CHAPTER 11: Unit 3. Shapes & Isomerism: Alkanes and Cycloalkanes
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| Conformational isomers: Rotation can occur around C-C single bond that gives rise to different conformation. This allows hydrogens associated to carbon to adopt a new orientation in space than the previous arrangement. Since this arrangement is three-dimensionally different, it gives rise to stereoisomers. This is a characteristics property of all carbon-carbon single bonds.For example, in ethane, the molecule is seen from the front, when two carbon atoms are superimposed. That produces another type of projection formula called Newman project formula. |
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| Here are two forms of Newman Projection formula of ethane. In left side figure all hydrogens are located on top of each other. That is called eclipsed conformation. In the right hand side diagram, C-C single bond is rotated by 600 and another conformational isomer (staggered). |
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| Constitutional isomers: Structural isomers are always possible for alkane with branches.For example, 1-butane and 2-methylpropane are constitutional isomers. |
| Cis-trans isomerism: Cis-trans isomers that have the same molecular formulas but different orientations of atoms in space due to restricted rotation about bonds. Since cyclic compounds have restricted bonds, they exhibit cis-trans isomerism. |
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| Cis TransTwo methyl groups Two methyl groupsOn the same side of ring Opposite side of ringCis & Trans 1,2-dimethylcyclopropane |
| PRACTICE PROBLEMS: |
How many structural isomers are possible for the molecular formula C5H12? Draw each of them. HOMEWORK EXERCISEDraw the structure for each compound.a. ethylcyclobutaneb. propylcyclopropaneDraw the structure for each compound.a. methylcyclohexaneb. butylcyclobutaneCycloalkyl groups can be derived from cycloalkanes in the same way that alkyl groups are derived from alkanes. These groups are named as cyclopropyl, cyclobutyl, and so on. Name each cycloalkyl halide.a. b. Halogenated cycloalkanes can be named by the IUPAC system. As with alkyl derivatives, monosubstituted derivatives need no number to indicate the position of the halogen. To name disubstituted derivatives, the carbon atoms are numbered starting at the position of one substituent (C1) and proceeding to the second substituted atom by the shortest route. Name each compound.a. b. Answers 1.a. b. 3.a. cyclopentyl bromideb. cyclohexyl chloride |
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