Question

A confused student was doing an isomer problem and listed the following six names as different structural isomers of C7H16. a. 1-sec-butylpropane b. 4-methylhexane c. 2-ethylpentane d. 1-ethyl-1-methylbutane e. 3-methylhexane f. 4-ethylpentane How many different structural isomers are actually present in these six names?

Answer 1

Answer:

One different structural formula is actually correct/present in the six names

Explanation:

Structural isomers are compounds with the same molecular formula but different structural formulas. The major problem this student had was with the naming of the isomers. For example

4-methylhexane is wrong because the methyl supposedly on the 4th carbon is too close to the end of the carbon chain and should have been counted in the reversed direction as 3-methylhexane.

2-ethylpentane is also wrong because ethyl cannot be found on the second carbon as a branch, hence, the branch will be the last carbon attached to the supposed second carbon (which is actually the third carbon), making the supposed ethyl part of the longest continuous carbon chain. The actual name ought to be 3-methylhexane.

1-ethyl-1-methylbutane is also wrong because the first/last carbon of the carbon chain does not take a branch as it forms part of the longest continuous carbon chain. Hence, the right name here is 3-methylhexane

4-ethylpentane is wrong. Again, the ethyl group is supposedly on the fourth carbon which is too close to the end of the carbon chain and hence should have been counted in the reverse direction. The correct name is 3-methylhexane.

The only correct structural isomer the student presented is 3-methylhexane (e).

Answer 2

In case of heptane (C7H16) the following structural isomers are possible

shown in figure

a. 1-sec-butylpropane : this is actually 3-methyl hexane

b. 4-methylhexane : this is actually 3-methylhexane

c. 2-ethylpentane : this is actually 3-methyl hexane

d. 1-ethyl-1-methylbutane: 3-methylhexane

e. 3-methylhexane: correct IUPAC

f. 4-ethylpentane: This is actually 3-methylhexane

Hence all represent single isomer