Alkenes, Cycloalkenes, and Alkadienes

A few very common alkenes also are called alkylenes by appending the suffix -ene to the name of the hydrocarbon radical with the same carbon skeleton. Examples are shown below with their alkylene names in parentheses. We shall continue to use the IUPAC names whenever possible. 

The hydrocarbon groups derived from alkenes have the suffix -enyl, as in alkenyl, and numbering of the group starts with the carbon atom attached to the main chain  

A few alkenyl groups have trivial names that commonly are used in place of systematic names. These are vinyl, allyl, and isopropenyl. And again we shall 

hydrogen atoms that are bonded directly to the unsaturated carbon atoms of a double bond often are called vinyl hydrogens, although the term alkenic hydrogens is more accurate and therefore preferable. 

Cycloalkenes are named by the system used for the open-chain alkenes, except that the numbering always is started at one of the carbons of the double bond and continued around the ring through the double bond so as to keep the index numbers as small as possible  

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