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
