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 C_nH_2n+2.

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

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)

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 C_nH_2n+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:

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:

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

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.

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.

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.

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:

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:

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

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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