1, 3-butadiene reacts with ethylene to form
A. Benzene
B. Cyclohexane
C. Cyclohexene
D. 2, 3 dimethyl butane

Hint: Hydrocarbons containing two double bonds are called dienes. These are also called alkadienes. The reaction of 1,3-butadiene with ethylene involves the formation of a six-membered ring by the 1,4-addition of an alkene to a conjugated diene.

Complete Step by Step Answer:
Hydrocarbons containing two double bonds are called dienes. These are also called alkadienes.
These compounds are isomeric with alkynes.

1,3-butadiene is a conjugated diene or a single bond that intervenes with two double bonds. They have an arrangement of alternate single and double bonds.

The reaction of 1,3-butadiene with ethylene involves the formation of a six-membered ring by the 1,4-addition of an alkene to a conjugated diene. This reaction is known as the Diels-Alder reaction.

Alkene used is known as a dienophile and the product is known as Diels-Alder adduct. It is a cycloaddition reaction. Diels-Alder reaction is a 1,4 addition of ethene to 1,3-butadiene and also a 1,2 addition of butadiene to ethene. It is known as [4 + 2] cycloaddition leading to the formation of a six-membered ring. This six-membered ring is cyclohexene.

The reaction happens as follows:

Image: Diels-Alder reaction
So, 1, 3-butadiene reacts with ethylene to form cyclohexane.
So, option C is correct.

Note: Ethene and other simple alkenes normally are weak dienophiles and react with 1,3-butadiene only under drastic conditions and in low yield. But, when the hydrogen atoms in the diene are replaced with various electron-attracting groups such as chlorine or bromine, electron-donating groups the reaction happens on its own.