6_alcohols_and_ethers_anotations

6_alcohols_and_ethers_anotations - Any time a Carbocation...

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Sec. 6: Alcohols, Ethers 1 Any time a Carbocation is formed Rearrangements can occur. C CH 3 H 3 C CH 3 C H CH 3 methanide migration C H 3 C CH 3 C H CH 3 CH 3 + + C C C H 3 C H 3 C H H H H C H H H Lewis base Lewis acid Why does this happen? C CH 3 H 3 C CH 3 CH 2 methanide migration + C H 3 C CH 3 CH 2 CH 3 + CH 3 CH 2 CH H CH 2 hydride migration + CH 3 CH 2 CH CH 2 H + C CH 3 C H 2 C H 2 C H C H H H H H + +
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Sec. 6: Alcohols, Ethers 2 Hydride and methanide shifts are very fast (faster than S N 1 or E1) which is partially due to hyperconjugation in the carbocation weakening the C-H, or C-C bond: Primary carbocations are too unstable to be formed by rearrangement. Secondary or tertiary carbocations equilibrate readily, leading to a mixture of products when trapped by a nucleophile.
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Sec. 6: Alcohols, Ethers 3 C C CH 3 H 3 C CH 3 Br CH 3 H CH 3 OH C C CH 3 H 3 C CH 3 CH 3 H + + Br methanide migration C C H 3 C CH 3 CH 3 H CH 3 + CH 3 OH -H + C C H 3 C CH 3 CH 3 H CH 3 OCH 3 What is the product of the following S N 1 reaction? acid acid base weak base poor nucleophile base C C CH 3 H 3 C CH 3 OCH 3 CH 3 H C C OCH 3 H 3 C CH 3 CH 3 CH 3 H C CH 2 CH 3 H 3 C CH 3 CH 2 C C CH 3 H 3 C CH 3 CH 3 O CH 3 H A) B) C) D) H 3 CO Mechanism 2 o 3 o
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Sec. 6: Alcohols, Ethers 4 What is the major product formed in the following reaction? 85% H 3 PO 4 60°C C CH 3 H 3 C CH 3 C H CH 3 OH C H 3 C H 3 C C CH 3 CH 3 C H 2 C H 3 C C CH 3 CH 3 H C CH 3 H 3 C CH 3 CH CH 2 C H 3 C H 3 C C CH 3 H C CH 3 H 3 C CH 3 C H CH 3 A) C) B) E) D) Also look in Vollhardt Chapter 11-11 (preparation of alkenes by dehydration)
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Sec. 6: Alcohols, Ethers 5 What is the Mechanism of the previous reaction? 85% H 3 PO 4 + + 60°C C CH 3 H 3 C CH 3 C H CH 3 OH P O OH OH O H acid base C CH 3 H 3 C CH 3 C H CH 3 O H H P O OH OH O + -H 2 O C CH 3 H 3 C CH 3 C H CH 3 methanide shift C C CH 3 C CH 3 CH 3 H H H H + β β β P O OH OH O C H 2 C H 3 C C CH 3 CH 3 H a a b C H 3 C H 3 C C CH 3 CH 3 major Zaitsev’s Rule + H 3 PO 4
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Sec. 6: Alcohols, Ethers 6 What is the major product formed in the following reaction? 85% H 2 SO 4 C H H 3 C CH 3 C H CH 3 OH NaBr C H H 3 C CH 3 C H CH 3 OH 85% H 2 SO 4 A) C) B) D) Br H Br H A) C) B) D)
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Sec. 6: Alcohols, Ethers 7 The mechanism of 3-methyl-2-butanol with concentrated H 2 SO 4 and NaBr? 85% H 2 SO 4 + + C H H 3 C CH 3 C H CH 3 OH acid base C H H 3 C CH 3 C H CH 3 O H H + -H 2 O C H H 3 C CH 3 C H CH 3 hydride shift C C CH 3 C H CH 3 H H H H + β β HSO 4 Na Br + Br C H 3 C CH 3 C H CH 3 H Br weak base good nucleophile If no NaBr present the only species present are H 2 SO 4 , HSO 4 ¯ and H 2 O, which are weak bases and poor nucleophiles. E1 reaction S N 1 C H 3 C H 3 C C CH 3 H C H 2 C H 3 C C H CH 3 H + major
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Sec. 6: Alcohols, Ethers 8 Primary alcohols may undergo rearrangement.
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This note was uploaded on 07/10/2010 for the course CHEM 266 taught by Professor Pols during the Spring '07 term at Waterloo.

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6_alcohols_and_ethers_anotations - Any time a Carbocation...

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