Rules of the Day 2-16-07

Click here for a copy of the Wolff-Kishner mechanism sheet used in lecture today.

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1. Metal hydride reagents can be used to reduce aldehydes and ketones to alcohols, but in this case carbon-carbon double or triple bonds are NOT reduced. In these reagents, NaBH4 (less reactive) and LiAlH4 (more reactive), the hydrogen atom is the nucleophile, that attacks a carbonyl carbon atom to give a tetrahedral intermediate, followed by addition of water to the reaction flask that protonates the oxyanion to give the alcohol product. Mechanism A.

2. The Clemmensen reduction uses Zn(Hg) and HCl to convert the carbonyl groups of ketones and aldehydes into CH2 groups. The mechanism of this reaction is complex and will not be covered in this class. This reaction cannot be used if there are acid sensitive groups such as tertiary alcohols (they can lose H2O to give an alkene) or acetals present.

3. The Wolff-Kishner reduction, the one with the "deadly" mechanism, uses NH2-NH2 and HO- to convert the carbonyl groups of ketones and aldehydes into CH2 groups. This reaction can be used when acid sensitive groups are present. The mechanism involves hydrazone formation, two detprotonations by HO-, loss of N2 and two protonations of the carbon atom that used to be the carbonyl carbon atom.

Homework: Begin studying for the exam. You are responsible for all material covered up through today's lecture. Keto-enol equiolibria will not be on the first exam. What we cover next week will be on the second exam. Problems: Good review problems 16.44, 16.52, 16.55