Carey - Organic Chemistry - chapt05

Carey - Organic Chemistry - chapt05

(Parte 1 de 6)

CHAPTER 5

Alkenesare hydrocarbons that contain a carbon–carbon double bond. Acarbon–carbon double bond is both an important structural unit and an important functional group in organic chemistry. The shape of an organic molecule is influenced by the presence of this bond, and the double bond is the site of most of the chemical reactions that alkenes undergo. Some representative alkenes include isobutylene(an industrial chemical), -pinene(a fragrant liquid obtained from pine trees), and farnesene (a naturally occurring alkene with three double bonds).

This chapter is the first of two dealing with alkenes; it describes their structure, bonding, and preparation. Chapter 6 discusses their chemical reactions.

5.1 ALKENE NOMENCLATURE

We give alkenes IUPAC names by replacing the -aneending of the corresponding alkane with -ene.The two simplest alkenes are etheneand propene.Both are also well known by their common names ethyleneand propylene.

Isobutylene (used in the production of synthetic rubber)

-Pinene (a major constituent of turpentine)

CH3

Farnesene (present in the waxy coating found on apple skins)

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168CHAPTER FIVEStructure and Preparation of Alkenes: Elimination Reactions

Ethyleneis an acceptable synonym for ethenein the IUPAC system. Propylene, isobutylene,and other common names ending in -yleneare not acceptable IUPAC names.

IUPAC name: ethene Common name: ethylene

CH3CHœCH2

IUPAC name: propene Common name: propylene

Ethylene was known to chemists in the eighteenth century and isolated in pure form in 1795. An early name for ethylene was gaz oléfiant(French for “oil-forming gas”), a term suggested to describe the fact that an oily liquid product is formed when two gases—ethylene and chlorine—react with each other.

The term gaz oléfiantwas the forerunner of the general term olefin,formerly used as the name of the class of compounds we now call alkenes.

Ethylene occurs naturally in small amounts as a plant hormone. Hormones are substances that act as messengers and play regulatory roles in biological processes. Ethylene is involved in the ripening of many fruits, in which it is formed in a complex series of steps from a compound containing a cyclopropane ring:

Even minute amounts of ethylene can stimulate ripening, and the rate of ripening increases with the concentration of ethylene. This property is used to advantage, for example, in the marketing of bananas. Bananas are picked green in the tropics, kept green by being stored with adequate ventilation to limit the amount of ethylene present, and then induced to ripen at their destination by passing ethylene over the fruit.* severalstepsNH3

CO2

1-Aminocyclopropanecarboxylic acid

CH2 CH2Ethylene other products

Ethylene (bp: 104°C)

Cl2

Chlorine (bp: 34°C)

ClCH2CH2Cl

1,2-Dichloroethane (bp: 83°C)

Ethylene is the cornerstone of the world’s mammoth petrochemical industry and is produced in vast quantities. In a typical year the amount of ethylene produced in the United States (5 1010lb) exceeds the combined weight of all of its people. In one process, ethane from natural gas is heated to bring about its dissociation into ethylene and hydrogen:

This reaction is known as dehydrogenationand is simultaneously both a source of ethylene and one of the methods by which hydrogen is prepared on an industrial scale. Most of the hydrogen so generated is subsequently used to reduce nitrogen to ammonia for the preparation of fertilizer.

Similarly, dehydrogenation of propane gives propene:

Propene is the second most important petrochemical and is produced on a scale about half that of ethylene.

Almost any hydrocarbon can serve as a starting material for production of ethylene and propene. Cracking of petroleum (Section 2.13) gives ethylene and propene by processes involving cleavage of carbon–carbon bonds of higher molecular weight hydrocarbons.

The major uses of ethylene and propene are as starting materials for the preparation of polyethylene and polypropylene plastics, fibers, and films. These and other applications will be described in Chapter 6.

CH3CH2CH3Propane H2Hydrogen

CH3CHœCH2 Propene

CH3CH3Ethane H2Hydrogen

CH2œCH2 Ethylene

*For a review, see “Ethylene—An Unusual Plant Hormone” in the April 1992 issue of the Journal of Chemical Education(p. 315–318). BackForwardMain MenuTOCStudy Guide TOCStudent OLCMHHE Website

The longest continuous chain that includes the double bond forms the base name of the alkene, and the chain is numbered in the direction that gives the doubly bonded carbons their lower numbers. The locant (or numerical position) of only one of the doubly bonded carbons is specified in the name; it is understood that the other doubly bonded carbon must follow in sequence.

Carbon–carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered.

Hydroxyl groups, however, outrank the double bond. Compounds that contain both a double bond and a hydroxyl group use the combined suffix -en -olto signify that both functional groups are present.

SAMPLE SOLUTION(a)The longest continuous chain in this alkene contains four carbon atoms. The double bond is between C-2 and C-3, and so it is named as a derivative of 2-butene.

Identifying the alkene as a derivative of 2-butene leaves two methyl groups to be accounted for as substituents attached to the main chain. This alkene is 2,3- dimethyl-2-butene. (It is sometimes called tetramethylethylene,but that is a common name, not an IUPAC name.)

We noted in Section 2.10 that the common names of certain frequently encountered alkylgroups, such as isopropyl and tert-butyl, are acceptable in the IUPAC system. Three alkenylgroups—vinyl, allyl,and isopropenyl—are treated the same way.

C 2,3-Dimethyl-2-butene

CH3HOCH2CH2CH2

C 5-Methyl-4-hexen-1-ol (not 2-methyl-2-hexen-6-ol)

CH3CHCHœCH2 W

CH3

3-Methyl-1-butene (not 2-methyl-3-butene)

BrCH2CH2CH2CHCH2CH2CH3

6-Bromo-3-propyl-1-hexene (longest chain that contains double bond is six carbons)

1-Butene (not 1,2-butene)

2-Hexene (not 4-hexene)

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When a CH2group is doubly bonded to a ring, the prefix methyleneis added to the name of the ring.

Cycloalkenesand their derivatives are named by adapting cycloalkane terminology to the principles of alkene nomenclature.

No locants are needed in the absence of substituents; it is understood that the double bond connects C-1 and C-2. Substituted cycloalkenes are numbered beginning with the double bond, proceeding through it, and continuing in sequence around the ring. The direction of numbering is chosen so as to give the lower of two possible locants to the substituent.

PROBLEM 5.2Write structural formulas or build molecular models and give the IUPAC names of all the monochloro-substituted derivatives of cyclopentene.

5.2STRUCTURE AND BONDING IN ALKENES

The structure of ethylene and the orbital hybridization model for the double bond were presented in Section 1.17. To review, Figure 5.1 depicts the planar structure of ethylene, its bond distances, and its bond angles. Each of the carbon atoms is sp2-hybridized, and the double bond possesses a component and a component. The component results when an sp2orbital of one carbon, oriented so that its axis lies along the internuclear axis, overlaps with a similarly disposed sp2orbital of the other carbon. Each sp2orbital contains one electron, and the resulting bond contains two of the four electrons of the double bond. The bond contributes the other two electrons and is formed by a “sideby-side” overlap of singly occupied porbitals of the two carbons.

Cyclopentene

Cl1

3-Chlorocycloheptene (not 1-chloro-2-cycloheptene)

1-Methylcyclohexene

Methylenecyclohexane CH2

CH2œCH±Vinyl as inCH2œCHCl Vinyl chloride

CH2œCHCH2±Allyl as inCH2œCHCH2OH Allyl alcohol as inCH2œC± W

CH3

Isopropenyl

CH2œCCl

Isopropenyl chloride

170CHAPTER FIVEStructure and Preparation of Alkenes: Elimination Reactions

Vinyl chloride is an industrial chemical produced in large amounts (1010lb/year in the United States) and is used in the preparation of poly(vinyl chloride). Poly(vinyl chloride), often called simply vinyl,has many applications, including siding for houses, wall coverings, and PVC piping.

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The double bond in ethylene is stronger than the C±C single bond in ethane, but it is not twice as strong. The CœC bond energy is 605 kJ/mol (144.5 kcal/mol) in ethylene versus 368 kJ/mol (8 kcal/mol) for the C±C bond in ethane. Chemists do not agree on exactly how to apportion the total CœC bond energy between its and components, but all agree that the bond is weaker than the bond.

formed by sp3–sp2overlap.

PROBLEM 5.3We can use bond-line formulas to represent alkenes in much the same way that we use them to represent alkanes. Consider the following alkene:

(a)What is the molecular formula of this alkene? (b)What is its IUPAC name?

(c)How many carbon atoms are sp2-hybridized in this alkene? How many are sp3- hybridized?

(d)How many bonds are of the sp2–sp3type? How many are of the sp3–sp3 type?

SAMPLE SOLUTION(a)Recall when writing bond-line formulas for hydrocarbons that a carbon occurs at each end and at each bend in a carbon chain. The appropriate number of hydrogens are attached so that each carbon has four bonds. Thus the compound shown is

H C±C bond length 150 pm

CœC bond length 134 pm sp3 hybridized carbon

C C sp2 hybridized carbon

FIGURE 5.1(a) The framework of bonds in ethylene showing bond distances in picometers and bond angles in degrees. All six atoms are coplanar. The carbon–carbon bond is a double bond made up of the component shown and the component illustrated in b. (b) The porbitals of two sp2 hybridized carbons overlap to produce a bond. An electron pair in the bond is shared by the two carbons.

The simplest arithmetic approach subtracts the C±C bond energy of ethane (368 kJ/mol; 8 kcal/mol) from the CœC bond energy of ethylene (605 kJ/mol; 144.5 kcal/mol). This gives a value of 237 kJ/mol (56.5 kcal/mol) for the bond energy.

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5.3ISOMERISM IN ALKENES

Although ethylene is the only two-carbon alkene, and propene the only three-carbon alkene, there are fourisomeric alkenes of molecular formula C4H8:

1-Butene has an unbranched carbon chain with a double bond between C-1 and C-2. It is a constitutional isomer of the other three. Similarly, 2-methylpropene, with a branched carbon chain, is a constitutional isomer of the other three.

The pair of isomers designated cis-and trans-2-butene have the same constitution; both have an unbranched carbon chain with a double bond connecting C-2 and C-3. They differ from each other, however, in that the cis isomer has both of its methyl groups on the same side of the double bond, but the methyl groups in the trans isomer are on opposite sides of the double bond. Recall from Section 3.12 that isomers that have the same constitution but differ in the arrangement of their atoms in space are classified as stereoisomers. cis-2-Butene and trans-2-butene are stereoisomers, and the terms “cis” and “trans” specify the configurationof the double bond.

Cis–trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. Thus, neither 1-butene nor 2-methylpropene can have stereoisomers.

PROBLEM 5.4How many alkenes have the molecular formula C5H10? Write their structures and give their IUPAC names. Specify the configuration of stereoisomers as cis or trans as appropriate.

In principle, cis-2-butene and trans-2-butene may be interconverted by rotation about the C-2œC-3 doublebond. However, unlike rotation about the C-2±C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal circumstances. It is sometimes said that rotation about a carbon–carbon double bond is restricted,but this is an understatement. Conventional laboratory sources of heat do not provide enough thermal energy for rotation about the double bond in alkenes to take place. As shown in Figure 5.2, rotation about a double bond requires the porbitals of C-2 and C-3 to be twisted from their stable parallel alignment— in effect, the component of the double bond must be broken at the transition state.

Identical C H

CH2CH3

1-Butene (no stereoisomers possible)

Identical CH3

CH3

2-Methylpropene (no stereoisomers possible)

Identical

CH2CH3

1-Butene

CH3

CH3

2-Methylpropene cis-2-Butene CH3 H

CH3 trans-2-Butene

CH3

CH3

172CHAPTER FIVEStructure and Preparation of Alkenes: Elimination Reactions

Stereoisomeric alkenes are sometimes referred to as geometric isomers.

The activation energy for rotation about a typical carbon–carbon double bond is very high—on the order of 250 kJ/mol (about 60 kcal/mol). This quantity may be taken as a measure of the bond contribution to the total CœC bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 171.

Make molecular models of cis-and trans-2-butene to verify that they are different.

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5.4NAMING STEREOISOMERIC ALKENES BY THE E–Z NOTATIONAL SYSTEM

When the groups on either end of a double bond are the same or are structurally similar to each other, it is a simple matter to describe the configuration of the double bond as cis or trans. Oleic acid, for example, a material that can be obtained from olive oil, has a cis double bond. Cinnamaldehyde, responsible for the characteristic odor of cinnamon, has a trans double bond.

PROBLEM 5.5Female houseflies attract males by sending a chemical signal known as a pheromone.The substance emitted by the female housefly that attracts the male has been identified as cis-9-tricosene, C23H46. Write a structural formula, including stereochemistry, for this compound.

The terms “cis” and “trans” are ambiguous, however, when it is not obvious which substituent on one carbon is “similar” or “analogous” to a reference substituent on the other. Fortunately, a completely unambiguous system for specifying double bond stereochemistry has been developed based on an atomic numbercriterion for ranking substituents on the doubly bonded carbons. When atoms of higher atomic number are on the sameside of the double bond, we say that the double bond has the Zconfiguration, where Zstands for the German word zusammen,meaning “together.” When atoms of higher atomic number are on oppositesides of the double bond, we say that the configuration is E.The symbol Estands for the German word entgegen,meaning “opposite.”

CinnamaldehydeOleic acid trans-2-Butene p orbitals aligned:

Optimal geometry for π bond formation cis-2-Butene p orbitals aligned:

Optimal geometry for π bond formation p orbitals perpendicular: Worst geometry for π bond formation

FIGURE 5.2Interconversion of cis-and trans-2-butene proceeds by cleavage of the component of the double bond. The red balls represent the two methyl groups.

(Parte 1 de 6)

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