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 doublebond 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
To see this new kind of isomerism, look at the following C 4 H 8 compounds: Copyright 2010 Pearson Education, Inc. Chapter Thirteen 11
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 different substituent groups. If one of the double-bond carbons is attached to two identical groups, cis trans isomerism is not possible. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 12
Properties of Alkenes and Alkynes 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 Copyright 2010 Pearson Education, Inc. Chapter Thirteen 13
Types of Organic Reactions 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. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 14
Elimination reaction: A general reaction type in which a saturated reactant yields an unsaturated product by losing groups from two adjacent carbons. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 15
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. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 16
Rearrangement reaction: A general reaction type in which a molecule undergoes bond reorganization to yield an isomer. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 17
Reactions of Alkenes and Alkynes 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. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 18
Hydrogenation: The addition of H 2 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: Copyright 2010 Pearson Education, Inc. Chapter Thirteen 19
Halogenation: The addition of Cl 2 or Br 2 to a multiple bond to give a dihalide product. Copyright 2010 Pearson Education, Inc. Chapter Thirteen 20