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Aldehydes and Ketones notes

MDCAT Chemistry

Aldehydes and ketones are carbonyl compounds containing the C=O functional group. These notes cover their preparation, physical and chemical reactivity, nucleophilic addition, oxidation and reduction, important tests, aldol condensation, Cannizzaro reaction, and iodoform test.

Structure and Classification of Aldehydes and Ketones

Aldehydes and ketones are organic compounds containing a carbonyl group, C=O. In an aldehyde, the carbonyl carbon is attached to at least one hydrogen atom. In a ketone, the carbonyl carbon is attached to two carbon groups.

The carbonyl carbon is sp2 hybridized. The carbonyl group is planar, and the carbon atom carries a partial positive charge while oxygen carries a partial negative charge. This polarity explains many reactions of aldehydes and ketones.

  • General formula of an aldehyde is RCHO. Formaldehyde is HCHO and ethanal is CH3CHO.
  • General formula of a ketone is RCOR. Propanone, or acetone, is CH3COCH3.
  • A ketone having two similar alkyl groups is called a symmetrical ketone. Acetone is an example.
  • A ketone having two different groups attached to the carbonyl carbon is called a mixed ketone. Acetophenone, C6H5COCH3, is a mixed ketone.
  • Aldehydes contain the formyl group, CHO. Ketones contain the carbonyl group between two carbon atoms.
  • Aldehydes and ketones show addition reactions at the carbonyl group. They do not normally show beta-elimination reactions as a characteristic reaction type.

Preparation from Alcohols

Aldehydes are prepared by controlled oxidation of primary alcohols. The aldehyde must be removed by distillation as soon as it forms because further oxidation changes it into a carboxylic acid.

Ketones are prepared by oxidation of secondary alcohols. Oxidation of a secondary alcohol produces a ketone, while further oxidation usually requires breaking carbon-carbon bonds.

  • Primary alcohols on oxidation give aldehydes: RCH2OH + [O] → RCHO + H2O.
  • Secondary alcohols on oxidation give ketones: R2CHOH + [O] → R2CO + H2O.
  • Ethanol on oxidation first gives ethanal and then ethanoic acid.
  • 2-propanol on oxidation gives propanone, which is a ketone.
  • Aldehydes can be obtained by passing vapours of a primary alcohol over heated copper at about 573 K: RCH2OH → RCHO + H2.
  • Ketones can be obtained by passing vapours of a secondary alcohol over heated copper: R2CHOH → R2CO + H2.
  • Primary alcohols and secondary alcohols can be distinguished by the products formed on oxidation.

Preparation from Carboxylic Acids and Their Derivatives

Carboxylic acids react with phosphorus pentachloride to form acyl chlorides. This reaction replaces the hydroxyl group of the acid by chlorine.

Acyl chlorides can be reduced to aldehydes by controlled reduction. Strong reduction of carboxylic acids or their derivatives generally continues to the alcohol stage.

  • Reaction of ethanoic acid with PCl5 is: CH3COOH + PCl5 → CH3COCl + POCl3 + HCl.
  • The product CH3COCl is acetyl chloride, an acyl chloride.
  • Aldehydes may be prepared by controlled reduction of acyl chlorides, for example by Rosenmund reduction.
  • Ketones can be prepared by treating an acyl chloride with a suitable organocadmium compound or by reactions involving acid derivatives.
  • Carboxylic acids on reduction with LiAlH4 give primary alcohols, not aldehydes.
  • LiAlH4 is a strong reducing agent and reacts violently with water. Therefore, water is added only after the reduction reaction has been completed.

Reactivity of the Carbonyl Group

The C=O bond is polar because oxygen is more electronegative than carbon. The carbonyl carbon is electrophilic, so it is attacked by nucleophiles. The oxygen atom can accept a proton or interact with acidic reagents.

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions. Aldehydes have less steric hindrance and only one alkyl group, while ketones have two alkyl groups that donate electron density to the carbonyl carbon.

  • Reactivity of carbonyl compounds is due to the polarity of the C=O bond, the positive character of carbonyl carbon, and the availability of a low-energy pi bond.
  • General reactivity order is HCHO > other aldehydes > ketones.
  • Formaldehyde is most reactive because it has no alkyl group and has minimum steric hindrance.
  • Aldehydes are more easily oxidized than ketones.
  • Electron-donating alkyl groups decrease the positive character of the carbonyl carbon and reduce reactivity.
  • Bulky groups around the carbonyl carbon reduce nucleophilic attack by steric hindrance.
  • Both acids and bases can catalyze nucleophilic addition reactions.

Nucleophilic Addition Reactions

In nucleophilic addition, a nucleophile attacks the positively polarized carbonyl carbon. The pi bond of C=O breaks temporarily, and the nucleophile becomes attached to the carbonyl carbon. The oxygen atom is then protonated when necessary.

Acid catalysis increases the positive character of the carbonyl carbon by protonating the oxygen. Base catalysis produces a stronger nucleophile, which attacks the carbonyl carbon directly.

  • Hydrogen cyanide adds to aldehydes and ketones to form cyanohydrins.
  • Reaction of formaldehyde with HCN is a nucleophilic addition reaction.
  • Ethanal reacts with HCN in the presence of dilute mineral acid and sodium cyanide to form acetaldehyde cyanohydrin: CH3CHO + HCN → CH3CH(OH)CN.
  • The cyanohydrin contains both a hydroxyl group and a cyano group.
  • Grignard reagents react with both aldehydes and ketones by nucleophilic addition.
  • Formaldehyde with a Grignard reagent gives a primary alcohol after hydrolysis. Other aldehydes give secondary alcohols, while ketones give tertiary alcohols.
  • Aldehydes and ketones can also add sodium bisulphite to form crystalline addition compounds.

Reduction and Oxidation Reactions

Reduction of the carbonyl group changes the C=O group into an alcohol group. Aldehydes give primary alcohols, whereas ketones give secondary alcohols.

Aldehydes are readily oxidized to carboxylic acids. Ketones resist mild oxidation because oxidation would require breaking carbon-carbon bonds.

  • Aldehyde reduction: RCHO + 2[H] → RCH2OH.
  • Ketone reduction: RCOR + 2[H] → RCHOH R.
  • LiAlH4 reduces aldehydes to primary alcohols and ketones to secondary alcohols.
  • NaBH4 is also used for the reduction of aldehydes and ketones under suitable conditions.
  • Ammoniacal AgNO3 is a mild oxidizing agent and is used in Tollens' test.
  • Aldehydes give a silver mirror with Tollens' reagent, while ordinary ketones do not.
  • Fehling's reagent is reduced by many aliphatic aldehydes to red copper(I) oxide, but ketones generally do not react.
  • An aldehyde is oxidized to a carboxylic acid: RCHO + [O] → RCOOH.

Tollens' Test, Fehling's Test and Separation by NaHSO3

Tollens' reagent is an ammoniacal solution of silver nitrate containing the diamminesilver(I) complex. On warming with an aldehyde, silver ions are reduced to metallic silver, producing a silver mirror.

Sodium bisulphite reacts with aldehydes and ketones to form crystalline bisulphite addition compounds. This reaction can be used to separate a carbonyl compound from substances that do not form such compounds under the given conditions.

  • The silver mirror test is also called Tollens' reagent test.
  • Aldehydes usually give a positive Tollens' test. Ketones usually do not.
  • Fehling's reagent gives a brick-red precipitate of Cu2O with many aliphatic aldehydes.
  • Ethanal gives a positive Tollens' test and a positive Fehling's test.
  • Acetone does not normally give Tollens' or Fehling's test.
  • NaHSO3 can be used to separate ethanal from a mixture containing ethanol because ethanal forms a bisulphite addition compound.
  • The bisulphite compound can be separated and then decomposed to regenerate the aldehyde.
  • The reaction with NaHSO3 is an addition reaction at the carbonyl group.

Aldol Condensation and Cannizzaro Reaction

Aldol condensation is shown by aldehydes or ketones having at least one alpha hydrogen. Two similar carbonyl molecules combine in the presence of a dilute base to form a beta-hydroxy carbonyl compound called an aldol. On heating, the aldol may lose water to form an unsaturated compound.

Cannizzaro reaction is shown by aldehydes that do not have an alpha hydrogen. In concentrated alkali, two molecules of the aldehyde undergo self oxidation-reduction. One molecule is oxidized to a carboxylate salt and the other is reduced to an alcohol.

  • The product of aldol addition contains both an aldehyde or ketone group and a hydroxyl group.
  • The word aldol comes from aldehyde plus alcohol.
  • The reaction of two similar carbonyl compounds to give an aldol product is called condensation.
  • Ethanal gives aldol condensation because it has alpha hydrogen atoms.
  • General aldol reaction: 2CH3CHO → CH3CH(OH)CH2CHO.
  • Cannizzaro reaction is not given by acetaldehyde because acetaldehyde has alpha hydrogen.
  • Formaldehyde and benzaldehyde can give Cannizzaro reaction because they have no alpha hydrogen.
  • General Cannizzaro reaction: 2RCHO + concentrated NaOH → RCH2OH + RCOONa.

Iodoform Test and Important Applications

The iodoform test is given by compounds containing the CH3CO group or compounds that are oxidized to a CH3CO group. The test uses iodine and sodium hydroxide and produces a yellow precipitate of iodoform.

Ethanol also gives the iodoform test because it is oxidized to ethanal under the test conditions. Compounds that give a positive iodoform test are not necessarily aldehydes or ketones.

  • Formula of haloform is CHX3. The iodoform formed in the test is CHI3.
  • Iodoform is a yellow precipitate with a characteristic smell.
  • Ethanal, ethanol, propanone and secondary alcohols of the type CH3CH(OH)R can give the iodoform test.
  • Ethanal gives the iodoform test and is also oxidized to ethanoic acid.
  • n-Butyl alcohol does not give the iodoform test because it does not contain the required CH3CH(OH) group and does not form ethanal on oxidation.
  • A compound giving a positive iodoform test will most probably not give Cannizzaro reaction if it is an aldehyde with alpha hydrogen, such as ethanal.
  • Acetophenone, C6H5COCH3, gives the iodoform test because it contains the CH3CO group.
  • Iodoform testing should be interpreted with the structure and other tests, because ethanol can also give a positive result.

Key terms

Carbonyl group
The functional group C=O present in aldehydes, ketones and several other organic compounds.
Aldehyde
An organic compound containing the group RCHO, with at least one hydrogen attached to the carbonyl carbon.
Ketone
An organic compound containing the group RCOR, with two carbon groups attached to the carbonyl carbon.
Nucleophile
An electron-rich species that donates an electron pair to an electron-deficient atom.
Nucleophilic addition
Addition of a nucleophile across the polar carbon-oxygen double bond.
Cyanohydrin
A compound formed by addition of HCN to an aldehyde or ketone, containing both OH and CN groups.
Grignard reagent
An organomagnesium compound with the general formula RMgX that adds to carbonyl compounds.
Tollens' reagent
Ammoniacal silver nitrate used to oxidize aldehydes and produce a silver mirror.
Fehling's reagent
An alkaline copper(II) solution reduced by many aliphatic aldehydes to red Cu2O.
Aldol
A beta-hydroxy aldehyde or ketone formed by aldol addition.
Aldol condensation
Combination of two carbonyl compounds having alpha hydrogen to form an aldol, often followed by loss of water.
Cannizzaro reaction
Self oxidation-reduction of an aldehyde without alpha hydrogen in the presence of concentrated alkali.
Alpha hydrogen
A hydrogen attached to the carbon atom directly next to a carbonyl carbon.
Iodoform test
A test producing yellow CHI3 from compounds containing or forming a CH3CO group.
Haloform
A compound with the general formula CHX3, where X is a halogen.
Acyl chloride
A carboxylic acid derivative containing the group RCOCl.
Mixed ketone
A ketone with two different groups attached to the carbonyl carbon, such as acetophenone.

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