Chapter 12 Alkenes, Alkynes, and Aromatic Compounds General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 12 Alkenes, Alkynes, and Aromatic Compounds 12.1 Alkenes and Alkynes Lectures © 2013 Pearson Education, Inc.

Unsaturated Hydrocarbons do not contain the maximum number of hydrogen atoms that could be attached to each carbon atom. are alkenes when they contain double C=C bonds. are alkynes when they contain triple C≡C bonds. react with hydrogen gas to form saturated hydrocarbons.

Alkenes Alkenes contain a carbon–carbon double bond that forms when adjacent carbon atoms share two pairs of valence electrons. are flat because the atoms in a double bond lie in the same plane. have a trigonal planar arrangement around the double bonded carbon atoms.

Alkynes Alkynes have a triple bond that forms when two carbon atoms share three pairs of valence electrons. have a 180˚ angle and are linear.

Guide to Naming Alkenes and Alkynes

Naming Alkenes and Alkynes Write the IUPAC name for the following. A. B. Step 1 Name the longest carbon chain that contains the double or triple bond. Analyze the Problem. Functional Group Class IUPAC Naming Name A. Double bond Alkene Replace –ane of alkane with –ene B. Triple bond Alkyne Replace –ane of alkane with –yne

Naming Alkenes and Alkynes Write the IUPAC name for the following. A. B. Step 1 Name the longest carbon chain that contains the double or triple bond. A. B. butene heptyne

Naming Alkenes and Alkynes Write the IUPAC name for the following. A. B. Step 2 Number the carbon chain from the end nearer the double or triple bond. A B. 2−butene 2−heptyne 1 2 3 4 1 2 3 4 5 6 7

Naming Alkenes and Alkynes Write the IUPAC name for A. B. Step 3 Give the location and name of each substituent (alphabetical order) as a prefix to the alkene or alkyne name. A. B. 2−butene 4-methyl−2−heptyne 1 2 3 4 1 2 3 4 5 6 7

Naming Cycloalkenes When naming a cycloalkene, if there are no substituents, the double bond does not need a number. if there is a substituent, the carbons in the double bond are numbered as 1 and 2, and the ring is numbered from carbon 2 in the direction that will give the lower number to the substituent. cyclopentene 3-methylcyclohexane

Cis and Trans Isomers Cis and trans isomers have the same formula, but different physical and chemical properties. occur because double bonds in alkenes are rigid and cannot rotate. We add the prefix cis or trans to denote whether the atoms bonded to the carbon atoms are on the same side or the opposite sides of the double bond.

Cis–Trans Isomers Cis–trans isomers occur when different groups are attached to the carbon atoms in a double bond. In a cis isomer, groups are attached on the same side of the double bond. In the trans isomer, the groups are attached on opposite sides of the double bond.

Cis–Trans Isomerism Cis–trans isomers do not occur if one of the carbon atoms in the double bond has identical groups. (neither cis nor trans) (neither cis nor trans)

Naming Cis–Trans Isomers The prefixes cis- or trans- are placed in front of the alkene name when there are cis–trans isomers. cis- trans- cis-1,2-Dibromoethene trans-1,2-Dibromoethene

Alkenes: Drawing Cis–Trans Isomers Draw the cis and trans isomers of 2-hexene.

Learning Check Name each, using cis–trans prefixes when needed. A.

Solution Name each, using cis–trans prefixes when needed. cis-1,2-Dibromoethene trans-2-Butene 1,1-Dichloropropene A. A. Identical atoms; no cis or trans

Addition Reactions There are different addition reactions as shown in Table 12.2.

Hydrogenation In hydrogenation, hydrogen atoms add to the carbon atoms of a double or triple bond. a catalyst, such as Pt, Pd or Ni, is used to speed up the reaction.

Halogenation Halogenation reactions of alkenes and alkynes occur rapidly without a catalyst.

Hydrohalogenation Hydrohalogenation reactions occur in two steps.

Markovnikov’s Rule In assymetrical alkenes, the H in HX adds to the carbon in the double bond that has the greater number of H atoms.

Hydration In hydration, a strong acid (H+) catalyst is required. water (HOH) adds to a double bond. an −H atom bonds to one C in the double C═C bond. an −OH bonds to the other C.

Hydration When hydration occurs with a double bond that has an unequal number of H atoms, the −H atom bonds to the C in the double C═C bond that has more hydrogen atoms. the −OH bonds to the C in the double C═C bond that has the fewer hydrogen atoms.

Polymers Polymers are large, molecules that consist of small repeating units called monomers. found in nature, including cellulose in plants, starches in food, and proteins and DNA in the body. also made synthetically, such as polyethylene and polystyrene, Teflon, and nylon. often made by reaction of small alkenes at high temperature and pressure.

Polymerization In polymerization, small repeating units called monomers join to form a long chain polymer.

Common Synthetic Polymers

Polymers from Alkenes

Polymers from Alkenes

Recycling Plastics Recycling is simplified by using codes found on plastic items.

Benzene Structure Benzene has 6 electrons shared equally among the 6 C atoms. behaves more like an alkane; it does not undergo addition reactions. is also represented as a hexagon with a circle drawn inside.

Aromatic Compounds At the time of its discovery, many compounds containing benzene had fragrant odors, so the family of benzene compounds became known as aromatic compounds.

Common Aromatic Compounds

Naming Aromatic Compounds Aromatic compounds are named with benzene as the parent chain. with one side group named in front of benzene. Methylbenzene Chlorobenzene

Some Common Names Some substituted benzene rings have common names that have been used for many years. with a single substituent use a common name or are named as a benzene derivative.

Aromatic Compounds with Two Groups When two groups are attached to a benzene ring, number the ring to give the lowest numbers to the side groups (IUPAC), or use prefixes to show the arrangement of the substituents (common) on the ring, as shown below. ortho (o) for 1,2- meta (m) for 1,3- para (p) for 1,4-

Aromatic Compounds with Two Substituents 3-Chlorotoluene 1,4-Dichlorobenzene 2-Chlorophenol (m-chlorotoluene) (p-dichlorobenzene) (o-chlorophenol) Substituents on 1st and 3rd carbons 1st and 4th carbons 1st and 2nd carbons

Xylene For isomers of dimethylbenzene, the common name, xylene may be used.

Aromatic Compounds with Three Substituents 1,3,5-Trichlorobenzene 2,6-Dibromo-4-chlorotoluene 4-Bromo-2-chlorotoluene

Properties of Aromatic Compounds have higher melting and boiling points due to the flat symmetrical structure of the rings. are not soluble in water unless they contain –OH or –COOH substituents. have a stable aromatic bonding system. are resistant to many reactions that break up the aromatic system. are flammable like other hydrocarbons.