topic 5c alkanes hydrocarbons saturated hydrocarbons — carbon skeletons saturated with hydrogen no...
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Topic 5C
Alkanes
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Hydrocarbons
• Saturated hydrocarbons — carbon skeletons saturated with hydrogenNo double bonds or triple bonds in the compoundNo other groups (oxygen, nitrogen etc).
23
• Paraffins — “parum affinis meaning ‘little affinity’ ”Aliphatics “aliphar meaning ‘fat or oil’ ”.
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Constitutional Isomerism
• Straight chains are formed by successive replacement of hydrogen by a “methyl” group.
• Simplest are:
23
CH4
CH3CH3
CH3CH2CH3
C2H6
C3H8
Methane
Ethane
Propane
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Constitutional Isomers(Structural isomers)
• Next member of series is C4H10.
• Two structural isomers are possible:
24
CH3CH2CH2CH3 butane or n-butane
CH3
CH3CHCH3 isobutane
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Constitutional isomers of C5H12
• Three constitutional isomers for C5H12:
24
CH3 C CH3
CH3
CH3
2,2-dimethylpropane (neopentane)
and
2-methylbutane (isopentane)
CH3
CH3CHCH2CH3
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Constitutional isomers of C5H12
24
CH3
CH3CHCH2CH3
isopentane
CH3 C CH3
CH3
CH3
neopentane
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Numbers of isomers
• C6H14 has five isomers
• C7H16 has nine isomers
• C10H22 has seventy five isomers
• C40H82 has 62x1012 isomers!
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Note the general formula — CnH2n+2
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Naming alkanes• IUPAC — International Union of Pure and Applied Chemistry.• Systematic Nomenclature enables accurate description.• Three parts:
prefix — describes substituents stem — identifies longest chain suffix — identifies type of compound
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1C —Stem name is meth- CH4
2C — eth- CH3 CH3
3C — prop- CH3 CH2 CH3
4C — but- CH3(CH2)2CH3
5C — pent- CH3(CH2)3CH3
6C — hex- CH3(CH2) 4CH3
7C — hept- CH3(CH2) 5CH3
8C — oct- CH3(CH2)6CH3
9C — non- CH3(CH2) 7CH3
10C — dec- CH3(CH2)8CH3
26Suffix— “-ane” indicates an alkane.Stem — indicates number of carbons
in longest chain.
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• Prefix—indicates nature and positionof a substituent on the chain.
• Number chain to give position with the substituent the lowest number.
26
methyl
Name is “2-methylhexane (not 5-methylhexane)
CH3CH2CH2CH2CH2CH3
CH3
CHCH2CH2CH2CH3CH3
CH3
1 2 3 4 5 6
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Examples: 26
1 2 3 4 5 6
3-methylhexane
CH3
CH3CH2CHCH 2CH2CH3
74 631 52
CH3 CH3
CH3CHCH 2CH2CHCH 2CH3 2,5-dimethylheptane(not 3,6-dimethylheptane)
Note the use of “di-” — “tri-, tetra-, penta- etc also used
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• Different substituents are listed alphabetically with their locants.
26
CH3 CHCH2CH2CHCH2CH2CH3
CH2CH3
CH2CH3
543
2 1
986 7
Name is 6-ethyl-3-methylnonane(e comes before m alphabetically)
Thus
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2
3
5
Multipliers “di-”, “tri-”, “tetra-” are disregarded.
27
CH3
CH3 CH2CH3
CH3CHCHCH2CHCH2CH3
1 7
Name is 5-ethyl-2,3-dimethylheptane
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Locant is repeated with multiple substituents at same carbon.
27
CH3 C CH2CH2CHCH2CH3
CH3
CH3 CH2CH3
1 7
Name is 5-ethyl-2,2-dimethylheptane
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Straight chain substituents:
27
CH4 Methane CH3- Methyl
CH3CH3 Ethane CH3CH2- Ethyl
CH3CH2CH3 Propane CH3CH2CH2- Propyl
CH3(CH2)2CH3 Butane CH3(CH2)2CH2- Butyl
CH3(CH2)3CH3 Pentane CH3(CH2)3CH2- Pentyl
R-H Alkane R- Alk yl
Parent Substituent
Substituent names
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Branched substituents: 28
CH
CH3
CH3an “isopropyl” group
CH3CH2CH2CHCH2CH2CH3
CH(CH3)2
4-Isopropylheptane
CH3CH2CHCHCH2CH2CH3
CH(CH3)2
4-Isopropy-3-methyllheptane
CH3
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Branched substituents: 29
CH CH2
CH3
CH3
(CH3) 2CHCH2— isobutyl
CH3CH2CHCH3 secondary-butyl (also s-butyl orsec-butyl)
CCH3
CH3
CH3
(CH3)3C— tertiary-butyl (also t-butyl or tert-butyl)
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Substituent groups: 29
CCH3
H
CH3
secondary (2°) : 2 carbons bondedto carbon attached to chain
CCH3
CH3
CH3
tertiary (3°) : 3 carbons bondedto carbon attached to chain
CCH3
H
H
primary (1°) : 1 carbon bondedto carbon attached to chain.
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Cycloalkanes
Note the general formula — CnH2n
30
Cyclobutane C4H8
CH2
CH2 CH2
CH2CH3
CH2 CH2
CH3 –2H
CH2
CH2
CH2
CH2
CH2
CH2CH3
CH2
CH2
CH2
CH2
CH3
–2H
Cyclohexane C6H12
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Prefix alkane name with “cyclo”
31Cycloalkanes
H2C CH2
CH2Cyclopropane
CH2
CH2
CH2 CH2
CHCH3
54
3
2
1Methylcyclopentane
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• In monosubstituted rings, the carbon bearing the substituent is by convention 1.
• In disubstituted rings, number towards nearest substituent.
31Numbering rings
CH2
CH2
CH2 CH2
CHCH3
54
3
2
1
CH2
CH2CH2
CH
CH2
CH
CH2CH3
CH
CH3
CH3
4
5
6
12
3
• Highest alphabetical substituent in position 1.
1-ethyl-3-isopropylcyclohexane
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Skeletal structures are often used for rings:
31Ring presentations
CH3
CH2CH3
CH(CH3)2
or
or
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Cyclopropane CyclopropylCyclobutane CyclobutylCyclopentane CyclopentylCyclohexane Cyclohexyl
32Rings as substituents(where chain is bigger than the ring)
CH3 CH CH2 CH2 CH CH2 CH3
1 2 7
2-cyclopentylheptane
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Conformation in alkanes
• Study of three-dimensional shape of molecules and how this affects their chemical and physical properties• Very important in biology• Isomers that have the same formula and connectivity but differ only in the way the atoms are arranged in space are STEREOISOMERS• Constitutional isomers having different connectivity and are joined up in a different way are NOT stereoisomers
33
CH3CH2
CH2CH2
CH3 CH3C
CH2CH3 CH3
CCH3
CH3 CH3H CH3
2,2-dimethylpropanen-pentane 2-methylbutane
Stereochemistry:
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Conformational isomerism• Conformational isomers (conformers): isomers that differ because of rotation about single bonds.
Conformers are generally interconvertible without bond breaking.
33
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Conformational isomerismAlkanes
• Rotation about single bonds leads to different conformations.
33
H
HH
H H
H
H
HHH
HH
Staggeredconformation
60°
H
HH
HH
H
H
HH
H
HH
Eclipsedconformation
0°
greater electron repulsion raises energy
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C
CHH
H
H
HH
Staggeredconformer
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C
C
HH
HH
HH
Eclipsedconformer
Conformational isomerismAlkanes
33
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Energetics of rotation• The energy varies. In the eclipsed conformation there is more
repulsion than in the staggered conformation.
34
H
HHH
HH
Staggered
Rotation
Pot
enti
al e
ner
gy
12.6kJmol-1
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H
HH
H
HH
Eclipsed
H
HHH
HH
Staggered
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Energetics of rotation
• Butane rotation about the middle bond• Eclipsed methyls raise the rotation barrier
34
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Butane:
Movie from SaundersGeneral Chemistry CD-ROM
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34Conformation in cycloalkanes
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C6H6 – "chair" shape
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Flat carbon ring
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Almost flat ring
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Conformation in cyclohexane
• Chair conformation involves fully staggered C—C bonds
• This is the lowest energy conformation
35
H
H
HH
HH
Staggered ethane
H
H
H
H
H
H
H
H
HH
H
HH
H
Chair cyclohexane
H
H
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Chair form of cyclohexane 35
axial bonds
e
e
e
e
a
a
a
a
equatorialbonds
axial bonds
• Axial and equatorial bond
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Two chair conformations
• Interchanging chair conformations
• Axial and equatorial atoms are interchanged
35
a
e
a
e
a
ea
e
a
e
a
e
e
a
aa
e
e
a
a
e
a
ee
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Boat cyclohexane
• In the “boat” conformer the sides of the boat are eclipsedHigh energy conformation
35
H
H
H
H
HH
HH
H
H
H
H
H
H
Boat cyclohexane
HH
H HH H
Eclipsed ethane
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High energy boat• The “flagpole” hydrogens strongly interfere, raising the energy further
36
H H H H
Twist-boat conformations
Flagpole hydrogensfurther apart
• Twisting can reduce repulsion slightly
H
H
H
H
HH
HH
H
H
H
H
Flagpole hydrogens ouch!
Space filling modelboat conformation
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Energetics• As cyclohexane moves through chair to boat to chair, the energy
varies
36
Chair IITwist IIBoatTwist IChair I
Conformers
Pot
entia
l ene
rgy
kJ/m
ol
6.7
23
46
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Monosubstituted cycloalkanes
• Two chair forms are possible• The substituent is more stable in the equatorial position• Axial-axial interactions destabilise the chair form with the substituent in an axial position
C(CH3)3
H
H
H
tert-butylcyclohexane
37
H
H
H
CH3
CH3
H
H
H
Methylcyclohexane
HH
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O
HO
Estrone
OH
HO
Estradiol
Estrogens(Female)
Importance of chairs• Steroidal hormones all contain chair-shaped cyclohexanes
OH
OTestosterone
O
HOH
Androsterone
Androgens(Male)
Cholesterol HO
H
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Geometrical Isomers 37.• Cycloalkanes cis/trans
• There are two faces to cyclic alkanes• When two substituents are on the same face the
isomer is termed cis. When on opposite faces, we have the trans isomer:
CH3
H
H
CH3
CH3
H
CH3
H
Cl
H
H
Cl
Cl
H
Cl
H
trans-dimethylcyclopropane
cis-dimethylcyclopropane
trans-1,3-dichlorocyclopentane
cis-1,3-dichlorocyclopentane
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37Geometrical isomerism in
Cycloalkanes
CH3
H
CH3
H
Cis-1,2-dimethylcyclopropane
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H
CH3
CH3
H
Trans-1,2-dimethylcyclopropane
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38Cycloalkanes
H
CH2CH3
CH3
H2
31
Trans-1-ethyl-3-methylcyclohexane
CH3
CH2CH3
HH Or
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Conformations in geometrical isomers
38
• Di-equatorial favoured over di-axial conformations
CH3
H
H
CH3
CH3
H
H
CH3
cis-1,4-dimethyl-cyclohexane
H
HO
OH
HH
OH
HOH
cis-1,2-dihydroxy-cyclohexane
H
CH3
H
CH3
CH3
H
CH3
H
trans-1,4-dimethyl-cyclohexane
HO
H
H
HOH
OH
HOH
trans-1,2-dihydroxy-cyclohexane
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Properties of alkanes
• Boiling point increases with molecular weight:
39
CH4 n-C4H10 n-C7H16 n-C10H22
bp (˚C)– 162 0 98 174
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Physical properties of alkanes
• Low molecular weight alkanes are gases.• Boiling point (and melting point) increases with
molecular weight.
39
Notes:
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39
• Unbranched hydrocarbons have higher boiling points than branched alkanes
• They can align themselves more closely:
Physical properties
CH3 C
CH3
CH3
CH3
bp 10° 36bp °
CH3(CH2)3CH3
n-pentane neopentane
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Unbranched Branched
• Unbranched can align themselves more closely• Stronger intermolecular attractive forces are possible
Physical properties of alkanes
Nonane2,3,4-Trimethylhexane
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Chemical properties of alkanes
• Generally unreactive towards:Strong acidsStrong alkalisMild oxidising agents Halogens (F, Cl, Br, I) in the dark
• Oxidation (addition of O or removal of H).• Halogenation (using light).
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Oxidation
Combustion:
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CH4 + 2O 2
Methane(Natural gas)
CO2 + 2H 2O + 886 kJmol-1
CH3CH2CH3 + 5O 2
Propane3CO2 + 4H 2O + 2209 kJmol-1
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Oxidation
Dehydrogenation:
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(Removal of hydrogen is also oxidation)
CH3 CH3 C C
H
H
H
Hcatalyst
heat
ethene
2H.