General Chemistry Principles, Patterns, and Applications, 2011

produced from the different hydrocarbon fuels. Recall that n-heptane, which does not burn smoothly, has
an octane rating of 0, and 2,2,4-trimethylpentane (“isooctane”), which burns quite smoothly, has a rating
of 100 (Figure 2.25 "The Octane Ratings of Some Hydrocarbons and Common Additives"). Isooctane has
a branched structure and is capable of forming tertiary radicals that are comparatively stable.
In contrast, the radicals formed
during the combustion ofn-heptane, whether primary or secondary, are less stable and hence more reactive, which
partly explains why burning n-heptane causes premature ignition and engine knocking.
In Section 24.2 "Isomers of Organic Compounds", we explained that rotation about the carbon–carbon
multiple bonds of alkenes and alkynes cannot occur without breaking a π bond, which therefore
constitutes a large energy barrier to rotation (Figure 24.15 "Carbon–Carbon Bonding in Alkenes and
Interconversion of "). Consequently, the cis and trans isomers of alkenes generally behave as distinct
compounds with different chemical and physical properties. A four-carbon alkene has four possible
isomeric forms: three structural isomers, which differ in their connectivity, plus a pair of geometric
isomers from one structural isomer (2-butene). These two geometric isomers are cis-2-butene andtrans-
2-butene. The four isomers have significantly different physical properties.
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