Symmetrical alkanes yield one monobromo product in bromination

Steps:

• Bromine reacts with alkanes via free radical substitution, replacing one H with Br.
• A single product requires all hydrogens to be chemically equivalent.
• Ethane (C₂H₆) has six identical primary hydrogens attached to equivalent carbons.
• Substitution yields only CH₃CH₂Br (bromoethane).

Why B is correct:

• C₂H₆ has equivalent hydrogens, so free radical substitution follows Markovnikov's principle selectively to give one product.

Why the others are wrong:

• A. CH₄ has equivalent hydrogens but bromination is violent at high temperatures, often leading to polybromination rather than a single controlled product.
• C. C₂H₂ undergoes electrophilic addition with Br₂, forming multiple products like cis/trans-1,2-dibromoethene and 1,1,2,2-tetrabromoethane.
• D. CH₃OH is an alcohol, not a hydrocarbon, and does not undergo addition with Br₂ to form a single product.

Final answer: B