Lecture 3 Unsaturated Hydrocarbons Lecture Outline Class odds and ends Defining unsaturation in hydrocarbons Naming alkenes and alkynes Cis and trans isomers Reactions of alkenes and alkynes Hydrogenation Halogenation Hydrohalogenation hydration Polymers Aromatic compounds Properties of aromatic compounds

Unsaturated Hydrocarbons Compare the following structures of ethane, ethene and ethyne. What difference do you notice among the first and the last two structures? (Ethane is saturated and ethene and ethyne are unsaturated? How would you define unsaturated hydrocarbons? Ethane

Bond Angles in Alkenes and Alkynes According to VSEPR theory: Three groups in a double bond are bonded at 120° angles. Alkenes are flat, because the atoms in a double bond lie in the same plane. The groups attached to a triple bond are at 180° angles. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Naming Alkenes Look and study the names of the following alkenes Alkene IUPAC Common H2C=CH2 ethene ethylene H2C=CH─CH3 propene propylene cyclohexene CH2=CH─CH2─CH3 1-butene CH3─CH=CH─CH3 2-butene How is the naming of alkenes done? Talk over with your group members

Naming Alkynes Now, let’s look at the case of alkynes. Naming is done in a fashion similar to that of alkenes. Alkyne IUPAC Common HC≡CH ethyne acetylene HC≡C─CH3 propyne 4-Bromo-2-pentyne cycloheptyne

Naming Alkenes and Alkynes with Substituents Now what if the alkene or alkyne has a substituent? Here is an example. Write the IUPAC name for CH3 │ CH3─CH─CH=CH─CH3 STEP 1 Name the longest carbon chain pentene STEP 2 Number the chain from the double bond STEP 3 Give the location of each substituent: 4-methyl- 2-pentene 2-pentene

Naming Alkynes with Substituents Write the IUPAC name for CH3 │ HC≡C─CH─CH3

Summary: Naming Alkenes and Alkynes Okay, so let’s summarize the rules for naming alkenes and alkynes. Here is what we do: Name the longest carbon chain with a double or triple bond Indicate the location of the double or triple bond in the main chain by number, starting at the end closer to the double or triple bond Cycloalkenes do not require the numerical prefix but the bonds are given numbers 1 and 2 Give the location and name of each substituent (alphabetical order) as a prefix to the name

Learning Check Write the IUPAC name for each of the following: 1. CH2=CH─CH2─CH3 2. CH3─CH=CH─CH3 CH3 | 3. CH3─CH=C─CH3 4. CH3─CC─CH3 Solution ______________

Learning Check Write the structural formula for each of the following: A. 2-pentyne 3-methyl-2-pentene Solution _______________ ________________

Cis and Trans Isomers In an alkene, cis and trans isomers are possible because the double bond Is rigid. Cannot rotate. Has groups attached to the carbons of the double bond that are fixed relative to each other. CH3 CH3 CH3 CH = CH CH = CH cis trans CH3

Cis-Trans Isomers Cis-trans isomers occur when different groups are attached to the 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. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Cis-Trans Isomerism Cis-trans isomers do not occur if a carbon atom in the double bond is attached to identical groups. Identical Identical 2-bromopropene 1,1-dibromoethene (not cis or trans) (not cis or trans) H Br H H

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

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

Lecture 3 Unsaturated Hydrocarbons Reactions of Alkenes and Alkynes Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Addition Reactions There are four addition reactions we will study, summarized in table 12.2 TABLE 12.2

Hydrogenation Question: what happens in hydrogenation? Study the examples given below to answer this question? (Side note: a catalyst such as Pt or Ni is used to speed up the reaction) Your conclusion about hydrogenation:

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. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Write an equation for the hydrogenation of 1-butene using a platinum catalyst.

Trans Fats In vegetable oils, the unsaturated fats usually contain cis double bonds. During hydrogenation, some cis double bonds are converted to trans double bonds (more stable) causing a change in the fatty acid structure If a label states “partially” or “fully hydrogenated”, the fats contain trans fatty acids.

Learning Check Write the product of each the following reactions: Pt CH3─CH=CH─CH3 + H2 + H2

Halogenation Question: What happens in halogenation reaction? Think about the following reactions. Your conclusion about halogenation of alkenes and alkynes is that …

Learning Check Write the product of the following addition reactions: 1. CH3─CH=CH─CH3 + Cl2 2.

Testing for Unsaturation Br2 When bromine (Br2) is added to an alkane, the red color of bromine persists. When bromine (Br2) is added to an alkene or alkyne, the red color of bromine disappears immediately. Br2 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 15

Hydrohalogenation What happens in hydrohalogenation reactions? Again, think about where atoms of a hydrogen halide end up.

Markovnikov’s Rule When an unsymmetrical alkene undergoes hydrohalogenation, the H in HX adds to the carbon in the double bond that has the greater number of H atoms .

Hydration In the addition reaction called hydration An acid H+ catalyst is required. Water (HOH) adds to a double bond. An H atom bonds to one C in the double bond. An OH bonds to the other C. H OH H+ │ │ CH3─CH=CH─CH3 + H─OH CH3─CH─CH─CH3

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 bond with the most H. The OH bonds to the C in the double bond with the fewest H atoms. OH H H+ │ │ CH3─CH=CH2 + H─OH CH3─CH─CH2 (1H) (2H)

Learning Check Write the product for the hydration of each of the following: H+ 1. CH3─CH2─CH=CH─CH2─CH3 + HOH CH3 │ H+ 2. CH3─C=CH─CH2─CH3 + HOH 3. + HOH

Learning Check Write the products of each reaction

Polymers Polymers are Large, long-chain molecules. Found in nature, including cellulose in plants, starches in food, proteins and DNA in the body. Also synthetic such as polyethylene and polystyrene, Teflon, and nylon. Made up of small repeating units called monomers. Made by reaction of small alkenes.

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

Common Synthetic Polymers Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Polymers from Addition Reactions TABLE 12.3 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

More Monomers and Polymers Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check What is the starting monomer for polyethylene?

Learning Check Name the monomer used to make Teflon and write a portion of a Teflon polymer using four monomers.

Recycling Plastics Recycling is simplified by using codes found on plastic items. 1 PETE Polyethyleneterephtalate 2 HDPE High-density polyethylene 3 PV Polyvinyl chloride 4 LDPE Low-density polyethylene 5 PP Polypropylene PS Polystyrene Copyright © 2007 by Pearson Education, Inc . Publishing as Benjamin Cummings

Lecture 3 Unsaturated Hydrocarbons Aromatic Compounds Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Benzene Structure Benzene Has 6 electrons shared equally among the 6 C atoms. Is also represented as a hexagon with a circle drawn inside. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Aromatic Compounds in Nature and Health Vanillin Aspirin Ibuprofen Acetaminophen

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 used for many years. With a single substituent use a common name or are named as a benzene derivative. toluene aniline phenol (methylbenzene) (benzenamine) (hydroxybenzene)

Aromatic Compounds with Two Groups Two naming systems are used when two groups are attached to a benzene ring. Number the ring to give the lowest numbers to the side groups. Use prefixes to show the arrangement: ortho(o-) for 1,2- meta(m-) for 1,3- para(p-) for 1,4-

Aromatic Compounds with Two Groups 3-chlorotoluene 1,4-dichlorobenzene 2-chlorophenol m-chlorotoluene p-dichlorobenzene o-chlorophenol

Learning Check Select the correct name for each compound: 1) chlorocyclohexane 2) chlorobenzene 3) 1-chlorobenzene 1) 1,2-dimethylbenzene 2) m-xylene 3) 1,3-dimethylbenzene

Learning Check Write the structural formulas for each of the following: A. 1,3-dichlorobenzene B. o-chlorotoluene

Learning Check Identify the organic family for each: A. CH3─CH2─CH=CH2 B. C. CH3─C≡CH D.

Properties of Aromatic Compounds Have a stable aromatic bonding system. Are resistant to many reactions. Undergo substitution reactions, which retain the stability of the aromatic bonding system.

Substitution Reactions In a substitution reaction, a hydrogen atom on a benzene ring is replaced by an atom or group of atoms. Type of substitution H on benzene replaced by Halogenation chlorine or bromine atom Nitration nitro group (—NO2) Sulfonation —SO3H group

Halogenation In a halogenation An H atom of benzene is replaced by a chlorine or bromine atom. A catalyst such as FeCl3 is needed in chlorination. A catalyst such as FeBr3 is needed in bromination.

Nitration In the nitration of benzene An H atom of benzene is replaced by a nitro (-NO2) group from HNO3. An acid catalyst such as H2SO4 is needed.

Sulfonation In a sulfonation An H atom on benzene is replaced by a —SO3H group from SO3. An acid catalyst such as H2SO4 is needed.

Learning Check Write the equation for the bromination of benzene including catalyst.