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