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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
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