Unsaturated Hydrocarbons Alkenes are compounds that contain a carbon−carbon double bond. Alkynes are compounds that contain a carbon−carbon triple bond. The multiple bond is always drawn in a condensed structure. 1

Naming  use the corresponding alkane name  change the ending from -ane to –ene or -yne Alkene IUPAC Common H2C=CH2 Ethene Ethylene H2C=CH─CH3 Propene Propylene Cyclohexene Alkyne IUPAC Common HC≡CH Ethyne Acetylene HC≡C─CH3 Propyne

2 2 1 1 CH3 CH3 CH3 6 1-methylcyclopentene 1,6-dimethylcyclohexene When naming cycloalkenes, the double bond is located between C1 and C2. The ring is numbered to give the first substituent the lower number. 2 2 1 1 CH3 CH3 CH3 1-methylcyclopentene 6 1,6-dimethylcyclohexene 3

12.2 Cis–Trans Isomers 2-butene There is restricted rotation around the C atoms of a double bond. Therefore, 2-butene has two possible arrangements: 2-butene two CH3 groups on the same side two CH3 groups on opposite sides In naming, the prefixes cis- or trans- are placed in front of the alkene name.

Whenever the two groups on each end of a C═C are different from each other, two isomers are possible. When the two groups on one end of the double bond are identical (e.g., both H or both CH3), no cis and trans isomers are possible.

Stereoisomers are isomers that differ only in the 3-D arrangement of atoms. Constitutional isomers differ in the way the atoms are bonded to each other. 6

12.3 Addition Reactions Double Bond Alkenes undergo addition reactions wherein new groups X and Y are added to the alkene. One bond of the double bond is broken and two new single bonds are formed. Double Bond The two bonds are different: sigma bond  pi bond  The sigma bond uses sp2 orbitals (one from each carbon) The pi bond uses pure p orbitals (one from each carbon) Because the C-C  bond is relatively weak, alkene chemistry is basically the addition of a reagent, A-B, to give a saturated compound.

alkane dihalide Haloalkane Alkyl halide alcohol 8

Addition of Hydrogen—Hydrogenation Hydrogenation is the addition of H2 to an alkene. The metal catalyst (Pd, Pt, Ni) speed up the rate of the reaction. The product of hydrogenation is an alkane.

Hydrogenation of Oils Adding H2 to double bonds in vegetable oils produces compounds with higher melting points solids at room temperature, such as margarine, soft margarine, and shortening Have a long shelf life Stable during deep frying Trans fatty acids Evidence that consumption of trans fatty acids increases the risk of coronary heart disease. TFAs raises levels of low-density lipoproteins (LDL) and reduces high-density lipoproteins (HDL), and increases the ratio of total cholesterol to HDL cholesterol, a powerful predictor of the risk of CHD.

Addition of Halogen—Halogenation Halogenation is the addition of halogen (X2) to an alkene (X2 is usually Cl2 or Br2). Halogenation occurs readily and does not require a catalyst. The product of halogenation is a dihalide. Testing for Unsaturation When bromine (Br2) is added to an alkane, the red color of bromine persists. When added to an alkene or alkyne, the red color of bromine disappears immediately. 11

Addition of Hydrogen Halides— Hydrohalogenation Hydrohalogenation is the addition of HX (HCl or HBr) to an alkene. The product of hydrohalogenation is an alkyl halide. If the reactant is an asymmetrical alkene, two possible products can be formed in theory. To determine which of the two products will actually form, we use Markovnikov’s rule. 12

Markovnikov’s rule Markovnikov’s rule states that the H atom of H–X will bond to the less substituted C atom in the C═C double bond. the C in the double bond with the most H’s will bond to the H atom of H–X.

Mechanism Figure 12-03-10UN Title: Addition Reaction Caption: This reaction produces a carbocation (a carbon cation) with a positive charge. In the second step, a pair of electrons from the bromide ion Br- reacts rapidly with the carbocation.

Addition of Water—Hydration Hydration requires a strong acid, H2SO4. The product is an alcohol. If the reactant is an asymmetrical alkene, the product will be determined by Markovnikov’s rule.

Aromatic compounds are compounds that contain a benzene ring. Resonance structures have the same arrangement of atoms, but different locations of electrons. The true structure of benzene is a combination of both resonance structures, called a hybrid. This can be drawn with a circle inside the hexagon

Nomenclature of Benzene Derivatives  Monosubstituted Benzenes Name the substituent first Then add the word benzene at the end Some monosubstituted benzenes have common names that you must learn.

Nomenclature of Benzene Derivatives  Disubstituted Benzenes Some disubstituted benzenes have common names that you must learn. Xylene

If there are two groups on the benzene ring and they are different, alphabetize the two substituent names. If one of the two substituents is part of a common root, then name the molecule as a derivative of that monosubstituted benzene. When benzene is considered a branch, it is called phenyl. Aromatic Compounds with More than One Ring

Aromatic Compounds in Nature and Health

Reactions of Aromatic Compounds Substitution is a reaction in which an atom is replaced by another atom or group of atoms. Substitution of H by X keeps the stable aromatic ring intact. In chlorination. In nitration, nitric acid (HNO3) in the presence of sulfuric acid (H2SO4) forms nitrobenzene. In sulfonation, SO3 in the presence of H2SO4 such that a SO3H group.

Polymers Polymers are large molecules made up of repeating units of smaller molecules (monomers) covalently bonded together. In polymerization, the monomer C═C double bonds are broken and single bonds linking the monomers together are formed.

PVC is fairly hard and brittle. It can be softened by adding a plasticizer (carboxylic acid ester). Garden hoses Plastic covers Vinyl leather

Extra Credit Due right at the end of the chapter. 1. Write the IUPAC name for each of the following: 2. Name each, using cis–trans prefixes when needed.

3. Write the IUPAC name for each of the following:

Br2 4. Write the condensed structural formula and name of the product:

6. Predict the organic product for each of the following reactions: 7. Write the condensed structural formulas and name of the products:

8. provide the condensed structure & name of the product. Figure 12-05-60UN Title: Addition Reactions Caption: Answers to Selected Questions and Problems 12.17