Hydrocarbons: Alkanes and Alkenes: Question 5

Worked solution

Step 1: Recall what an addition reaction does to the C=C bond

But-2-ene is unsaturated because it contains a carbon–carbon double bond. In an addition reaction, this double bond opens up and the atoms of the second reactant (bromine, hydrogen or steam) attach across it, so that only single bonds remain between the carbon atoms. Each addition reaction here forms only one product:

• $CH_3CH=CHCH_3 + Br_2 \rightarrow CH_3CHBrCHBrCH_3$
• $CH_3CH=CHCH_3 + H_2 \rightarrow CH_3CH_2CH_2CH_3$ (nickel catalyst)
• $CH_3CH=CHCH_3 + H_2O \rightarrow CH_3CH_2CH(OH)CH_3$ (acid catalyst)

Step 2: Check option C against all three products

In every product above, the carbon–carbon double bond has been replaced by a single bond, because addition uses up the double bond. So all three products contain only single covalent bonds between carbon atoms. Option C is correct.

Step 3: Check why the other options are wrong

• A: Wrong, addition converts the unsaturated alkene into saturated products; none of the products is unsaturated.
• B: Wrong. Each product’s molecular formula differs, since atoms from the second reactant are added on: the dibromide is $C_4H_8Br_2$, the alkane product is $C_4H_{10}$, and the alcohol product is $C_4H_{10}O$. None of them is $C_4H_8$ (that is but-2-ene’s own formula).
• D: Wrong. Decolourising aqueous bromine is a test for the C=C double bond. Since all three products are saturated, none of them will decolourise aqueous bromine.

Final answers

• All three products are saturated, with only single covalent bonds between carbon atoms, $\boxed{\text{option C}}$.