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Advanced Organic Chemistry: Aromatic aldehydes/ketones: preparation, properties, uses

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 electrophilic substitution in benzene and methylbenzene to yield aromatic ketones and their physical properties

Part 7.8 Electrophilic substitution - acylation of arenes: benzene, methylbenzene and naphthalene (Friedel-Crafts reactions), properties & uses of aromatic ketones & aldehydes

Sub-index for this page

7.8.1 Preparation of aromatic ketones - reagents, conditions, equations

7.8.2 The electrophilic substitution mechanism for the Friedel-Crafts acylation of arenes

7.8.3 The physical properties of aromatic aldehydes and aromatic ketones

7.8.4 The chemical reactions of aromatic aldehydes and aromatic ketones

7.8.5 The uses of aromatic aldehydes and aromatic ketones

See also Part 5 Index of ALL My revision notes on aldehydes and ketones


INDEX of AROMATIC CHEMISTRY NOTES

All Advanced A Level Organic Chemistry Notes

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7.8.1 Preparation of aromatic ketones - reagents, conditions, equations

A Friedel-Crafts synthesis of aromatic ketones.

The arene (e.g. benzene or methylbenzene) is refluxed with an acid chloride and anhydrous aluminium chloride catalyst AlCl3 and an aromatic ketone is formed.

Aluminium bromide AlBr3, Iron(III) chloride FeCl3 and iron(III) bromide FeBr3 can also be used as catalysts.

You can think of the catalyst as an acyl group carrier.

Despite their catalytic status, these compounds are not easy to recycle, and relatively high temperatures may be required, so the search is on to find much 'greener' catalysts which can be more readily recycled and operate at a lower temperature by offering a lower activation energy mechanistic route.

Examples of aromatic Friedel Crafts acylation substitution reactions

(i)  +    ===>  +  HCl

benzene + ethanoyl chloride ===> 1-phenylethanone + hydrogen chloride

 

(ii)    +   ===>  + HCl

benzene + benzoyl chloride ==> diphenylmethanone + hydrogen chloride

for R = H, benzene: C6H6  +  R'COCl  ===> C6H5COR'  +  HCl

 

(iii) The acylation of methylbenzene e.g. with ethanoyl chloride gives three possible isomeric aromatic ketones.

(b) (c) doc b  +     ===> structural formula 1-(2-methylphenyl)ethanone 2-methylacetophenone molecular structure of molecule structural formula 1-(3-methylphenyl)ethanone 3-methylacetophenone molecular structure of molecule structural formula 1-(4-methylphenyl)ethanone 4-methylacetophenone molecular structure of molecule  +  HCl

For acylation of methylbenzene typical yields are 11%, 4% and 85% for substitution at the 2, 3 and 4 positions (from left to right).

The compounds are called: 1-(2/3/4-methylphenyl)ethanone (2/3/4-methylacetophenone)

Little, if any, further substitution takes place because the ketone group deactivates the benzene ring.


The acylation of naphthalene another Friedel-Crafts reaction

Friedel-Crafts reaction naphthalene readily reacts with ethanoyl chloride (acetyl chloride) to yield mixture of 1-acetylnaphthalene and 2-acetylnaphthalene molecular structure aluminium chloride catalyst structural formula electrophilic substitution mechanism

At room temperature, in the presence of aluminium chloride catalyst, naphthalene readily reacts with ethanoyl chloride (acetyl chloride) to yield a mixture of 1-acetylnaphthalene and 2-acetylnaphthalene.

The products are aromatic ketones.

This reaction is in principle the same electrophilic substitution reaction undergone by benzene and methylbenzene.


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7.8.2 The electrophilic substitution mechanism for the Friedel-Crafts acylation of arenes

A Friedel-Crafts synthesis of aromatic ketones using an acid/acyl chloride.

Note that the catalyst aluminium chloride (AlCl3) has a vacant orbital and can act as a Lewis acid, accept a pair of electrons, and, in this context, facilitate the formation of a more powerful electrophile [RC=O]+ in step (1) of this acylation of benzene electrophilic substitution mechanism. Its the same situation for the catalysts AlBr3, FeCl3 and FeBr3.

diagram of electrophilic substitution reaction mechanisms Friedel Crafts acylation of benzene ring in aromatic compounds syntheis of aromatic ketones

mechanism 25 - electrophilic substitution by an acyl group in the benzene ring

for R = H, benzene: C6H6  +  R'COCl  ===> C6H5COR'  +  HCl   [see mechanism 25 below]

[mechanism 25 above] If ethanoyl chloride, CH3COCl, was used (R=CH3-), benzene forms phenylethanone, C6H5-CO-CH3.

Step (1) Although the acid chloride molecule is polar, it is still not a strong enough electrophile to disrupt the pi electron system of the benzene ring.

The aluminium chloride reacts with an acid chloride molecule to form an acylium ion, RCO+ (another type of carbocation), which is a much stronger electron pair accepting positive electrophile than the original acid chloride (either RCO+ or an AlCl3-RCOCl complex - details not needed for A level).

Step (2) An electron pair from the delocalised pi electrons of the benzene ring forms a C-C bond with the electron pair accepting carbocation forming a second highly unstable carbocation.

It is very unstable because the stable electron arrangement of the benzene ring pi electrons is partially broken to give a 'saturated' C (top right of ring.

Step 2 has the highest activation energy and is the rate determining step (see mechanism diagram 81E below).

Step (3) is a proton transfer, as the tetrachloroaluminate(III) ion [formed in step (1)], abstracts a proton from the second highly unstable intermediate carbocation to give the ketone product, 'waste' hydrogen chloride gas and reforming the aluminium chloride catalyst.

In forming the aromatic ketone product, you retain the stabilising effect of the complete pi electron system of the benzene ring.

for R = CH3, methylbenzene: C6H5CH3  +  R'COCl  ===>  R'COC6H4CH3  +  HCl

and again there is the potential to form three position isomers by substituting in the 2, 3 or 4 position on the ring.

The overall acylation reaction is the substitution of -H by RC=O

diagram of electrophilic substitution reaction mechanisms Friedel Crafts acylation of benzene with ethanoyl chloride

Mechanism diagram 81A: The acylation of benzene using ethanoyl chloride

In the first step the aluminium chloride abstracts a chlorine atom from the ethanoyl chloride to give the acylium ion CH3CO+ (a type of carbocation).

The acylium carbocation is a much more powerful electrophile (electron pair acceptor) than the original acyl chloride.

A pair of pi electrons are donated to the attacking CH3CO+ electrophile forming a ring carbon - carbon bond in the benzene molecule.

The intermediate is unstable because the stabilising ring of pi electrons is broken - one of the benzene ring carbons is temporarily saturated.

A tetrachloroaluminate ion abstracts a proton from the intermediate to yield the final product of 1-phenylethanone.

With the formation of the aromatic ketone product, the stable and complete pi electron ring system is reformed in the benzene ring - overall electrophilic substitution instead of electrophilic addition.

 

 

reaction progress profile diagram of electrophilic substitution reaction mechanisms for acylation of benzene ring in aromatic compounds Friedel Crafts synthesis of aromatic ketones

Mechanism diagram 81E shows the reaction progress profile for the acylation of benzene with an acid chloride.

The first step here, forming the unstable intermediate acylium ion (a type of carbocation), has the highest activation energy and is the slower rate determining step.

The final step yielding the final product, with the reformed stable benzene ring of the aromatic ketone, has the lower activation energy and is much faster.

 

diagram of electrophilic substitution reaction mechanisms Friedel Crafts acylation of methylbenzene with ethanoyl chloride

Mechanism diagram 81B: The acylation of methylbenzene using ethanoyl chloride

In the first step the aluminium chloride abstracts a chlorine atom from the ethanoyl chloride to give the acylium ion CH3CO+ (a type of carbocation).

The acylium carbocation is a much more powerful electrophile (electron pair acceptor) than the original acyl chloride.

A pair of pi electrons are donated to the attacking CH3CO+ electrophile forming a 2nd ring carbon - carbon bond in the methylbenzene molecule.

The intermediate is unstable because the stabilising ring of pi electrons is broken - one of the benzene ring carbons is temporarily saturated.

A tetrachloroaluminate ion abstracts a proton from the intermediate to yield the final aromatic ketone product of 1-(2/3/4-methylphenyl)ethanone.

With the formation of the aromatic ketone product, the stable and complete pi electron ring system is reformed in the benzene ring - overall electrophilic substitution instead of electrophilic addition.

I drawn the mechanism for substitution at the 2 and 4 ring positions of the benzene ring, since these are the two principal products.


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7.8.3 The physical properties of aromatic aldehydes and aromatic ketones

Abbreviations used:

mpt = melting point oC;   bpt = boiling point oC;   sub. = sublimes

dec. = thermally decomposes;   liq. = liquid at room temperature ~20oC

Name of aromatic aldehyde Structure Mpt/oC Bpt/oC Comments
benzaldehyde    structural formula benzaldehyde molecular structure -26 179 Insoluble in water.
2-hydroxybenzaldehyde   structural formula of 2-hydroxybenzaldehyde molecular structure Salicylaldehyde -7 196 Very slightly soluble in water.

Salicylaldehyde - fragrant oil in meadowsweet and other plants

3-hydroxybenzaldehyde  structural formula of 3-hydroxybenzaldehyde molecular structure  structural formula of 4-hydroxybenzaldehyde molecular structure 108    
4-hydroxybenzaldehyde 116    
2-methylbenzaldehyde  structural formula of 2-methylbenzaldehyde molecular structure  structural formula of 3-methylbenzaldehyde molecular structure structural formula of 4-methylbenzaldehyde molecular structure liq. >208  
3-methylbenzaldehyde liq. 199  
4-methylbenzaldehyde liq. 204  
2-chlorobenzaldehyde  structural formula of 2-chlorobenzaldehyde molecular structure structural formula of 3-chlorobenzaldehyde molecular structure structural formula of 4-chlorobenzaldehyde molecular structure liq. >208  
3-chlorobenzaldehyde      
4-chlorobenzaldehyde 49 >213  
2-nitrobenzaldehyde structural formula of 2-nitrobenzaldehyde molecular structure  structural formula of 3-nitrobenzaldehyde molecular structure  structural formula of 4-nitrobenzaldehyde molecular structure 44    
3-nitrobenzaldehyde 58    
4-nitrobenzaldehyde 106    
2-aminobenzaldehyde  structural formula of 2-aminobenzaldehyde molecular structure structural formula of 3-aminobenzaldehyde molecular structure  structural formula of 4-aminobenzaldehyde molecular structure 40    
3-aminobenzaldehyde      
4-aminobenzaldehyde 71    
2-methoxybenzaldehyde  structural formula of 2-methoxybenzaldehyde molecular structure  structural formula of 3-methoxybenzaldehyde molecular structure  structural formula of 4-methoxybenzaldehyde molecular structure 3 244  
3-methoxybenzaldehyde 35    
4-methoxybenzaldehyde

(Anisaldehyde)

0 248 'Anisaldehyde' has a sweet, floral and strong aniseed odour
         
Name of aromatic ketone Structure Mpt/oC Bpt/oC Comments
1-phenylethanone

(acetophenone)

 20 202 IUPAC preferred name is 1-phenylethan-1-one.

Insoluble in water.

1-(2-methylphenyl)ethanone

2-methylacetophenone

structural formula 1-(2-methylphenyl)ethanone 2-methylacetophenone molecular structure of molecule structural formula 1-(3-methylphenyl)ethanone 3-methylacetophenone molecular structure of molecule structural formula 1-(4-methylphenyl)ethanone 4-methylacetophenone molecular structure of molecule      Pale yellow liquid with a nutty coconut aroma, insoluble in water.
1-(3-methylphenyl)ethanone

3-methylacetophenone

-9 219  
1-(4-methylphenyl)ethanone

4-methylacetophenone

23 226 Used in flavours and scents
diphenylmethanone

(diphenylketone)

 48 306 Insoluble in water.
1-(2-hydroxyphenyl)ethanone

Preferred IUPAC name

structural formula of 2-hydroxyphenylethanone 2-hydroxy-1-phenylethanone acetophenone molecular structure 19 213 Very slightly soluble in water.
         

Further comments on the data table

(a) Most of those listed above are colourless or pale yellow liquids at room temperature.

(b) Those with a hydroxy group in the ring can hydrogen bond and that usually raises the melting point above room temperature.

Others, with another highly polar group e.g. nitro NO2, are also usually solids at room temperature with the increased contribution to the intermolecular attractive forces of the permanent dipole - permanent dipole interactions.

(c)

 


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7.8.4 The chemical reactions of aromatic aldehydes and aromatic ketones

(a) The reaction with 2,4-dinitrophenylhydrazone  (24DNPH for short)

diagram equation reaction of benzaldehyde & 1-phenylethanone reacting with 2,4-dintrophenylhydrazine

The above diagram gives the equations for benzaldehyde and 1-phenylethanone reacting with 2,4-dintrophenylhydrazine to give orange-yellow precipitates of the 2,4-dinitrophenylhydrazones.

This is a test for a carbonyl compound, but does not distinguish between aldehydes and ketones because they both undergo the same condensation reaction.

 

(b) The reaction with hydrogen cyanide

diagram equation 1-phenylethanone reaction with hydrogen cyanide giving 2-hydroxy-2-phenylpropanenitrile hydrolysis to 2-hydroxy-2-phenylpropanoic acid

1-phenylethanone undergoes nucleophilic addition of hydrogen cyanide to form a hydroxynitrile.

The hydroxynitrile, 2-hydroxy-2-phenylpropanenitrile, can hydrolysed by refluxing with strong acid or alkali solution to give a hydroxy carboxylic acid, 2-hydroxy-2-phenylpropanoic acid.

The ammonia would not be free, the reaction is far to slow with water, and the ammonium salt of the acid would form i.e. RCOO-NH4+. You need a strong acid or base to effect the hydrolysis.

Refluxing the nitrile with sodium hydroxide produces free ammonia, which is boiled off, leaving the sodium salt of the carboxylic acid in solution, RCOO-Na+.

The organic acid is freed by adding stronger mineral acid. RCOO-  +  H+  ==> RCOOH

Refluxing the nitrile with dilute hydrochloric/sulfuric acid yields the free acid and the corresponding ammonium salt.

 

(c) Reduction of aromatic aldehydes and ketones using lithium tetrahydridoaluminate(III), LiAlH4

diagram equation reaction of reduction of benzaldehyde & 1-phenylethanone reacting with lithium tetrahydridoaluminate(III) reducing agent

The reaction is carried out in dry ethoxyethane ('ether') and the product hydrolysed with dilute mineral acid.

Benzaldehyde is reduced to phenylmethanol (primary alcohol).

1-phenylethanone is reduced to 1-phenylethanol (secondary alcohol(.

 

(d) The iodoform reaction

diagram equation for the iodoform recation of 1-phenylethanone

Only aromatic methyl-ketones can give the iodoform reaction.

If 1-phenylethanone is gently warmed with iodine and sodium hydroxide solution, a yellow precipitate of triiodomethane forms. Sodium benzoate is left in solution.

structural formula 1-(2-methylphenyl)ethanone 2-methylacetophenone molecular structure of molecule structural formula 1-(3-methylphenyl)ethanone 3-methylacetophenone molecular structure of molecule structural formula 1-(4-methylphenyl)ethanone 4-methylacetophenone molecular structure of molecule would all give the iodoform reaction.

 

(e) Other tests for aldehydes and ketones

Tests for aromatic aldehydes and aromatic ketones (apart from 24DNPH reaction)

structural formula benzaldehyde molecular structureBenzaldehyde gives a silver mirror with ammoniacal silver nitrate (Tollen's reagent), which would distinguish it from other aromatic ketones.

BUT, Fehling's solution is a weaker oxidising agent and no red-brown precipitate is given by benzaldehyde.

The pi electrons of the benzene ring overlap with the pi electrons of the carbonyl bond, stabilising benzaldehyde and lowering the reducing power of benzaldehyde - enough to give a negative test with Fehling's solution.

 structural formula of 2-methylbenzaldehyde molecular structure  structural formula of 3-methylbenzaldehyde molecular structure structural formula of 4-methylbenzaldehyde molecular structure would give similar results.

diagram showing overlap of electron clouds of benzene ring and carbonyl group of the aldehyde function group in benzaldehyde

The diagram shows the overlap of the electron clouds of the benzene ring (C6H5) and the carbonyl group (C=O) of the aldehyde function group in benzaldehyde.


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7.8.5 The uses of aromatic aldehydes and aromatic ketones

The aromatic nature of these compounds gives them characteristic odours and many are used in scents and fragrances.

Some are found in nature and are used as flavourings in the food industry.

Many of such useful compounds are synthetically manufactured and the 'collection' added to by synthetic analogues to those found in nature.

Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of AROMATIC CHEMISTRY NOTES

 All Advanced Organic Chemistry Notes

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