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General, Organic, and General, Organic, and Biochemistry, 7eBiochemistry, 7e

Bettelheim,Bettelheim,

Brown, and MarchBrown, and March

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Chapter 11Chapter 11

Alkanes and Alkanes and CycloalkanesCycloalkanes

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HydrocarbonsHydrocarbons• Hydrocarbon:Hydrocarbon: a compound composed of only

carbon and hydrogen

H-C C-HH-C-C-HH

H

H

H HC C

H

H H

Hydrocarbons

Alkanes(Chapter 11)

Alkenes(Chapter 12)

Alkynes(Chapter 12)

Arenes(Chapter 13)

Only carbon-carbon single

bonds

One or more carbon-carbondouble bonds

One or morecarbon-carbontriple bonds

One or morebenzene-like

rings

Ethane Ethene(Ethylene)

Ethyne(Acetylene)

Benzene

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AlkanesAlkanes• Alkanes:Alkanes: hydrocarbons containing only carbon-

carbon single bonds• the first two alkanes are methane and ethane

H-C-HH

HH-C-C-HH

H

H

H

Methane Ethane

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AlkanesAlkanes• line-angle formula:line-angle formula:

• a line represents a carbon-carbon bond and an angle represents a carbon atom

• a line ending in space represents a -CH3 group

• hydrogen atoms are not shown in line-angle formulas

CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3PentaneButanePropane PentaneButanePropane

Condensedstructural

formula

Line-angleformula

Ball-and-stick model

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AlkanesAlkanes• the first 10 alkanes with unbranched chains

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18 CH3(CH2)6CH3C9H20 CH3(CH2)7CH3C10H22 CH3(CH2)8CH3

CH4 CH4C2H6 CH3CH3C3H8 CH3CH2CH3C4H10 CH3(CH2)2CH3C5H12 CH3(CH2)3CH3

C6H14 CH3(CH2)4CH3C7H16 CH3(CH2)5CH3C8H18C9H20C10H22

CondensedStructural Formula

MolecularFormulaName

decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane

CondensedStructural Formula

MolecularFormulaName

CondensedStructural Formula

MolecularFormulaName

decane

nonane

octane

heptane

hexane

pentane

butane

propane

ethanemethane

CondensedStructural FormulaName

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Constitutional IsomerismConstitutional Isomerism• Constitutional isomers:Constitutional isomers: compounds that have the

same molecular formula but different structural formulas• for the molecular formulas CH4, C2H6, and C3H8, only

one structural formula is possible; there are no constitutional isomers for these molecular formulas

• for the molecular formula C4H10, two constitutional isomers are possible

CH3CH2CH2CH3 CH3CHCH3

CH3

Butane(bp -0.5°C)

2-Methylpropane(bp -11.6°C)

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Constitutional IsomerismConstitutional Isomerism• Problem:Problem: do the structural formulas in each set

represent the same compound or constitutional isomers?

CH3CH2CH2CH2CH2CH3 CH3CH2CH2CH2CH2CH3

CH3CHCH2CHCH3

CH3

CH3

CH3CH2CHCHCH3

CH3

CH3

(a) and (each is C6H14)

(b) and (each is C7H16)

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Constitutional IsomerismConstitutional Isomerism• Solution:Solution:

(a) they represent the same compound

• Solution:Solution:

(b) they represent constitutional isomers

CH3CHCH2CH

CH3

CH3

CH3CH3CH2CHCHCH3

CH3

CH35

1 12 23 3

4

455

2 5 4 3

2 1

431 and

CH3CH2CH2CH2CH2CH3 CH3CH2CH2

CH2CH2CH3

654321

654

321

and

21

12

3 3

4

45

5

6

6

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Constitutional IsomerismConstitutional Isomerism• Problem:Problem: draw structural formulas for the five

constitutional isomers of molecular formula C6H14

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Constitutional IsomerismConstitutional Isomerism• Problem:Problem: draw structural formulas for the five

constitutional isomers of molecular formula C6H14

• Solution:Solution:

Six carbons in an unbranched chain

Five carbons in a chain; one carbon as a branch

1 32 4

56

1 12

23 3

4 4

5 5

Four carbons in a chain; two carbons as branches

1 12 2

3 34 4

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IUPAC NamesIUPAC Names• The IUPAC name of an alkane with an

unbranched chain of carbon atoms consists of two parts: • (1) a prefix: the number of carbon atoms in the chain• (2) the suffix -ane-ane: shows that the compound is a

saturated hydrocarbon

Prefixmeth-eth-prop-but-pent-

hex-

oct-non-dec-

12345

67hept-8910

Number ofCarbon Atoms

Number ofCarbon AtomsPrefix

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IUPAC NamesIUPAC Names• The name of an alkane with a branched chain of

carbon atom consists of • a parent name: the longest chain of carbon atoms• substituent names: the groups bonded to the parent

chain

CH3

CH3CH2CH2CHCH2CH2CH2CH38

substituent

4-Methyloctane

1 2 3 4 5 6 7 8

parent chain

12

34

5

6

7

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IUPAC NamesIUPAC Names• Alkyl group:Alkyl group: a substituent group derived from an

alkane by removal of a hydrogen atom• commonly represented by the symbol R-• named by dropping the -aneane from the name of the

parent alkane and adding the suffix -ylyl

-CH2CH3

-CH3

-CH2CH2CH3

-CHCH3CH3

-CH2CH2CH2CH3

-CH2CHCH3CH3

-CHCH2CH3CH3

-CCH3

CH3

CH3

tert-butyl

sec-butyl

isobutyl

butyl

isopropyl

propyl

ethyl

methylName

CondensedStructural Formula

CondensedStructural FormulaName

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IUPAC NamesIUPAC Names1. The name for an alkane with an unbranched

chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending -aneane

2. For branched-chain alkanes, longest chain of carbon atoms is the parent chain and its name is the root name

3. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name

CH3CHCH3

CH3

2-Methylpropane

12

3

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IUPAC NamesIUPAC Names4. If there is one substituent, number the parent

chain from the end that gives the substituent the lower number

CH3

CH3CH2CH2CHCH35

2-Methylpentane(not 4-methylpentane)

123

4

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IUPAC NamesIUPAC Names5. If the same substituent occurs more than once,

• number the parent chain from the end that gives the lower number to the substituent encountered first

• indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on

• use a comma to separate position numbers

CH3CH2CHCH2CHCH3

CH3 CH3

2,4-Dimethylhexane(not 3,5-dimethylhexane)

12

34

56

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IUPAC NamesIUPAC Names6. If there are two or more different substituents

• list them in alphabetical order• number the chain from the end that gives the lower

number to the substituent encountered first• if there are different substituents in equivalent

positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number

CH3CH2CHCH2CHCH2CH3

CH3

CH2CH3

12

34

56

7

3-Ethyl-5-methylheptane(not 3-methyl-5-ethylheptane)

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IUPAC NamesIUPAC Names7. Do not include the prefixes di-, tri-, tetra-, and so

on, or the hyphenated prefixes sec- and tert- in alphabetizing; • alphabetize the names of substituents first, and then

insert these prefixes

CH2CH3

CH3CCH2CHCH2CH3

CH3

CH34-Ethyl-2,2-dimethylhexane

(not 2,2-dimethyl-4-ethylhexane)

23

45

61

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Common NamesCommon Names• Common names; in this older system

• the number of carbon atoms determines the name• the first three alkanes are methane, ethane, and

propane

• all alkanes of formula C4H10 are called butanes, all those of formula C5H12 are called pentanes, etc.

• for alkanes beyond propane, isoiso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH-

• for more complex alkanes, use the IUPAC system

CH3CHCH3

CH3

CH3CH2CHCH3

CH3

IsopentaneIsobutane

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CycloalkanesCycloalkanes• Cyclic hydrocarbon: Cyclic hydrocarbon: a hydrocarbon that contains

carbon atoms joined to form a ring• Cycloalkane:Cycloalkane: a cyclic hydrocarbon in which all

carbons of the ring are saturated • cycloalkanes of ring sizes ranging from 3 to over 30

carbon atoms are found in nature• five-membered (cyclopentane) and six-membered

(cyclohexane) rings are especially abundant in nature

Cyclopentane Cyclohexane

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CycloalkanesCycloalkanes• Nomenclature

• to name a cycloalkane, prefix the name of the corresponding open-chain alkane with cyclo-,cyclo-, and name each substituent on the ring

• if there is only one substituent on the ring, there is no need to give it a location number

• if there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

Isopropylcyclopentane 1-tert-butyl-4-methylcyclohexane

1 4

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Conformations - AlkanesConformations - Alkanes• Conformation:Conformation: any three-dimensional

arrangement of atoms in a molecule that results by rotation about a single bond• following are three conformations for a butane

molecule

Most crowdedconformation

rotate by 120°

rotate by 60°

Least crowdedconformation

Intermediatecrowding

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CyclopentaneCyclopentane• The most stable conformation of a cyclopentane

ring is an envelope conformation

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CyclohexaneCyclohexane• The most stable conformation of a cyclohexane

ring is the chair conformation• all bond angles are approximately 109.5°

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CyclohexaneCyclohexane• In a chair conformation,

• six C-H bonds are equatorialequatorial• six C-H bonds are axialaxial

HH

H

HHH

(a) Ball-and-stick modelshowing all 12 hydrogens

axis through thecenter of the ring

H H

H

H

H

H

(b) The six equatorialC-H bonds

(c) The six axial C-H bonds

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CyclohexaneCyclohexane• the more stable conformation of a substituted

cyclohexane ring has substituent group(s) equatorial rather than axial

CH3

Equatorial methylcyclohexane

CH3

Axial methylcyclohexane

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Cis-Trans IsomersCis-Trans Isomers• Cis:Cis: on the same side• Trans:Trans: across from

• we can view a cyclopentane ring edge-on

trans-1,2-Dimethyl-cyclopentane

cis-1,2-Dimethyl-cyclopentane

CH3

H

CH3

H

H

HH

H

HH

H

H

CH3

H3C

H

HH

HH

H

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Cis-Trans IsomersCis-Trans Isomers• alternatively, we can view it from above

CH3trans-1,2-Dimethyl-

cyclopentanecis-1,2-Dimethyl-

cyclopentane

H3C CH3 H3C

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Cis-Trans IsomersCis-Trans Isomers• to determine cis-trans isomers if cis-trans isomers are

possible, we can view a cyclohexane ring as a planar hexagon

• because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomersstereoisomers

• cis-trans isomers are one type of stereoisomers • in Ch 15, we study another type called enantiomers

trans-1,4-Dimethylcyclohexane cis-1,4-Dimethylcyclohexane

H

H3C

CH3

H

H

H3C

H

CH3

or or

CH3

CH3

CH3

CH3

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Physical PropertiesPhysical Properties• The most important physical property of alkanes

and cycloalkanes is their almost complete lack of polarity• the electronegativity difference between carbon and

hydrogen is 2.5 - 2.1 = 0.4 on the Pauling scale• given this small difference, we classify a C-H bond as

nonpolar covalent• alkanes are nonpolar compounds and the only

interaction between their molecules are the very weak London dispersion forces

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Physical PropertiesPhysical Properties• Melting and boiling points

• boiling points of alkanes are lower than those of almost any other type of compound of the same molecular weight

• in general, both boiling and melting points of alkanes increase with increasing molecular weight

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Physical PropertiesPhysical Properties

CH4CH3CH3CH3CH2CH3CH3(CH2)2CH3

CH3(CH2)3CH3CH3(CH2)4CH3CH3(CH2)5CH3CH3(CH2)6CH3CH3(CH2)7CH3CH3(CH2)8CH3

methane

ethanepropane

butane

pentane

hexane

heptane

octanenonane

decane

Name

CondensedStructrualFormula

mp(°C)

bp(°C)

-182

-183

-190-138

-130

-95

-90

-57-51

-30

-164

-88

-420

36

69

98

126151

174

(a gas)

(a gas)

(a gas)(a gas)

0.626

0.659

0.684

0.7030.718

0.730

*For comparison, the density of H2O is 1 g/mL at 4°C.

Mol wt(amu)

16.0

30.144.1

58.1

72.2

86.2

100.2

114.2128.3142.3

Density of Liquid

(g/mL at 0° C)*

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Physical PropertiesPhysical Properties• alkanes that are constitutional isomers are different

compounds and have different physical and chemical properties

bp (°C)Namehexane

2-methylpentane3-methylpentane

2,3-dimethylbutane

2,2-dimethylbutane

68.7

60.363.3

58.0

49.7

Hexane

2,2-Dimethylbutane

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Physical PropertiesPhysical Properties• Solubility: a case of “like dissolves like”

• alkanes are not soluble in water; they are unable to form hydrogen bonds with water

• alkanes are soluble in each other• alkanes are also soluble in other nonpolar organic

compounds, such as toluene and diethyl ether

• Density• the average density of the liquid alkanes listed in Table

11.4 is about 0.7 g/mL; that of higher-molecular-weight alkanes is about 0.8 g/mL

• all liquid and solid alkanes are less dense than water (1.0 g/mL) and, because they are insoluble in water, they float on water

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ReactionsReactions• Oxidation (combustion)

• oxidation of hydrocarbons, including alkanes and cycloalkanes, is the basis for their use as energy sources for heat [natural gas, liquefied petroleum gas (LPG), and fuel oil] and power (gasoline, diesel fuel, and aviation fuel)

CH3CH2CH3 5O2 3CO2 4H2O++Propane

530 kcal/mol+

CH4 2O2 CO2 2H2OMethane

++ 212 kcal/mol+

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ReactionsReactions• Reaction with halogens (halogenation)

• halogenation of an alkane is a substitution reaction

CH4 Cl2 CH3Cl HCl+ heator light

+Methane Cloromethane

(Methyl chloride)

CH3Cl Cl2 CH2Cl2 HCl+heat

+Dichloromethane

(Methylene chloride)

CH2Cl2Cl2

CHCl3Cl2

CCl4heat heatTrichloromethane

(Chloroform)Tetrachloromethane

(Carbon tetrachloride)

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The ChlorofluorocarbonsThe Chlorofluorocarbons• Chlorofluorocarbons (CFCs)

• manufactured under the trade name Freon• CFCs are nontoxic, nonflammable, odorless, and

noncorrosive

• among the CFCs most widely used were CCl3F (Freon-11) and CCl2F2 (Freon-12)

• CFCs were used as • heat-transfer agents in refrigeration systems• industrial cleaning solvents to prepare surfaces for

coatings and to remove cutting oils from millings• propellants for aerosol sprays

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CFC ReplacementsCFC Replacements• Chlorofluorocarbons (CFCs) cause destruction of

the Earth’s stratospheric ozone layer• The most prominent replacements are the

hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)• these compounds are chemically more reactive that

CFCs and are destroyed before they reach the stratosphere

H-C–C-FHCl

H ClF-C-C-HF

F H

F

HFC-134a HCFC-141b

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Sources of AlkanesSources of Alkanes• Natural gas

• 90 to 95 percent methane, • 5 to 10 percent ethane, and • a mixture of other relatively low-boiling alkanes, chiefly

propane, butane, and 2-methylpropane

• Petroleum• a thick, viscous liquid mixture of thousands of

compounds, most of them hydrocarbons formed from the decomposition of marine plants and animals

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End End Chapter 11Chapter 11

Chapter 11Chapter 11