Maximum number of isomeric monochloro derivatives which can be obtained from 2,2,5,5-tetramethylhexane by chlorination is _____

The structural formula of 2,2,5,5-tetramethylhexane can be written as:

$$\text{(CH}_3)_3\text{C} - \text{CH}_2 - \text{CH}_2 - \text{C(CH}_3)_3$$

This molecule is highly symmetrical, with a plane of symmetry right down the middle of the C3-C4 bond.

There are only two positions containing hydrogen atoms where chlorination can occur:

• Position A (Methyl groups): The 12 hydrogens on the four terminal $$-\text{CH}_3$$ groups attached to the quaternary carbons (C2 and C5) are completely equivalent.
• Position B (Methylene groups): The 4 hydrogens on the two $$-\text{CH}_2-$$ groups (C3 and C4) are also completely equivalent.

Chlorination at Position A (Methyl Groups)

Substituting one hydrogen from any of the methyl groups gives:

$$\text{Cl-CH}_2 - \text{C(CH}_3)_2 - \text{CH}_2 - \text{CH}_2 - \text{C(CH}_3)_3$$

• Chirality: The carbon attached to the $$-\text{CH}_2\text{Cl}$$ group is bonded to two identical methyl groups ($$-\text{CH}_3$$). Therefore, this molecule has no chiral center and is achiral.
• Isomer Count: 1 (structural isomer)
  

Chlorination at Position B (Methylene Groups)

Substituting one hydrogen from either C3 or C4 gives:

$$\text{(CH}_3)_3\text{C} - \text{CH(Cl)} - \text{CH}_2 - \text{C(CH}_3)_3$$

• Chirality: Let's look at the groups attached to the chlorinated carbon (C3):
  1. $$-\text{H}$$
  2. $$-\text{Cl}$$
  3. $$-\text{C(CH}_3)_3$$ (tert-butyl group)
  4. $$-\text{CH}_2-\text{C(CH}_3)_3$$ (neopentyl group)

  Since all 4 groups attached to C3 are completely different, C3 is a chiral (asymmetric) carbon center.

• Isomer Count: Because it contains one chiral center, it exists as a pair of enantiomers ($d$- and $l$-forms). This gives 2 stereoisomers.

$$\text{Total Isomeric Monochloro Derivatives} = 1 \text{ (achiral)} + 2 \text{ (enantiomers)} = \mathbf{3}$$