http://www.ck12.org Chapter 25. Organic Chemistry
Practice Problems- Name the following molecule.
- Draw the structural formula for 3,3-diethyl-2,6-dimethylnonane.
Alkenes
Analkeneis a hydrocarbon with one or more carbon-carbon double covalent bonds. The simplest alkene is
composed of two carbon atoms and is called ethene. Each carbon is bonded to two hydrogen atoms in addition
to the double bond between them.
Each carbon atom is sp^2 hybridized and exhibits a trigonal planar geometry. All the atoms of the molecule lay in
one plane. Like the alkane series, the names of alkenes are based on the number of carbon atoms in the parent
chain. Propene (C 3 H 6 ) has three carbons total, while butene (C 4 H 8 ) has four. The general formula for an alkene
with one double bond is CnH 2 n. Alkenes are called unsaturated hydrocarbons. Anunsaturated hydrocarbonis a
hydrocarbon that contains less than the maximum number of hydrogen atoms that can possibly bond with the number
of carbon atoms present.
Starting with butene, there are multiple structural isomers based on where in the chain the double bond occurs. This
is illustrated by the condensed structural formulas of 1-butene and 2-butene.
The number in the name of the alkene refers to the lowest numbered carbon in the chain that is part of the double
bond.
A different type of isomerism is exhibited by the two different molecules that would both be called 2-butene.
Geometric isomersare isomers in which all of the atoms are still bonded to the same bonding partners, but the
arrangement of atoms in space is different. Geometric isomers are also referred to as stereoisomers. The double
bond in an alkene is not free to rotate because of the nature of the pi bond. Therefore, there are two different ways
to construct the 2-butene molecule. Shown below (Figure25.3) are the two geometric isomers, calledcis-2-butene
andtrans-2-butene.
Thecisisomer has the two single hydrogen atoms on the same side of the double bond, while thetransisomer has
them on opposite sides. In both molecules, each atom has all of the same bonding partners. In order for geometric