{"id":574,"date":"2017-10-04T20:59:59","date_gmt":"2017-10-04T20:59:59","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/?post_type=chapter&p=574"},"modified":"2017-10-24T15:05:29","modified_gmt":"2017-10-24T15:05:29","slug":"conformers-of-cycloalkanes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-organicchemistry\/chapter\/conformers-of-cycloalkanes\/","title":{"raw":"Conformers of Cycloalkanes","rendered":"Conformers of Cycloalkanes"},"content":{"raw":"
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Objectives<\/h3>\r\nAfter completing this section, you should be able to\r\n
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  1. describe, and sketch the conformation of, cyclopropane, cyclobutane and cyclopentane.<\/li>\r\n \t
  2. describe the bonding in cyclopropane, and hence account for the high reactivity of this compound.<\/li>\r\n \t
  3. analyse the stability of cyclobutane, cyclopentane and their substituted derivatives in terms of angular strain, torsional strain and steric interactions.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
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    Study Notes<\/h3>\r\nNotice that in both cyclobutane and cyclopentane, torsional strain is reduced at the cost of increasing angular (angle) strain.\r\n\r\n<\/div>\r\n<\/div>\r\n\r\n

    Although the customary line drawings of simple cycloalkanes are geometrical polygons, the actual shape of these compounds in most cases is very different.<\/p>\r\n\r\n<\/div>\r\n\"image\"\r\n\r\nCyclopropane is necessarily planar (flat), with the carbon atoms at the corners of an equilateral triangle. The 60\u00ba bond angles are much smaller than the optimum 109.5\u00ba angles of a normal tetrahedral carbon atom, and the resulting angle strain dramatically influences the chemical behavior of this cycloalkane. Cyclopropane also suffers substantial eclipsing strain, since all the carbon-carbon bonds are fully eclipsed. Cyclobutane reduces some bond-eclipsing strain by folding (the out-of-plane dihedral angle is about 25\u00ba), but the total eclipsing and angle strain remains high. Cyclopentane has very little angle strain (the angles of a pentagon are 108\u00ba), but its eclipsing strain would be large (about 10 kcal\/mol) if it remained planar. Consequently, the five-membered ring adopts non-planar puckered conformations whenever possible.\r\n\r\nRings larger than cyclopentane would have angle strain if they were planar. However, this strain, together with the eclipsing strain inherent in a planar structure, can be relieved by puckering the ring. Cyclohexane is a good example of a carbocyclic system that virtually eliminates eclipsing and angle strain by adopting non-planar conformations. Cycloheptane and cyclooctane have greater strain than cyclohexane, in large part due to transannular crowding (steric hindrance by groups on opposite sides of the ring).\r\n

    Cyclic systems are a little different from open-chain systems. In an open chain, any bond can be rotated 360 degrees, going through many different conformations. That complete rotation isn't possible in a cyclic system, because the parts that you would be trying to twist away from each other would still be connected together. Cyclic systems have fewer \"degrees of freedom\" than aliphatic systems; they have \"restricted rotation\".<\/p>\r\n

    Because of the restricted rotation of cyclic systems, most of them have much more well-defined shapes than their aliphatic counterparts. Let's take a look at the basic shapes of some common rings.<\/p>\r\n\r\n