5-1 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Introduction to Organic Chemistry 2 ed William H. Brown

5-2 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Alkenes & Alkynes Chapter 5

5-3 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Unsaturated Hydrocarbons Unsaturated hydrocarbon Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds Alkene Alkene: contains a carbon-carbon double bond and has the general formula C n H 2n

5-4 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Unsaturated Hydrocarbons Alkyne Alkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2

5-5 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Unsaturated Hydrocarbons Arenes Arenes: benzene and its derivatives (Ch 9)

5-6 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Benzene & Phenyl Group We do not study benzene and its derivatives until Chapter 9 But, we show structural formulas of compounds containing the phenyl group before that time the phenyl group is not reactive under any of the conditions we describe in Ch 3-8

5-7 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Structure of Alkenes The two carbon atoms of a double bond and the four atoms attached to them lie in a plane, with bond angles of approximately 120° According to the orbital overlap model, a double bond consists of one sigma bond formed by overlap of sp 2 hybrid orbitals one pi bond formed by overlap of parallel 2p orbitals

5-8 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Structure of Alkenes Length of C-C bonds: single > double > triple Strength of C-C bonds:triple > double > single

5-9 5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Cis-Trans Isomerism Because of restricted rotation about a C-C double bond, groups on adjacent carbons are either cis or trans to each other

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Cis,Trans Isomerism Trans alkenes are more stable than cis alkenes because of nonbonded interaction strain between alkyl substituents of the same side of the double bond

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Structure of Alkynes The functional group of an alkyne is a carbon-carbon triple bond A triple bond consists of one sigma bond formed by the overlap of sp hybrid orbitals two pi bonds formed by the overlap of sets of parallel 2p orbitals

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkenes en Use the infix -en- to show the presence of a carbon-carbon double bond Number the parent chain to give the 1st carbon of the double bond the lower number Follow IUPAC rules for numbering and naming substituents For a cycloalkene, the double bond must be numbered 1,2

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkenes

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkenes

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkenes Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkynes yn IUPAC: use the infix -yn- to show the presence of a carbon-carbon triple bond

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Nomenclature - Alkynes Common names: prefix the substituents on the triple bond to the name “acetylene”

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Configuration - cis, trans The cis-trans system The cis-trans system: configuration is determined by the orientation of atoms of the main chain

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Configuration - E,Z The E,Z system uses the priority rules of the R,S system to assign to the groups on each carbon of a carbon-carbon double bond 1. Each atom bonded to the C-C double bond is assigned a priority 2. If groups of higher priority are on the same side, configuration is Z (German, zusammen) 3. If groups of higher priority are on opposite sides, configuration is E (German, entgegen)

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Configuration - E,Z Example Example: name each alkene and specify its configuration by the E,Z system

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Cis-Trans in Cycloalkenes Configuration of the double bond in cyclopropene through cycloheptene must be cis

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Cis-Trans in Cycloalkenes trans-Cyclooctene is the smallest trans- cycloalkene stable at 25°C the cis isomer is 9.1 kcal/mol more stable than the trans isomer.

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Physical Properties Alkenes are nonpolar compounds The only attractive forces between their molecules are dispersion forces The physical properties of alkenes are similar to those of alkanes

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Terpenes Terpene Terpene: a compound whose carbon skeleton can be divided into two or more units identical with the carbon skeleton of isoprene

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Terpenes Myrcene, C 10 H 16, a component of bayberry wax and oils of bay and verbena

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Terpenes Menthol, from peppermint

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Terpenes  -Pinene, from turpentine, shown as a ball-and-stick model, first without hydrogens and then with them

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Terpenes Camphor, from the camphor tree

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Vitamin A (Retinol) How many stereoisomers are possible for vitamin A?

Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. Alkenes & Alkynes End Chapter 5