Question

Ethene when treated with Br₂ in the presence of CCl₄ which compound is formed.
(a) 1, 2-dibromoethane
(b) 1-bromo-2-chloroethane
(c) Both (a) and (b)
(d) 1, 1, 1 - tribromoethane

Answer 1

Hint: Bromine (\[B{r_2}\]) is a reddish-brown colour liquid. When it is treated with an alkene in the presence of carbon tetrachloride (\[CC{l_4}\]) the reddish-brown colour disappears with the formation of vicinal-dibromo (a colourless compound). This reaction is used for the detection of double bonds in organic compounds.

Complete Step by Step Answer:
When the unsaturated hydrocarbon like ethene is treated with bromine and chlorine in the presence of a non-nucleophilic solvent such as \[CC{l_4}\]this led to the isolation of vicinal dibromo or dichloroalkene respectively.

Figure 1: Bromination of alkene via bromine in the presence of carbon tetrachloride (\[CC{l_4}\]) as a solvent.

The solvent carbon tetrachloride (\[CC{l_4}\]) is used because of its non-reactive nature i.e., it does not interfere in the reaction pathway unlike other solvents such as., water and alcohol, etc.

During the bromination, the alkene is activated by the attack of the bromonium (\[B{r^ + }\]) ion, which is later attacked by the bromide (\[B{r^ - }\]) ion, hence, as a result, the formation of a trans-dibromo compound is occurs. Therefore, we can say bromination delivers bromine in the trans fashion to the alkene, or on the other hand the bromination of alkene is an anti-addition.

Figure 2: Reaction mechanism of bromination of an alkene.
In the question, option (b), (c), and (d) will be the incorrect option because the bromination of ethene do not involve the formation of 1-bromo-2-chloro or 1-bromo-2-chloro and 1, 2-dibromoethane together.
Therefore, from the above discussion, it is quite clear that option (a) will be the correct answer.

Note: The alkane, alkene and alkyne are saturated (alkane) and unsaturated (alkene and alkyne) hydrocarbons. Alkane is a saturated hydrocarbon, in which the carbon atoms contain the single bonds Alkene and alkyne belong to the family of unsaturated hydrocarbons in which the double and triple bond is present between the carbon atoms.