chapter 6 clayden organics 大学有机化学.pdf-全文可读

来自 : max.book118.com/html/2018/0806 发布时间：2021-03-25

The aims of this chapter are to learn ….. • How and why the C=O group reacts with nucleophiles; • Explaining the reactivity of the C=O group using molecular orbitals and curly arrows; • What sorts of molecules can be made by reactions of C=O groups; • How acid or base catalysts improve the reactivity of the C=O group. Addition to carbonyl group Additions to carbonyl groups generally consist of two mechanistic steps: - Nucleophilic attack on the carbonyl group - Protonation of the anion that results Why do nucleophiles attack the carbonyl group?  carbonyl double bond consists of two parts: one σ bond and one π bond.  σ bond between carbon and oxygen is formed from two sp2 orbitals. The other sp2 orbitals on carbon form the two σ bonds to the substituents while those on oxygen are filled by the two lone pairs.  sp2 hybridization means that the carbonyl group has to be planar, and the angle between the substituents is close to 120°.  π bond then results from overlap of the remaining p orbitals Electronegativities, bond lengths, and bond strengths Representative bond energies (kJ mol– 1): C–O 351; C=O 720 Representative bond lengths (Å): C–O 1.43; C=O 1.21 Electronegativity: C 2.5; O 3.5 - Because there are two types of bonding between C and O, the C=O shorter than a typical C–O single bond, and also over twice as strong So why is it so reactive? - polarized C=O bond gives the C atom some positive charge, and this charge attracts negatively charged nucleophiles; - polarization of the antibonding π* orbital towards carbon is also important - when the carbonyl group reacts with a nucleophile, electrons move from HOMO of the nucleophile into LUMO of the electrophile – the greater coefficient of the π* orbital at C means a better HOMO–LUMO interaction. Attack o