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IB DP Chemistry: HL复习笔记20.1.4 Reduction Reactions

Category: IB课程, 教材笔记, 福利干货 Date: 2022年7月29日下午3:07

Reduction Reactions

Carbonyl compounds

Alcohols can be oxidised to carbonyl compounds in the presence of a suitable oxidising agent
- Primary alcohol → aldehyde → carboxylic acid
- Secondary alcohol → ketone
- Tertiary alcohol - no reaction
These reactions can be reversed in the presence of a suitable reducing agent
- Carboxylic acid → aldehyde → primary alcohol
- Ketone → secondary alcohol
The two most common reducing agents for carbonyl compounds are:
1. Lithium aluminium hydride, LiAlH₄, in anhydrous conditions, commonly dry ether, followed by the addition of aqueous acid
  - This is the stronger of these reducing agents and can reduce carboxylic acids
2. Sodium borohydride, NaBH₄, in aqueous or alcoholic solutions
  - This is the less hazardous of these reducing agents but it cannot reduce carboxylic acids
Both of these reagents produce the nucleophilic hydride ion, H^-

Exam Tip

You can be expected to know typical conditions and reagents of all reactions, e.g. catalysts, reducing agents, reflux, etc.However, you do not need to know more precise details such as specific temperatures

Reduction Reactions

Equations for reduction reactions can be written using [H] to represent the reducing agent
Carboxylic acid to a primary alcohol (using LiAlH₄ refluxed in dry ether, followed by dilute acid)
- Remember that NaBH₄ cannot reduce carboxylic acids

7.2.2-Reduction-of-a-carboxylic-acid

Aldehyde to a primary alcohol (using LiAlH₄ or NaBlH₄)
Ketone to a secondary alcohol (using LiAlH₄ or NaBlH₄)

Exam Tip

Take care if you are asked about the formation of an aldehyde from a carboxylic acidYou have to use LiAlH₄ refluxed in dry ether, followed by dilute acid but this reaction cannot be stopped at the aldehyde because the LiAlH₄ is too powerfulTo form an aldehyde from a carboxylic acid, you have to reduce the carboxylic acid down to a primary alcohol and then oxidise it back up to the aldehyde

Reduction of nitrobenzene

Nitrobenzene, C₆H₅NO₂, can be reduced to phenylamine, C₆H₅NH₂, according to the following two-stage reaction:

20.1.4-The-two-stage-reduction-reaction-of-nitrobenzene-to-phenylamine

The two-stage reduction reaction of nitrobenzene to phenylamine

Stage 1 - Reduction of nitrobenzene

- C₆H₅NO₂ (l) + 3Sn (s) + 7H⁺ (aq) → C₆H₅NH₃⁺ (aq) + 3Sn²⁺ (aq) + 2H₂O (l)
- Nitrobenzene, C₆H₅NO₂, is reacted with tin, Sn, and concentrated hydrochloric acid, HCl
- The reaction mixture is heated under reflux in a boiling water bath
- The phenylammonium ions, C₆H₅NH₃⁺, are protonated due to the acidic conditions

Stage 2 - Formation of phenylamine

- C₆H₅NH₃⁺ (aq) + OH^- (aq) → C₆H₅NH₂ (l) + H₂O (l)
- The phenylammonium ions, C₆H₅NH₃⁺, are deprotonated by the addition of sodium hydroxide solution, NaOH (aq)