Isomers | NemoQuiz

Isomers: chemicals with the same chemical formula and different structure.

Consider the following examples:

$\definesubmol{a}{C(-[2])(-[6])} \definesubmol{d}{C(-[6])=[,.6]C(-[6])} \chemfig{[,.5]-!a-[,.6]!a-[,.6]!a-[,.6]!d-}$

$\definesubmol{a}{C(-[2])(-[6])} \definesubmol{d}{C(-[6])=[,.6]C(-[6])} \chemfig{[,.5]-!a-[,.6]!a-[,.6]!d-[,.6]!a-}$

$\definesubmol{a}{C(-[2])(-[6])} \chemfig{[,.45]-!a-[,0.6]!a-[,.6]C(-[6.5,.6]C(-)(-[4])(-[6]))=[,.6]C(-[2])-}$

$\definesubmol{a}{-C(-[::-12,0.5])(-[::-96,0.5])} \chemfig[,0.5]{C(-[:-138,.5])(-[:-222,.5])*5(!a!a!a!a-)}$

All four chemicals have the same chemical formula, C₅H₁₀. All of the structures are different, though, so these are isomers.

Identical Structures

Often you can draw the same structure in multiple ways. For example, all of the following structures show the same exact molecule. These are not isomers, because they are identical.

$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c([6]!c-)!c!c-}$	$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c!c([6]!c-)!c-}$
$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c([2]!c-)!c!c-}$	$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c!c([2]!c-)!c-}$

Straight Chain versus Branched

Straight chain alkanes have no branches. None of the carbons are connected to 3 or 4 other carbons.

$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c!c!c!c!c-}$

$\definesubmol{h}{(-[0,.4])(-[2,.4])(-[4,.4])(-[6,.4])} \chemfig{[,.6]-C!h-[6]C!h-C!h-C!h-[2]C!h-C!h}$

Even though the second example is bent, there are no carbons that branch (connect to 3 other carbons), so it is still a straight chain hydrocarbon.

Branched alkanes have at least one carbon connected to 3 other carbons. They have one or more “forks” in the carbon chain.

$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c([2]!c-)!c([6]!c-)!c!c-}$

$\definesubmol{c}{-[,.6]C(-[::+90])(-[::-90])} \chemfig{[,.4]-C(-[2])(-[6])!c!c([6]!c!c-)!c!c-}$

The two examples above have the same formula, C₇H₁₆, and different structures, so they are isomers.