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 during the addition of a hydrogen halide (H-X) to cyclohexene is a carbocation. The π-bond electrons of cyclohexene bond with the electrophilic hydrogen atom of the polar H-X bond, breaking the double bond and generating a positive charge on the carbon atom where the hydrogen attaches. The carbocation intermediate structure is represented as \(\text{(H-)}\text{Cyclohexyl}^+(\text{-X})\). The subsequent step involves the X^- ion donating its lone pair of electrons to the carbocation, ultimately forming a substituted cyclohexane with the halogen atom.

Step-by-step solution

Write the reaction with the alkene and hydrogen halide

In this step, write down the general reaction of an alkene with a hydrogen halide. We'll be using cyclohexene as the alkene and H-X as the hydrogen halide. The reaction would look like this: Cyclohexene + H-X → Product

Identify the electrophilic addition mechanism

An electrophilic addition reaction involves the initial addition of an electrophile (in this case, the hydrogen atom from the hydrogen halide) to the double bond of the alkene. This initial addition step creates a charge-separation and results in the formation of an intermediate carbocation.

Describe the formation of the carbocation intermediate

The hydrogen atom of the hydrogen halide (H-X) is bonded to a halogen atom, which is more electronegative than hydrogen. As a result, the H-X bond is polar and the hydrogen carries a partial positive charge. The double bond in cyclohexene consists of electron-rich π-bond, which acts as a nucleophile and can be easily attracted by the electrophilic hydrogen. As the π-bond electrons of cyclohexene bond with the hydrogen atom, the electrons from the H-X bond move towards the halide, forming a halide ion X^-. The process breaks the double bond and generates a positive charge on the carbon atom where the hydrogen atom attaches, creating a carbocation intermediate.

Illustrate the carbocation intermediate's structure

During the formation of the carbocation intermediate, one of the double-bonded carbons in cyclohexene forms a single bond with hydrogen from the hydrogen halide, leading to the formation of a carbocation. The structure of the carbocation intermediate in this reaction involving cyclohexene and H-X can be depicted as: \[ \text{(H-)}\text{Cyclohexyl}^+(\text{-X}) \] Keep in mind that there might be various resonance forms, depending on the carbon adjacent to the positively charged carbon. Finally, in a fast and reversible step, the X^- ion (formed in the first step of the reaction) donates its lone pair of electrons to the carbocation, attaching itself to the positively charged carbon. This forms the final product, a substituted cyclohexane with the halogen atom at the position where the double bond was broken.