Chapter 5 Part 1 Notes PDF

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Alkenes: Bonding, Nomenclature, and Properties Outline 5.1 Structure of Alkenes How To: Calculate the Index of Hydrogen Deficiency 5.2 Nomenclature of Alkenes 5.3 Physical Properties of Alkenes 5.4 Naturally Occurring Alkenes—Terpene Hydrocarbons An unsaturated hydrocarbon contains one or more carbon-carbon double or triple bonds. The term unsaturation indicates that fewer hydrogens are bonded to carbon than in an alkane, CnH2n12. The three most important classes of unsaturated hydro- carbons are alkenes, alkynes, and arenes. Alkenes contain a carbon-carbon double bond and, with one double bond and no rings, have the general formula CnH2n. Alkynes contain a carbon-carbon triple bond and, with one triple bond and no rings, have the general formula CnH2n22. The simplest alkene is ethylene, and the simplest alkyne is acetylene. Haze in the Blue Ridge Mountains. The aerosolization of hydrocarbons emitted by trees and other plants causes light to scatter and appear as haze. Many naturally occurring hydrocarbons are formed from iso- prene units and are oxidized by light (see Section 5.4). (© Zack Frank/ Shutterstock.com) Unsaturated hydrocarbon A hydrocarbon containing one or more carbon-carbon double or triple bonds. The three most important classes of unsaturated hydrocarbons are alkenes, alkynes, and arenes.

206 Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied,

scanned, or duplicated, in whole or in part. WCN 02-200-203 " Ethylene 9#9 Acetylene In this chapter, we study the structure, nomenclature, and physical properties of alkenes. Alkynes are discussed separately in Chapter 7. Arenes are a third class of unsaturated hydrocarbons. The Lewis structure of benzene, the simplest arene, is shown below. H HCH CC CC HCH H Benzene Just as a group derived by removal of an H from an alkane is called an alkyl group and is given the symbol R! (Section 2.3A), a group derived by removal of an H from an arene is called an aryl group and is given the symbol Ar!. When the aryl group substituent on a parent chain is a benzene ring, it is given the special name phenyl group. Although ethane becomes ethyl in its substituent form, the derivative name for benzene stems from“phene,”a now-obsolete term for benzene. Throughout this text, we represent benzene by a hexagon with three inscribed double bonds. It is also common to represent it by a hexagon with an inscribed circle. The struc- tural formula for the phenyl group and two alternative representations follow. Benzene The chemistry of benzene and its derivatives is quite different from that of alkenes and alkynes, but even though we do not study the chemistry of arenes until Chapters 21 and 22, we will show structural formulas of compounds containing aryl groups be- fore then. The three double bonds in a six-membered ring create a special stabiliza- tion called aromaticity, which lowers the reactivity of benzene relative to other alkenes. What you need to remember at this point is that an aryl group is not chemically reac- tive under any of the conditions we describe in

Chapters 6 through 20. Arene A term used to classify benzene and its derivatives. Alkenes: Bonding, Nomenclature, and Properties 207

Aryl group (Ar!) A group derived from an arene by removal of an H. Phenyl group A group derived by removing an H from benzene; abbreviated C6H5! or Ph!.

Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 208 Chapter 5: Alkenes: Bonding, Nomenclature, and Properties 5.1 Structure of Alkenes A. Shapes of Alkenes Using valence-shell electron-pair repulsion (Section 1.4) for a carbon-carbon double bond, we predict a value of 1208 for the bond angles about each carbon. The observed H!C!C bond angle in ethylene is 121.18, close to that predicted. In other alkenes, deviations from the predicted angle of 1208 may be somewhat larger because of the strain introduced by nonbonded interactions created by groups bonded to the carbons of the double bond. The C!C!C bond angle in propene, for example, is 123.98.

121.1° 123.9° HH H3CH CC CC HH HH

Ethylene Propene

HOW TO Calculate the Index of Hydrogen Deficiency Valuable information about the structural formula with the same number of carbon atoms and of an unknown compound can be obtained by make the following adjustments to the number of inspecting its molecular formula. In addition to learning the number of atoms of carbon, hydrogen, oxygen, nitrogen, and so forth, in a molecule of the compound, we can also determine what is called its index of hydrogen deficiency. For each ring and p bond, the molecular formula has two fewer hydrogens. The index of hydrogen deficiency is the sum of the number of rings and p bonds in a molecule. It is determined by comparing the number of hydrogens in the molecular formula of a compound whose structure is to be determined (Hmolecule) with the number of hydrogens in a reference alkane of the same number of carbon atoms (Hreference). sHreference 2 Hmoleculed Index of hydrogen deficiency 5 2 1. The molecular formula of a reference acyclic alkane is CnH2n12 (Section 2.1). 2. To compare the molecular formula for a compound containing elements besides carbon and hydrogen, write the formula of the reference hydrocarbon hydrogen atoms in the unknown. a. Replace each monovalent atom of a Group 7 element (F, Cl, Br, I) with one hydrogen; halogen substitutes for hydrogen and reduces the number of hydrogens by one per halogen. The general formula of an acyclic monochloroalkane, for example, is CnH2n11Cl; the general formula of the

corresponding acyclic alkane is CnH2n12. b. No correction is necessary for the addition of divalent atoms of Group 6 elements (O, S, Se). Insertion of a divalent Group 6 element into a hydrocarbon does not change the number of hydrogens. c. For each atom of a trivalent Group 5 element (N, P, As) present, add one hydrogen, because insertion of a trivalent Group 5 element adds one hydrogen to the molecular formula. The general molecular formula for an acyclic alkylamine, for example, is CnH2n13N. Index of hydrogen deficiency The sum of the number of rings and p bonds in a molecule. B. Carbon-Carbon Double Bond Orbitals In Section 1.7C, we described the formation of a carbon-carbon double bond in terms of the overlap of atomic orbitals. A carbon-carbon double bond consists of one s bond and one p bond (Figure 5.1). Each carbon of the double bond uses its three sp2 hybrid orbitals to form s bonds to three atoms. The unhybridized 2p atomic orbitals, which lie perpendicular to the plane created by the axes of the three sp2 hybrid orbitals, combine to form two p molecular orbitals: one bonding and the other antibonding. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 For the unhybridized 2p orbitals to be parallel, thus giving maximum overlap, the two carbon atoms of the double bond and the four bonded atoms must lie in a plane. (a) (b) s sp s s sp

5.1 Structure of Alkenes 209

It takes approximately 264 kJ (63 kcal)/mol to break the p bond in ethylene [i.e., to rotate one carbon by 908 with respect to the other where zero overlap occurs between 2p orbitals on adjacent carbons (Figure 5.2)]. This energy is considerably greater than the thermal energy available at room temperature; consequently, rotation about a carbon-carbon double bond does not occur under normal conditions.You might compare rotation about a carbon-carbon double bond, such as in ethylene, with that about a carbon-carbon single bond, such as in ethane (Section 2.5A). Whereas rotation about the carbon-carbon single bond in ethane is relatively free [rotation barrier approximately 12.5 kJ (3.0 kcal)/mol], rotation about the carbon-carbon double bond in ethylene is severely restricted. ( ( (a ) ) ) ( ( (b ) ) ) p Figure 5.1 Covalent bonding in ethylene. (a) Lewis structure and (b) orbital overlap model showing the s and p bonds.

Figure 5.2 Restricted rotation about a carbon-carbon double bond. (a) Orbital overlap model showing the p bond. (b) The p bond is broken by rotating the plane of one H!C!H group by 90° with respect to the plane of the other H!C!H group.

example 5.1 Index of Hydrogen Deficiency I Calculate the index of hydrogen deficiency for 1-hexene, C6H12, and account for this deficiency by reference to its structural formula. Solution The molecular formula of the reference acyclic alkane of six carbon atoms is C6H14. The index of hydrogen deficiency of 1-hexene (14 2 12)/2 5 1 and is accounted for by the one p bond in 1-hexene. Problem 5.1 Calculate the index of hydrogen deficiency of cyclohexene, C6H10, and account for this deficiency by reference to its structural formula. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 210 Chapter 5: Alkenes: Bonding, Nomenclature, and Properties example 5.2 Index of Hydrogen Deficiency II Isopentyl acetate, a compound with a banana-like odor, is a component of the alarm pheromone of honeybees. The molecular formula of isopentyl acetate is C7H14O2. Calculate the index of hydrogen deficiency of this compound. Solution The molecular formula of the reference hydrocarbon is C7H16. Adding oxygens does not require any correction in the number of hydrogens. The index of hydrogen deficiency is (16 2 14)/2 5 1, indicating either one ring or one p bond. Following is the structural formula of isopentyl acetate. It contains one p bond!in this case, a carbon-oxygen p bond.

Isopentyl acetate Problem 5.2 The index of hydrogen deficiency of niacin is 5. Account for this index of hydrogen deficiency by reference to the structural formula of niacin. Nicotinamide (Niacin)

Cis,trans isomers Isomers that have the same order of attachment of their atoms but a different arrangement of their atoms in space owing to the presence of either a ring (Section 2.6) or a carbon-carbon double bond (Section 5.1C). H3C C. Cis,Trans Isomerism in Alkenes Because of restricted rotation about a carbon-carbon double bond, any alkene in which each carbon of the double bond has two different groups bonded to it shows cis,trans isomerism. For example, 2-butene has two stereoisomers. In cis-2butene, the two methyl groups are on one side of the double bond and the two hydrogens are on the other side. In trans-2-butene, the two methyl groups are on opposite sides of the double bond. These two compounds cannot be converted into one an- other at room temperature because of the restricted rotation about the double bond; they are different compounds (diastereomers), with different physical and chemical properties. CH3 CC CC

CH3 cis -2-Butene H3C steric strain

Cis alkenes with double bonds in open chains are less stable than their trans isomers because of steric strain between alkyl substituents on the same side of the double bond, as can be seen in space-filling models of the cis and trans isomers Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 trans -2-Butene of 2-butene. This is the same type of strain that results in the preference for equatorial methylcyclohexane over axial methylcyclohexane (Section 2.5B). trans-2Butene is more stable than the cis isomer by about 4.2 kJ (1.0 kcal)/mol because of the sum of steric strain and angle strain that results from the two methyls moving apart. At the energy minimum, the C"C!CH3 angle for cis-2-butene is about 1278. 5.2 Nomenclature of Alkenes Alkenes are named using the IUPAC system, but as we shall see, some are usually referred to by their common names. A. IUPAC Names To form IUPAC names for alkenes, change the -an- infix of the parent alkane to -en- (Section 2.3C). Hence, CH2"CH2 is named ethene and CH3CH"CH2 is named propene. In higher alkenes, where isomers exist that differ in location of the double bond, a numbering system must be used. According to the IUPAC system: 1. Number the longest carbon chain that contains the double bond in the direction that gives the carbon atoms of the double bond the lowest possible numbers. 2. Indicate the location of the double bond by the number of its first carbon. 3. Name branched or substituted alkenes in a manner similar to alkanes. 4. Number the carbon atoms, locate and name substituent groups, locate the double bond, and name the main chain. 1-Hexene 4-Methyl-1-hexene 2-Ethyl-4-methyl-1-pentene Note that there is a chain of six carbon atoms in 2-ethyl-4-methyl-1-pentene. However, because the longest chain that contains the double bond has only five carbons, the parent hydrocarbon is pentane, and the molecule is named as a disubstituted 1-pentene.

5.2 Nomenclature of Alkenes 211

example 5.3 Nomenclature of Alkenes Write the IUPAC name of each alkene. Solution (a) 4-Ethyl-3,3-dimethyl-1-octene (b) 2-Methyl-2-butene (Continued) Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 212 Chapter 5: Alkenes: Bonding, Nomenclature, and Properties Problem 5.3 Write the IUPAC name of each alkene. (a) (a) (b) (b) Methylene A CH2" group. Vinyl A CH2"CH! group. Allyl A CH2"CHCH2! group. Ethene Ethylene Propene Propylene 2-Methylpropene Isobutylene cis In cis,trans-alkene nomenclature, it refers to molecules in which the carbonatoms of the main chain are on the same side of the double bond. trans

In cis,trans-alkene nomenclature, it refers to molecules in which the carbon atoms of the main chain are onopposite sides of the double bond. C. Systems for Designating Configuration in Alkenes The Cis,Trans System The most common method for specifying the configuration in alkenes uses the prefixes cis and trans. There is no doubt which isomers are intended by the names trans-3-hexene and cis-3-hexene. For more complex alkenes, the orientation of the atoms of the parent chain determines whether the alkene is cis or trans. On the right is a structural formula for the cis isomer of 3,4-dimethyl-2-pentene. In this example, carbon atoms of the main chain (carbons 1 and 4) are on the same side of the double bond; therefore, this alkene is cis. B. Common Names Some alkenes, particularly those of low molecular weight, are known almost exclusively by their common names, as illustrated by the common names of these alkenes.

Furthermore, the common names methylene (a CH2 group), vinyl, and allyl are often used to show the presence of the following alkenyl groups: Alkenyl Group CH2" CH2"CH! CH2"CHCH2! IUPAC Name Methylidene Ethenyl 2-Propenyl Common Name Methylene Vinyl " Allyl " Example

IUPAC Name (Common Name) Methylidenecyclopentane (Methylenecyclopentane) Ethenylcyclopentane ( Vinylcyclopentane) 2-Propenylcyclopentane (Allylcyclopentane)

Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 5.2 Nomenclature of Alkenes 213

trans -3-Hexene cis -3-Hexene cis -3,4-Dimethyl-2-pentene example 5.4 Trans Versus Cis Alkene Nomenclature Name each alkene and show the configuration about each double bond using the cis,trans system. (a) (a) (b) (b) Solution 

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(a) The chain contains seven carbon atoms and is numbered from the end that gives the lower number to the first carbon of the double bond. Its name is trans-3-heptene. (b) The longest chain contains eight carbon atoms and is numbered from either end of the chain so that the first carbon of the double bond is carbon 4 of the chain. Its name is cis-4-octene.

Problem 5.4 Which alkenes show cis,trans isomerism? For each alkene that does, draw the

trans isomer. (a) 2-Pentene (b) 2-Methyl-2-pentene (c) 3-Methyl-2-pentene

The E,Z System E,Z system A system to specify the configuration of groups about a carbon-carbon double bond. Z From the German zusammen, together. Specifies that groups of higher priority on the carbons of a double bond are on the same side. E From the German entgegen, opposite. Specifies that groups of higher priority on the carbons of a double bond are on opposite sides. Watch a video explanation Because the cis,trans system becomes confusing with tri- and tetrasubstituted alkenes and is not detailed enough to name all alkenes, chemists developed the E,Z system. This system uses the priority rules of the R,S system (Section 3.3) to assign priority to the substituents on each carbon of a double bond. Using these rules, we decide which group on each carbon has the higher priority. If the groups of higher priority are on the same side of the double bond, the configu- ration of the alkene is Z (German: zusammen, together). If they are on opposite sides of the double bond, the alkene is E (German: entgegen, opposite). "" Z zusammen E entgegen Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 214

Chapter 5: Alkenes: Bonding, Nomenclature, and Properties Throughout this text, we use the cis,trans system for alkenes in which each carbon of the C"C bond bears a hydrogen. We use the E,Z system in all other cases. The E,Z system should always be used if confusion is possible. example 5.5 E,Z Nomenclature Name each alkene and specify its configuration by the E,Z system. (a) (b) (c) Solution 

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(a) The group of higher priority on carbon 2 is methyl; that of higher priority on carbon 3 is isopropyl. Because the groups of higher priority are on the same side of the double bond, the alkene has the Z configuration. Its name is (Z)-3,4-dimethyl-2-pentene. (b) Groups of higher priority on carbons 2 and 3 are !Cl and !CH2CH3. Because these groups are on opposite sides of the double bond, the configuration of this alkene is E. Its name is (E)-2-chloro-2-pentene. (c) The groups of higher priority are on opposite sides of the double bond; the configuration is E. The name of this bromoalkene is (E)-1-bromo-4isopropyl-5-methyl-4-octene.

Problem 5.5 Name each alkene and specify its configuration by the E,Z system. (a) (b) (c)

Watch a video explanation D. Cycloalkenes In naming cycloalkenes, the carbon atoms of the ring double bond are numbered 1 and 2 in the direction that gives the substituent encountered first the smaller number. 3-Methylcyclopentene 4-Ethyl-1-methylcyclohexene 1,6-Dimethylcyclohexene

Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 E. Cis,Trans Isomerism in Cycloalkenes Following are structural formulas for four cycloalkenes: Cyclopentene Cyclohexene Cycloheptene Cyclooctene In these representations, the configuration about each double bond is cis. Is it possible to have a trans configuration in these and larger cycloalkenes? To date, transcyclooctene is the smallest trans cycloalkene that has been prepared in pure form and is stable at room temperature.Yet, even in this trans cycloalkene, there is considerable angle strain; the double bond’s 2p orbitals make an angle of 448 to each other. Cis- cyclooctene is more stable than its trans isomer by 38 kJ (9.1 kcal)/mol. Note that the trans isomer is chiral even though it has no chiral center. hn 5.2 Nomenclature of Alkenes 215 example 5.6 Nomenclature for Cyclic Alkenes Write the IUPAC name of each cycloalkene. (a) (b) (c) Solution (a) 3,3-Dimethylcyclohexene (b) 1,2-Dimethylcyclopentene (c) 4-(1-Methylethyl)1-methylcyclohexene Problem 5.6 Write the IUPAC name of each cycloalkene. (a) (b) (c)

cis -Cyclooctene F. Bridgehead Alkenes trans -Cyclooctene trans-Cyclooctene

Another manner in which alkenes become strained is when one of the carbons in the double bond is placed at a bridgehead carbon of a bicyclic ring system. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-200-203 216 Chapter 5: Alkenes: Bonding, Nomen...