Question

Glycerol does not contain __________ alcoholic group.
(A) $1^{\circ}$
(B) $2^{\circ}$
(C) $3^{\circ}$
(D) $1^{\circ}$ and $2^{\circ}$

Answer 1

Hint: The IUPAC name for glycerol is propane-$1,2,3$-triol, represented by the chemical formula $CH_2OH-CH_2OH-CH_2OH$. The degree of alcohol is determined by the degree of each carbon atom present in it. The degree of a carbon atom is the number of other carbon atoms surrounding it.

Complete step by step solution:
A carbon atom can be of three types - $1^{\circ}, 2^{\circ} , 3^{\circ}$.
$A 1^{\circ}$ carbon is a carbon atom that is not attached to any other carbon atom.
$A 2^{\circ}$ carbon is a carbon atom that is attached to one carbon atom and three other atoms.
$A 3^{\circ}$ carbon is a carbon atom that is attached to two other carbon atoms and another atom.
The structure of propane-1,2,3-triol can be represented as under:

The first carbon atom is attached to the second carbon atom, two hydrogen atoms, and one $OH$ group. Since it is attached to only one carbon atom, the first carbon atom is of $1^{\circ}$.
The second carbon atom is attached to the first and three carbon atoms, one hydrogen atom, and one $OH$ group. Since it is attached to two adjacent carbon atoms, the second carbon atom is of $2^{\circ}$.
The third carbon atom is attached to the second carbon atom, one hydrogen atom, and one $OH$ group. Since it is attached to only one adjacent carbon atom, the third carbon atom is of $1^{\circ}$.
From the above observations, it can be concluded that glycerol contains only $1^{\circ}$ and $2^{\circ}$ carbon atoms. Therefore, $3^{\circ}$ carbon atoms are absent in glycerol.

So, the correct answer is Option c.

Note: Glycerol should not be confused with glycol, whose IUPAC name is ethane-$1,2$-diol. The chemical formula for a glycol is $CH_2OH-CH_2OH$.
A $1^{\circ}$ carbon atom is also called a primary carbon atom.
A $2^{\circ}4$ carbon atom is also called a secondary carbon atom.
A $3^{\circ}$ carbon atom is also called a tertiary carbon atom.