Notes on Stereochemistry of Nucleophilic Addition Reactions | CHEM 343, Study notes of Organic Chemistry

Download Notes on Stereochemistry of Nucleophilic Addition Reactions | CHEM 343 and more Study notes Organic Chemistry in PDF only on Docsity! 17 February Stereochemistry of nucleophilic addition reactions: re and si faces Prochiral carbonyl carbon – a carbonyl carbon bonded to two different substituents The two faces may be classified as re or si according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re; if counterclockwise, then si. Nucleophilic addition to α,β-unsaturated aldehydes and ketonesunsaturated aldehydes and ketones The reaction pattern (direct addition vs conjugated addition) depends on the nature of the nucleophile. Nucleophiles that are strong bases (such as RMgX, H-unsaturated aldehydes and ketones) tend to form direct addition products. Nucleophiles that are relatively weak bases (such as cyanide ions, amines, thiols, and halide ions) usually form conjugate addition products. Direct addition occurs more rapidly than conjugate addition, therefore, the direct addition product is formed first. When nucleophile is a strong base, the direct addition is irreversible and the resulting product will be the final product of the reaction. When nucleophile is a relatively weak base, the direct addition is reversible and the initially formed direct addition product can collapse back to the starting material, allowing conjugated addition to occur. Since conjugate addition is irreversible, the conjugate addition product will accumulate and eventually become the major product of the reaction.