organic chemistry - Isomer Identification Using Condensed Structural Formulae

Which of the following pairs are isomers?

a) $\ce{C5H10}$ and $\ce{C10H20}$
b) $\ce{CH3(CH2)4CH3}$ and $\ce{CH3(CH2)3CH3}$
c) $\ce{CH3CH(CH3)(CH2)2CH3}$ and $\ce{CH3(CH2)2CH(CH3)2}$
d) $\ce{(CH3)3CH}$ and $\ce{CH3CH2CH2CH3}$

My textbook says the answer is C). I beg to differ. I think it is D). C) is just a different way of writing 2-methylpentane.

Answer

isomer

One of several species (or molecular entities ) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties.

Source:
Pure and Applied Chemistry, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
Pure and Applied Chemistry, 1996, 68, 2193 (Basic terminology of stereochemistry (IUPAC Recommendations 1996))

a)
C₅H₁₀
and
C₁₀H₂₀

different chemical formula → not isomeric

b)
CH₃(CH₂)₄CH₃ = hexane (C₆H₁₄)
and
CH₃(CH₂)₃CH₃ = pentane (C₅H₁₂)

different chemical formula → not isomeric

c)
CH₃CH(CH₃)(CH₂)₂CH₃ = 2-methylpentane (C₆H₁₄)
and
CH₃(CH₂)₂CH(CH₃)₂ = 2-methylpentane (C₆H₁₄)

same chemical formula and same structural formula → identical, not isomeric

d)
(CH₃)₃CH = isobutane (C₄H₁₀)
and
CH₃CH₂CH₂CH₃ = butane (C₄H₁₀)

same chemical formula and different structural formula → isomeric