Nucleophilic Addition Mechanism

Aldehydes and ketones react by nucleophilic addition. The C=O bond is polar: the carbon is δ+ and the oxygen is δ−. A nucleophile (an electron pair donor) is attracted to the δ+ carbon, and two species join to make one product, so it is an addition.The mechanism, using NaBH₄ (nucleophile H⁻):Step 1. A curly arrow goes from the lone pair on the hydride ion, :H⁻, to the δ+ carbonyl carbon. At the same time, a curly arrow goes from the C=O double bond to the oxygen atom: the π bond breaks and its electrons move onto the oxygen. This gives a negatively charged intermediate with O⁻ (an alkoxide ion).Step 2. A curly arrow goes from a lone pair on the O⁻ to an H⁺ ion (from water or dilute acid). The oxygen is protonated and the alcohol forms.Getting the marks: show the δ+ and δ− on C=O, start the first arrow from the lone pair (or negative charge) on the nucleophile, show the arrow from the C=O bond to O, draw the intermediate with its negative charge on oxygen, and start the last arrow from a lone pair on O⁻.The same mechanism works with the cyanide ion, :CN⁻, as the nucleophile, which the next lesson covers.Example: H⁻ adds to propanone, CH₃COCH₃, giving the intermediate CH₃CH(O⁻)CH₃, which is protonated to give propan-2-ol, CH₃CH(OH)CH₃.