8.5 Hydrohalogenation

What does treatment of an alkene with HX (HCl, HBr, HI) produce?

An addition reaction where H and X add across the π bond.

What is the name of this reaction?

Hydrohalogenation

What must be considered when the alkene is unsymmetrical?

Which carbon gets H and which carbon gets X

What are the two carbons involved in the decision?

The vinylic carbons (the two carbons of the double bond).

Who discovered the regioselectivity of hydrohalogenation?

Vladimir Markovnikov (1869).

According to Markovnikov, where does H add?

To the vinylic carbon already bearing more hydrogens.

According to Markovnikov, where does X add?

To the more substituted carbon.

What type of reaction is one that favors one regioisomer?

A regioselective reaction.

What unusual result was sometimes seen with HBr?

Bromine added to the less substituted carbon (anti-Markovnikov addition).

What impurity caused this behavior?

Alkyl peroxides (ROOR)

What does peroxide do to the mechanism?

Causes a radical mechanism → anti-Markovnikov addition.

Does peroxide affect HCl or HI in the same way?

No, only HBr undergoes anti-Markovnikov addition with peroxides.

How can the regiochemical outcome of HBr addition be controlled?

Use peroxides for anti-Markovnikov; avoid peroxides for Markovnikov.

How many steps are in the ionic hydrohalogenation mechanism?

Two steps.

What is the first step?

Protonation of the π bond to form a carbocation.

What is the second step?

Nucleophilic attack by the halide ion (X⁻).

Which step is rate-determining?

Proton transfer (step 1).

What two carbocations can form during protonation?

A less substituted secondary carbocation or a more substituted tertiary carbocation

Which carbocation is more stable?

The tertiary carbocation.

What effect does carbocation stability have on the reaction?

The reaction proceeds through the pathway that forms the most stable carbocation.

Why is the tertiary pathway faster?

It has a lower-energy transition state (Hammond postulate).

What explains the regioselectivity of hydrohalogenation?

Formation of the more stable carbocation intermediate.