October 15 - 3 o alkyl halides best substrates in S N 1...

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Lecture #22, October 15, 2008 S N 1: Unimolecular Nucleophilic Substitution Br H 2 O OH + Br + H 3 O Mechanism Step 1: bromine leaves, and a carbocation forms Step 2: water attacks the carbocation Step 3: deprotonation to give the alcohol Rate = k [alkyl halide] Ionization of alkyl halide Br slow + Br 3º carbocation Nucleophilic attack on carbocation 3º carbocation + O H H fast O H H O H H + O H H Note: H 2 O is a relatively stronger base than the bromide ion and functions as the weak base here.
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Energetics of S N 1 solvolysis of t-butyl bromide potential energy reaction coordinate Br + H 2 O + Br O H H + H 2 O + H 2 O OH + Br + H 3 O Rate-limiting step is carbocation formation .... CH 3 Br Br Br Br Refer to Figure 8.5 on page 332 Methyl and primary carbocations are high energy species; methyl and primary alkyl halides DO NOT undergo S N 1 reactions Tertiary carbocations are most stable; tertiary halides undergo S N 1 reactions
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Unformatted text preview: 3 o alkyl halides best substrates in S N 1, then 2 o ... but never 1 o or methyl Any substrate that can form a stable carbocation can react via S N 1 mechanism: Br allylic cation stabilized by resonance allylic bromide Br benzylic cation stabilized by resonance H 3 C CH 3 H 3 C Br F 3 C CF 3 F 3 C Br H 3 C CH 3 CH 3 + Br F 3 C CF 3 CF 3 + Br Which of these carbocations forms faster? K 1 K 2 A B Carbocation A is stabilized by the inductive effect of electron-releasing methyl groups and by hyperconjugation (refer to page 157); hence, it forms at a faster rate than B. Carbocation B is de-stabilized due to juxtaposition of charges; the highly electronegative fluorine leads to a polar covalent C-F bond with a fractional positive charge on the carbon of the CF 3 group....
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