Question

Describe the intermediate that is thought to form in the addition of a hydrogen halide to an alkene, using cyclohexene as the alkene in your description.

Short answer

The intermediate formed in the addition of a hydrogen halide to cyclohexene is a carbocation with a positive charge on one of the carbon atoms in the cyclohexene ring. This occurs after the double bond of cyclohexene attacks the hydrogen atom of the hydrogen halide, partially breaking the double bond and bonding with the hydrogen atom. The carbocation intermediate is highly reactive and unstable due to its incomplete octet.

Step-by-step solution

Understand the reaction mechanism

The reaction of an alkene with a hydrogen halide follows the electrophilic addition mechanism. In this reaction, the alkene acts as a nucleophile, attacking the electrophilic hydrogen of the hydrogen halide. As a result, the double bond (π bond) of the alkene is partially broken, and a carbocation intermediate is formed.

Describe the reaction between cyclohexene and a hydrogen halide

Cyclohexene is an alkene with a six-membered cyclic structure which contains one double bond. When a hydrogen halide (HX, where X is a halogen) reacts with cyclohexene, the alkene's double bond attacks the hydrogen atom of the hydrogen halide. This results in the formation of a new bond between the carbon atom of the alkene and the hydrogen atom. This step also releases a halide ion (X-).

Identify the intermediate species in this reaction

After the first step, a carbocation intermediate is formed. This intermediate has a positive charge on the carbon atom, where the double bond has been partially broken (the bond with the halogen atom has not yet been formed). This carbocation is highly reactive and unstable, as it has an incomplete octet.

Complete the reaction mechanism

Finally, the halide ion (X-) that was previously released will attack the carbocation intermediate. It forms a bond with the positively charged carbon atom, resulting in the formation of the final product, a substituted cyclohexane (a halo-substituted cyclohexane, i.e., halocyclohexane). So, the intermediate species formed in the addition of a hydrogen halide to cyclohexene is a carbocation with a positive charge on one of the carbon atoms of the cyclohexene ring, after it has bonded with the hydrogen atom.