Ch 10 - Alkanes

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2
Hydrocarbons
Chapter 10
2-1
2 Structure
Hydrocarbon: a compound composed only of
carbon and hydrogen
Saturated hydrocarbon: a hydrocarbon
containing only single bonds
Alkane: a saturated hydrocarbon whose
carbons are arranged in a open chain
Aliphatic hydrocarbon: another name for an
alkane
2-2
2 Structure
Shape
• tetrahedral about carbon
•all bond angles are approximately
109.5°
2-3
2 Nomenclature
Alkanes have the general formula CnH 2n+2
Name
Molecular
Formula
Condensed
Structural Formula
methane
ethane
propane
CH4
C2 H6
C3 H8
CH4
butane
C4 H10
CH3 (CH2 )2 CH3
pentane
C5 H12
CH3 (CH2 )3 CH3
hexane
C6 H14
CH3 (CH2 )4 CH3
heptane
C7 H16
CH3 (CH2 )5 CH3
CH3 CH3
CH3 CH2 CH3
2-4
2 Nomenclature
Alkanes (contd.)
octane
nonane
C8 H18
C9 H20
CH3 (CH2 )6 CH3
decane
C10 H22
CH3 (CH2 )8 CH3
dodecane
C12 H26
CH3 (CH2 )10 CH3
tetradecane
C14 H30
CH3 (CH2 )12 CH3
hexadecane
octadecane
eicosane
C16 H34
C18 H38
C20 H42
CH3 (CH2 )14 CH3
CH3 (CH2 )7 CH3
CH3 (CH2 )16 CH3
CH3 (CH2 )18 CH3
2-5
2 Constitutional Isomerism
Constitutional isomers: compounds with the
same molecular formula but a different
connectivity (order of attachment of their
atoms)
•example: C4H10
CH3
CH3 CH2 CH2 CH3
Butane
CH3 CHCH3
2-Methylpropane
2-6
2 Prob 1
Which sets represent pairs of constitutional
isomers?
O
(a)CH3 CH2 OH and CH3 OCH3
O
(b) CH3 CCH3 and CH3 CH2 CH
O
O
(c) CH3 COCH3 and CH3 CH2 COH
OH
O
(d) CH3 CHCH2 CH3 and CH3 CCH2 CH3
(e)
(f)
and CH3CH2 CH2 CH2 CH3
and CH2 = CHCH2 CH2 CH3
2-7
2 Constitutional
Isomerism
Molecular Constitutional
Formula
Isomers
1
CH4
C5 H12
3
C10 H22
75
C15 H32
4,347
C25 H52
36,797,588
C30 H62
4,111,846,763
2-8
2 Nomenclature - IUPAC
Suffix -ane specifies an alkane
Prefix tells the number of carbon atoms
Prefix Carbons
meth1
eth2
prop3
but4
pent5
hex6
7
heptoct8
non9
dec10
Carbons
Prefix
undec11
dodec12
tridec13
tetradec14
pentadec- 15
hexadec16
heptadec- 17
octadec18
nonadec19
eicos20
2-9
2 Nomenclature - IUPAC
Parent name: the longest carbon chain
Substituent: a group attached to the parent chain
•alkyl group: a substituent derived by removal of an H
from an alkane and given the symbol RAlkane
Name
Alkyl group Name
CH4
Methane
CH3 -
Methyl group
CH3 CH3
Ethane
CH3 CH2 -
Ethyl group
2-10
2 Nomenclature
1.The name of a saturated hydrocarbon with an
unbranched chain consists of a prefix and suffix
2. For branched alkanes, the parent chain is the
longest chain of carbon atoms
3. Each substituent is given a name and a number
CH3
CH3 CHCH3
2-Methylpropane
2
1
3
4. If there is one substituent, number the chain
from the end that gives it the lower number
CH3
CH3 CH2 CH2 CHCH3
2-Methylpentane
5
4 3
2
1
2-11
2 Nomenclature
6. If there are two or more different substituents,
• list them in alphabetical order
• number from the end of the chain that gives the
substituent encountered first the lower number
CH3
CH3 CH2 CHCH2 CHCH2 CH3
2
1
3
4
5
6
7
CH2 CH3
3-Ethyl-5-methylheptane
2-12
2 Nomenclature
7. The prefixes di-, tri-, tetra-, etc. are not
included in alphabetization
CH3 CH2 CH3
CH3 CCH2 CHCH2 CH3
CH3
1
2 3
4
5
6
4-Ethyl-2,2-dimethylhexane
8. Substituents are named by the same set
of rules.
2-13
2 Nomenclature
Alkyl groups
Name
Condensed
Structural Formula
Name
Condensed
Structural Formula
butyl
- CH2 CH2 CH2 CH3
methyl
- CH3
2-methylpropyl
(isobutyl)
- CH2CHCH3
ethyl
- CH2 CH3
propyl
- CH2 CH2 CH 3
1-methylpropyl
(sec-butyl)
- CHCH 2 CH 3
CH3
CH3
1-methylethyl - CHCH3
(isopropyl)
CH3
CH3
1,1-dimethylethyl - CCH3
(tert-butyl)
CH3
2-14
2 Classification of C & H
Primary (1° ) C: a carbon bonded to one
other carbon
•1° H: a hydrogen bonded to a 1° carbon
Secondary (2° ) C: a carbon bonded to two
other carbons
•2° H: a hydrogen bonded to a 2° carbon
Tertiary (3° ) C: a carbon bonded to three
other carbons
•3° H: a hydrogen bonded to a 3° carbon
Quaternary (4° ) C: a carbon bonded to four
other carbons
2-15
2 Physical Properties
Intermolecular forces of attraction
• ion-ion (Na+ and Cl - in NaCl)
• ion-dipole (Na+ and Cl - solvated in aqueous
solution)
• dipole-dipole and hydrogen bonding
• dispersion forces (very weak electrostatic
attraction between temporary dipoles)
2-16
2 Physical Properties
Low-molecular-weight alkanes
(methane
butane) are gases at room
temperature
Higher molecular-weight alkanes (pentane,
decane, gasoline, kerosene) are liquids at
room temperature
High-molecular-weight alkanes (paraffin wax)
are semisolids or solids at room temperature
2-17
2 Physical Properties
Constitutional isomers have different
physical properties
Name
mp
(°C)
bp
Density
(°C)
(g/mL)
hexane
2-methylpentane
3-methylpentane
2,3-dimethylbutane
2,2-dimethylbutane
-95
-154
-118
-129
-98
68.7
60.3
63.3
58.0
49.7
0.659
0.653
0.664
0.661
0.649
2-18
2 Oxidation of Alkanes
Oxidation is the basis for their use as energy
sources for heat and power
• heat of combustion: heat released when one mole
of a substance in its standard state is oxidized to
carbon dioxide and water
DH°
kJ (kcal)/mol
CH 4 + 2O2
CO2
+2H2O
-890 (-212)
Methane
CH3 CH2 CH3 + 5O2
Propane
3CO2
+ 4H2O
-2220 (-530)
2-19
2 Sources of Alkanes
Natural gas
•90-95% methane
Petroleum
• gases (bp below 20° C)
• naphthas, including gasoline (bp 20 - 200° C)
• kerosene (bp 175 - 275° C)
• fuel oil (bp 250 - 400° C)
• lubricating oils (bp above 350° C)
•asphalt (residue after distillation)
Coal
2-20
2 Gasoline
Octane rating: the percent 2,2,4-trimethylpentane
(isooctane) in a mixture of isooctane and heptane
that has equivalent antiknock properties
CH3 CH3
CH3 (CH2 )5CH3
CH3 CCH2 CHCH3
CH3
Heptane
(octane rating 0)
2,2,4-Trimethylpentane
(octane rating 100)
2-21
2 Synthesis Gas
A mixture of carbon monoxide and hydrogen
in varying proportions which depend on the
means by which it is produced
C + H2O
Coal
CH4 +
Methane
1
O2
2
heat
catalyst
CO + H2
CO + 2H2
2-22
2 Synthesis Gas
Synthesis gas is a feedstock for the
industrial production of methanol and acetic
catalyst
acid
+2H2
CO
CH3 OH
Methanol
CH3 OH + CO
Methanol
catalyst
O
CH3 COH
Acetic acid
• it is likely that industrial routes to other organic
chemicals from coal via methanol will also be
developed
2-23
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