)

2-methylbenzoic acid structural formula 2-methylbenzoic acid molecular structure  structural formula 3-methylbenzoic acid molecular structure  structural formula 4-methylbenzoic acid molecular structure 3.91 1.23 x 10-4 Methyl group slightly increases strength.
3-methylbenzoic acid 4.24 5.75 x 10-5 Little effect on strength.
4-methylbenzoic acid 4.34 4.57 x 10-5 Decreases strength.
2-hydroxybenzoic acid structural formula 2-hydroxybenzoic acid molecular structure Salicylic acid structural formula 3-hydroxybenzoic acid molecular structure structural formula 4-hydroxybenzoic acid molecular structure

Note the extra acidic phenol group, but it's pKa will be much higher, a much smaller Ka.

2.99 1.02 x 10-3 The presence of the extra electronegative oxygen atom (-I effect), increases the strength of the 2- and 3- acid compared to benzene.

The effect decreases as the electronegative OH is further from the COOH (same for -Cl and - NO2 substituted acids below.

3-hydroxybenzoic acid 4.08 8.32 x 10-5
4-hydroxybenzoic acid 4.58 2.63 x 10-5
2-chlorobenzoic acid molecular structure 2-chlorobenzoic acid structural formula  molecular structure 3-chlorobenzoic acid structural formula  molecular structure 4-chlorobenzoic acid structural formula 2.94 1.48 x 10-3 The presence of the electronegative chlorine atom (-I effect), increases the strength of the acid compared to benzene.

The effect decreases as the electronegative Cl is further from the COOH (same for NO2 acids below)

3-chlorobenzoic acid 3.83 1.48 x 10-4
4-chlorobenzoic acid 3.99 1.02 x 10-4
2-nitrobenzoic acid molecular structure 2-nitrobenzoic acid structural formula advanced A level organic chemistry  molecular structure 3-nitrobenzoic acid structural formula advanced A level organic chemistry  molecular structure 4-nitrobenzoic acid structural formula advanced A level organic chemistry   2.17 6.76 x 10-3 The presence of the strongly electronegative nitro group -NO2 (-I effect) increases the strength of the acid compared to benzene, more so than the a single chlorine atom.
3-nitrobenzoic acid 3.45 3.55 x 10-4
4-nitrobenzoic acid 3.43 3.72 x 10-4
benzene-1,2-dicarboxylic acid structural formula benzene-1,2-dicarboxylic acid molecular structure     pKa1 = 2.98

pKa2 = 5.41

Ka1 = 1.05 x 10-3

Ka2 = 3.89 x 10-6

Two ionisations possible to yield hydrated protons H3O+.

 Note the order  Ka1 > Ka2  and  pKa1 < pKa2 for dibasic acids.

The extra electronegative group ensures the 1st ionisation is more so than in benzene, but the 2nd ionisation constant is smaller than for benzoic acid.

benzene-1,3-dicarboxylic acid structural formula benzene-1,3-dicarboxylic acid molecular structure pKa1 = 3.46

pKa2 = 4.60

Ka1 = 3.47 x 10-4

Ka2 = 2.51 x 10-5

benzene-1,4-dicarboxylic acid structural formula benzene-1,4-dicarboxylic acid molecular structure pKa1 = 3.51

pKa2 = 4.82

Ka1 = 3.09 x 10-4

Ka2 = 1.51 x 10-5

         

 


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7.11.4 The reactions of aromatic acids with alkalis and carbonates/hydrogencarbonate

(a) Reaction with alkalis

(i) benzoic acid  +  sodium hydroxide  ===>  sodium benzoate  +  water

(c) doc b (aq)  +  NaOH(aq)  ===>  (aq)  +  H2O(l)

This reaction ensures benzoic acid is much more soluble in alkali than water because of the salt formation - ions much readily solvated in water than the original acid - see Aspirin next.

 

(ii) Making soluble aspirin

from: (aq)  +   NaOH(aq)  ===> (aq)  +  H2O(l)

sodium 2-acetoxybenzoate (sodium acetylsalicylate)

A couple of more complicated examples - using simplified equations

 

(iii) Neutralisation of aromatic dicarboxylic acids (dibasic acids)

This will be a 2 stage neutralisation and both salts can be crystallised from aqueous solution

HOOC-C6H4-COOH(aq)  +  NaOH(aq)  ===> HOOC-C6H4-COO-Na+(aq)  +  H2O(l)

HOOC-C6H4-COO-Na+(aq)  +  NaOH(aq)  ===> Na+-OOC-C6H4-COO-Na+(aq)  +  H2O(aq)

 

(iv) The neutralisation of hydroxybenzoic acids

Again, there is a two stage neutralisation, but the carboxylic acid group will be neutralised first, because the phenol group is a weaker acid - but strong enough to react will alkali, but contrast with reaction with carbonates in (b).

HOC-C6H4-COOH(aq)  +  NaOH(aq)  ===> HOC-C6H4-COO-Na+(aq)  +  H2O(l)

HOC-C6H4-COO-Na+(aq)  +  NaOH(aq)  ===> Na+-OC-C6H4-COO-Na+(aq)  +  H2O(aq)

 

(b) The reaction of aromatic carboxylic acids with carbonates and hydrogencarbonates

(i) Benzoic acid will readily react with sodium carbonate solution to give sodium benzoate and carbon dioxide gas.

2C6H5COOH(s/aq)  +  Na2CO3(aq)  ===>  2C6H5COO-Na+(aq)  +  H2O(l)  +  CO2(g)

 

(ii) Watch out with hydroxybenzoic acids - the phenol group is not strongly acidic enough to liberate carbon dioxide from sodium hydrogencarbonate, so the equations can only be a single step neutralisation reaction, as with benzoic acid itself.

C6H5COOH(s/aq)  +  NaHCO3(aq)  ===>  C6H5COO-Na+(aq)  +  H2O(l)  +  CO2(g)

HOC6H4COOH(s/aq)  +  NaHCO3(aq)  ===>  HOC6H4COO-Na+(aq)  +  H2O(l)  +  CO2(g)

The salts formed are sodium benzoate and sodium 2-/3-/4-hydroxybenzoate


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7.11.5 The reduction of aromatic acids

Sodium tetrahydridoborate(III) NaBH4, (sodium borohydride), is not a powerful enough reducing agent to reduce carboxylic acids.

Lithium tetrahydridoaluminate(III), LiAlH4, (lithium aluminium tetrahydride), is a much more powerful reducing agent than NaBH4, and in a DRY ether solvent (ethoxyethane), readily reduces carboxylic acids to primary alcohols.

The LiAlH4 effectively releases a hydride ion, :H-, a powerful nucleophile - electron pair donor, which can attacks the δ+ carbon of polarised carbonyl bond δ+C=Oδ-. (electronegativity of O > C).

The reaction must be carried with a dry solvent because LiAlH4 reacts rapidly with water (and ethanol too).

The reaction is complex and goes through various stages and can be summarised as:

RCOOH  +  4[H]  ===>  RCH2OH  +  H2O  (R = H, alkyl or aryl)

The initial product must then be hydrolysed with water to release the primary alcohol.

e.g. benzoic acid, an aromatic carboxylic acid, is reduced to phenylmethanol, a primary aliphatic alcohol:

C6H5COOH + 4[H] ==> C6H5CH2OH + H2O

(c) doc b  +  4[H]  ===>  (c) doc b  +  H2O

 


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7.11.6 Aromatic acid chlorides - preparation, properties and reactions

(a) Preparation with thionyl chloride from benzoic acid

RCOOH  +  SOCl2  ===> RCOCl  +  SO2  +  HCl

(c) doc b  +  SOCl2  ===>  (c) doc b   +  SO2  +  HCl

Benzoyl chloride is a colourless liquid, boiling point 197oC

This method has the advantage of producing gaseous waste products and just leaving the acid chloride behind for further purification.

(b) Hydrolysis by water

Aromatic acid chlorides are more table to nucleophilic attack (e.g. from water), but they will slowly hydrolyse in contact with water e.g. benzoyl chloride slowly hydrolyses to benzoic acid.

 (c) doc b  +  H2O  ===>  (c) doc b  +  HCl

benzoyl chloride  + water ===> /benzoic acid + hydrogen chloride (fuming in damp air)

or  C6H5COCl(l)  +  2H2O(l)  ===>  C6H5COOH(aq)  +  H3O+(aq)  +  Cl-(aq)

 

(c) The reaction of aromatic acid chlorides with phenols

The acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic.

However, phenols may require the presence of aqueous sodium hydroxide to facilitate the reaction, especially if the acyl chloride itself is itself a less reactive (than aliphatic) aromatic e.g. C6H5COCl.

The alkali generates a negative phenoxide ion (e.g. C6H5O from phenol C6H5OH), which is a more powerful nucleophile than the original neutral phenol molecule.

benzoyl chloride  +  phenol  ===>  phenyl benzoate  +  hydrochloric acid

(c) doc b  +  (c) doc b  ===>  (c) doc b  +  H+  +  Cl-

Notes:

Phenols are distinguished from alcohols by having the OH hydroxy group directly to the benzene ring.

You can use aqueous conditions because benzoyl chloride is more stable in water than the aliphatic acid chlorides like ethanoyl chloride.

If you shake benzoyl chloride with phenol dissolved in sodium hydroxide, you get an immediate white precipitate of the ester phenyl benzoate.

The reaction is particularly fast because the sodium hydroxide reacts with phenol (a very weak acid) to form the negative phenate ion - a much more powerful nucleophile than a neutral alcohol OR water molecules. The structure of sodium phenoxide is shown on the left.

 

(d) Amide formation from conc. ammonia solution or amine

 (i) benzoyl chloride  +  ammonia  ===>  benzamide  +  hydrogen chloride

(c) doc b +  2NH3  ===>  (c) doc b +  NH4Cl

This equation illustrates the formation of the primary aromatic amide, benzamide

 

(ii)  benzoyl chloride  +  phenylamine  ===> N-phenylbenzamide  + hydrogen chloride

(c) doc b  +  (c) doc b  ===>  (c) doc b   +  HCl

This equation illustrates the formation of the secondary aromatic amide, N-phenylbenzamide

 

(e) The reduction of acid/acyl chlorides - conversion to primary alcohol

Acyl/acid chlorides, like carboxylic acids esters, are reduced by the powerful reducing agent lithium tetrahydridoaluminate(III), LiAlH4, giving the corresponding primary alcohol.

LiAlH4 reacts with water and ethanol, therefore the reaction must be carried out in a dry inert solvent e.g. ethoxyethane ('ether') and NOT water or ethanol.

The general equation is:

RCOCl  +  4[H]  ===>  RCH2OH  +  HCl

The LiAlH4 effectively generates the equivalent of a hydride ion (:H-) which is a powerful nucleophile - lone pair of electrons donor.

e.g. benzoyl chloride  +  hydrogen  ===>  phenylmethanol ('benzyl alcohol')  + hydrogen chloride

(c) doc b  +  4[H]  ===>  (c) doc b   +  HCl

 

(f) The reaction of acyl/acid chlorides with aromatic hydrocarbons - ketone formation

Acid chlorides will react with aromatic hydrocarbons like benzene and methylbenzene to form ketones.

This is an example of a electrophilic substitution Friedel-Crafts acylation reaction

The reaction must be carried out in dry conditions using an aluminium chloride catalyst.

You reflux the mixture of the acid chloride, aromatic hydrocarbon and aluminium chloride in a fume cupboard.

Fumes of hydrogen chloride are given off. e.g.

(ii) benzene  +  benzoyl chloride  == AlCl3  ==>  diphenylmethanone  + hydrogen chloride

(c) doc b  +  (c) doc b ===>  (c) doc b  +  HCl

The product is also called 'diphenyl ketone'.  You can describe this product as a completely aromatic ketone.

 


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7.11.7 Examples of electrophilic substitution in the benzene ring of benzoic acid

electron pi orbital map of benzoic acid showing reduced electron density in the benzene ring less so at 3 and 5 positions favoured orientation of substitution

If the carbon atom that is bonded by a sigma bond to the benzene ring also has a pi bond (e.g. >C-C=O), the electron density of the ring is reduced and substitution is more difficult, but less so in the 3 and 5 positions - illustrated in the 'electron density' picture above.

The same orientation argument is backed up by considering the secondary resonance structures on the diagram below. The three on the right show the electron density is more reduced on carbon atoms 2, 4 and 6 of the benzene ring.

resonance structures of benzoic acid resonance hybrid between benzene ring and carboxylic acid functional group

Therefore the main product in electrophilic substitution of benzoic acid is the 3- ....benzoic acid if one more substituent is placed in the benzene ring of benzoic acid - see the three examples described below.

 

See also 7.14 Explaining orientation of products of a 2nd substituent into a monosubstituted benzene derivative

 

Examples of electrophilic substitution in the benzene ring of  benzoic acid

 

(a) The ring halogenation of aromatic benzoic acid

You can replace the hydrogen atoms in the benzene ring using chlorine and a catalyst such as aluminium chloride (AlCl3) or iron(III) chloride (FeCl3).

(c) doc b  +  Cl2  == AlCl3 or FeCl3 ==>  2-chlorobenzoic acid structural formula  +  HCl

The principal product is 3-chlorobenzoic acid, with much smaller amounts of 2-chlorobenzoic acid and 4-chlorobenzoic acid.

Note the major product is substitution at the 3 position of the benzene ring.

 

You can brominate i.e. substitute bromine into the benzene ring using bromine and a catalyst of aluminium bromide (AlBr3)  or iron(III) bromide (FeBr3) to yield mainly 3-bromobenzoic acid - equation as above, just swap Br for Cl.

Note again, that the major product is substitution at the 3 position of the benzene ring.

 

(b) Nitration with a mixture of concentrated nitric and sulfuric acids

(c) doc b  +  HNO3  ===>  3-nitrobenzoic acid structural formula advanced A level organic chemistry  +  H2O

The aromatic acid (benzoic acid) is heated with a mixture of concentrated nitric and sulfuric acid.

The principal product is 3-nitrobenzoic acid (78%), with much smaller amounts of 2-nitrobenzoic acid (22%) and 4-nitrobenzoic acid (2%).

Again, the major product is substitution at the 3 position of the benzene ring.


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7.11.8 Esters of aromatic carboxylic acids - preparation, properties and reactions

Brief descriptions of the methods with equations

Preparation of methyl benzoate ethyl benzoate refux condenser flask alcohol plus benzoic acid methanol ethanol conc. sulfuric acid catalyst(a) Esters from carboxylic acid plus alcohol using an acid catalyst

Concentrated sulphuric acid acts as a catalyst in this reaction.

(i) Esterification using an acid catalyst

Refluxing benzoic acid with methanol in the presence of conc. sulfuric acid yields the ester methyl benzoate.

General word equation for esterification:

carboxylic acid  +  alcohol  == acid catalyst ==>  ester  +  water

e.g. benzoic acid  +  methanol    methyl benzoate  +  water

(c) doc b + CH3OH (c) doc b + H2O

sometimes more simply written as

C6H5COOH  +  CH3OH    C6H5COOCH3 + H2O

The reaction is reversible and the mixture reaches equilibrium, and only about 2/3rds of the carboxylic acid and alcohol have been converted to the ester.

Without a strong acid catalyst e.g. conc. sulfuric acid, the reaction is very slow and the mixture is heated to further increase the rate of reaction (see details of the method below).

 

(ii) Esterification reaction between an acid chloride and an alcohol/phenol

The acyl chloride and alcohol usually readily react at room temperature, especially if both are aliphatic.

Examples of nucleophilic addition of an alcohol to acid/acyl chlorides, followed by elimination to give the ester and hydrogen chloride.

(c) doc b  +  CH3OH  ===>  (c) doc b  +  HCl

Yields methyl benzoate

(c) doc b  +  (c) doc b  ===>  (c) doc b  +  H+  +  Cl-

Yields phenyl benzoate, rapid if sodium hydroxide added to the mixture - phenoxide ion is a stronger nucleophile

Phenyl benzoate C6H5COOC6H5 is a white solid, insoluble in water, melting point 71oC and boiling point 314oC.

 

(b) Hydrolysis of an ester with acid or alkali - the opposite of esterification

Esters can be hydrolysed by refluxing with strong mineral acids or alkali, it is very slow with pure water.

(i) Hydrolysis of an ester with dilute hydrochloric or sulfuric acid

RCOOR'  +  H2O   RCOOH  +  R'OH

This reaction cannot go to completion because it is a reversible reaction, despite the use of a catalyst and heating the mixture under reflux.

You can go to a high % yield to the right using an aqueous solution with lots of excess water.

e.g. (c) doc b +  H2O  ==  H+ ==> (c) doc b  +  CH3CH2OH

 

(ii) Hydrolysis of an ester with aqueous sodium hydroxide

Hydrolysing an ester with strong alkali e.g. aqueous or ethanolic sodium/potassium hydroxide is called saponification, i.e. its a specific name for a particular type of hydrolysis reaction.

RCOOR'  +  NaOH ===>  RCOONa  +  R'OH

RCOOR'  +  OH- ===>  RCOO-  +  R'OH

e.g. (c) doc b +  NaOH  ===>   +  CH3CH2OH

or (c) doc b +   OH-  ===>   +  CH3CH2OH

This reaction is not reversible and goes to 100% completion forming the sodium salt of the acid.

The carboxylic acid is freed by adding excess of a strong hydrochloric or sulfuric mineral acid.

e.g.   +  H+  ===> (c) doc b

 


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7.11.9 Brief notes on aromatic amides - physical and chemical properties of benzamide

Benzamide amide of the parent benzoic acid

Aromatic amide -CONH2 group directly attached to the benzene ring, slightly soluble in water.

C6H5CONH2 : melting point 132oC; boiling point 290oC; both quite high due to intramolecular force of hydrogen bonding via C=O and -NH2 groups.

 

(a) Preparation of benzamide

Reaction between benzoyl chloride and concentrated ammonia solution.

C6H5COCl +  2NH3  ===>  C6H5CONH2  +  NH4Cl

 

(b) Hydrolysis of benzamide

(i) Refluxing with strong mineral acid e.g. hydrochloric acid yielding benzoic acid and ammonium chloride

C6H5CONH2  +  H2O  +  H+  ===>  C6H5COOH  +  NH4+

(ii) Refluxing with strong alkali e.g. sodium hydroxide yielding sodium benzoate and ammonia

C6H5CONH2  +  OH-  ===>  C6H5COO-  +  NH3

You then add strong mineral acid to free the weak benzoic acid.

C6H5COO-  +  H+  ===>  C6H5COOH

 

need x-ref to Kevlar and Paracetamol


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7.11.10 Brief notes on aromatic nitriles - physical and chemical properties of benzonitrile

Aromatic nitriles e.g. benzonitrile

Benzonitrile, C6H5CN, is a colourless liquid at room temperature, mpt. 13oC and bpt. 191oC

An aromatic nitrile has the functional group -C≡N (abbrev. -CN) directly attached to the benzene ring.

Reaction (a) Reduction

Sodium tetrahydridoborate(III), NaBH4 (sodium borohydride), is not a powerful enough reducing agent to reduce nitriles to primary amines.

LiAlH4 is a more powerful reducing agent than NaBH4 and reacts violently with water (and reacts with ethanol too), so the reaction must be carried out in an inert non-aqueous solvent like dry ethoxyethane ('ether').

The reduction reaction of nitriles can be summarised as:

RCtripbondN + 4[H] ===> RCH2NH2 (R = H, alkyl or aryl)

e.g. + 4[H] ===> (c) doc b

The product benzylamine, C7H9N, (c) doc b an aliphatic primary amine, NOT aromatic, compare with one of its isomers 2-methylphenylamine

 

Reaction (b) The hydrolysis of an aromatic nitriles

 If the nitrile is refluxed with dilute hydrochloric/sulfuric acid or sodium hydroxide (strong base - alkali) the corresponding carboxylic acid or its sodium salt is formed.

The hydrolysis with pure water is to slow, but the reaction is speeded up by a strong acid or strong alkali.

Strictly speaking all the reactants and products should be suffixed by (aq)

 

(c) doc b (i) Equations for the dilute mineral acid hydrolysis of a nitrile to give the free acid

In this case converting benzonitrile to benzoic acid.

+  2H2O  +  H+  ===> (c) doc b  +  NH4+

Here the free acid and an ammonium ion are formed.

 

(ii) Equations for the dilute mineral acid hydrolysis of a nitrile to give the sodium salt of the  acid

In this case converting benzonitrile to sodium benzoate.

+  2H2O  +  OH-  ===>   +  NH3

You then add strong mineral acid to free the benzoic acid from the sodium salt solution

+  OH+  ===> (c) doc b   

 


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7.11.11 Brief notes on aromatic aldehydes

Benzaldehyde is the simplest aromatic aldehyde where the -CHO aldehyde functional group is directly attached to the benzene ring.

At room temperature it is a colourless liquid, boiling point 179oC, insoluble in water and has a strong smell of almonds and is synthetically produced as a food flavouring and a fragrance component in perfumes.

Note the oxidation sequence

(c) doc b  == [O] ==>  (c) doc b  == [O] ==>  (c) doc b

Phenylmethanol (aliphatic primary alcohol)  ==> benzaldehyde (aromatic aldehyde)  ==> benzoic acid (aromatic carboxylic acid)

You normally come across disproportionation in the redox reactions of transition metal ion chemistry.

The Cannizzaro Reaction (shown below) is an organic example of disproportionation involving the three molecules above.

2 (c) doc b +  OH-  ===> (c) doc b  + 

When benzaldehyde is mixed with a strong base like potassium hydroxide, one molecule is reduced to phenylmethanol and another is oxidised to benzoic acid.

The benzoic acid can be freed by adding hydrochloric acid, filtered off and recrystallised from hot water.

The phenylmethanol can be extracted with ether and purified by fractional distillation.


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