Alkenes, Alkynes, and Aromatic Compounds

Alkenes, Alkynes, and

Aromatic Compounds

Chapter Thirteen

Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 2

► Saturated: A molecule whose carbon atoms bond to the maximum number of hydrogen atoms.

► Unsaturated: A molecule that contains a carbon–carbon multiple bond, to which more hydrogen atoms can be added.


► In the IUPAC system, alkenes and alkynes are named by a series of rules similar to those used for alkanes. The parent names indicating the number of carbon atoms in the main chain are the same as those for alkanes, with the -ene suffix used in place of -ane for alkenes and the -yne suffix used for alkynes.

► STEP 1: Name the parent compound. Find the longest chain containing the double or triple bond, and name the parent compound by adding the suffix -ene or -yne to the name for the main chain.

Naming Alkenes and Alkynes


The number of multiple bonds is indicated using a numerical prefix (diene = 2 double bonds, triene = 3 double bonds, and so on) when there is more than one.


STEP 2: Number the carbon atoms in the main

chain, beginning at the end nearer the multiple bond.

If the multiple bond is an equal distance from both

ends, begin numbering at the end nearer the first

branch point.


Cyclic alkenes are called cycloalkenes. The double-

bond carbon atoms in substituted cycloalkenes are

numbered 1 and 2 so as to give the first substituent

the lower number:


► STEP 3: Write the full name. Assign numbers to the branching substituents, and list the substituentsalphabetically.

► Use commas to separate numbers and hyphens to separate words from numbers.

►

► Indicate the position of the multiple bond in the chain by giving the number of the first multiple-bonded carbon. If more than one double bond is present, identify the position of each and use the appropriate name ending (for example, 1,3-butadiene and 1,3,6-heptatriene).

► For historical reasons, there are a few alkenes and alkynes whose names do not conform strictly to the rules.

Copyright © 2010 Pearson Education, Inc. 8

The two-carbon alkene should be called ethene, but the name ethylene has been used for so long that it is now accepted by IUPAC. The three-carbon alkene, propene, is usually called propylene. The simplest alkyne, ethyne, is more often called acetylene.


The Structure of Alkenes: Cis-Trans Isomerism


Alkenes and alkynes differ from alkanes in shape because of their

multiple bonds.

Methane is tetrahedral, ethylene is flat and acetylene is linear, as

predicted by the VSEPR model.

Unlike the situation in alkanes, where free rotation around the

single bond occurs, there is no rotation around the double

bonds. As a consequence, a new kind of isomerism is possible

for alkenes.


To see this new kind of isomerism, look at the following C4H8 compounds:


► The two 2-butenes are called cis–trans isomers. They have the same formula and connections between atoms but different structures.

► Cis–trans isomerism occurs in an alkene whenever each double-bond carbon is bonded to two differentsubstituent groups. If one of the double-bond carbons is attached to two identical groups, cis–trans isomerism is not possible.


Like the alkanes, alkenes and alkynes are: Nonpolar; insoluble in

water; soluble in nonpolar organic solvents; less dense than

water

Flammable; nontoxic

Unlike the alkanes:

Alkenes display cis–trans isomerism when each double-bond

carbon atom has different substituents

Alkenes and alkynes are chemically reactive at the multiple bond

Properties of Alkenes and Alkynes


Addition reaction: A general reaction type in which a

substance X-Y adds to the multiple bond of an unsaturated

reactant to yield a saturated product that has only single

bonds.

Types of Organic Reactions


Elimination reaction: A general reaction type in which a

saturated reactant yields an unsaturated product by losing

groups from two adjacent carbons.


Substitution reaction: A general reaction type in

which an atom or group of atoms in a molecule is

replaced by another atom or group of atoms.


Rearrangement reaction: A general reaction type

in which a molecule undergoes bond reorganization

to yield an isomer.


Most of the reactions of carbon–carbon multiple bonds are addition reactions. A generalized reagent we might write as X-Y adds to the multiple bond in the unsaturated reactant to

yield a saturated product that has only single bonds.

Reactions of Alkenes and Alkynes


► Hydrogenation: The addition of H2 to a multiple bond to

give a saturated product.

► Alkenes and alkynes react with hydrogen in the presence of a

metal catalyst such as palladium to yield the corresponding

alkane product:


Halogenation: The addition of Cl2 or Br2 to a

multiple bond to give a dihalide product.

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Alkenes, Alkynes, and Aromatic Compounds

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