RELATIVE STABILITIES OF ALKENES

Consider the heats of combustion of the four isomeric alkenes of molecular formula C4H8. All undergo combustion according to the equation

When the heats of combustion of the isomers are plotted on a common scale as in the figure, we see that the isomer of highest energy (the least stable one) is 1-butene, CH2=CHCH2CH3. The isomer of lowest energy (most stable) is 2-methylpropene (CH3)2C=CH2.
Analogous data for a host of alkenes tell us that the most important factors governing alkene stability are:
1. Degree of substitution (alkyl substituents stabilize a double bond)
2. Van der Waals strain (destabilizing when alkyl groups are cis to each other)
Degree of substitution. We classify double bonds as monosubstituted, disubstituted, trisubstituted, or tetrasubstituted according to the number of carbon atoms that are directly attached to the CoeC structural unit.

Monosubstituted alkenes:
 Disubstituted alkenes:
(R and R may be the same or different)
 Trisubstituted alkenes:
(R, R , and R may be the same or different)
 Tetrasubstituted alkenes:
(R, R , R , and R may be the same or different)
Like the sp2-hybridized carbons of carbocations and free radicals, the sp2-hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and are, therefore, better able to stabilize an alkene.
An effect that results when two or more atoms or groups interact so as to alter the electron distribution in a system is called an electronic effect. The greater stability of more highly substituted alkenes is an example of an electronic effect.
van der Waals strain. Alkenes are more stable when large substituents are trans to each other than when they are cis. As was seen in Figure above , trans-2-butene has a lower heat of combustion and is more stable than cis-2-butene. The energy difference between the two is 3 kJ/mol (0.7 kcal/mol). The source of this energy difference is due to the fact that methyl groups approach each other very closely in cis-2-butene, but the trans isomer is free of strain. An effect that results when two or more atoms are close enough in space that a repulsion occurs between them is one type of steric effect. The greater stability of trans alkenes compared with their cis counterparts is an example of a steric effect.
The difference in stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond. A particularly striking example compares cis- and trans-2,2,5,5-tetramethyl-3-hexene, in which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10.5 kcal/mol) higher than that of the trans. The cis isomer is destabilized by the large van der Waals strain between the bulky tert-butyl groups on the same side of the double bond.

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