Is alkylation a double displacement reaction?
1 Answer
It depends on your definition.
Some people define a double displacement reaction as a reaction of the type
AB + CD → AD + CB
Others say that it must be a reaction between ions:
A⁺B⁻ + C⁺D⁻ → AD + CB
Let's look at some alkylation reactions.
Organolithium Alkylations
CH₃-C≡C-Li + CH₃CH₂-Br → CH₃-C≡C-CH₂CH₃ + LiBr
The C-Li bond is highly polar, but it is not ionic. This is a reaction between molecules.
It is a double displacement reaction by the first, but not by the second definition. Organic chemists call this an
Alkylation of Ketones
This reaction occurs in two steps.
CH₃COCH₃ +NaOH ⇌ CH₃COCH₂⁻Na⁺ + H₂O
CH₃COCH₂⁻Na⁺ + C₆H₅CH₂-I → CH₃COCH₂-CH₂C₆H₅ + I⁻
The second step is the alkylation step. This is a double displacement reaction by the second definition. This is also an
Friedel-Crafts Alkylation
This is a three-step reaction.
(CH₃)₃C-Cl + FeCl₃ → (CH₃)₃C⁺ + FeCl₄⁻
(CH₃)₃C⁺ + C₆H₆ → (CH₃)₃C-C₆H₆⁺
(CH₃)₃C-C₆H₆⁺ + FeCl₄⁻ → (CH₃)₃C-C₆H₅ + HCl + FeCl₃
The alkylation step is the second step. It is not a double displacement reaction. It is more like a synthesis reaction: A + B → C.
