Chapter 16 (Lecture #12)

Chapter 16 (Lecture #12) - Nucleophilic Addition to the...

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Nucleophilic Addition to the Carbon-Oxygen Double Bond The reactivity of the carbon-oxygen double bond is determined by the polarization : C = O : : !" ! + Because of the permanent dipole, nucleophiles add to the electropositive carbon and electrophiles add to the electronegative oxygen. Two General Mechanisms (1) When the reagent is a strong nucleophile ( Nu: - ) such as an organometallic or metal hydride reagent, addition usually occurs first to the carbon center forming an oxide ion. Electrophilic addition then occurs to the oxide.
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C = O : : Nu: - sp 2 -hybridized carbon trigonal planar C O: : : R R' Nu R R' - sp 3 -hybridized carbon tetrahedral E + C O- E : : Nu R R' addition product tetrahedral In these additions, E + is often a proton or metal ion.
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(2) A second general mechanism is acid-catalyzed nucleophilic addition . This mechanism often involves weaker oxygen nucleophiles adding to the carbonyl group in the presence of catalytic amounts of acid. C = O : : R R' + H-A fast and reversible protonation acid catalyst C = O-H : R R' + C - O-H : R R' + : + A: - protonated carbonyl is more reactive towards nucleophilic addition C = O-H : R R' + + :Nu-H nucleophilic addition C R R' Nu-H + OH : : reversible addition C R R' Nu-H + OH : : + A: - deprotonation C R R' Nu: OH : : + H-A acid is regenerated addition product
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Structural Effects: Aldehydes versus Ketones Generally, aldehydes are more reactive than ketones in nucleophilic addition reactions. This reactivity difference arises from both steric and electronic factors. Steric Factors C = O R R' 120 o + Nu: - trigonal planar C R R' Nu O - tetrahedral 109 o In the TS, the R and R' groups are moving towards each other. C O Nu !" !" R R' In the TS of ketones, where R and R' are alkyl or aryl groups, there is some degree of buildup of steric strain as the groups move closer together.There is less steric strain in the TS of aldehydes.
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Electronic Factors The starting state of a ketone is generally stabilized relative to an aldehyde by the presence of two electron-releasing groups that electronically interact with the carbonyl. C :O: = R H C :O: = R R' !" !" ! + ! + more stable less stable This difference in the energy of the starting states contributes to the greater reactivity of aldehydes compared with ketones.
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