Day_Lecture_9 - Chemistry 307 Chapter 9 Alcohols Ethers You...

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1 Chemistry 307 Chapter 9 Alcohols Ethers You are familiar with the ability of alcohols to form either alkoxide or alkoxonium ions and with some of the reactions that may occur. 1. Alkoxide ions a) Reaction with strong bases generate alkoxide ions. We use reagents whose conjugate acids have pK a values greater than the pK a of the alcohols; examples include i) lithium diisopropylamide, conj. acid pK a = 40 ii) butyllithium, conj. acid pK a = 50 iii) potassium hydride, conj. acid pK a = 38 b) Reaction with alkali metals also generates alkoxide ions. The hydride ion, H , combines with the partially positive hydrogen of the OH function to form molecular hydrogen. i) 2 CH 3 CH 2 CH 2 OH + 2 Na 2 CH 3 CH 2 CH 2 O Na + + H 2 ii) 2 (CH 3 ) 3 COH + 2 K 2 (CH 3 ) 3 CO Na + + H 2 The reactivity shows the trend: CH 3 OH > CH 3 CH 2 OH > (CH 3 ) 2 CHOH > (CH 3 ) 3 COH 2. Reaction of alcohols with strong acids, HBr or HI, generates alkoxonium ions. The alkoxonium ions are strong acids (Table 8.3); therefore the H 2 O is a good leaving group. Depending on the nature of the alcohol protonation may be followed by S N 2 or S N 1 reactions.
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2 i) Primary alcohols react by the S N 2 mechanism: CH 3 (CH 2 ) 2 OH + HBr CH 3 (CH 2 ) 2 + OH 2 + Br CH 3 (CH 2 ) 2 Br + H 2 O ii) Secondary or tertiary alcohols react by S N 1 or E1 mechanism: (CH 3 ) 3 C OH + HBr (CH 3 ) 3 C + OH 2 + Br (CH 3 ) 3 C + OH 2 (CH 3 ) 3 C + + OH 2 (CH 3 ) 3 C + + Br (CH 3 ) 3 C Br In summary 3. Carbocation rearrangements Some reactions proceeding via carbocations lead to unexpected products; we had discussed these earlier. The “normal” substituted product is obtained in low yield and an unexpected product in higher yield. 2° carbocations formed by loss of H 2 O from an oxonium ion may form more stable carbocations by rearrangement, migration of H or R (1,2-shift), yielding "unexpected” substitution products ( signifies rearrangement).
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3 Both carbocations react with Br but the intramolecular rearrangement is faster than the intermolecular “capture” by the nucleophile. In order to understand the mechanism by which these rearrangements occur, we recall the concept of hyperconjugation stabilizing a free radical or a carbocation (cf., Figure 9.2). Hyperconjugation works by distorting or shifting the electrons of a C H bond in the direction of the electron-poor carbon. For the rearrangement the hydrogen migrates along with the electron pair; this can be described as a hydride shift. The positive charge ends up at the carbon from where the hydride left.
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