Question

The alkane which has only $ {1^ \circ }{\text{ }}H $ atoms is
A. Neopenatne
B. Isopentane
C. Pentane
D. 2,2- dimethylbutane

Answer 1

Hint: Here we have to find the degree of the carbon atom and thus the degree of the carbon atom is equal to the degree of hydrogen which is attached to it. Thus we will draw the structure of each given compound and hence find the degree of hydrogen atoms respectively by looking at the degree of carbon atom.

Complete answer:
The degree of carbon atom is equal to the number of carbon atoms attached to it. Therefore we will find the degree of carbon atom and thus predict the degree of hydrogen attached to that carbon atom. We will draw structure of each compound as:
A.Neopentane
The structure of Neopentane can be drawn as:

Here we can observe that there are four one degree carbon atoms and one four degree carbon atoms. At one degree carbons all hydrogen atoms are also one degree. Thus here all hydrogen atoms are one degree.
B. Isopentane
The structure of Isopentane can be drawn as:

Here we can observe that there are three $ {1^ \circ } $ carbon atoms ,one $ {2^ \circ } $ and one $ {3^ \circ } $ carbon atom. Thus we can say that there are hydrogen atoms which are $ {1^ \circ }{\text{ , }}{2^ \circ } $ and $ {3^ \circ } $ .
C. Pentane

Here we can say that two carbon atoms are $ {1^ \circ } $ and the rest are $ {2^ \circ } $ carbon atoms. Therefore it does not contain any one degree carbon atom. Thus no hydrogen of one hydrogen.
D. 2,2- dimethylbutane

Here we can observe that there are four $ {1^ \circ } $ carbon atoms ,one $ {2^ \circ } $ carbon atom and one $ {4^ \circ } $ carbon atom. Thus it contains $ {1^ \circ } $ and $ {2^ \circ } $ hydrogen atoms.
Hence on the basis of structure, the correct answer is option A.

Note:
We cannot predict the degree of hydrogen atom directly, therefore we will find the degree of carbon atom first and then we will predict the degree of hydrogen atom attached to that carbon atom. We can also say that the degree of a carbon atom gives us knowledge of the number of carbon atoms attached to that carbon atom.