Alkenes, Alkynes, and Aromatic Compounds

20 Pages·2010·1.75 MB·English

by  James E. Mayhugh

Chapter Thirteen
Alkenes, Alkynes, and
Aromatic Compounds
► 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.
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 2
Naming Alkenes and Alkynes
► 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.
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 3
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.
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 4
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.
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 5
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:
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 6
► STEP 3: Write the full name. Assign numbers to the
branching substituents, and list the substituents
alphabetically.
► 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. Chapter Thirteen 7
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.
Copyright © 2010 Pearson Education, Inc. 8
The Structure of Alkenes: Cis-Trans Isomerism
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 9
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.
Copyright © 2010 Pearson Education, Inc. Chapter Thirteen 10