Alkane is an open-chain saturated hydrocarbon chain (saturated group), the carbon atoms in the molecule are linked to a single bond, and the remaining valences are bonded hydrogen compound. Alkane Functional Group

The formula is C n H 2n+2, which is the simplest organic compound. The main sources of alkanes are oil and natural gas, which are important chemical raw materials and energy materials.


A compound composed only of two hydrocarbon elements is called a hydrocarbon and is simply referred to as a hydrocarbon.

Hydrocarbons can be classified into two major classes of chain hydrocarbons (aliphatic hydrocarbons) and cyclic hydrocarbons (alicyclic hydrocarbons) depending on the molecular skeleton of the hydrocarbon.


Chain hydrocarbons can be further divided into saturated hydrocarbons and unsaturated hydrocarbons. Where saturated hydrocarbons are alkanes.

Most of the overall configuration solely of a carbon-hydrogen organic compound with the atoms to carbon-carbon single bond consisting of hydrocarbons means a saturated carbon atom binding and other atoms in the molecule reaches the maximum.

As can be seen from the above, strictly speaking, alkanes do not include cycloalkanes, so this entry focuses on the inclusion of cyclic alkanes.

Physical properties

  1. When the number of carbon atoms is less than or equal to 4, the alkane is in a gaseous state at normal temperature, and the other alkane is solid or liquid at normal temperature (new pentane is gaseous at normal temperature)
  2. It is insoluble in water and soluble in organic solvents.
  3. The boiling point gradually increases as the number of carbon atoms increases.
  4. As the number of carbon atoms increases, the relative density gradually increases. The density of alkanes is generally less than the density of water.


The alkane is not a planar structure drawn by the structural formula, but a three-dimensional shape. All carbon atoms are sp3 hybridized, and each atom is connected by a σ bond.


The bond angle is close to 109°28′, and the average bond of the CC bond. With a length of 154 pm, the average bond length of the CH bond is 109 pm. Since the σ bond electron cloud is axially symmetric along the bond axis, the two bond atoms can be “freely” rotated about the bond axis.

Chemical formula

Starting from methane, each additional carbon atom increases two hydrogen atoms accordingly. Therefore, the alkane has the general formula C n H 2n+2, and n represents the number of carbon atoms (n=1, 2, 3 . . .) in theory, n can be large, but the known alkane n is within about 100.

A series of compounds having the same molecular formula and structural characteristics are called the same series, the alkane is the same as the CH2, and the alkane having different C atoms is a homologue.

The homologues in the same series have similar structures, similar chemical properties, and physical properties change regularly with the increase of carbon atoms.

Physical properties


Low boiling point (boiling point) alkane as a colorless liquid, with special smell; high boiling alkane’s viscous oily liquid, odorless.

Physical state

The physical properties of alkanes change regularly as the number of carbon atoms in the molecule increases. The alkane having 1 to 4 carbon atoms is a gas at room temperature of 25 °.

An alkane having 5 to 17 carbon atoms is a liquid. However, an alkane having 10 to 19 carbon atoms may actually be solid at normal temperature. Alkane Functional Group

Containing 18 or more carbon atoms as solid n-alkanes, but the n-alkanes containing up to 60 carbon atoms (melting point of 99 deg.] C) Melting point (melting point) does not exceed 100 ℃.

Alkane is non-polar molecules (non-polar molecule), dipole moments (dipole moment) is zero, but the molecular charge distribution is not uniform, may be generated in the movement transient dipole moment, the dipole moment has between instantaneous Interaction force ( dispersion force ).

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In addition, there are van der Waals forces between the molecules. The forces between these molecules are one or two orders of magnitude smaller than the chemical bonds.

The energy required to overcome these forces is also low. Therefore, the melting point and boiling point of general organic compounds rarely exceed 300 °C.

Boiling Point

What alkane has the highest boiling point? The boiling point of normal paraffin’s increases with the increase of carbon atoms, because the energy required for molecular motion increases, the inter-molecular contact surface increases, and van der Waals forces increase.

For each additional CH 2 of lower alkanes, the relative molecular mass changes greatly, and the boiling points also differ greatly; the boiling point difference of higher alkanes gradually decreases.


Therefore, lower alkanes are easier to separate and higher alkane separation is much more difficult.

In the isomers, the molecular structure is different, the molecular contact area is different, and the interaction force is also different.

The boiling point of n-pentane is 36.1 ° C, the boiling point of 2-methyl butane is 25 ° C, and the boiling point of 2, 2-dimethylpropane is only 9 ° C. The cross-chain molecules tend to be spherical, and the contact area is reduced due to the steric hindrance of the branches, thereby reducing the intermolecular forces and lower boiling points.

Melting Point

The melting point of solid molecules also increases with the increase of carbon atoms, but it is not as regular as the change of boiling point. Alkane Functional Group

The same series C 1 -C 3 is not so regular, but C 4 or higher is increased with the increase of carbon number.

This is due to the interaction between the crystal molecules, not only depending on the relative molecular mass but also on the arrangement of the molecules in the crystal lattice. High molecular symmetry, tighter alignment, intermolecular absorption

Gravity is large and the melting point is higher.  In n-alkanes, alkanes containing odd carbon atoms have a lower melting point than those containing even carbon atoms.

Thus, in the melting point curve of a linear alkane, the alkane containing an odd number and an even number of carbon atoms each constitute a melting point curve, with an even number and an odd number below.

As analyzed by X-ray diffraction, the solid n-alkane crystals are zigzagged, and the methyl groups at both ends of the odd-numbered carbon atom dentate chain are on the same side, such as n-pentane.

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In the even carbon chain, the methyl groups at both ends are not on the same side, such as n-hexane, the even carbon chains are closer to each other, and the interaction force is large, so the melting point rise value is larger than that of the single carbon chain.


The density of alkane increases with the increase of relative molecular mass, which is also the result of intermolecular interaction force. Alkane Functional Group

The intermolecular attraction increases, the distance between molecules decreases, the relative density increases, and the density increases.

After the value, the relative molecular mass increases and the density changes little. The maximum is close to 0.8 g·cm -3, so all alkanes are lighter than water.


Why are alkanes unreactive? The sigma bond in the alkane has a small polarity and its molecular dipole moment is zero, which is a non-polar molecule.

According to the principle of similar compatibility, alkanes are soluble in non-polar solvents such as carbon tetrachloride, hydrocarbons (ether, benzene) and are insoluble in polar solvents such as water.

The boiling point, melting point and density of the cycloalkane are somewhat lower than those of the paraffin having the same number of carbon atoms.

This is because the chain compound can be shaken relatively freely, the “pull” between the molecules is not tight, and it is easy to volatilize, so the boiling point is lower.

Because of this shaking, it is more difficult to make an orderly arrangement in the crystal lattice, so the melting point is also lower. Alkane Functional Group

Since there is no ring pinning, the arrangement of the chain compound is looser than that of the ring compound, so the density is also lower.

Isomers and the cis and Trans isomers have different physical properties. The table below is the physical constants for several alkanes and naphthenes.

Physical constants of some paraffin’s and naphthenes


Molecular formula

Boiling point / °C

Melting point / °C

Relative density

MethaneCH 4-161.7-182.6——
Ethane2 H 6-88.6-182.8——
Propane ( cyclopropane )3 H 8 (C 3 H 6 )-42.2 (-32.7)-187.1 (-127.6)0.5005
Butane ( cyclobutane )4 H 10 (C 4 H 8 )-0.5 (12.5)-138.4(-80)0.5788
Pentane ( cyclopentane )5 H 12 (C 5 H 10 )36.1 (49.3)-129.3 (-93.9)0.6264 (0.7457)
Hexane ( cyclohexane )6 H 14 (C 6 H 12 )68.7 (80.7)-94.0 (6.6)0.6594 (0.7786)
Heptane ( cycloheptane )7 H 16 (C 7 H 14 )98.4 (118.5)-90.5 (-12.0)0.6837 (0.8098)
Octane ( cyclooctane )8 H 18 (C 8 H 16 )125.6 (150)-56.8 (14.3)0.7028 (0.8349)
Decane9 H 20150.7-53.70.7179
Decane10 H 22174.0-29.70.7298
Undecane11 H 24195.8-25.60.7404
Dodecane12 H 26216.3-9.60.7493
Tridecane13 H 28245.4-60.7568
Tetradecane14 H 302515.50.7636
Pentadecane15 H 32268100.7688
Hexadecane16 H 3428018.10.7749
Heptadecane17 H 3630322.00.7767
Octadecane18 H 3830828.00.7767
Ninecane19 H 4033032.00.7776
Eicosane20 H 4234336.40.7886
Triacontane30 H 62449.7660.7750
Tetradecane40 H 82——81——