• Cycloalkenes are preferred to be cis unless the ring is large enough (at least eight carbon atoms) to be trans .

• Cis - trans nomenclature cannot be used for the alkenes having four different groups. For example, 1-bromo-1-chloropropene is not clearly cis or trans as there is no similar groups. Such alkene can be named using E-Z Nomenclature.

• Each end of the double bond should be considered separately. Assign first and second priorities to the two substituent groups on one end of the double bond. Do the same for the other end. If the two first-priority atoms are on the same side of the double bond, it is called as Z isomer. If the two first-priority atoms are on opposite sides of the double bond, it is then called as E isomer.

4.3 Stability of Alkenes

The stability of an alkene depends on its structure. The heat released in a hydrogenation reaction is called the heat of hydrogenation . When an alkene is treated with hydrogen in the presence of a platinum catalyst, hydrogen adds to the double bond, reducing the alkene to an alkane. Hydrogenation is exothermic, evolving about 20 to 30 kcal of heat per mole of hydrogen consumed.

The difference in the stabilities of alkenes is the difference in their heats of hydrogenation. While considering the hydrogenation of 1-butene (a monosubstituted alkene), 2-butene (a disubstituted alkene) and 2-methyl-2-butene (a trisubstituted alkene), 2-methyl-2-butene is more stable by 3.4 kcal/mol and 2-butene is stable by 2.7 kcal/mol (Scheme 1). More substituted double bonds are usually more stable. In other words, the alkyl groups attached to the double bonded carbons stabilize the alkene.