Why do aldehydes add nucleophiles faster?

A nucleophile approaching the planar carbonyl carbon perpendicular to the plane, with δ+ size and the steric cone of the two groups drawn for each substrate.

Find the most reactive carbonyl among propanone, propanal, benzaldehyde and p-nitrobenzaldehyde.

Ketones are more reactive than aldehydes.

• Unlike alkenes, which add electrophiles, aldehydes and ketones add nucleophiles across C=O.
• Mechanism: the nucleophile attacks the carbonyl carbon from a direction roughly perpendicular to the plane of its sp² orbitals. The carbon becomes sp³, giving a tetrahedral alkoxide intermediate, which takes a proton from the medium. Net result: Nu⁻ and H⁺ add across C=O.
• Aldehydes are generally more reactive than ketones. Sterically, a ketone's two larger groups crowd the approach to the carbon; electronically, two alkyl groups lower the carbon's positive character more than one does.
• Benzaldehyde is less reactive than propanal: resonance with the ring reduces the polarity of its C=O, so its carbonyl carbon is less electrophilic.
• Worked order of reactivity, lowest first: butanone < propanone < propanal < ethanal; and acetophenone < p-tolualdehyde, benzaldehyde < p-nitrobenzaldehyde.
• HCN adds to give cyanohydrins. Pure HCN reacts very slowly, so a base is added to generate CN⁻, a stronger nucleophile. Cyanohydrins are useful intermediates in synthesis.
• Sodium hydrogensulphite (NaHSO₃) adds to give water-soluble addition products. For most aldehydes the equilibrium lies to the right, for most ketones to the left (steric reasons). Dilute mineral acid or alkali regenerates the carbonyl compound, so the reaction is used to separate and purify aldehydes.
• With dry HCl, an aldehyde adds one molecule of a monohydric alcohol to give a hemiacetal (an alkoxyalcohol) and a second to give an acetal, a gem-dialkoxy compound. Ketones with ethylene glycol give cyclic ethylene glycol ketals. Dry HCl protonates the carbonyl oxygen and makes the carbon more electrophilic.
• Acetals and ketals are hydrolysed by aqueous mineral acid back to the aldehyde or ketone. Grignard reagents also add to C=O (Unit 7).

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