C
H
H
C
H
H
H
C
H
H
C
H
H
H
C
H
H
C
C
H
H
C
H
H
H
H
H
H
n-Butane (bp −0.6 o C)
Isobutane (bp −10.2 o C)
or
or
Alkanes have similar chemical properties, but their physical properties vary
with molecular weight and the shape of the molecule. The low polarity of all
the bonds in alkanes means that the only intermolecular forces between
molecules of alkanes are the weak dipole–dipole forces (see 2.5.1), which
are easily overcome. As a result, compared with other functional groups,
alkanes have low melting and boiling points, and low solubility in polar
solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and
dichloromethane. Most cycloalkanes also have low polarity.
Number of
Molecular
Condensed
Name
carbons
formula
structure
bp (
C)
mp (
C)
Methane
1
CH 4
CH 4
À164
À182.5
Ethane
2
C 2 H 6
CH 3 CH 3
À88.6
À183.3
Propane
3
C 3 H 8
CH 3 CH 2 CH 3
À42.1
À189.7
Butane
4
C 4 H 10
CH 3 (CH 2 ) 2 CH 3
À0.60
À138.4
Pentane
5
C 5 H 12
CH 3 (CH 2 ) 3 CH 3
36.1
À129.7
Hexane
6
C 6 H 14
CH 3 (CH 2 ) 4 CH 3
68.9
À93.5
Heptane
7
C 7 H 16
CH 3 (CH 2 ) 5 CH 3
98.4
À90.6
Octane
8
C 8 H 18
CH 3 (CH 2 ) 6 CH 3
125.7
À56.8
Nonane
9
C 9 H 20
CH 3 (CH 2 ) 7 CH 3
150.8
À51.0
Decane
10
C 10 H 22
CH 3 (CH 2 ) 8 CH 3
174.1
À29.7
Undecane
11
C 11 H 24
CH 3 (CH 2 ) 9 CH 3
196
À26
Dodecane
12
C 12 H 26
CH 3 (CH 2 ) 10 CH 3
216
À10
The boiling points of alkanes increase steadily with increasing molecular
weights, as shown in the above table. Alkanes from methane to butane are
gases at room temperature.
Structure and conformation of alkanes
Alkanes have only sp
3 -hybridized carbons. The conformation of alkanes is
discussed in Chapter 3 (see Section 3.2.2). Methane (CH 4 ) is a nonpolar
molecule, and has four covalent carbon–hydrogen bonds. In methane, all
four CÀ ÀH bonds have the same length (1.10 A ˚ ), and all the bond angles
(109.5
) are the same. Therefore, all four covalent bonds in methane are
identical. Three different ways to represent a methane molecule are shown
here. In a perspective formula, bonds in the plane of the paper are drawn as
solid lines, bonds sticking out of the plane of the paper towards you are
64
CH4 ORGANIC FUNCTIONAL GROUPS
H
H
C
H
H
H
C
H
H
C
H
H
H
C
H
H
C
C
H
H
C
H
H
H
H
H
H
n-Butane (bp −0.6 o C)
Isobutane (bp −10.2 o C)
or
or
Alkanes have similar chemical properties, but their physical properties vary
with molecular weight and the shape of the molecule. The low polarity of all
the bonds in alkanes means that the only intermolecular forces between
molecules of alkanes are the weak dipole–dipole forces (see 2.5.1), which
are easily overcome. As a result, compared with other functional groups,
alkanes have low melting and boiling points, and low solubility in polar
solvents, e.g. water, but high solubility in nonpolar solvents, e.g. hexane and
dichloromethane. Most cycloalkanes also have low polarity.
Number of
Molecular
Condensed
Name
carbons
formula
structure
bp (
C)
mp (
C)
Methane
1
CH 4
CH 4
À164
À182.5
Ethane
2
C 2 H 6
CH 3 CH 3
À88.6
À183.3
Propane
3
C 3 H 8
CH 3 CH 2 CH 3
À42.1
À189.7
Butane
4
C 4 H 10
CH 3 (CH 2 ) 2 CH 3
À0.60
À138.4
Pentane
5
C 5 H 12
CH 3 (CH 2 ) 3 CH 3
36.1
À129.7
Hexane
6
C 6 H 14
CH 3 (CH 2 ) 4 CH 3
68.9
À93.5
Heptane
7
C 7 H 16
CH 3 (CH 2 ) 5 CH 3
98.4
À90.6
Octane
8
C 8 H 18
CH 3 (CH 2 ) 6 CH 3
125.7
À56.8
Nonane
9
C 9 H 20
CH 3 (CH 2 ) 7 CH 3
150.8
À51.0
Decane
10
C 10 H 22
CH 3 (CH 2 ) 8 CH 3
174.1
À29.7
Undecane
11
C 11 H 24
CH 3 (CH 2 ) 9 CH 3
196
À26
Dodecane
12
C 12 H 26
CH 3 (CH 2 ) 10 CH 3
216
À10
The boiling points of alkanes increase steadily with increasing molecular
weights, as shown in the above table. Alkanes from methane to butane are
gases at room temperature.
Structure and conformation of alkanes
Alkanes have only sp
3 -hybridized carbons. The conformation of alkanes is
discussed in Chapter 3 (see Section 3.2.2). Methane (CH 4 ) is a nonpolar
molecule, and has four covalent carbon–hydrogen bonds. In methane, all
four CÀ ÀH bonds have the same length (1.10 A ˚ ), and all the bond angles
(109.5
) are the same. Therefore, all four covalent bonds in methane are
identical. Three different ways to represent a methane molecule are shown
here. In a perspective formula, bonds in the plane of the paper are drawn as
solid lines, bonds sticking out of the plane of the paper towards you are
64
CH4 ORGANIC FUNCTIONAL GROUPS