Alkenes and Alkynes

Alkenes are organic molecules, both hydrocarbons and molecules with other elements substitute for the hydrogens, with at least one carbon-carbon double bond somewhere in the structure.  Because at least one carbon bears a double bond, alkenes are said to be unsaturated. Alkenes are named using the -ene suffix. Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond and are also unsaturated as a result.  Alkynes are named using the -yne suffix. For both alkenes and alkenes when there are two or more double or triple bonds in the molecule the prefix di, tri etc. as appropriate is used in front of the root name.  The carbons along the chain are numbered and the location of the multiple bond(s) is identified in the name by placing a number in front of the whole molecule name.  For example, 1,3 pentadiene.
 

Isomers are two or more molecules that have the same molecular formula but with their individual atoms in a different arrangement. The word "isomer" comes from the Greek meaning "equal" parts.  You might find it useful to think of isomers as different molecules made from the same kit or parts or atoms but with those parts attached in different ways.

Some environmentally important alkenes have chlorine substituted for hydrogens in the molecule.  When a chlorine atom is substitute it's location is also signaled by the carbon number to which it is attached in the name.  For example, 1,1-chloroethene is distinguished from its isomer, 1,2-dichloroethene.

The double and triple bonds make alkenes and alkynes more rigid than alkanes.  As a result, they cannot pack together as closely as the alkanes do an in general, multiple bonds lead to substances with lower melting points.