High school9 min

Addition, substitution, elimination: classifying a reaction

Three categories, two questions to tell them apart — how many species, what happens to the double bond — and Markovnikov's rule for placing the H.

The essentials

1

Addition

Two reactants, a single product: an unsaturation disappears. The added molecule attaches across the double bond. Examples: CHX2=CHX2+HBr→CHX3−CHX2−Br\ce{CH2=CH2 + HBr -> CH3-CH2-Br} (bromoethane); catalytic hydrogenation CHX2=CHX2+HX2→CHX3−CHX3\ce{CH2=CH2 + H2 -> CH3-CH3}, over palladium or platinum.
2

Substitution

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3

Elimination

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4

Markovnikov's rule

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5

What the starting molecule lets you expect

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