Alkenes and Alkynes PDF

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER FOUR Alkenes and Alkynes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-1 Unsaturated Hydrocarbons Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon-carbon double or triple bonds...

WILLIAM H. BROWN THOMAS POON www.wiley.com/college/brown CHAPTER FOUR Alkenes and Alkynes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-1 Unsaturated Hydrocarbons Unsaturated Hydrocarbon: A hydrocarbon that contains one or more carbon-carbon double or triple bonds or benzene-like rings. – Alkene: contains a carbon-carbon double bond and has the general formula CnH2n. – Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-2 Unsaturated Hydrocarbons Arene: benzene and its derivatives (Ch 9) Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-3 Benzene and Phenyl Groups We do not study benzene and its derivatives until Chapter 9. – However, we show structural formulas of compounds containing a phenyl group before that time. – The phenyl group is not reactive under any of the conditions we describe in chapters 5-8. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-4 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°. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-5 Structure of Alkenes Figure 4.1 According to the orbital overlap model, a double bond consists of one s bond formed by overlap of sp2 hybrid orbitals and one p bond formed by overlap of parallel 2p orbitals. Rotating by 90°breaks the pi bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-6 Cis-Trans Isomerism Because of restricted rotation about a C–C double bond, groups on the carbons of a double bond are either cis or trans to each other. – Because of nonbonded interaction strain between alkyl substituents on the same side of the double bond, a trans alkene is more stable than an isomeric cis alkene. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-7 Structure of Alkynes The functional group of an alkyne is a carbon-carbon triple bond. A triple bond consists of: – One s bond formed by the overlap of sp hybrid orbitals. – Two p bonds formed by the overlap of sets of parallel 2p orbitals. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-8 Nomenclature of Alkenes IUPAC Nomenclature of alkenes – Use the infix -en- to show the presence of a carbon-carbon double bond. – Number the parent chain to give the first carbon of the double bond the lower number. – Follow IUPAC rules for numbering and naming substituents. – For a cycloalkene, number the atoms of the ring beginning with the two carbons of the double bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-9 Nomenclature of Alkenes IUPAC nomenclature of alkenes Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-10 Nomenclature of Alkenes Some alkenes, particularly low-molecular- weight alkenes, are known almost exclusively by their common names. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-11 Nomenclature of Alkynes IUPAC nomenclature of alkynes – Use the infix -yne to show the presence of a carbon-carbon triple bond. – Number the parent chain to give the first carbon of the triple bond the lower number. – Follow IUPAC rules for numbering and naming substituents. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-12 Configuration: Cis-Trans The cis-trans system: Configuration is determined by the orientation of atoms of the main chain. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-13 Configuration: E,Z To Assign an E,Z configuration, first assign a priority to the substituents on each carbon of the double bond. – If the groups of higher priority are on the same side of the double bond, the configuration is Z (German: zusammen, together). – If the groups of higher priority are on opposite sides of the double bond, the configuration is E (German: entgegen, opposite). Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-14 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-15 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. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-16 Configuration - E,Z Example: Name each alkene by the E,Z system and specify its configuration. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-17 Cis-Trans Isomerism – The configuration of the double bond in cyclopropene through cycloheptene must be cis. These rings are not large enough to accommodate a trans double bond. – Cyclooctene is the smallest cycloalkene that can accommodate a trans double bond. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-18 Cis-Trans Isomerism Dienes, trienes, and polyenes – For an alkene with n carbon-carbon double bonds, each of which can show cis-trans isomerism, 2n cis- trans isomers are possible. – Consider 2,4-heptadiene; it has four cis-trans isomers, two of which are drawn here. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-19 Cis-Trans Isomerism – Vitamin A has five C-C double bonds, four of which can show cis-trans isomerism. – Vitamin A is the all-trans isomer. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-20 Physical Properties Alkenes and alkynes are nonpolar compounds. – The only attractive forces between their molecules are dispersion forces. The physical properties of alkenes and alkynes are similar to those of alkanes with similar carbon skeletons. – Those that are liquid at room temperature are less dense than water (1.0 g/mL). – They dissolve in each other and in nonpolar organic solvents. – They are insoluble in water. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-21 The Acidity of Terminal Alkynes One of the major differences between the chemistry of alkenes and alkynes is that the hydrogen atom of a terminal alkyne is sufficiently acidic (pKa 25) that it an be removed by a strong base such as sodium amide, NaNH2, to give an alkyne anion. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-22 The Acidity of Terminal Alkynes Because water (pKa 15.7) is a stronger acid than acetylene or a terminal alkyne, hydroxide ion is not a strong enough base for an acid-base reaction with a terminal alkyne. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-23 The Acidity of Terminal Alkynes The pKa values for alkane and alkene hydrogens are so large that no base is strong enough to remove a hydrogen from either of these classes of compounds. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-24 Problem 4.39 Zoapatanol is found in the leaves and twigs of Montanoa tomentoa. (a) Specify the configuration of each carbon-carbon double bond. (b) How many cis-trans isomers are possible for this compound. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-25 Problem 4.40 Pyrethrin II and pyrethrosin are derived from plants of the chrysanthemum family. (a) In each label all C—C double bonds about which cis/trans isomerism is possible. (b) How many cis-trans isomers are possible for each structural formula? Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-26 The Acidity of Terminal Alkynes How do we account for the acidity of a terminal alkyne compared to an alkane or alkene? To answer this question, we must focus on the stability of the anion derived from each class of hydrocarbon. The lone pair of electrons of an alkane anion lies in an sp3 hybrid orbital, which has 25% s character. For an alkene anion, the lone pair lies in an sp2 hybrid orbital, which has 33% s character. For a terminal alkyne anion, the lone pair lies in an sp hybrid orbital, which has 50% s character. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-27 The Acidity of Terminal Alkynes The greater the s character of the orbital bearing the negative charge, the greater the stability of the anion, and thus the greater acidic the hydrogen removed. Of the series of compounds alkane, alkene, and alkyne, the carbon of a terminal alkyne is the most electronegative (50% s character). Therefore, a terminal alkyne anion is the most stable of the series, and a terminal alkyne is the most acidic. Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-28 Alkenes and Alkynes End Chapter 4 Copyright © 2017 John Wiley & Sons, Inc. All rights reserved. 4-29