Organic Chemistry I - Cyclohydrocarbons - Quick Guides | Chemistry

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Study GuideOrganic Chemistry I–Cyclohydrocarbons1.Cyclohydrocarbons: NomenclatureHow Are Substituted Cycloalkanes Named?Substituted cycloalkanes are named similarly to open-chain or aliphatic alkanes. TheInternationalUnion of Pure and Applied Chemistry (IUPAC)nomenclature rules for substituted cycloalkanesfollow these steps:Step 1: Number the Carbon Atoms in the Ring and Substituent1.Determine the number of carbon atomsin both the ring and the largest substituent.oIf thering has more carbonsthan the substituent, it is named as analkyl-substituted cycloalkane.oIf thesubstituent has more carbonsthan the ring, it is named as acycloalkylalkane.Step 2: Number the Substituents for the Lowest Sum2.If an alkyl-substituted cycloalkane hasmore than one substituent, number the ring so thatthe substituents have thelowest sum of numbers.

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Study GuideStep 3: Use Alphabetical Priority for Different Substituents3.If the molecule hastwo or more different substituent groups, thenumber one positionisdetermined byalphabetical priority.Key Takeaway•Number the ringbased on the largest substituent or if the substituent has more carbons thanthe ring.•Lowest sum numberingis used for multiple substituents.•When there aredifferent substituents,alphabetical prioritydetermines the number oneposition.

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Study Guide2.Cyclohydrocarbons: StereochemistryWhat is Stereochemistry in Cycloalkanes?Cycloalkanes haverestricted rotationaround theircarbon-carbon single bonds, meaning that theycan havecisandtransstereoisomers when they contain two or more substituents. For example,1,2-dibromobutanecan exist ascisandtransisomers.2.1Cycloalkanes with Fewer Carbon AtomsCycloalkane rings withfive or fewer carbon atomstend to beplanarornearly planar. For suchmolecules,cisandtransisomerism refers to the location of substituents:•Cis: Substituents are on thesame sideof the ring plane.•Trans: Substituents are onopposite sidesof the ring plane.2.2Cycloalkanes with More Carbon AtomsCycloalkanes containingsix or more carbonsbend out of the plane, creatingpuckerin thestructure. For example,cyclohexaneexists inthree forms.2.3Chair and Boat Forms of CyclohexaneThe two end structures in cyclohexane are calledchair forms, while the middle structure is called theboat form. The chair form of cyclohexane ismore stablethan the boat form.

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Study Guide•The carbon atoms ofcyclohexaneformsingle bondswith each other. Since the bonds mustbesp³ hybridized, the shape at each atom istetrahedral, and the bond angle isabout 110°.2.4Axial and Equatorial PositionsThe hydrogen atoms on cyclohexane occupy eitheraxialorequatorial positions:•Axialbonds are roughly parallel to animaginary axisthrough the center of the carbon ring.•Equatorialbonds are roughly110°away from the axial bond on the same carbon.Cisandtranssubstitution positions are defined based on the location of these axial and equatorialbonds.2.5Cis and Trans IsomerismForcis-1,2-disubstitution, the substituents are on thesame sideof the ring plane.Fortrans-1,2-disubstitution, the substituents are onopposite sidesof the ring plane.

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