Relative Ease of Hydrolysis (Cambridge (CIE) A Level Chemistry): Revision Note

Hydrolysis of Acyl Chlorides, Alkyl Chlorides & Halogenoarenes

• Hydrolysis is the breakdown of a compound using water

• The ease of hydrolysis for different organic compounds may differ

• For example, the ease of hydrolysis, starting with the compounds most readily broken down, is: acyl chloride > alkyl chloride > aryl chloride

• This trend can be explained by looking at the strength of the C-Cl

Strength of C-Cl bond in acyl chlorides

• Acyl chlorides are hydrolysed most readily at room temperature

• This is because the carbon bonded to the chlorine atom is also attached to an oxygen atom

• There are two strong electronegative atoms pulling electrons away from the carbonyl carbon, leaving it very δ⁺

• The C-Cl bond is therefore weakened and nucleophilic attack of the carbonyl carbon is much more rapid

Hydrolysis of acyl chlorides

The hydrolysis of acyl chlorides occurs most readily

Strength of C-Cl bond in alkyl chlorides

• The carbonyl carbon in alkyl chlorides is only attached to one electronegative atom which pulls electrons away from it

• This carbon atom is therefore not very δ⁺ and the C-Cl bond is stronger than the C-Cl bond in acyl chlorides

• The hydrolysis of alkyl chlorides, therefore, requires a strong alkali (such as OH^-) to be refluxed with it

• An OH^- ion will hydrolyse the alkyl chloride as it is a stronger nucleophile than H₂O

Hydrolysis of alkyl chlorides

The hydrolysis of alkyl chlorides requires a strong nucleophile

Strength of C-Cl bond in aryl chlorides

• In aryl chlorides, the carbon atom bonded to the chlorine atom is part of the delocalised π bonding system of the benzene ring

• One of the lone pairs of electrons of the Cl atom overlaps with this delocalised system

• The C-Cl bond, therefore, has some double-bond character causing it to become stronger

• As a result, the C-Cl bond is difficult to break and hydrolysis will not occur

Hydrolysis of aryl chlorides

Due to the strong C-Cl bond in aryl chlorides, these compounds will not undergo hydrolysis