© 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e Bettelheim, Brown, and March

© 2003 Thomson Learning, Inc. All rights reserved Chapter 12 Alkenes and Cycloalkenes Alkenes and Cycloalkenes

© 2003 Thomson Learning, Inc. All rights reserved Alkenes and Alkynes Alkene: Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds ethylene is the simplest alkene Alkyne: Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds acetylene is the simplest alkyne

© 2003 Thomson Learning, Inc. All rights reserved Alkenes Structure: the VSEPR model predicts bond angles of 120° about each carbon of a double bond in ethylene, the actual angles are close to 120° in substituted alkenes, angles about each carbon of the double bond may be greater than 120° because of repulsion of alkyl groups bonded to the double bond

© 2003 Thomson Learning, Inc. All rights reserved Alkenes Cis-trans isomerism because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism

© 2003 Thomson Learning, Inc. All rights reserved Alkenes - IUPAC Names To name an alkene the parent name is that of the longest chain that contains the C=C number the chain from the end that gives the lower numbers to the carbons of the C=C locate the C=C by the number of its first carbon eneuse the ending -ene to show the presence of the C=C branched-chain alkenes are named in a manner similar to alkanes; substituted groups are located and named

© 2003 Thomson Learning, Inc. All rights reserved Alkenes - IUPAC Names Examples

© 2003 Thomson Learning, Inc. All rights reserved Alkynes - IUPAC Names ynefollow the same rules as for alkenes, but use the ending -yne to show the presence of the triple bond

© 2003 Thomson Learning, Inc. All rights reserved Common Names Common names are still used for some alkenes and alkynes, particularly those of low molecular weight

© 2003 Thomson Learning, Inc. All rights reserved Cycloalkenes To name a cycloalkene number the carbon atoms of the ring double bond 1 and 2 in the direction that gives the lower number to the substituent encountered first number and list substituents in alphabetical order

© 2003 Thomson Learning, Inc. All rights reserved Dienes, Trienes, Polyenes alkenes that contain more than one double bond are named as alkadienes, alkatrienes, and so on those that contain several double bonds are referred to more generally as polyenes (Greek: poly, many)

© 2003 Thomson Learning, Inc. All rights reserved Physical Properties alkenes and alkynes are nonpolar compounds the only attractive forces between their molecules are London dispersion forces their physical properties are similar to those of alkanes with the same carbon skeletons alkenes and alkynes are insoluble in water but soluble in one another and in nonpolar organic liquids alkenes and alkynes that are liquid or solid at room temperature have densities less than 1 g/mL; they float on water

© 2003 Thomson Learning, Inc. All rights reserved Terpenes Terpene: Terpene: a compound whose carbon skeleton can be divided into five-carbon units identical with the carbon skeleton of isoprene Terpenes illustrate an important principle of the molecular logic of living systems in building large molecules, small subunits are bonded together by a series of enzyme-catalyzed reactions and then chemically modified by additional enzyme-catalyzed reactions

© 2003 Thomson Learning, Inc. All rights reserved Terpenes

© 2003 Thomson Learning, Inc. All rights reserved Reactions of Alkenes The most common reaction is addition

© 2003 Thomson Learning, Inc. All rights reserved Reactions of Alkenes Most alkene addition reactions are exothermic the products are more stable (lower in energy) than the reactants just because they are exothermic doesn’t mean that they occur rapidly reaction rate depends on activation energy many alkene addition reactions require a catalyst

© 2003 Thomson Learning, Inc. All rights reserved Addition of HX Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane H adds to one carbon of the C=C and X to the other regioselectivereaction is regioselective Markovnikov’s rule:Markovnikov’s rule: H adds to the less substituted carbon and X to the more substituted carbon

© 2003 Thomson Learning, Inc. All rights reserved Addition of HX reaction mechanism Chemists account for the addition of HX to an alkene by a two-step reaction mechanism we use curved arrows to show the repositioning of electron pairs during a chemical reaction the tail of an arrow shows the origin of the electron pair (either on an atom of in a bond) the head of the arrow shows its new position curved arrows show us which bonds break and which new ones form

© 2003 Thomson Learning, Inc. All rights reserved Addition of HCl to 2 Butene Step 1: reaction of the carbon-carbon double bond with H + gives a secondary carbocation intermediate Step 2: reaction of the carbocation intermediate with chloride ion completes the addition

© 2003 Thomson Learning, Inc. All rights reserved Addition of H 2 O hydration Addition of water is called hydration hydration is acid catalyzed, most commonly by H 2 SO 4 hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon

© 2003 Thomson Learning, Inc. All rights reserved Addition of H 2 O Step 1: Step 2: Step 3:

© 2003 Thomson Learning, Inc. All rights reserved Addition of Cl 2 and Br 2 Addition takes place readily at room temperature reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent addition of Br 2 is a useful qualitative test for the presence of a carbon-carbon double bond Br 2 has a deep red color; dibromoalkanes are colorless

© 2003 Thomson Learning, Inc. All rights reserved Addition of H 2 - Reduction Virtually all alkenes add H 2 in the presence of a transition metal catalyst, commonly Pd, Pt, or Ni

© 2003 Thomson Learning, Inc. All rights reserved Polymerization From the perspective of the organic chemical industry, the single most important reaction of alkenes is polymerization polymer:polymer: Greek: poly, many and meros, part monomer:monomer: Greek: mono, single and meros, part

© 2003 Thomson Learning, Inc. All rights reserved Polymerization show the structure of a polymer by placing parentheses around the repeating monomer unit place a subscript, n, outside the parentheses to indicate that this unit repeats n times the structure of a polymer chain can be reproduced by repeating the enclosed structure in both directions following a section of polypropene (polypropylene)

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© 2003 Thomson Learning, Inc. All rights reserved Polyethylene Low-density polyethylene (LDPE) a highly branched polymer; polymer chains do not pack well and London dispersion forces between them are weak softens and melts above 115°C approximately 65% used for the production of films for packaging and for trash bags High-density polyethylene (HDPE) only minimal chain branching; chains pack well and London dispersion forces between then are strong has higher melting point than LDPE and is stronger can be blow molded to squeezable jugs and bottles

© 2003 Thomson Learning, Inc. All rights reserved Codes for Plastics

© 2003 Thomson Learning, Inc. All rights reserved Chapter 12 End Chapter 12 Chapter 12