Acid-catalyst dehydration of tert- butanol is faster than that of n-butanol. Explain.?
回答 (1)
2016-06-15 8:54 am
The mechanism of acid-catalyzed dehydration of tert-butanol :
Step 1 : (CH₃)₃COH + H⁺ ⇌ (CH₃)₃COH₂⁺ → (CH₃)₃C⁺ + H₂O (slow)
Step 2 : (CH₃)₃C⁺ → (CH₃)₂C=CH₂ + H⁺
The mechanism of acid-catalyzed dehydration of n-butanol :
Step 1 : CH₃CH₂CH₂CH₂OH + H⁺ ⇌ CH₃CH₂CH₂CH₂OH₂⁺ →CH₃CH₂CH₂CH₂⁺ + H₂O (slow)
Step 2 : CH₃CH₂CH₂CH₂⁺ → CH₃CH₂CH=CH₂ + H⁺
Alkyl groups are electron releasing which can stabilize carbonium ions by inductive effect. The tertiary carbonium ion (CH₃)₃C⁺ is stabilized by 3 alkyl groups, and it is more stable than the primary carbonium ion CH₃CH₂CH₂CH₂⁺ which is stabilized by only 1 alkyl group.
Because (CH₃)₃C⁺ is more stable, it is formed in a faster rate in step 1 of the mechanism. Hence, acid-catalyzed dehydration of tert-butanol is faster than that of n-butanol.
Step 1 : (CH₃)₃COH + H⁺ ⇌ (CH₃)₃COH₂⁺ → (CH₃)₃C⁺ + H₂O (slow)
Step 2 : (CH₃)₃C⁺ → (CH₃)₂C=CH₂ + H⁺
The mechanism of acid-catalyzed dehydration of n-butanol :
Step 1 : CH₃CH₂CH₂CH₂OH + H⁺ ⇌ CH₃CH₂CH₂CH₂OH₂⁺ →CH₃CH₂CH₂CH₂⁺ + H₂O (slow)
Step 2 : CH₃CH₂CH₂CH₂⁺ → CH₃CH₂CH=CH₂ + H⁺
Alkyl groups are electron releasing which can stabilize carbonium ions by inductive effect. The tertiary carbonium ion (CH₃)₃C⁺ is stabilized by 3 alkyl groups, and it is more stable than the primary carbonium ion CH₃CH₂CH₂CH₂⁺ which is stabilized by only 1 alkyl group.
Because (CH₃)₃C⁺ is more stable, it is formed in a faster rate in step 1 of the mechanism. Hence, acid-catalyzed dehydration of tert-butanol is faster than that of n-butanol.
收錄日期: 2021-04-18 15:13:03
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