Introduction to the Alkanes

James Richard Fromm


Hydrocarbons are simple organic compounds which contain only carbon and hydrogen.  If the carbon atoms are linked in chains, the compounds are called aliphatic compounds; if the atoms are linked in rings, the compounds are called alicyclic.

The chain compounds, or aliphatic compounds, may be further classified on the basis of the individual carbon-to-carbon bonds.  Every carbon atom can form four bonds to other atoms thus the noble gas configuration is reached (8 outer electrons).  Every hydrogen atom forms one bond producing 2 outer electrons, the most stable state for hydrogen.

Chain compounds in which all carbon-to-carbon bonds are only simple single bonds are called ALKANES.  These compounds are also called saturated hydrocarbons, because each carbon-to-carbon bond is a single bond, and the valence of the carbon atom is, therefore, saturated.   No more atoms can be bonded to the atoms in the compound, without breaking the compound into two or more fragments.  If it contains one or more bonds which can react with hydrogen it is called an unsaturated hydrocarbon.  Almost all other organic compounds can be named as derivatives of these simple hydrocarbons.   Alkanes which have long carbon chains are often called paraffins in chemical industry.  The most simple alkane is methane with the formula CH4. The second alkane is ethane with the formula C2H6.   The general formula of alkanes is CnH2n+2.

alkanetable.gif (5196 bytes)
The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10).

The simplest alkane is the gas methane, whose molecular formula is CH4.   Methane exists as a tetrahedral shape, but it is often represented by a flattened structure as are most organic compounds.  Flattened structures for the three simplest alkanes are given in the Figure below. In many cases the structures can be further simplified without loss of information by omitting all single bonds and writing the letter symbol of the element close to the letter symbol of the element to which it is attached.   Thus the representation of methane as CH4, ethane as H3CCH3 (rather than as C2H6), and propane as H3CCH2CH3 (rather than as C3H8) is a representation of structure as well as of molecular composition.  For many simple organic compounds representations like this are adequate for discussion and identification purposes.  When they are not adequate, all organic chemists resort to more elaborately drawn structures which convey the necessary information.

The structure of ethane can be derived from that of methane by substitution of a -CH3 group, called a methyl group, for one of the hydrogens of methane.   The structure of propane can be derived either by substitution of a methyl group upon ethane or by substitution of an ethyl group upon methane; either method yields the same product, propane.  Likewise, the structure of the next member of the series, butane, can be derived by the substitution of a methyl group upon propane.  The alkane series of compounds can be extended indefinitely by this method.  The names of the compounds and their substituent groups are given in the Table below.  For even longer chains, the name is simply the number (given as a Greek prefix).

The alkanes above propane are named by giving the number of carbons (in Greek) with the ending -ane added.  If an alkane is not a straight chain, then the longest straight chain in it is used as the basis of the name and the shorter side chains are considered to be substituents; thus names such as methylpropane and methylbutane are derived.

Positional Isomers of Alkanes

Two four-carbon alkanes are known, and they have measurably different chemical and physical properties.  Their structures are H3CCH2CH2CH3 and H3CCH(CH3)2.  Therefore they were named normal butane, often abbreviated as n-butane, and isobutane.  The straight-chain form is considered the "normal" form.  Alternatively, they could be named using the systematic IUPAC method as 1-methylpropane and 2-methylpropane, or even as butane and 2-methylpropane.  The IUPAC method names the longest straight carbon chain in the usual way and then numbers the carbons; the location of a substituent group is given by the number of its carbon.  The IUPAC method is always used for more complicated molecules, but many of the simpler ones still use non-systematic names, called trivial names, because these are less cumbersome to use.

The two forms of butane are described as two positional isomers of butane because they differ only in the position of the substituent group.  The pentanes have three positional isomers: H3CCH2CH2CH2CH3, normal pentane or n-pentane; H3CCH2CH2(CH3)2, isopentane; and H3CC(CH3)3, neopentane.   This older nomenclature is still used to distinguish normal or n-compounds, which have straight chains, from iso-compounds, which branch at the carbon next to the end carbon, but neo-compounds are now named using the IUPAC system.  The IUPAC systematic names for the three positional isomers of pentane would be pentane, 2-methylbutane, and 2,2-dimethylpropane.  Note that the appearance of two substituents on the same carbon causes no problem; the same location is simply given twice.

Table: Structural Formulas of the First Ten Continuous-chain Alkanes
Name Molecular Formula Structural Formula Boiling Point (oC)
Methane CH4 CH4 -161.0
Ethane C2H6 CH3CH3 -88.5
Propane C3H8 CH3CH2CH3 -42.0
Butane C4H10 CH3CH2CH2CH3 0.5
Pentane C5H12 CH3CH2CH2CH2CH3 36.0
Hexane C6H14 CH3CH2CH2CH2CH2CH3 68.7
Heptane C7H16 CH3CH2CH2CH2CH2CH2CH3 98.5
Octane C8H18 CH3(CH2)6CH3 125.6
Nonane C9H20 CH3(CH2)7CH3 150.7
Decane C10H22 CH3(CH2)8CH3 174.1
Note: In the table above octane, nonane, and decane have (CH2) groups followed by a subscript designating the number of (CH2) groups attached to the carbons between the end CH3's of each chain.

Names of the higher members of this series consist of a numerical term, followed by "-ane".  Examples of these names are shown in the table below.   The generic name of saturated aliphatic (acyclic) hydrocarbons (branched or unbranched) is "alkane".

Examples of names:

(n = total number of carbon atoms)

n n n
11 Undecane 22 Docosane 33 Tritriacontane
12 Dodecane 23 Tricosane 40 Tetracontane
13 Tridecane 24 Tetracosane 50 Pentacontane
14 Tetradecane 25 Pentacosane 60 Hexacontane
15 Pentadecane 26 Hexacosane 70 Heptacontane
16 Hexadecane 27 Heptacosane 80 Octacontane
17 Heptadecane 28 Octacosane 90 Nonacontane
18 Octadecane 29 Nonacosane 100 Hectane
19 Nonadecane 30 Triacontane 132 Dotriacontahectane
20 Icosane 31 Hentriacontane  
21 Henicosane 32 Dotriacontane  

Each compound differs from the next by a multiple of the -CH2- (methylene group).  Essentially we are building the series of compounds by removing a hydrogen atom from one of the carbon atoms and adding - CH2- to the chain and then replacing the hydrogen.

A series of compounds whose structures differ from each other by a specific structural unit (such as -CH2- in the case of alkanes) is called a Homologous Series.  A general formula can be written for all of the members of a homologous series such as the alkanes.  For the alkanes, the formula CnH2n+2, where n is the number of carbon atoms in the compound.

There are general trends in physical and chemical properties within homologous families which can be used to study these families as a whole.  For instance, as the molecular weight of the compounds in a family increases, the boiling point increases. This can be seen from the table listing the first ten members of the alkane family above.

It is interesting to note that the first four alkanes, which all exist in the vapor state under normal atmospheric conditions, are the principle ingredients in natural gas.   The members of the alkane family use the Greek (sometimes Latin) prefix for the number of carbon atoms, and the characteristic ending -ane to identify and describe their position and structure within their respective family.

If one hydrogen atom, with its associated electron, is removed from a hydrocarbon molecule, a Radical is left:

CH4 CH3-
Methane Methyl
CH3-CH3 CH3-CH2-
Ethane Ethyl
CH3-CH2-CH3 CH3-CH2-CH2-
Propane Propyl

Radicals are named by substituting the ending -yl for the normal -ane ending of the parent compound.

For convenience in naming organic compounds, carbon atoms in a structural formula are given position numbers.  In an unbranched chain molecule, the numbering of carbon atoms can begin at either end of the chain:

CH3-CH2-CH2-CH3 CH3-CH2-CH2-CH2-CH3
Butane Pentane

Not all alkanes have unbranched chains of carbon atoms.  Complex alkanes are named by using the longest chain of carbon atoms as the basis of the compound name. The PARENT CHAIN does not necessarily occur in a straight line.   This compound

CH3CH2CH(CH3)CH2CH3
3-methylpentane

has pentane (C5H12) as the parent chain since the longest chain contains five carbon atoms.  The carbon atoms of the longest chain are given position numbers beginning at one end of the parent chain.  The CH3- group which is attached to the main chain is called a side chain or substituent.  The side chain is named as a radical.  We indicate, by number, the position of the carbon atom of the parent chain to which the side chain (radical) is attached.   Thus, the name 3-methylpentane for the above saturated alkane.  The parent compound is pentane, the radical is methyl which attached to the number-3 carbon atom of the parent chain.  The name is written with a hyphen between the substituent (radical) position number and name.  The radical and parent are written as one word.   Numbering of the carbon atoms of the parent chain begins at the end which will give the lowest position numbers to the radical.

Note: Be aware that 3-methylpentane is written with a methyl radical (CH3) in parenthesis immediately to the right of the carbon to which it is attached.  It is not part of the parent chain but rather attached as a side branch.
CH3CH2C(CH3CH2)(CH3)CH2CH3
3-ethyl-3-methylhexane

It is relatively easy to see that if one does not follow the established rules the previous structure may have been mistakenly named 4-ethyl-4-methylhexane.  ONE MUST BEGIN NUMBERING THE CARBONS OF THE PARENT CHAIN FROM THAT END CLOSEST TO THE ATTACHED RADICAL. You may have wondered why the structure was not named 3-methyl-3-ethylhexane. The fact of the matter is that both 3-ethyl-3-methylhexane and 3-methyl-3-ethylhexane are correct.  It is permissible to choose one of two rules.

  1. When there is more than one radical attached, and the number is not a factor, list them in alphabetical order.
  2. When there is more than one radical attached, and the number is not a factor, list them in order of their mass (smaller to larger).

The first rule listed is generally the accepted method of representing the attached radicals and functional groups.  Whatever method is chosen it is important to be consistent.

If there are two or more radicals attached which are alike, it is convenient to use prefixes (di-, tri-, tetra-, penta-, etc.) instead of writing each group separately.  A comma is placed between the position numbers of the substituents which are alike.

CH3CH(CH3)CH(CH3)CH2CH3
2,3-dimethylpentane

The parent chain is the five carbon chain-pentane.  The carbons of the parent chain must be numbered starting from the left because the radicals (methyl) are located closest to that end.  The two similar radicals are positioned on the second and third carbons of the parent chain, thus, the 2,3- designation.  Because there are two radicals of the same kind the prefix di- is utilized to indicate the number present.  Again, note the presence of the comma and hyphen.   For structures such as methane (CH4), ethane (CH3-CH3), and propane (CH3-CH2-CH3) only one structural diagram needs to be drawn.  There is, however, an alternative structure for butane.

CH3-CH2-CH2-CH3 CH3CH(CH3)CH3
Butane (C4H10) Methylpropane (C4H10)

As noted in section 10.1, the existence of two or more substances with the same molecular formula (C4H10 in this instance), but different arrangements of atoms and bonds, is called ISOMERISM.   The two structures of butane diagrammed above are called isomers of butane.   Most organic compounds have isomers but there is no known way of predicting exactly how many isomers most compounds can form. Pentane (C5H12), the next member of the alkane family, has three isomers:

CH3-CH2-CH2-CH2-CH3 CH3CH(CH3)CH2CH3 CH2C(CH3)2CH3
Pentane Methylbutane Dimethylpropane

Because there is only one form of methylpropane, methylbutane, and dimethylpropane it is not necessary to include the position numbers when writing their name.

Hexane (C6H14), the next member of the alkane family, has five isomers, and heptane (C7H16) has nine.  Isomers are named according to the longest chain, and not according to the total number of carbon atoms in the molecule.  Thus, the isomers of heptane are:

  1. heptane
  2. 2-methylhexane
  3. 3-methylhexane
  4. 2,2-dimethylpentane
  5. 3,3-dimethylpentane
  6. 2,3-dimethylpentane
  7. 2,4-dimethylpentane
  8. ethylpentane
  9. trimethylbutane

The number of isomers increases dramatically with the number of carbons in the parent chain as indicated by octane with eighteen (18), nonane with thirty-five (35), and decane with seventy-five (75).

Table: Fractions Obtained from Crude Oils
Fraction Composition of carbon chains Boiling range (oC) Percent of crude oil
Natural Gas C1 to C4 Below 20 10%
Petroleum ether (solvent) C5 to C6 30 to 60 10%
Naphtha (solvent) C7 to C8 60 to 90 10%
Gasoline C6 to C12 75 to 200 40%
Kerosene C12 to C15 200 to 300 10%
Fuel oils, mineral oil C15 to C18 300 to 400 30%
Lubricating oil, petroleum jelly, greases, paraffin wax, asphalt C16 to C24 Over 400 10%

All alkanes are inflammable, the product of the burning being carbon dioxide (CO2) and water (H2O).  Methane, ethane, propane and butane are gaseous at room temperature and are collectively referred to as natural gas.  Hydrocarbons are nonpolar allowing van der Waals forces to act between them.  Short alkanes have weak van der Walls forces. With an increase of carbon atoms the van der Waals forces increase.  As the molecular chains become longer the molecules become liquid and eventually solid.


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Copyright 1997 James R. Fromm - Revised May, 1998