Identify the Alkane (molecular formula $\mathrm{C}_8 \mathrm{H}_{18}$ ) which yields only a single monochloride on Chlorination in the presence of sunlight.

The formation of only a single monochloride upon chlorination implies that all hydrogen atoms in the alkane molecule are equivalent. This means the molecule must possess a high degree of symmetry such that replacing any hydrogen atom with a chlorine atom results in the same product.

Analyzing the options:

2,4-Dimethylhexane has multiple types of hydrogen atoms (primary, secondary, and tertiary), leading to multiple monochlorinated isomers.

3,4-Dimethylhexane also has multiple types of hydrogen atoms, leading to multiple monochlorinated isomers.

2,2,3,3-Tetramethylbutane has a structure where all 18 hydrogen atoms are equivalent. Specifically, it consists of two tert-butyl groups connected by a central C-C bond. Each of the four methyl groups attached to the quaternary carbons is equivalent, and there are no other types of hydrogens. Thus, chlorination yields only one monochloride, 1-chloro-2,2,3,3-tetramethylbutane.

2,2,4-Trimethylpentane (isooctane) has several distinct types of hydrogen atoms (primary, secondary, and tertiary), leading to multiple monochlorinated products.

Therefore, 2,2,3,3-tetramethylbutane is the correct alkane.

Answer: 2, 2, 3, 3-Tetramethylbutane