What does Markovnikov's rule help to determine in an addition reaction?

Markovnikov's rule is a principle that guides the outcome of addition reactions involving unsymmetrical alkenes and reagents. Specifically, it states that when hydrogen halides or water are added to an alkene, the more substituted carbon atom in the double bond will bear the positive part of the reagent, while the less substituted carbon will be bonded to the hydrogen atom or the negative part of the reagent.

This rule is essential for predicting the major and minor products of an addition reaction. For example, in the case of adding hydrochloric acid (HCl) to propene, the hydrogen atom will add to the carbon with more hydrogen atoms already attached (the terminal carbon), while the chloride will attach to the more substituted carbon (the middle carbon), leading to the predominant formation of isopropyl chloride rather than n-propyl chloride.

Understanding Markovnikov's rule thus directly informs chemists on what products to expect from a reaction, allowing for the prediction that leads to successful synthetic strategies in organic chemistry.