Chapter 2 Alkanes - Seattle Central College
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Transcript Chapter 2 Alkanes - Seattle Central College
Chapter 2
Alkanes
Hydrocarbon: a compound composed of
only carbon and hydrogen.
Alkanes: hydrocarbons containing only
carbon-carbon single bonds.
Hydrocarbons
Alkanes
(Ch apter 11)
Alkenes
(Ch apter 12)
Alkynes
(Chapter 12)
Arenes
(Chap ter 13)
Only carb on carb on single
bonds
HH
H-C-C-H
HH
Ethan e
One or more
carb on -carbon
double b on ds
One or more
carbon-carbon
trip le bonds
One or more
ben zene-like
rings
H
H
C C
H
H
Eth ene
(Ethylene)
H-C C-H
Ethyne
(A cetylene)
Benzen e
The first two alkanes are methane and
ethane.
H
H-C-H
H
Methane
HH
H-C-C-H
HH
Eth ane
•
Table 2.1 The first 10 alkanes with
unbranched chains
CH3 CH2 CH3
Conden sed
Molecu lar Stru ctural
Name Formula Formula
hexane C6 H1 4
CH3 ( CH2 ) 4 CH3
heptane C7 H1 6
CH3 ( CH2 ) 5 CH3
octan e C8 H1 8
CH3 ( CH2 ) 6 CH3
C4 H1 0
CH3 ( CH2 ) 2 CH3
non ane C9 H2 0
pentane C5 H1 2
CH3 ( CH2 ) 3 CH3
decane
Name
Conden sed
Molecu lar Stru ctural
Formula Formula
methan e CH4
eth ane
C2 H6
propan e C3 H8
butane
CH4
CH3 CH3
C1 0 H2 2
CH3 ( CH2 ) 7 CH3
CH3 ( CH2 ) 8 CH3
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: shows that the compound
is a saturated hydrocarbon.
Prefix
methethpropbu tpen t-
N umber of
Carb on Atoms
1
2
3
4
5
Prefix
hexheptoctnon dec-
N umber of
Carb on Atoms
6
7
8
9
10
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
su bstituen t
paren t chain
CH3
1
2
3
4
5
6
7
6
8
CH3 CH2 CH2 CHCH2 CH2 CH2 CH3
1
4-Methyloctane
2
4
3
8
5
7
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 -ane from the name of
the parent alkane and adding the suffix -yl.
N ame
methyl
Con dens ed
Structu ral Formula
-CH3
ethyl
-CH2 CH3
propyl
-CH2 CH2 CH3
isopropyl -CHCH3
CH3
bu tyl
-CH2 CH2 CH2 CH3
N ame
isobu tyl
sec-butyl
Con dens ed
Structu ral Formula
-CH2 CHCH3
CH3
-CHCH2 CH3
CH3
CH3
t ert-bu tyl -CCH3
CH3
If there is one substituent, number the
parent chain from the end that gives the
substituent the lower number.
CH3
CH3 CH2 CH2 CHCH3
4
5
3
2
2-Methylpen tane
(n ot 4-methylpentan e)
1
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-, hexaand so on.
Use a comma to separate position numbers.
CH3 CH3
CH3 CH2 CHCH2 CHCH3
6
4
5
2
3
2,4-Dimethylhexan e
(n ot 3,5-d imethylhexan e)
1
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.
CH3
CH3 CH2 CHCH2 CHCH2 CH3
2
1
4
3
CH2 CH3
3-Ethyl-5-methylhep tane
(not 3-methyl-5-ethylheptan e)
6
5
7
Do not include the prefixes di-, tri-, tetra-,
and so on, or the hyphenated prefixes secand tert- in alphabetizing;
alphabetize
the names of substituents first,
and then insert these prefixes
CH3 CH2 CH3
CH3 CCH2 CHCH2 CH3
1
2
4
3
6
5
CH3
4-Eth yl-2,2-d imethylhexan e
(n ot 2,2-d imethyl-4-ethylhexane)
Halogens as substituent groups
Same
priority as alkyl groups
Fluoro, chloro, bromo, iodo
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, iso shows that
one end of an otherwise unbranched chain
terminates in (CH3)2CH-
•
For more complex alkanes, use the IUPAC
system.
Cycloalkanes
Cyclic hydrocarbon: a hydrocarbon that
contains carbon atoms joined to form a
ring.
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 sixmembered (cyclohexane) rings are especially
abundant in nature.
Cyclopentane
Cycloh exane
Nomenclature
To
name a cycloalkane, prefix the name of the
corresponding open-chain alkane with 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.
1
4
Isopropylcycl open tan e 1-tert- Bu tyl -4-me th ylcycloh e xan e
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.
rotate
by 120°
Least crow ded
conformation
rotate
by 60°
Intermed iate
crow din g
Most crow d ed
conformation
Cyclopentane
The most stable conformation of a
cyclopentane ring is an envelope
conformation.
Cyclohexane
The most stable conformation of a
cyclohexane ring is the chair conformation.
all
bond angles are approximately 109.5°.
In a chair conformation,
six
C-H bonds are equatorial.
six C-H bonds are axial.
axis th rough the
cen ter of th e rin g
H
H
H
H H
H
H
H
(a) Ball-and-s tick mod el
sh ow ing all 12 hydrogen s
H
(b) The s ix eq uatorial
C-H bond s
H
H
H
(c) The s ix axial
C-H b on ds
•
The more stable conformation of a substituted
cyclohexane ring has substituent group(s)
equatorial rather than axial.
CH3
CH3
Equatorial methylcyclohexane
Axial methylcyclohexane
Cis-trans isomers
Cis: on the same side.
Trans: across from each other.
Same molecular formula, same
connectivity—different orientation of atoms
in space. Stereoisomers.
Viewing a cyclopentane ring edge-on:
H
H
H
H H
H
H
H
H
H
H H H3 C
H
CH 3
CH3
cis-1,2-D imeth ylcyclop entane
H
H
CH3
H
trans -1,2-D imethylcyclop entane
View from above:
H3 C
CH3
cis-1,2-D imethylcyclopentan e
H3 C
CH3
t rans-1,2-D imeth ylcyclopentan e
View of the cyclohexane ring as a planar
hexagon:
CH3
H
CH3
H
CH3
H
or
H3 C
H
CH3
trans -1,4-Dimethylcyclohexane
or
H3 C
CH3
CH3
cis-1,4-Dimethylcyclohexane
Physical 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.
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.
Conden sed
Stru ctrual
Formula
Mol w t
(amu)
mp
(°C)
CH4
CH3 CH3
CH3 CH2 CH3
CH3 ( CH2 ) 2 CH3
16.0
30.1
44.1
-182
-183
hexane
CH3 ( CH2 ) 3 CH3
CH3 ( CH2 ) 4 CH3
heptane
octan e
non ane
Name
methan e
eth ane
propan e
butane
pentane
decane
D ensity
of Liquid
bp
(°C) (g/mL at 0° C)*
(a gas)
-164
-88
(a gas)
58.1
-190
-138
-42
0
(a gas)
(a gas)
72.2
-130
36
0.626
-95
69
0.659
CH3 ( CH2 ) 5 CH3
86.2
100.2
-90
98
0.684
CH3 ( CH2 ) 6 CH3
CH3 ( CH2 ) 7 CH3
CH3 ( CH2 ) 8 CH3
114.2
128.3
142.3
-57
-51
-30
126
151
174
0.703
0.718
0.730
*For comp aris on , th e dens ity of H2 O is 1 g/mL at 4°C.
•
Alkanes that are constitutional isomers are
different compounds and have different
physical and chemical properties.
N ame
hexan e
3-methylp entane
2-methylp entane
bp (°C)
68.7
63.3
60.3
2,3-dimethylbutan e 58.0
2,2-dimethylbutan e 49.7
Hexan e
2,2-D imeth ylb utane
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.
Reactions
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).
CH4 + 2O2
Methane
CH3 CH2 CH3 + 5O2
Propane
CO2 + 2H2 O + 212 kcal/mol
3CO2 + 4H2 O + 530 kcal/mol
Reaction with halogens (halogenation)
Halogenation
of an alkane is a substitution
reaction.
CH4 + Cl 2
Me th an e
h e at
CH3 Cl + HCl
or li gh t
Ch l orome than e
(Me th yl chl ori de )
CH3 Cl + Cl2
CH2 Cl2
Cl2
heat
CH2 Cl2 + HCl
Dichloromethane
(Methylene chloride)
Cl2
CHCl3
CCl4
heat
heat
Trichloromethane
Tetrachloromethane
(Chloroform)
(Carbon tetrachloride)
Summary of Topics: Chapter 2
Nomenclature
Cis-trans
isomers
Conformations—particularly cyclohexane
Properties (mp/bp; solubility)
Reactions:
Combustion
Radical
halogenation