aldehydes and ketones

what is the general formula of aldehydes and ketones? (1)

C_nH_2nO

how do aldehydes and ketones differ in the placement of the C=O functional group? (2)

• ketones have the C=O group in the middle of the carbon chain

• aldehydes have the C=O group at the end of the carbon chain

what are aldehydes and ketones classified as in terms of isomerism? (1)

they are functional group isomers of each other

how can aldehydes and ketones be distinguished in oxidation reactions? (2)

• aldehydes are further oxidised to carboxylic acids

• ketones are not easily oxidised using potassium dichromate(VI)

why can't potassium dichromate(VI) distinguish between primary and secondary alcohols? (1)

both show the orange to green colour change during oxidation

draw and label the structural formulas of propanone and propanal, and explain their functional group isomerism (2)

what is the observation when aldehydes react with Tollens' reagent? (1)

a silver mirror is formed

what is the observation when ketones react with Tollens' reagent? (1)

no observable change

what is the observation when aldehydes react with Fehling's solution? (1)

a brick-red precipitate is formed

what is the observation when ketones react with Fehling's solution? (1)

no observable change

draw a general mechanism for nucleophilic addition of aldehydes and ketones (3)

what is the condition for reduction of aldehydes and ketones? (1)

aqueous 

what is the role of NaBH₄ (sodium tetrahydridoborate) in reduction reactions? (2)

• NaBH₄ acts as a reducing agent

• providing hydride (H⁻) ions to reduce aldehydes and ketones to alcohols

what are aldehydes reduced to in the presence of a reducing agent? (1)

aldehydes are reduced to primary alcohols

write the equation for the reduction of ethanal to ethanol (1)

CH₃CHO + 2[H] → CH₃CH₂OH

what are ketones reduced to in the presence of a reducing agent? (1)

ketones are reduced to secondary alcohols

write the equation for the reduction of propanone to propan-2-ol (1)

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

draw and label the nucleophilic addition mechanism for ethanal reacting with NaBH₄

what happens when aldehydes and ketones react with KCN (potassium cyanide) and dilute HCl? (2)

• aldehydes and ketones undergo nucleophilic addition with KCN

• forming an alcohol with a nitrile functional group

write the overall equation for the reaction of ethanal with KCN and dilute HCl (1)

CH₃CHO + KCN + HCl → CH₃CH(OH)CN + KCl

write the ionic equation for the nucleophilic addition reaction of ethanal with KCN (1)

CH₃CHO + CN⁻ + H⁺ → CH₃CH(OH)CN

draw and label the mechanism for the reaction of ethanal with KCN and dilute HCl (2)