8Chapter 04

bonds dr wolfs chm 201 202 4 87 figure 416

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Unformatted text preview: fore, alkyl groups therefore, stabilize carbocations better than H. Electronic effects transmitted through Electronic σ bonds are called "inductive effects." bonds Dr. Wolf's CHM 201 & 202 4- 87 Figure 4.16 Stabilization of carbocations Figure 4.16 Stabilization of carbocations Figure Figure via hyperconjugation via hyperconjugation via via + Dr. Wolf's CHM 201 & 202 electrons in this σ electrons bond can be shared by positively charged carbon because the σ orbital can overlap with the empty 2p with orbital of positively charged carbon 4- 88 Figure 4.16 Stabilization of carbocations Figure 4.16 Stabilization of carbocations Figure Figure via hyperconjugation via hyperconjugation via via δ+ δ+ Dr. Wolf's CHM 201 & 202 electrons in this σ electrons bond can be shared by positively charged carbon because the σ orbital can overlap with the empty 2p with orbital of positively charged carbon 4- 89 Figure 4.16 Stabilization of carbocations Figure 4.16 Stabilization of carbocations Figure Figure via hyperconjugation via hyperconjugation via via δ+ δ+ Dr. Wolf's CHM 201 & 202 Notice that an occupied orbital of this type is available when sp3 available sp hybridized carbon is hybridized attached to C+, but is attached but not available when H not is attached to C+. Therefore,alkyl groups Therefore,alkyl stabilize carbocations better than H does. 4- 90 4.11 Effect of Alcohol Structure on Effect Reaction Rate Reaction Dr. Wolf's CHM 201 & 202 4- 91 Slow step is: + R O• H •H + R H + • O• • •H The more stable the carbocation, the faster it is formed. Tertiary carbocations are more stable than secondary, which are more stable than primary, which are more stable than methyl. Tertiary alcohols react faster than secondary, Tertiary which react faster than primary, which react faster which than methanol. Dr. Wolf's CHM 201 & 202 4- 92 High activation energy for formation of methyl cation. δ+ CH3 δ+ H O• •H Eact H + CH3 + • O • • + CH3 O • •H H •H Dr. Wolf's CHM 201 & 202 4- 93 Smaller activation energy for formation of primary carbocation. δ+ RCH2 δ+ O• •H Eact + RCH2 O • H + RCH2 + • O • • H •H H •H Dr. Wolf's CHM 201 & 202 4- 94 Activation energy for formation of secondary Activation secondary carbocation is less than that for formation of carbocation primary carbocation. δ+ R2CH R2CH + H δ+ H O• •H Eact + R2CH + • O • • H •H O• •H Dr. Wolf's CHM 201 & 202 4- 95 Activation energy for formation of tertiary Activation tertiary carbocation is less than that for formation of carbocation secondary carbocation. δ+ R3C δ+ H O• •H R3C + H O• •H Dr. Wolf's CHM 201 & 202 Eact + R3C + • O • • H •H 4- 96 4.12 Reaction of Primary Alcohols with Reaction Hydrogen Halides. Hydrogen The SN2 Mechanism Dr. Wolf's CHM 201 & 202 4- 97 Preparation of Alkyl Halides Preparation of Alkyl Halides 25°C (CH3)3COH + HCl (CH3)3CCl + H2O 78-88% OH + HBr OH 80-100°C Br + H2O Br 73% CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O Dr. Wolf's CHM 201 & 202 87-90% 4- 98 Preparation of Alkyl Halides Preparation of Alkyl Halides Primary carbocations are too high in energy to Primary allow SN1 mechanism. Yet, primary alcohols allow mechanism. are converted to alkyl halides. Primary alcohols react by a mechanism called SN2 (substitution-nucleophilic-bimolecular). CH3(CH2)5CH2OH + HBr 120°C CH3(CH2)5CH2Br + H2O Dr. Wolf's CHM 201 & 202 87-90...
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