Chapter Thirteen

Fundamentals of General, Organic, and Biological Chemistry 5th Edition Chapter Thirteen Alkenes, Alkynes, and Aromatic Compounds James E. Mayhugh Oklahoma City University 2007 Prentice Hall, Inc.

Outline • 13.1 Alkenes and Alkynes • 13.2 Naming Alkenes and Alkynes • 13.3 The Structure of Alkenes: Cis–Trans Isomerism • 13.4 Properties of Alkenes and Alkynes • 13.5 Types of Organic Reactions • 13.6 Reactions of Alkenes and Alkynes • 13.7 How Alkene Addition Reactions Occur • 13.8 Alkene Polymers • 13.9 Aromatic Compounds and the Structure of Benzene • 13.10 Naming Aromatic Compounds • 13.11 Reactions of Aromatic Compounds Chapter Thirteen

13.1 Alkenes and Alkynes • Alkene: A hydrocarbon that contains a C=C double bond. • Alkyne: A hydrocarbon that contains a CC triple bond. • Simple alkenes are made in vast quantities in the petroleum industry by thermal “cracking” of the alkanes in petroleum. • Most of the organic chemicals used in making drugs, explosives, paints, plastics, and pesticides are synthesized by routes that begin with alkenes. Chapter Thirteen

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. Chapter Thirteen

13.2 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. Chapter Thirteen

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. Chapter Thirteen

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. Chapter Thirteen

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: Chapter Thirteen

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. Chapter Thirteen

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. Chapter Thirteen

13.3 The Structure of Alkenes: Cis-Trans Isomerism Chapter Thirteen

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. Chapter Thirteen

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

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. Chapter Thirteen

13.4 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 Chapter Thirteen

13.5 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. Chapter Thirteen

Elimination reaction: A general reaction type in which a saturated reactant yields an unsaturated product by losing groups from two adjacent carbons. Chapter Thirteen

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. Chapter Thirteen

Rearrangement reaction: A general reaction type in which a molecule undergoes bond reorganization to yield an isomer. Chapter Thirteen

13.6 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. Chapter Thirteen

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: Chapter Thirteen

Halogenation: The addition of Cl2 or Br2 to a multiple bond to give a dihalide product. Chapter Thirteen

Alkenes react with hydrogen bromide (HBr) to yield alkyl bromidesand with hydrogen chloride (HCl) to yield alkyl chlorides in what are called hydrohalogenation reactions: Chapter Thirteen

2-Methylpropene couldadd HBr to give 1-bromo-2- methylpropane, but it does not; it gives only 2-bromo-2-methylpropane. Only one of the two possible addition products is obtained. Chapter Thirteen

Markovnikov’s rule: In the addition of HX to an alkene, the H attaches to the carbon that already has the most H’s, and the X attaches to the carbon that has fewer H’s. Chapter Thirteen

Hydration: The addition of water, in the presence of a strong acid catalyst, to a multiple bond to give an alcohol product. Chapter Thirteen

As with the addition of HBr and HCl, we can use Markovnikov’s rule to predict the product when water adds to an unsymmetrically substituted alkene. Hydration of 2-methylpropene, for example, gives 2-methyl-2-propanol: Chapter Thirteen

13.7 How Alkene Addition Reactions Occur • Reaction mechanism: A description of the individual steps by which old bonds are broken and new bonds are formed in a reaction. • Detailed studies show that alkene addition reactions take place in two distinct steps and involve a carbocation intermediate. • The addition of HBr to ethylene is an example. Chapter Thirteen

In the first step, two electrons move from the double bond to form a C-H bond. In the second step, Br- uses two electrons to form a bond to the carbocation. Chapter Thirteen

13.8 Alkene Polymers A polymer is a large molecule formed by the repetitive bonding together of many smaller molecules called monomers. Chapter Thirteen

Adding an initiatorto an alkene results in a break in the double bond, yielding a reactive intermediate that contains an unpaired electron. This reactive intermediate adds to a second alkene molecule to produce another reactive intermediate, and so on. Chapter Thirteen

Chapter Thirteen

13.9 Aromatic Compounds and the Structure of Benzene • Aromatic: The class of compounds containing benzene-like rings. • Benzene and other aromatic compounds are much less reactive than alkenes. Chapter Thirteen

Benzene’s relative lack of chemical reactivity is a consequence of its structure. • If you draw a six-membered ring with alternating single and double bonds, there are two equivalent possibilities (b), neither of which is fully correct by itself. Experimental evidence shows that all six C–C bonds in benzene are identical (c). Chapter Thirteen

The properties of benzene are best explained by assuming that its true structure is an averageof the two equivalent conventional structures. Each C-C bond is thus intermediate between a single bond and a double bond. • The name resonance is given to this phenomenon where the true structure of a molecule is an average among two or more possible conventional structures, and a special double-headed arrow is used to show the resonance relationship. • Simple aromatic hydrocarbons like benzene are nonpolar, insoluble in water, volatile, and flammable. Unlike alkanes and alkenes, however, several aromatic hydrocarbons are toxic. Chapter Thirteen

13.10 Naming Aromatic Compounds • Substituted benzenes are named using -benzene as the parent. • No number is needed for monosubstituted benzenes because all the ring positions are identical. Chapter Thirteen

Disubstituted aromatic compounds are named using one of the prefixes ortho-, meta-, or para-. • An ortho- or o-disubstituted benzene has its two substituents in a 1,2 relationship on the ring. • A meta- or m-disubstituted benzene has its two substituents in a 1,3 relationship on the ring. • A para- or p-disubstituted benzene has its substituents in a 1,4 relationship on the ring. Chapter Thirteen

Many substituted aromatic compounds have common names in addition to their systematic names. Chapter Thirteen

Occasionally, the benzene ring itself may be considered a substituent group attached to another parent compound. When this happens, the name phenyl is used for the unit: Chapter Thirteen

13.11 Reactions of Aromatic Compounds Unlike alkenes, which undergo addition reactions, aromatic compounds usually undergo substitution reactions. That is, a group Y substitutes for one hydrogen atom on the aromatic ring without changing the ring itself. Chapter Thirteen

Nitration is the substitution of a nitro groupfor one of the ring hydrogens. • The reaction occurs when benzene reacts with nitric acid in the presence of sulfuric acid as catalyst: Chapter Thirteen

Halogenation is the substitution of a halogen atom, usually bromine or chlorine, for one of the ring hydrogens. • The reaction occurs when benzene reacts with Br2 or Cl2 in the presence of iron as catalyst: Chapter Thirteen

Sulfonation is the substitution of a sulfonic acid group for one of the ring hydrogens. • The reaction occurs when benzene reacts with concentrated sulfuric acid and SO3. Chapter Thirteen

Chapter Summary • Alkenes are hydrocarbons that contain a C=C double bond, and alkynes are hydrocarbons that contain a CC triple bond. Aromatic compounds contain six-membered, benzene-like rings. All three families are said to be unsaturated because they have fewer hydrogens than corresponding alkanes. • Alkenes are named using the family ending -ene; alkynes use the family ending -yne. Disubstituted benzenes have the suffix -benzene as the parent name, and positions of the substituents are indicated with the prefixes ortho- (1,2 substitution), meta- (1,3 substitution), or para- (1,4 substitution). Chapter Thirteen

Chapter Summary Cont. • In a cis isomer, the two substituents are on the same side of the double bond; in a trans isomer, they are on opposite sides of the double bond. • Addition reactions occur when two reactants add to form a single product with no atoms left over. • In eliminations, a reactant splits into two products. • Substitution reactions occur when two reactants exchange parts to give two new products. • Rearrangement reactions occur when a single reactant undergoes a reorganization to yield a single isomeric product. Chapter Thirteen

Chapter Summary Cont. • Addition of H2 to an alkene yields an alkane; addition of Br2 or Cl2 yields a 1,2-dihaloalkane; addition of HBr and HCl yields an alkyl halide; and addition of water yields an alcohol. • Markovnikov’s rule predicts that in the addition of HX or H2O to a double bond, the H becomes attached to the carbon with more H’s, and the X or OH becomes attached to the carbon with fewer H’s. • Aromatic compounds are stable but can be made to undergo substitution reactions. Examples: nitration, halogenation, and sulfonation. • The addition of HX to an alkene takes place in two steps. First, the alkene uses two electrons to bond to H+ then the carbocation reacts with the halide ion. Chapter Thirteen

End of Chapter 13 Chapter Thirteen

Chapter Thirteen