Question

Which compound has total molecular mass of 72 with three primary carbons?

• A. n-pentane
• B. 2,2-dimethylbutane
• C. 1,1-dimethylcyclopropane
• D. 2-methylbutane

Answer 1

The Correct Option is D

Concept:
A primary carbon is a carbon atom attached to only one other carbon atom. In alkanes, terminal \(CH_3\) groups are primary carbons. The molecular mass requirement must also match the given value. Step 1: Determine molecular formula for molar mass 72. For alkanes: \[ C_nH_{2n+2} \] Check \(n=5\): \[ C_5H_{12} \] Molecular mass: \[ 5(12) + 12(1) = 60 + 12 = 72 \] Thus the compound must be an isomer of \(C_5H_{12}\). Possible isomers: \[ \text{n-pentane} \] \[ \text{2-methylbutane} \] \[ \text{2,2-dimethylpropane} \] Step 2: Count primary carbons. n-pentane \[ CH_3-CH_2-CH_2-CH_2-CH_3 \] Primary carbons = 2 2-methylbutane \[ CH_3-CH(CH_3)-CH_2-CH_3 \] Primary carbons = 3 2,2-dimethylpropane \[ C(CH_3)_4 \] Primary carbons = 4 Step 3: Select compound with three primary carbons. Only 2-methylbutane has three primary carbons. \[ \boxed{\text{2-methylbutane}} \]