User:Benjah-bmm27/degree/1/VKA

Molecular orbital picture of the attack of the HOMO of a nucleophile (chloride) on the LUMO (C-O π*) of a carbonyl compound (formaldehyde). The preferred angle of attack, termed the Bürgi-Dunitz angle, is about 107°.

Introduction

"Reactions and Synthesis with Carbonyl Compounds", Professor Varinder Aggarwal.

General features of carbonyl reactions

• The carbonyl group is electrophilic at carbon, so many nucleophiles can (and do) attack there
• The mechanisms of these reactions generally involve the following steps:
  • A nucleophile adds to the carbon atom of a carbonyl group. This step is called nucleophilic addition.
  • The product formed by nucleophilic addition to a carbonyl group is generally a tetrahedral carbonyl addition compound.
  • If the tetrahedral product is unstable, it can collapse back to a trigonal carbonyl compound by expelling a leaving group.
  • The overall process of nucleophilic addition followed by leaving group elimination is called nucleophilic acyl substitution, because the nucleophile replaces the leaving group as a substituent on the carbonyl carbon.

Particular examples of carbonyl reactions

• Reduction of aldehydes and ketones to alcohols with sodium borohydride, NaBH₄
• Reduction of esters to alcohols with lithium aluminium hydride, LiAlH₄
• Jones oxidation: oxidation of primary alcohols to carboxylic acids and secondary alcohols to ketones with chromium trioxide, CrO₃ (can also use e.g. pyridinium chlorochromate, PCC)
• Ester hydrolysis, such as saponification
• Alkylimino-de-oxo-bisubstitution
• RCO₂H + OSCl₂ → RCOCl + HCl + SO₂
  • goes via an S_Ni mechanism