Isomers are chemicals that have the same types and quantities of various atoms and yet are different compounds. One type of isomer is the structural isomer, where the same atoms are connected in different ways to form different molecules. For example, two carbons, six hydrogens and one oxygen could be arranged to form diethyl ether (CH3OCH3) or ethanol (CH3CH2OH). The six carbons and 12 hydrogens found in the formula C6H12 can be arranged to form a surprising 25 different structural isomers.
Draw the one possible six-carbon ring structure: cyclohexane. This structure is drawn by joining the six carbons one to the next in a circle so they form a ring. Draw two hydrogens attached to every carbon.
Draw the one possible substituted five-carbon ring structure: methylcyclopentane. This structure is drawn by joining five carbons in a ring. Draw the remaining carbon with three hydrogens on it (that is, the "methyl" group CH3-) attached to any one carbon in that ring.
Draw the four possible substituted four-carbon ring structures: 1,1-dimethylcyclobutane, 1,2-dimethylcyclobutane, 1,3-dimethylcyclobutane and ethylcyclobutane. These structures are drawn by drawing four carbons in a ring. Two CH3- groups are joined to the carbon(s) at the position numbers that appear at the start of the name. Any carbon in the ring can be chosen as No. 1, carbon two would be the one next to it. Continue in that pattern until all four carbons are complete. The exception is ethylcyclobutane, which has the "ethyl" group CH3CH2- attached to any one carbon in the ring.
Draw the six possible substituted three-carbon ring structures: 1,2,3-trimethylcyclopropane, 1,1,2-trimethylcyclopropane, 1-ethyl-1-methylcyclopropane, 1-ethyl-2-methylcyclopropane, propylcyclopropane and isopropylcyclopropane. These structures are drawn by drawing three carbons in a ring. As before, the appropriate groups are attached at the appropriate carbons as numbered around the ring. A CH3- group is drawn where methyl is found in the name, a CH3CH2- group when ethyl is found, CH3CH2CH2- for propyl and (CH3)2CH2- for isopropyl.
Draw the four linear four-carbon structures containing one double bond: 2-ethyl-1-butene [CH2=C(CH2CH3)CHCH2CH3], 2,3-dimethyl-2-butene [CH3C(CH3)=C(CH3)CH2CH3], 2,3-dimethyl-1-butene [CH2=C(CH3)CH(CH3)CH3] and 3,3-dimethyl-1-butene [CH2=CHC(CH3)(CH3)CH2CH3].
Draw the six linear five-carbon structures containing one double bond: 2-methyl-1-pentene [CH2=C(CH3)CH2CH2CH3], 3-methyl-1-pentene [CH2=CHCH(CH3)CH2CH3], 4-methyl-1-pentene [CH2=CHCH2CH(CH3)CH3], 2-methyl-2-pentene [CH3C(CH3)=CHCH2CH3], 3-methyl-2-pentene [CH3CH=C(CH3)CH2CH3] and 4-methyl-2-pentene [CH3CH=CHCH(CH3)CH3].
Draw the three linear six-carbon structures containing one double bond: 1-hexene [CH2=CHCH2CH2CH2CH3], 2-hexene [CH3CH=CHCH2CH2CH3] and 3-hexene [CH3CH2CH=CHCH2CH3].
Some of the isomers will produce additional isomers if so-called geometric isomerism is considered, where the placement of attached groups relative to the plane of a ring or double bond is taken into account.
Tips and warnings
- Some of the isomers will produce additional isomers if so-called geometric isomerism is considered, where the placement of attached groups relative to the plane of a ring or double bond is taken into account.