Identify substrate 'S' in the following reaction.
$\text{S} \xrightarrow{\large{\text{Na / dry ether}}}$ 3, 4 - diethyl - 3, 4 - dimethyl hexane

The given reaction is a Wurtz reaction, which involves the coupling of two alkyl halide molecules in the presence of sodium and dry ether to form a symmetrical alkane.

The structure of the product, 3,4-diethyl-3,4-dimethylhexane, is:
$\text{CH}_3\text{-CH}_2\text{-C(CH}_3\text{)(CH}_2\text{CH}_3\text{)-C(CH}_3\text{)(CH}_2\text{CH}_3\text{)-CH}_2\text{-CH}_3$

Since the Wurtz reaction produces a symmetrical alkane by the dimerization of alkyl radicals, we can determine the structure of the reactant by cleaving the central carbon-carbon bond of the product.

Cleaving the bond between C3 and C4 yields two identical radicals:
$\text{CH}_3\text{-CH}_2\text{-C}^{\bullet}\text{(CH}_3\text{)(CH}_2\text{CH}_3)$

The corresponding alkyl halide substrate 'S' is formed by attaching a halogen (e.g., chlorine) to the radical carbon:
$\text{CH}_3\text{-CH}_2\text{-C(Cl)(CH}_3\text{)-CH}_2\text{-CH}_3$

The longest carbon chain has 5 carbon atoms. Numbering the chain from either side gives the same locants, with a chloro group and a methyl group at the 3rd carbon.

Thus, the IUPAC name of the substrate 'S' is 3-chloro-3-methylpentane.

Answer: 3 - chloro - 3 - methylpentane