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 to cyclohexene is a carbocation, with the structure: \( \chemfig{ *6(=-=(-[+])-=-=) } \) This species is generated after the alkene's double bond attacks the electropositive hydrogen atom in the hydrogen halide, breaking its bond with the halogen atom. The carbocation then reacts with the negatively charged halide anion to form the final product of the reaction.

Step-by-step solution

Identify the reactants

In this reaction, the reactants are cyclohexene (an alkene) and a hydrogen halide (HX, where X could be a halogen atom like Cl, Br or I). Cyclohexene: \( \chemfig{*6(=-=-=-)} \) HX: H-X

Understand the reaction mechanism

The reaction mechanism for the addition of a hydrogen halide to an alkene is an electrophilic addition reaction. In this mechanism, the double bond of the alkene will attack the hydrogen atom of the hydrogen halide, breaking its bond with the halogen atom. This leads to the formation of a carbocation, which is a species with a positive charge on a carbon atom. The halogen anion then attacks the carbocation to form the final product.

Describe the attack of the alkene double bond on the hydrogen atom

Initially, the pi electrons in the alkene's double bond will be attracted to the electropositive hydrogen atom in the hydrogen halide, forming a new bond between the hydrogen atom and one of the carbons of the alkene. This breaks the bond between hydrogen and halogen, leaving a negatively charged halide anion (X^-) and generating a carbocation intermediate.

Describe the formation of the carbocation intermediate

When the pi bond in the cyclohexene attacks the hydrogen atom in the hydrogen halide, the cyclohexene loses its double bond and forms a carbocation intermediate. This intermediate has a positive charge on the carbon atom that was part of the double bond but did not form a new bond with the hydrogen atom. The carbocation intermediate formed looks like this: \( \chemfig{ *6(=-=(-[+])-=-=) } \)

Describe the attack of the halide anion on the carbocation

Finally, the negatively charged halide anion (X^-) that was produced earlier will attack the positively charged carbon atom in the carbocation intermediate. This leads to the formation of a new bond between the carbon and the halogen atom, yielding the final product of the addition reaction between cyclohexene and the hydrogen halide. In conclusion, the intermediate formed during the addition of a hydrogen halide to cyclohexene is a carbocation, which looks like this: \( \chemfig{ *6(=-=(-[+])-=-=) } \)