Aldehydes and ketones
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What reagents and conditions are used to reduce an aldehyde or ketone to an alcohol?
Aldehydes and ketones are reduced to alcohols using either NaBH₄ (sodium borohydride) in aqueous ethanol or LiAlH₄ (lithium aluminum hydride) in dry ether, followed by dilute acid. NaBH₄ is generally used for aldehydes and ketones; LiAlH₄ is a stronger reducing agent and more suitable for reducing carboxylic acids and other functionalities as well.
Describe the mechanism of the nucleophilic addition reaction of hydrogen cyanide (HCN) with an aldehyde or ketone.
The nucleophilic cyanide ion (CN⁻) attacks the partially positive carbon atom of the carbonyl group (C=O). The pi bond breaks, and electrons move to the oxygen, forming an alkoxide ion. This ion is then protonated by H⁺ from HCN to form a hydroxynitrile.
What reagent is used to detect the presence of a carbonyl group (C=O)?
2,4-Dinitrophenylhydrazine (2,4-DNPH) reagent is used to detect the presence of a carbonyl group. A positive result is indicated by the formation of a yellow, orange, or red precipitate.
How can you distinguish between an aldehyde and a ketone using simple chemical tests?
Aldehydes can be oxidized easily, whereas ketones require stronger conditions. Use Tollen's reagent (silver mirror test) and Fehling's solution (brick red precipitate) to distinguish. Aldehydes will react positively with both, while ketones will not.
What observation indicates the presence of a CH₃CO– group in an aldehyde or ketone?
A yellow precipitate of tri-iodomethane (CHI₃) forms when the aldehyde or ketone is reacted with alkaline iodine (I₂/NaOH). This reaction occurs because the CH₃CO– group is present, producing the characteristic yellow solid.
Give the reaction of ethanal with HCN and KCN (catalyst) with heat. Give name of the organic product.
Ethanal reacts with HCN in the presence of KCN (catalyst) and heat to produce 2-hydroxypropanenitrile (or ethanal cyanohydrin). The cyanide ion attacks the carbonyl carbon.
Give the reaction of propanone with HCN and KCN (catalyst) with heat. Give name of the organic product.
Propanone reacts with HCN in the presence of KCN (catalyst) and heat to produce 2-hydroxy-2-methylpropanenitrile (or propanone cyanohydrin). The cyanide ion attacks the carbonyl carbon.
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