This question involves identifying a branched-chain hydrocarbon isomer with molecular mass 72u that produces only one mono-substituted alkyl halide isomer upon halogenation. Let's break down the concept step by step.

Step 1: Determine the Molecular Formula

The molecular mass is given as 72u. For alkanes (saturated hydrocarbons), the general formula is $C_n{H}_{2n+2}$. We solve for n:

$12n+1(2n+2)=72$

$12n+2n+2=72$

$14n+2=72$

$14n=70$

$n=5$

So, the molecular formula is $C_5{H}_{12}$ (pentane).

Step 2: Identify the Isomers of Pentane

$C_5{H}_{12}$ has three isomers:

• n-pentane (straight chain)
• isopentane (2-methylbutane)
• neopentane (2,2-dimethylpropane)

Step 3: Understand Mono-Substituted Alkyl Halide Formation

Mono-substituted alkyl halide means replacing one hydrogen atom with a halogen (e.g., Cl). The number of possible isomers depends on the number of distinct types of hydrogen atoms in the hydrocarbon. Each unique hydrogen position gives a different alkyl halide isomer.

Step 4: Analyze Hydrogen Types in Each Isomer

• n-pentane (CH3-CH2-CH2-CH2-CH3): It has three types of hydrogens (terminal CH3, internal CH2, and middle CH2), so it gives three mono-substituted alkyl halide isomers.
• Isopentane (2-methylbutane): It has four types of hydrogens (three on the tertiary carbon, others on different CH3 and CH2 groups), so it gives multiple isomers.
• Neopentane (2,2-dimethylpropane): All 12 hydrogen atoms are equivalent (all are on equivalent primary carbons attached to the same quaternary carbon). So, replacing any hydrogen gives the same alkyl halide isomer. Thus, only one mono-substituted alkyl halide is possible.

Step 5: Match with the Options

Among the options:

• Isohexane and Neohexane are for C6H14 (hexane isomers), not C5H12.
• Tertiary butyl chloride is an alkyl halide (C4H9Cl), not a hydrocarbon.
• Neopentane is the correct branched-chain hydrocarbon (C5H12) that gives only one mono-substituted alkyl halide isomer.

Final Answer

Neopentane is the branched-chain isomer of C5H12 (molecular mass 72u) that gives only one isomer of mono-substituted alkyl halide because all its hydrogen atoms are equivalent.

Related Topics

Structural Isomerism: Compounds with the same molecular formula but different connectivity of atoms. Neopentane is an isomer of pentane.

Equivalent Hydrogens: Hydrogen atoms that are in identical chemical environments; replacing any of them yields the same product. Symmetry in molecules like neopentane leads to all hydrogens being equivalent.

Key Formulae

General formula for alkanes: $C_n{H}_{2n+2}$

Molecular mass calculation: $12n+1(2n+2)$