All alkanes are colorless. Alkanes with the lowest molecular weights are gases, those of intermediate molecular weight are liquids, and the heaviest are waxy solids.
Table of alkanes Boiling points Alkanes experiences intermolecular
van der Waals forces. The cumulative effects of these intermolecular forces give rise to greater boiling points of alkanes. Two factors influence the strength of the van der Waals forces: • the number of electrons surrounding the
molecule, which increases with the alkane's molecular weight • the surface area of the molecule Under
standard conditions, from CH4 to C4H10 alkanes are gaseous; from C5H12 to C17H36 they are liquids; and after C18H38 they are solids. As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has an almost linear relationship with the size (
molecular weight) of the molecule. As a rule of thumb, the boiling point rises 20–30 °C for each carbon added to the chain; this rule applies to other homologous series. A second difference in crystal structure is that even-numbered alkanes (from octane onwards) tend to form more rotationally ordered crystals compared to their odd-numbered neighbors. This causes them to have a greater
entropy of fusion (increase in disorder from the solid to the liquid state), lowering their melting point. While these effects operate in opposing directions, the first effect tends to be slightly stronger, leading even-numbered alkanes to have slightly higher melting points than the average of their odd-numbered neighbors. This trend does not apply to methane, which has an unusually high melting point, higher than both ethane and propane. This is because it has a very low entropy of fusion, attributable to its high molecular symmetry and the rotational disorder in solid methane near its melting point (
Methane I).
Molecular geometry The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from the
electron configuration of
carbon, which has four
valence electrons. The carbon atoms in alkanes are described as sp3 hybrids; that is to say that, to a good approximation, the valence electrons are in orbitals directed towards the corners of a tetrahedron which are derived from the combination of the 2s orbital and the three 2p orbitals. Geometrically, the angle between the bonds are cos−1(−) ≈ 109.47°. This is exact for the case of methane, while larger alkanes containing a combination of C–H and C–C bonds generally have bonds that are within several degrees of this idealized value.
Bond lengths and bond angles An alkane has only C–H and C–C single bonds. The former result from the overlap of an sp3 orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp3 orbitals on adjacent carbon atoms. The
bond lengths amount to 1.09 × 10−10 m for a C–H bond and 1.54 × 10−10 m for a C–C bond. The spatial arrangement of the bonds is similar to that of the four sp3 orbitals—they are tetrahedrally arranged, with an angle of 109.47° between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not accurately depict the geometry.
Conformation s of the two rotamers of ethane The spatial arrangement of the C-C and C-H bonds are described by the torsion angles of the molecule, known as its
conformation. In
ethane, the simplest case for studying the conformation of alkanes, there is nearly free rotation about a carbon–carbon single bond. Two limiting conformations are important:
eclipsed conformation and
staggered conformation. The staggered conformation is 12.6 kJ/mol (3.0 kcal/mol) lower in energy (more stable) than the eclipsed conformation (the least stable). In highly branched alkanes, the bond angle may differ from the optimal value (109.5°) to accommodate bulky groups. Such distortions introduce a tension in the molecule, known as
steric hindrance or strain. Strain substantially increases reactivity.
Spectroscopic properties Spectroscopic signatures for alkanes are obtainable by the major characterization techniques.
Infrared spectroscopy The C-H stretching mode gives strong absorptions between 2850 and 2960
cm−1 and weaker bands for the C-C stretching mode absorbs between 800 and 1300 cm−1. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450 cm−1 and 1375 cm−1, while methylene groups show bands at 1465 cm−1 and 1450 cm−1. Carbon chains with more than four carbon atoms show a weak absorption at around 725 cm−1.
NMR spectroscopy The proton resonances of alkanes are usually found at
δH = 0.5–1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon:
δC = 8–30 (primary, methyl, –CH3), 15–55 (secondary, methylene, –CH2–), 20–60 (tertiary, methyne, C–H) and quaternary. The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of
nuclear Overhauser effect and the long
relaxation time, and can be missed in weak samples, or samples that have not been run for a sufficiently long time.
Mass spectrometry Since alkanes have high
ionization energies, their
electron impact mass spectra show weak currents for their molecular ions. The fragmentation pattern can be difficult to interpret, but in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resulting
free radicals. The mass spectra for straight-chain alkanes is illustrated by that for
dodecane: the fragment resulting from the loss of a single methyl group (
M − 15) is absent, fragments are more intense than the molecular ion and are spaced by intervals of 14 mass units, corresponding to loss of CH2 groups. ==Chemical properties==