Question

Can an alkene have the same formula as a cycloalkane? Explain.

Answer 1

if you imagine a straight chain alkane and then imagine bending it into a circle, you would almost get a cycloalkane. But the two carbons on either end of the chain have no space left to bond with each other and form the circle because they already have four bonds (with 3 hydrogens and the next carbon). So they must lose a hydrogen each so that they can bond to form that circle. Hence, the Cn H2n+2 general formula of alkanes becomes Cn H2n. Alkenes have this same general formula as they have a double bond, and so bond to 2 less hydrogens than alkanes.

Alkenes and Cycloalkanes have the same general formula, C_nH_2n. Therefore, the general formula does not identify the structure as an alkene nor an cycloalkane. To further become problematic there are alkenes which contain more than one double bond. Those with two double bonds have the formula, C_nH_2n-2.

Cycloalkenes have the general formula C_nH_2(n-m). The letter m represents the number of double bonds. Thus, cyclopropene has the formula C₃H₄ while that of cyclobutene is C₄H₆. The properties of alkanes and alkenes are very similiar.