Concept 02 / 07Aldehydes, Ketones and Carboxylic Acids
Physical Properties and Nucleophilic Addition Reactions of Aldehydes and Ketones
Boiling points and solubility of carbonyl compounds, the mechanism of nucleophilic addition, relative reactivity of aldehydes and ketones, and addition of HCN, sodium hydrogensulfite, Grignard reagents and alcohols (hemiacetals, acetals, ketals), and addition-elimination with ammonia derivatives (oximes, hydrazones, 2,4-DNP derivatives, semicarbazones).
Physical properties. Methanal is a gas; ethanal is a volatile liquid; others are liquids or solids. Boiling points are higher than those of hydrocarbons and ethers of similar mass (dipole-dipole attractions) but lower than those of alcohols (no hydrogen bonding among themselves): n-butane , methoxyethane , propanal , acetone , propan-1-ol . Lower members are water-soluble (hydrogen bonds with water); solubility decreases with chain length.
Nucleophilic addition (see figure). A nucleophile attacks the electrophilic carbonyl carbon from a direction roughly perpendicular to the plane of the carbon; the carbon becomes and the oxygen becomes a tetrahedral alkoxide ion, which then picks up a proton (from the medium) to give the neutral addition product.
Reactivity: aldehydes are more reactive than ketones, for both steric reasons (two bulky groups in ketones hinder attack) and electronic reasons (two alkyl groups reduce the positive charge on carbon more than one). Aromatic aldehydes and ketones are less reactive than aliphatic ones because the ring's resonance reduces the electrophilicity of the carbonyl carbon. Order: .
Important additions.
Hydrogen cyanide: (cyanohydrin). The reaction is slow with pure HCN and is catalysed by base, which generates the stronger nucleophile . Cyanohydrins are useful synthetic intermediates.
Sodium hydrogensulfite: gives crystalline bisulfite addition compounds (with aldehydes and methyl ketones), which are decomposed back by dilute acid or alkali; used to separate and purify aldehydes.
Grignard reagents: give alcohols after hydrolysis (methanal: primary; other aldehydes: secondary; ketones: tertiary).
Alcohols: aldehydes react with one equivalent of an alcohol in dry HCl to form hemiacetals, and then with a second to form acetals, . Ketones react with ethane-1,2-diol to form cyclic ketals. Acetals and ketals are hydrolysed back by aqueous acid, so they are used to protect carbonyl groups.
Ammonia derivatives (): addition followed by loss of water (addition-elimination), in weakly acidic medium (optimum pH about ):
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With hydroxylamine: oxime; with hydrazine: hydrazone; with phenylhydrazine: phenylhydrazone; with 2,4-dinitrophenylhydrazine (2,4-DNP, Brady's reagent): 2,4-dinitrophenylhydrazone, an orange-red precipitate used to identify carbonyl compounds; with semicarbazide: semicarbazone.
Key Takeaways:
- Carbonyls: dipole-dipole forces; b.p. between ethers and alcohols.
- Nucleophilic addition: to via an alkoxide intermediate.
- Aldehydes > ketones; aliphatic > aromatic in reactivity.
- HCN (cyanohydrin, base catalysed), (purification), ROH (acetals, protection).
- derivatives: oxime, hydrazone, 2,4-DNP (orange ppt), semicarbazone.
- Pitfall: in reactions with ammonia derivatives, too much acid protonates the nucleophile and stops the reaction; hence the weakly acidic optimum.
Brain-Teaser 1: Arrange in increasing reactivity towards HCN: acetone, benzaldehyde, ethanal, di-tert-butyl ketone.
Solution: Di-tert-butyl ketone < acetone < benzaldehyde < ethanal. Steric hindrance and electron release from alkyl groups lower reactivity; benzaldehyde is less reactive than ethanal because of conjugation with the ring but more reactive than ketones.
Brain-Teaser 2: Why is the addition of HCN to carbonyl compounds catalysed by a base?
Solution: HCN is a weak acid and a poor nucleophile. A base converts it into , a strong nucleophile that attacks the carbonyl carbon readily; the resulting alkoxide takes a proton from HCN, regenerating .
Brain-Teaser 3: Write the product of propanone with (a) hydroxylamine, (b) 2,4-DNP, (c) ethane-1,2-diol in dry HCl.
Solution: (a) Propanone oxime, . (b) Propanone 2,4-dinitrophenylhydrazone, (orange precipitate). (c) The cyclic ketal, 2,2-dimethyl-1,3-dioxolane.
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