Question

The maximum number of isomers possible for an alkene with molecular formula C₄H₈ is:

• A. 5
• B. 4
• C. 2
• D. 3

Hint:

List the straight chain, branched chain, and cis-trans forms of C4H8 separately.

Answer 1

The Correct Option is B

Step 1: C4H8 as an alkene means a four carbon skeleton, straight or branched, that contains exactly one carbon-carbon double bond.

Step 2: List every possible structure.

• 1-butene: CH2=CH-CH2-CH3 (double bond between carbon 1 and carbon 2 of a straight chain)
• cis-2-butene: the two CH3 groups lie on the same side of the C2=C3 double bond
• trans-2-butene: the two CH3 groups lie on opposite sides of the C2=C3 double bond
• 2-methylpropene (isobutylene): (CH3)2C=CH2, a branched chain alkene

Step 3: 2-butene shows cis and trans geometrical isomerism because each carbon of the double bond carries two different groups, one H and one CH3, so it must be counted as two separate isomers, not one.

Step 4: Adding these up: 1-butene, cis-2-butene, trans-2-butene and 2-methylpropene gives 4 isomers in total.

Step 5: Hence the maximum number of isomers of C4H8 (alkene) is 4, option B.