Hydrolysis of Acyl Chlorides Compared to Alkyl and Acyl Chlorides

What type of reaction is hydrolysis?

Nucleophilic substitution

Relationship between the ease of hydrolysis and rate of formation of AgX ppt from rxn with AgNO3(aq)?

The faster the formation of AgX ppt, the greater the ease of hydrolysis.

Trend of ease of hydrolysis (grp 17)

C6H5X < RCl < RBr < RI

Why does the ease of hydrolysis (grp 17) increase down the group?

Strength of C-Cl > C-Br > C-I, so the ease of breaking is C-Cl < C-Br < C-I.

Describe the procedure to differentiate whether the compound is acyl, alkyl or aryl chloride

• Add water to the compound

• Add NaOH(aq) and heat with water bath

• Acidify with excess HNO3(aq) — removes excess OH- and prevent brown ppt of Ag2O from forming)

• Add dilute AgNO3(aq) — to precipitate out X-

Observation of Acyl Chloride

White ppt seen immediately

Why is the reaction rate so quick for Acyl Chlorides?

Carbonyl C is bonded to two highly electronegative atoms (Cl and O).

This causes carbonyl C to be highly electron deficient.

Hence, it is very susceptible to nucleophilic substitution

Observations for Alkyl Chloride

• No PPT upon adding water

• On heating with NaOH, followed by dilute HNO3, then AgNO3, white ppt forms

Why does ppt of alkyl chloride form slower than ppt of Acyl chloride

There is only one electronegative atom (Cl) bonded to it, thus alkyl C atom is less electron deficient and is less susceptible to nucleophilic addition unlike acyl chloride.

Observation for halogenoarene (aryl chloride)

No white ppt observed after prolonged heating with NaOH and addition of AgNO3

Why is there no ppt formed for aryl Chlorides?

• Lone pair of e- in Cl has delocalised into the benzene ring

• This strengthens the C-X bonds due to presence of partial double bond character

• Thus, nucleophilic substi does not occur.