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4-1 © 2005 John Wiley & Sons, Inc All rights reserved Chapter 4: Alkenes and Alkynes
4-2 © 2005 John Wiley & Sons, Inc All rights reserved Unsaturated Hydrocarbons Unsaturated hydrocarbon Unsaturated hydrocarbon: contains one or more carbon-carbon double or triple bonds alkenealkene: contains a carbon-carbon double bond and has the general formula C n H 2n alkynealkyne: contains a carbon-carbon triple bond and has the general formula C n H 2n-2
4-3 © 2005 John Wiley & Sons, Inc All rights reserved Unsaturated Hydrocarbons Arenes Arenes: benzene and its derivatives (Ch 9)
4-4 © 2005 John Wiley & Sons, Inc All rights reserved Benzene and Phenyl Groups 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
4-5 © 2005 John Wiley & Sons, Inc All rights reserved Structure of Alkenes The two carbon atoms of a double bond and the four atoms bonded to them lie in a plane, with bond angles of approximately 120°
4-6 © 2005 John Wiley & Sons, Inc All rights reserved Structure of Alkenes According to the orbital overlap model, a double bond consists of a bond formed by overlap of sp 2 hybrid orbitals a bond formed by overlap of parallel 2p orbital Rotating by 90°breaks the pi bond Rotating by 90°breaks the pi bond
4-7 © 2005 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 Remember same order of attachment, different arrangement in space
4-8 © 2005 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
4-9 © 2005 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 bond formed by the overlap of sp hybrid orbitals two bonds formed by the overlap of sets of parallel 2p orbitals
4-10 © 2005 John Wiley & Sons, Inc All rights reserved Nomenclature of Alkenes IUPAC Nomenclature enuse the -en- at the end 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
4-11 © 2005 John Wiley & Sons, Inc All rights reserved Nomenclature of Alkenes IUPAC nomenclature
4-12 © 2005 John Wiley & Sons, Inc All rights reserved Numbering System (cont) e.g.
4-13 © 2005 John Wiley & Sons, Inc All rights reserved IUPAC nomenclature
4-14 © 2005 John Wiley & Sons, Inc All rights reserved Nomenclature of Alkenes Some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names CH 2 = methylene CH 2 =CH─ vinyl CH 2 =CHCH 2 ─ allyl
4-15 © 2005 John Wiley & Sons, Inc All rights reserved Question Name this compound:
4-16 © 2005 John Wiley & Sons, Inc All rights reserved Answer 2-methyl-2-butene
4-17 © 2005 John Wiley & Sons, Inc All rights reserved Nomenclature of Alkynes IUPAC nomenclature ynuse the -yn- at the end to show the presence of a carbon-carbon triple bond number the parent chain to give the 1st carbon of the triple bond the lower number follow IUPAC rules for numbering and naming substituents
4-18 © 2005 John Wiley & Sons, Inc All rights reserved IUPAC nomenclature
4-19 © 2005 John Wiley & Sons, Inc All rights reserved Numbering System (cont) e.g. 3-Methyl-1-butyne
4-20 © 2005 John Wiley & Sons, Inc All rights reserved Question Name this compound:
4-21 © 2005 John Wiley & Sons, Inc All rights reserved Answer 2,2-dimethyl-3-heptyne
4-22 © 2005 John Wiley & Sons, Inc All rights reserved Dienes, Trienes, and Polyenes Alkenes that contain more than one double bond are alkadienes, alkatrienes, and so on. e.g. 2-methyl-1,4-pentadiene
4-23 © 2005 John Wiley & Sons, Inc All rights reserved Polyenes
4-24 © 2005 John Wiley & Sons, Inc All rights reserved Alkylidene Groups Double Bonds Fused to Rings
4-25 © 2005 John Wiley & Sons, Inc All rights reserved Name These
4-26 © 2005 John Wiley & Sons, Inc All rights reserved
4-27 © 2005 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 Draw cis-2- butene and trans-2-butene Cis and trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. Neither 1-butene and 2-methylpropene have no stereoisomers.
4-28 © 2005 John Wiley & Sons, Inc All rights reserved Stability of Alkenes Stability of cis- and trans isomers Interconversion does not occur spontaneously Interconversion can be brought about by treating the alkene with a strong acid Cis isomers are less stable than trans isomers because of the steric strain between the two larger substituents on the same side of the double-bond
4-29 © 2005 John Wiley & Sons, Inc All rights reserved Relative Stabilities of Alkenes
4-30 © 2005 John Wiley & Sons, Inc All rights reserved Sequence Rules: The E,Z Designation Z – for German zusammen “together” High priority groups on each carbon are on same side of the double-bond E – for German entgegen “opposite” High priority groups on each carbon are on opposite sides of the double-bond E,Z system Sequence rules used to assign priorities to the substituent groups on the double-bond carbons (alkenes)
4-31 © 2005 John Wiley & Sons, Inc All rights reserved E/Z System
4-32 © 2005 John Wiley & Sons, Inc All rights reserved Configuration: E,Z Priority rules 1. Priority is based on atomic number; the higher the atomic number, the higher the priority 2. If priority cannot be assigned on the basis of the atoms bonded directly to the double bond, look to the next set of atoms; priority is assigned at the first point of difference
4-33 © 2005 John Wiley & Sons, Inc All rights reserved Configuration - E,Z 3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds
4-34 © 2005 John Wiley & Sons, Inc All rights reserved “E” is More Stable than “Z”
4-35 © 2005 John Wiley & Sons, Inc All rights reserved Physical Properties Alkenes and alkynes are nonpolar compounds Their physical properties are similar to those of alkanes of similar carbon skeletons those that are liquid at room temperature are less dense than water (1.0 g/m L) they dissolve in each other and in nonpolar organic solvents they are insoluble in water
4-36 © 2005 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
4-37 © 2005 John Wiley & Sons, Inc All rights reserved Terpenes Vitamin A (retinol) the four isoprene units in vitamin A are shown in red they are linked head to tail, and cross linked at one point (the blue bond) to give the six-membered ring
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