CARBONYL COMPOUNDS

Carbonyl compounds

from your syllabus

Content

I Aldehydes (exemplified by ethanal)

(i) Oxidation to carboxylic acids
(ii) Reaction with hydrogen cyanide
(iii) Characteristic tests for aldehydes

II Ketones (exemplified by propanone and phenylethanone)

(i) Reaction with hydrogen cyanide
(ii) Characteristic tests for ketones Learning outcomes

Candidates should be able to:

(a) describe

(i) the formation of aldehydes and ketones from primary and secondary alcohols respectively using Cr2O7 2–/H+
(ii) the reduction of aldehydes and ketones, e.g. using NaBH4 or LiAlH4
(iii) the reaction of aldehydes and ketones with HCN and NaCN

(b) *describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with aldehydes and ketones

(c) describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) reagent to detect the presence of carbonyl compounds

(d) deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of simple tests (i.e. Fehling’s and Tollens’ reagents; ease of oxidation)

(e) describe the reaction of CH3CO– compounds with alkaline aqueous iodine to give tri-iodomethane

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 Carbonyl Compounds

Carbonyl Compounds – Nomenclature

(1) Aldehydes :

Aldehyde name’s are made up of two simple parts :

(i) prefix –

This depends on the number of carbon atoms in the longest chain, as with alkanes

(ii) suffix –

Add __anal onto the end of the prefix

Exemplar compounds –

ethanal –	butanal –

 

(2) Ketones :

Ketone name’s are generally made up of three parts :

(i) prefix

– this depends on the number of carbon atoms in the longest chain

(ii) position number

– count from the nearest chain end to the carbonyl carbon atom

(iii) suffix

– add –an__one around the position number at the end of the prefix

Exemplar compounds –

propanone –	pentan-2-one –

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Carbonyl Compounds – Formation

Both aldehydes and ketones are mainly formed by the oxidation of alcohols, for example with acidified dichromate(VI)(aq) ions, Cr2O72-.

Aldehydes are formed by the partial oxidation of primary alcohols,

and ketones are formed by the oxidation of secondary alcohols.

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Carbonyl Compounds – Reactions

(1) Aldehydes :

(i) Reduction –

The process of forming an aldehyde by the partial oxidation of a primary alcohol can be reversed, by reacting an aldehyde with an aqueous ethanolic solution of sodium borohydride (NaBH4) –

The actual mechanism is quite complex. It can be thought of as a direct addition of the hydrogen in sodium borohydride to the carbonyl C=O bond.

(ii) Oxidation –

Aldehydes can be further oxidised by refluxing with acidified dichromate(VI)(aq) ions to carboxylic acids –

(iii) With hydrogen cyanide –

The carbonyl group undergoes nucleophilic addition reactions, instead of electrophilic addition that other compounds with π-bonds do (e.g. alkenes).

They will react with hydrogen cyanide to form cyanohydrins –

These cyanohydrin compounds formed from aldehydes are all 2-hydroxy compounds.

(2) Ketones :

(i) Reduction –

The process of forming a ketone by the oxidation of a secondary alcohol can be reversed, by reacting a ketone with an aqueous ethanolic solution of sodium borohydride (NaBH4) –

(ii) With hydrogen cyanide –

The carbonyl group undergoes nucleophilic addition reactions, instead of electrophilic addition that other compounds with π-bonds do (e.g. alkenes).

They will react with hydrogen cyanide to form cyanohydrins –

These cyanohydrin compounds formed from ketones are generally 2-hydroxy-2-alkyl compounds.

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Carbonyl Compounds – Reaction Mechanism

The carbonyl group (C=O) is an extremely polar group – more so than a C-Halogen bond. This gives the carbon atom a strong δ+ charge thus making it susceptible to attack by nucleophiles (just as haloalkanes are). However, the carbonyl compounds do not undergo substitution reactions like haloalkanes they undergo addition reactions, just as alkenes do.

 

Therefore these reactions are called nucleophilic additions.

The reaction used as an example with this type of mechanism is the reaction of hydrogen cyanide with a carbonyl group -(the only one required for you)

 

R and R’ are any organic groups, e.g. H, CH3, etc..

The overall effect is that a molecule of HCN is added across the carbon-oxygen double bond.

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Carbonyl Compounds – Aldehyde tests

There are a number of tests that can be used to identify the presence of a carbonyl group in a compound. These rely on the oxidising nature of the various carbonyl compounds.

(1) Using Tollen’s reagent :

This reagent is prepared by warming a mixture of ammonia(aq) and silver nitrate(aq) to give ammoniacal silver(I) ions, [Ag(NH3)2]+.

These ions can be reduced by heating with an aldehyde. This produces silver metal which generally coats the inside of the test tube used to give a (silver) mirror,

Ketones give no change to the reagent when mixed, because of their inability to be oxidised.

(2) Using Fehling’s/Benedict’s reagent :

This is a two part reagent comprising of a solution of copper(II) sulphate and an alkaline solution, which, when mixed with an aldehyde and warmed gently, give a red precipitate.

The blue copper(II)(aq) ions are reduced to give a red precipitate of copper(I) oxide,

Again, this reagent has no effect on ketones.

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Carbonyl Compounds – Iodoform Reaction

The iodoform test is a test for the existence of the CH3-CO- group in a molecule. This could be part of an alcohol (C-O single bond) or part of a carbonyl compound (C=O double bond).

The hydrogen atoms on the methyl group are slightly acidic and can be removed with sodium hydroxide (stage 1).

The carbanion formed then react with iodine molecules to give an iodide ion and an organic iodo compound (stage 2).

This substitution continues until a triiodo group has been formed (stage 3) by repeated use of sodium hydroxide and iodine.

Another hydroxide ion can then attack the carbonyl carbon atom, giving a carboxylic acid and releasing the CI3 group which abstracts a proton from a water molecule to give CHI3(triiodomethane or iodoform) (stage 4).

 

Triiodomethane is a straw yellow solid, insoluble in water. This test works for ethanal and all methyl ketones,

ethanal :
methyl ketones : (R=any alkyl chain)