Relative Ease of Hydrolysis (CIE A Level Chemistry)

• 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

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 a stronger nucleophile than H₂O

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

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