Alkene, which releases the most heat, must be the least stable. In contrast, the alkene, which releases the least heat, must be the most stable.
The heats of hydrogenation show that trans- isomers are generally more stable than the corresponding cis- isomers. Because the alkyl substituents are separated farther in trans- isomers than they are in cis- isomers. The greater stability of the trans- isomer is evident in the following example, which shows that the trans -isomer is stable by 1.0 kcal/mol (Scheme 2).
Scheme 2
4.4 Stability of Cycloalkenes
Rings that are five-membered or larger can easily accommodate double bonds, and these cycloalkenes react much like acyclic alkenes. Three- and four-membered rings show evidence of ring strain. Cyclopropene is highly strained as it has bond angles of about 60°, compressing the
Scheme 3
bond angles of the carbon-carbon double bond to half their usual value of 120°. The double bond in cyclobutene, where the angle of sp2 hybrid carbons is 90° instead the usual value of 120°, has about 4.1 kcal/mol of extra ring strain in addition to the ring strain in cyclobutane (Scheme 3).
The extra ring strain in cyclopropene and cyclobutene makes the double bond more reactive than a typical double bond.
Cycloalkanes usually have cis -isomer especially for small rings but the trans- isomers are possible for rings having more than eight carbon atoms. Trans -cyclohexene is too strained to be isolated, but trans -cycloheptene can be isolated at low temperatures. Trans -cyclooctene is stable at room temperature and its cis -isomer is still more stable. Cycloalkene containing at least ten or more carbon atoms can easily accommodate a trans double bond. For cyclodecene and larger cycloalkenes, the trans -isomer is nearly as stable as the cis -isomer.
A bicyclic compound is one that contains two rings that share two carbons. A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms. This principle is called Bredt's rule . A bridged bicyclic compound has at least one carbon atom in each of the three links between the bridgehead carbons. If there is a double bond at the bridgehead carbon of a bridged bicyclic system, one of the two rings contains a cis double bond and the other must contain a trans double bond. For example, norbornane contains a five-membered ring and a six-membered ring. If there is a double bond at the bridgehead carbon atom, an unstable arrangement results that contains a cis double bond in the five-membered ring and trans double bond in the six-membered. If the larger ring contains at least eight carbon atoms, then stable arrangement results that contain a trans double bond at the bridgehead (Figure 3).
Figure 3