Question

The minimum number of carbon atoms in an alkane having four primary carbon atoms are:
(1) $4$
(2) $8$
(3) $5$
(4) $6$

Answer 1

Hint: As we know that a primary carbon atom is the one which is bound to only one other carbon atom and in case of alkane, a primary carbon is bound with three hydrogen atoms and results in making of an acyclic structure.

Complete Step by step answer:
- A primary carbon atom is the one which is bound to only one other carbon atom and in case of alkane, a primary carbon is bound with three hydrogen atoms and results in making of an acyclic structure and not a ring structure. We can easily identify a primary, secondary or tertiary carbon in a compound by just counting the number of carbons attached to the main carbon or one at least one carbon.
- Primary carbon is attached with one other carbon, a secondary carbon is the one attached to the two other carbons and a tertiary carbon is the one attached to the three other carbon atoms and a quaternary carbon has four carbons attached to one carbon. For instance, ethane with one simple primary carbon, propane with two carbons attached to one primary carbon etc .
- Similarly in the given question the alkane having four primary carbon atoms can be a neopentane where the main chain is having three carbon atoms and two methyl groups attached to central carbon atom and the structure can be given as follows:

Neopentane has the simplest structure and the primary carbon has five carbon atoms each attached with three hydrogen atoms giving a molecular formula as $$C_5{H}_{12}$$.

Therefore the correct answer is (3).

Note: Neopentane is a flammable gas substance at room temperature and pressure with a boiling point of only ${9.5}^{\circ }C$ and melting point of $-{16.5}^{\circ }C$. It can liquefy in cold weather or cool places. Its carbocation is one of the most stable among all the carbocations without any resonance.