In organic chemistry, which of the following is an empirical rule used to predict the regioselectivity of electrophilic addition reactions of alkanes and alkynes?

Markovnikov's rule

The correct answer is option 3. Markovnikov's rule.

In organic chemistry, the empirical rule used to predict the regioselectivity of electrophilic addition reactions of alkanes and alkynes is Markovnikov's rule.

Markovnikov's rule: This rule states that in the addition of an unsymmetrical hydrogen halide (HX) to an unsymmetrical alkene, the hydrogen atom from the HX molecule becomes bonded to the carbon atom in the alkene that already has the greater number of hydrogen atoms attached to it. This results in the formation of a more stable carbocation intermediate.

The other options you provided are used in different contexts:

Cornforth's rules: These rules are used to predict the stereochemistry of certain organic reactions, not the regioselectivity.

Bredt's rule: This rule states that stable molecules do not contain "strained" rings, such as rings with fewer than four or more than eight atoms. It is not specifically related to regioselectivity.

Baldwin's rules: These rules predict the preferred ring sizes formed in ring-closing reactions, not the regioselectivity of addition reactions.

Therefore, Markovnikov's rule is the only option that specifically addresses the prediction of regioselectivity in electrophilic addition reactions of alkanes and alkynes.