5-5-11

Organic Organic ChemistryChemistry

William H. BrownWilliam H. Brown

Christopher S. FooteChristopher S. Foote

Brent L. IversonBrent L. Iverson

William H. BrownWilliam H. Brown

Christopher S. FooteChristopher S. Foote

Brent L. IversonBrent L. Iverson

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Alkenes:Alkenes:Structure andStructure andNomenclatureNomenclature

Chapter 5Chapter 5

5-5-33

Unsaturated HydrocarbonsUnsaturated 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 CnH2n

Alkyne:Alkyne: contains a carbon-carbon triple bond and has the general formula CnH2n-2

Ethylene (an alkene)

H

C C

H

H H

Acetylene (an alkyne) .

H-C C-H

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Unsaturated HydrocarbonsUnsaturated Hydrocarbons Arenes:Arenes: benzene and its derivatives (Ch 21-22)

• we do not study arenes until Chapters 21 & 22• however, 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 6-20

Benzene Alternative representations for the phenyl group

CCC

CC C

C6H5- Ph-

H H

H

H H

H

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Structure of AlkenesStructure of Alkenes A double bond consists of • one sigma bond formed by the overlap of sp2 hybrid

orbitals and one pi bond formed by the overlap of parallel 2p orbitals

• 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°

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Structure of AlkenesStructure of Alkenes

• it takes approximately 264 kJ (63 kcal)/mol to break the pi bond in ethylene; that is, to rotate one carbon by 90° with respect to the other so that there is no overlap between 2p orbitals on adjacent carbons

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Cis,TransCis,Trans Isomerism in Alkenes Isomerism in Alkenes Cis,transCis,trans isomers: isomers: isomers that have the same

connectivity but a different arrangement of their atoms in space due to the presence of either a ring or a carbon-carbon double bond

cis-2-Butenemp -139°C, bp 4°C

trans-2-Butenemp -106°C, bp 1°C

C CCH3

HH3C

HC C

H

CH3H3C

H

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Index of Hydrogen DeficiencyIndex of Hydrogen Deficiency Index of hydrogen deficiency (IHD):Index of hydrogen deficiency (IHD): the sum of

the number of rings and pi bonds in a molecule To determine IHD, compare the number of

hydrogens in an unknown compound with the number in a reference hydrocarbon of the same number of carbons and with no rings or pi bonds• the molecular formula of the reference hydrocarbon is

CnH2n+2

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Index of Hydrogen DeficiencyIndex of Hydrogen Deficiency

• for each atom of a Group 7 element (F, Cl, Br, I), add one H

• no correction is necessary for the addition of atoms of Group 6 elements (O,S) to the reference hydrocarbon

• for each atom of a Group 5 element (N, P), subtract one hydrogen

IDH =2

(Hreference - Hmolecule)

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Index of Hydrogen DeficiencyIndex of Hydrogen Deficiency

Problem:Problem: isopentyl acetate has a molecular formula of C7H14O2. Calculate its IHD• reference hydrocarbon C7H16

• IHD = (16-14)/2 = 1

Problem:Problem: calculate the IHD for niacin, molecular formula C6H6N2O• reference hydrocarbon C6H16

• IHD = (16 - 6)/2 = 5

O

O

Isopentyl acetate

N

NH2

O

Niacin

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IUPAC NomenclatureIUPAC Nomenclature

1. Number the longest chain of carbon atoms that contains the double bond in the direction that gives the carbons of the double bond the lowest numbers

2. Locate the double bond by the number of its first carbon

3. Name substituents

4. Number the carbon, locate and name substituents, locate the double bond, and name the main chain

1-Hexene 4-Methyl-1-hexene 2-Ethyl-4-methyl-1-pentene

12

34

56

12

34

56

1234

5

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Common NamesCommon Names Despite the precision and universal acceptance

of IUPAC nomenclature, some alkenes, particularly low-molecular-weight ones, are known almost exclusively by their common names

CH2=CH2 CH3CH=CH2 CH3C=CH2

PropyleneEthyleneCommon:IUPAC: 2-MethylpropenePropeneEthene

Isobutylene

CH3

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Common NamesCommon Names

• the common names methylene, vinyl, and allyl are often used to show the presence of the following alkenyl groups

CH2=

CH2=CH-

CH2=CHCH2

H2C

CH2=CH

CH2=CHCH2Allyl

Methylidenecyclopentane(Methylenecyclopentane)

Ethenylcyclopentane(Vinylcyclopentane)

Vinyl

Methylene

ExampleCommon Name

AlkenylGroup

Methylidene

Ethenyl

3-Propenyl

3-Propenylcyclopentane(Allylcyclopentane)

IUPAC Name(Common name)

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The The CisCis,,TransTrans System System Configuration is determined by the orientation of

atoms of the main chain

C

H

C

CH3

CH(CH3)2H3C

C

H

C

CH2CH3

HCH3CH

2 cis-3,4-Dimethyl-2-pentene

1

2 3

4

trans-3-Hexene

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The The EE,,ZZ System System

• uses priority rules (Chapter 3) • if groups of higher priority are on the same side, the

configuration is Z (German, zusammen)• if groups of higher priority are on opposite sides, the

configuration is E (German, entgegen)

Z (zusammen) E (entgegen)

C

higher

C

higher

lowerlower

C

lower higher

C

lowerhigher

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The The EE,,ZZ System System Example:Example: name each alkene and specify its configuration

by the E,Z system

(a) (b)

(d)(c)

Cl

Cl

Br

Cl

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Cis,TransCis,Trans Isomerism Isomerism Cycloalkenes• in small-ring cycloalkenes, the configuration of the

double bond is cis • these rings are not large enough to accommodate a trans double bond

CyclohepteneCyclohexeneCyclopentene

H

H

H

H

H

H

H

HCyclobutene

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Cis,TransCis,Trans Isomerism Isomerism

• trans-cyclooctene is the smallest trans cyclooctene that has been prepared in pure form and is stable at room temperature

• the cis isomer is 38 kJ (9.1 kcal)/mol more stable than the trans isomer

• the trans isomer is chiral even though it has no chiral center

trans-Cyclooctene(a pair of enantiomers)

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Dienes, Trienes, and PolyenesDienes, Trienes, and Polyenes For alkenes containing two or more double

bonds, change the infix -enen- to -adienadien-, -atrienatrien-, etc.• those containing several double bonds are often

referred more generally as polyenes• following are three dienes

1,4-Pentadiene 2-Methyl-1,3-butadiene (Isoprene)

1,3-Cyclopentadiene

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Dienes, Trienes, and PolyenesDienes, Trienes, and Polyenes

• for alkenes with nn double bonds, each of which can show cis,trans isomerism, 22nn stereoisomers are possible

• example: 22 = 4 cis,trans isomers are possible for 2,4-heptadiene

C2-C3 C4-C5

Double bond

trans transtrans ciscis transcis cis

(2E,4E)-2,4-Heptadiene (2E,4Z)-2,4-Heptadiene

12 3

4 5

6 7

24

24

24

(2Z,4E)-2,4-Heptadiene (2Z,4Z)-2,4-Heptadiene

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Dienes, Trienes, and PolyenesDienes, Trienes, and Polyenes

• vitamin A, a biologically important compound for which a number of cis,trans isomers is possible

• there are four double bonds about which cis,trans isomerism is possible, for 24 = 16 stereoisomers

CH2OH

Vitamin A (retinol)

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Physical PropertiesPhysical 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

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TerpenesTerpenes Terpene:Terpene: a compound whose carbon skeleton

can be divided into two or more units identical with the carbon skeleton of isoprene

2-Methyl-1,3-butadiene (Isoprene)

12

34head tail

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TerpenesTerpenes

Myrcene, C10H16, a component of bayberry wax and oils of bay and verbena

Menthol, from peppermint

OH

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TerpenesTerpenes

• -Pinene, from turpentine• camphor, from the camphor tree

O

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Fatty AcidsFatty Acids Animal fats and vegetable oils are both triesters

of glycerol, hence the name triglyceridetriglyceride• hydrolysis of a triglyceride in aqueous base followed

by acidification gives glycerol and three fatty acids

• fatty acids with no C=C double bonds are called saturated fatty acid

• those with one or more C=C double bonds are called unsaturated fatty acids

OCH2OCR

CH2OCR''

R'COCH

O

O1. NaOH, H2O

2. HCl, H2O

CH2OH

CH2OH

HOCH

RCOOH

R'COOH

R''COOHA triglyceride

(a triester of glycerol) 1,2,3-Propanetriol

(Glycerol)

+

Fatty acids

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Fatty AcidsFatty Acids

• the most common fatty acids have an even number of carbons, and between 12 and 20 carbons in an unbranched chain

• the C=C double bonds in almost all naturally occurring fatty acids have a cis configuration

• the greater degree of unsaturation, the lower the melting point

• triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oilsoils

• triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fatsfats

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Fatty AcidsFatty Acids

• the four most abundant fatty acids

COOH

COOH

COOH

COOH

Stearic acid (18:0)(mp 70°C)

Oleic acid (18;1)(mp 16°C)

Linoleic acid (18:2)(mp-5°C)

Linolenic acid (18:3)(mp -11°C)

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Fatty AcidsFatty Acids

• carbon chains of saturated fatty acids exist largely in the staggered, anti-conformation

• because of their high degree of order, they pack together well and are held together by dispersion forces

• as a result both saturated fatty acids and triglycerides derived from them are solids at room temperature

• following is a saturated triglyceride

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Fatty AcidsFatty Acids

• cis double bonds place kinks in the chains of unsaturated fatty acids

• unsaturated fatty acids and the triglycerides derived from them do not pack as well in a crystal lattice as their saturated counterparts, and have weaker dispersion forces between their molecules

• butter fat, for example, has a high content of saturated fatty acids and is a solid at room temperature

• salad oils (from plant oils) have a high content of polyunsaturated fatty acids and are liquid at room temperature

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Alkenes: Alkenes: Structure andStructure andNomenclatureNomenclature

End Chapter 5End Chapter 5