How to predict regiochemistry for unsymmetrical alkenes?

The regiochemistry of addition to unsymmetrical alkenes is predicted by considering how stable the intermediate will be and the electronic effects of substituents, applying Markovnikov or anti-Markovnikov outcomes depending on the mechanism.

Think of the double bond like a fork in the road and the reaction chooses the path that gives the most stable in-between step, usually a more substituted carbocation or a stabilized radical or transition state; for reactions that form carbocations, the electrophile attaches to the carbon that leads to the more substituted carbocation, which is Markovnikov addition, while reactions that proceed through radicals or involve peroxides can give anti-Markovnikov products; electron-withdrawing groups near one carbon can pull electron density away and change where the positive character will develop, and resonance or adjacent heteroatoms can stabilize an intermediate so the new bond forms there.

Students often think Markovnikov always applies which is wrong because the mechanism matters, so the quick fix is to ask first whether a carbocation, radical, or concerted pathway is involved and then apply the right rule; another common mistake is ignoring electronic effects like resonance or inductive withdrawal from substituents, and the tip is to draw the possible intermediates and compare their stability rather than relying on memory; sometimes people forget that steric hindrance can steer addition away from the more substituted carbon, and you can check this by sketching both product possibilities and seeing which transition state is less crowded.

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Predicting Regiochemistry of Unsymmetrical Alkenes

1. Choose path giving most stable intermediate
2. Favor formation of more substituted carbocation
3. Consider radical or concerted pathways
4. Account for electron-withdrawing group effects
5. Use resonance to identify stabilized sites
6. Let substituents guide bond formation location

Common Regiochemistry Mistakes in Additions

1. Assuming Markovnikov always applies
2. Ignoring resonance and inductive effects
3. Overlooking steric hindrance effects

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