3.8 aldehydes and ketones

describe oxidation reactions of aldehydes 

• relatively easy to oxidise, even by weak oxidising agents like Cr₂O₇/H⁺ and Tollen’s reagent

• readily oxidised to carboxylic acids

  • aldehyde + [O] → carboxylic acid

  • during oxidation, a H atom is removed from the C atom double bonded to O

describe the conditions required when oxidising aldehydes

• heat under reflux with excess oxidant to ensure as much aldehyde as possible is oxidised to carboxylic acid

• heat and distil to purify carboxylic acid 

describe oxidation reactions of ketones

• cannot be oxidised easily, only oxidised by powerful oxidising agents

• the C atom double bonded to O is not bonded to any H atoms, so no H atoms are available to remove during oxidation

how can you distinguish between an aldehyde and a ketone?

• warm with Tollen’s reagent 

  • with aldehyde: colourless to silver mirror

  • with ketone: no visible change 

• warm with Fehling’s solution

  • with aldehyde: blue to brick red precipitate 

  • with ketone: no visible change 

describe the redox reactions that occur when an aldehyde is reacted with Tollen’s reagent

aldehyde is oxidised by Ag⁺ to carboxylic acid

• aldehyde + [O] → carboxylic acid 

Ag⁺ is reduced by aldehyde to metallic silver

• Ag⁺ + e^- → Ag

describe the redox reactions that occur when an aldehyde is reacted with Fehling’s solution 

aldehyde is oxidised by Cu²⁺ to carboxylate ion under alkaline solutions (the solution is alkaline)

• aldehyde + [O] → carboxylate ion^-

blue Cu²⁺ is reduced by aldehyde to brick-red Cu⁺ 

• Cu²⁺ + e^- → Cu⁺

why is Tollen’s reagent used over the less expensive acidified potassium dichromate (VI) as the oxidising agent to test for aldehydes?

dichromate(VI) will also oxidise alcohols

describe the structure and bonding in a carbonyl group

• C=O double bond consists of a sigma bond and a pi bond.

• the pi bond is formed from the sideways overlap of a 2p orbital from both the C atom and the O atom.

• C=O bond is polar: O is δ^- as O is more electronegative so attracts the electron density in the double bond towards itself, C is δ⁺

what is a nucleophile?

an electron pair donor 

why are aldehydes and ketones able to undergo nucleophilic addition?

• the C=O bond is polar, as O is more electronegative than C

• the lone pair on the nucleophile is attracted and donated to the δ⁺ C

what products are formed when aldehydes and ketones are reduced by nucleophilic addition using NaBH₄? 

• aldehydes reduced to primary alcohols

• ketones reduced to secondary alcohols

what conditions are needed to reduce aldehydes and ketones by NaBH₄?

warm with aqueous NaBH₄

• NaBH₄ provides hydride ion H^-

• H₂O provides hydrogen ion, H⁺

what are the equations for the reduction of aldehydes and ketones by nucleophilic addition using NaBH₄?

aldehyde + 2[H] -→ primary alcohol 

ketone + 2[H] → secondary alcohol

• above the arrow: NaBH₄/H₂O
  

[H] represents the reducing agent 

what is the chemical name for NaBH₄?

sodium borohydride

also sodium tetrahydridoborate (III)

why don’t alkenes react with aqueous NaBH₄? 

• H^- ion acts as a nucleophile and is attracted to δ+ C

• alkenes have electron rich C=C

• H^- is repelled by C=C / C=C only attacked by electrophiles 

what products are formed when aldehydes and ketones are reduced by nucleophilic addition using HCN? 

hydroxynitriles, hydroxyalkylnitriles

what conditions are needed to reduce aldehydes and ketones by HCN?

KCN and dilute acid, aqueous conditions

• KCN provides cyanide ion CN^-

• dilute acid provides hydrogen ion H⁺

what are the equations for the reduction of aldehydes and ketones by nucleophilic addition using HCN?

aldehyde or ketone + HCN —> hydroxynitrile or hydroxyalkylnitriles

• above the arrow: KCN/H₂SO₄

what are the hazards of using HCN?

HCN cannot be used directly as it:

• is very toxic / poisonous

• is hard to store as a gas 

• reacts to produce dangerous byproducts

KCN/H₂SO₄ used to generate HCN in the reaction mixture

what the pros and cons of using KCN instead of HCN?

• KCN dissociates better than HCN to provide CN^- nucleophile (HCN weak / [CN^-] too low)

  • reaction with HCN is very slow

• KCN is very toxic so the reaction is not carried out in the lab

why is this reaction extremely useful in organic synthesis? 

• it increases the carbon chain length

• the product contains 2 reactive functional groups: hydrogyl (OH) and nitrile (CN)

what happens when aldehydes and unsymmetrical ketones react with KCN/H⁺? 

why does this happen? 

• they form mixtures of enantiomers because the product has a chiral centre

  • carbonyl group is planar 

  • equal chance of CN^- nucleophile attacking from above or below 

  • equal amounts of both enantiomer is formed so a racemate is formed 

predict the product formed when propanone reacts with KCN/H₂SO₄

give the IUPAC name and the structural formula

2-hydroxy-2-methyl-propanenitrile

C(CH₃)₂(OH)CN

what is the structural formula of 2-hydroxybutanenitrile?

CH₃CH₂CH(OH)CN

how would the rate of reaction of propanone with HCN compare with the rate of reaction with HCN?

slower with propanone:

• C of C=O is less δ+ because alkyl groups are electron-releasing / have positive inductive effect so hinder attack by :CN^- nucleophile

faster with propanal:

• easier to attack end of the chain