radical chlorination is not as selective as radical bromination. the rate in which it reacts with primary, secondary or tertiary hydrogens is not that great of a difference.
So, in pentane, CH3CH2CH2CH2CH3, by the reaction with Cl2, the chlorine group can attach to any of the carbons which would form many isomers and not specifically 1-=chloropentane. In neopentane, (CH3)4C, it is different. each of the H atoms is equivalent. So no matter which one be substituted, only one product, (CH3)CCH2Cl, can be prepared.