SYSTEMATIC NAMING OF ORGANIC COMPOUNDS

Contents for this page	Related topics
Hydrocarbons Alkanes Alkyl groups The IUPAC nomenclature Alkenes Alkynes Alkanols Haloalkanes Carboxylic acid and esters Aldehydes and ketones Additional questions	Introduction to organic chemistry Addition, elimination, and substitution reactions	Data Glossary
Learning Outcomes
After studying this section, you will (a) know the principal characteristics of hydrocarbons, (b) know the difference between saturated and unsaturated hydrocarbons, (c) know about homologous series of hydrocarbons, (d) know the structures typical of alkanes, alkenes, alkynes and haloalkanes, and (e) know how to apply the IUPAC. system of nomenclature to simple saturated and unsaturated hydrocarbons

Hydrocarbons:

Hydrocarbons are the simplest organic compounds, and they are made up of atoms of carbon and hydrogen only. There are a great many such compounds (many of which play an important role as fuels and lubricants), and so it is necessary to classify them further.

Alkanes:

ALKANES are hydrocarbons (compounds containg only C and H) that have single covalent bonds joining the carbon atoms. The carbon atoms form open chains, which may have branches. The molecular formula of all alkanes fits the expression C_nH_2n+2, where n is the number of carbon atoms.

Check that the formula is C6H14!

An HOMOLOGOUS SERIES is an ordered series of organic compounds, each member of which differs from the preceeding one by the insertion of a -CH₂- group in the molecule. The first ten members of the homologous series of alkanes is shown below.

We note that there is an increase in the boiling points as we move from lower alkanes to higher alkanes. Each -CH₂ group that increases the chain length increases the van der Waals interactions between the molecules. More energy (the latent heat of vaporisation) is therefore needed to cause the molecules to leave the liquid phase and move into the gaseous phase. Since temperature is a measure of the kinetic energy of molecules, we understand therefore why the boiling point increases with increasing chain length.

Alkyl groups:

The names of the alkanes in the homologous series give rise to a way of naming any alkane, and with the application of certain rules, this can be applied to naming all organic compounds.

Imagine removing a hydrogen atom from a molecule of methane, CH₄. This gives a group of atoms CH₃-, called METHYL. Similarly, we can get a group C₂H₅- (called ETHYL from ethane), and so on. In general, any alkane can be used to generate alkyl groups. Alkyl groups are the key to naming complex organic molecules.

The IUPAC nomenclature:

IUPAC stands for the INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY. This is an organization which regulates various aspects of chemistry. In particular, it issues rules for the nomenclature (name-giving) of organic compounds.

The way some of these rules are applied can be seen as one works out a name for the compound shown below.

1. Take the longest chain of carbon atoms, and name the compound according to the alkane having the same number of carbon atoms. Since in this case the number is 4, the name will be based on but (from butane, which has four carbon atoms).
2. There is a methyl (CH₃-) group attached to the carbon chain. Hence the name will be methylbutane.

3. We give numbers to the carbon atoms of the longest chain, and in so doing, we see that the methyl group is attached to carbon #2. The name is thus 2-methylbutane.

4. Always number the carbon chain so as to have as low a number in the name as possible. If more than one alkyl group is attached, this is shown by adding the prefix di (for 2) or tri (for 3).

Alkenes:

ALKENES are hydrocarbons that have one or more C=C double bonds (two C atoms are linked by 4 shared electrons). The general formula is C_nH_2n, which is two hydrogen atoms less than the corresponding alkane. Note that if a compound has a molecular formula that fits the expression C_nH_2n, it does not necessarily mean that that compound is an alkene. (See under cycloalkanes).

Alkenes are said to be UNSATURATED, (since they do not have their full complement of hydrogen atoms).

Alkynes:

These are hydrocarbons that have one or more CºC triple bonds (two C atoms are joined by 6 shared electrons). The molecular formula fits the formula C_nH_2n-2 and has therefore four hydrogen atoms less than the corresponding alkane.

Alkynes, like alkenes, are also said to be UNSATURATED, (since they do not have their full complement of hydrogen atoms). The simplest alkyne, HCºCH, is commonly known as acetylene.

The naming of alkenes and alkynes follows the rules that are used for the naming of alkanes. Just remember that an alkene's name ends in ENE, while that of an alkyne ends in YNE. In addition, one must indicate the number of the carbon atom where the multiple bond starts, numbering the carbon chain in such a way as to give the atoms of the multiple bond the lowest numbers.

In this way, the carbon chain must be numbered (in the above example) from left to right, and not from right to left.

The compound shown is therefore called pent-2-ene, and not pent-3-ene.

Haloalkanes:

If one or more hydrogen atoms of an alkane is replaced by a halogen atom (chemists use the term SUBSTITUTION for this process), the compound is a HALOALKANE.

Alkanols:

These compounds arise when one or more hydrogen atoms of an alkane are substituted by the -OH (HYDROXYL) functional group.

They are named from the corresponding alkane, using the ending OL to indicate that we are dealing with an alkanol, and a number which locates the hydroxyl group on the carbon chain. The chain is numbered so as to give the OH group the lowest possible number:

We must name the alkanol shown above 3-methylpentan-2-ol. If the chain had been numbered from right to left the OH group would be on C-4, with the name 3-methylpentan-4-ol, which is incorrect.

Alkanols form their own homologous series, some of which are shown in the table below:

As we look at the above table, we note two things:

1. The boiling points of individual members of the series are much higher than the boiling points of the corresponding alkanes. This is because intermolecular forces due to hydrogen bonding exist in alkanols, but not in alkanes.
2. The boiling point of the alkanols increases with increasing size. The reason for this is that, in a given homologous series, boiling points increase with increasing molecular mass. In the case of alkanols, increased chain length increases van der Waals interactions between molecules, leading to stonger intermolecular forces and consequent increase in boiling point.

Polyhydric alcohols:

Alkanes that carry more than one hydroxyl (-OH) group are known as POLYHYDRIC ALCOHOLS. Two common examples of these are ethylene glycol, which is used as an anti-freeze agent, and glycerol, produced on a large scale in the manufacture of soap, and which is used to make nitroglycerine, the explosive component of the commercial explosives "dynamite" and "gelignite". It is also used as a food additive and sugar substitute.

What is the "alcohol" found in all sorts of alcoholic beverages? (Click here for a discussion)

Carboxylic acids:

These are compounds that have the CARBOXYL, -COOH functional group. They are sometimes called fatty acids, since (in combination with alkanols) they are normal constituents of fats and oils.

They are named by numbering the carbon atom of the carboxyl group as number 1 of the longest carbon chain in the molecule, and basing the name on the name of the alkane with the same number of carbon atoms. The ending -OIC ACID denotes that we are dealing with a carboxylic acid. For example, The compound whose structure is shown above is 2,3-dimethylbutanoic acid.

Water is also formed as a result of the reaction. All esters consist of two parts: one derived from a carboxylic acid, and one derived from an alkanol.

Esters are named as if they were salts (which they definitely are not!) of an alkyl group and the carboxylic acid. In the example above, ethanoic acid and ethanol react to form the ester ethyl ethanoate.

Aldehydes and ketones:

Both aldehydes and ketone contain the CARBONYL, >C=O group. Aldehydes have one carbon atom and one hydrogen atom attached to the carbonyl group, while in ketones, the carbonyl group is attached to two carbon atoms. Both aldehydes and ketones cannot form hydrogen bonds, and consequently have lower boiling points than the alcohols from which they can be derived by oxidation.

Naming aldehydes:

Aldehydes are named from the parent alkanes, using the suffix -AL. The carbon chain must be numbered by taking into account that the carbon atom of the -CHO group is numbered 1. The examples shown below should make this clear.

Naming ketones:

Ketones are named by taking the longest chain that contain the >CO group, and referring it to the corresponding straight-chain alkane. The name ends in -ONE, with a number indicating the position of the carbonyl group. This number must be as low as possible. Again, some examples will illustrate these rules:

Additional questions

Cycloalkanes:

Another group of hydrocarbons whose molecular formula obey the expression C_nH_2n are the CYCLOALKANES, which also form a homologous series, and whose first four members are shown below:

Their structural formulae are normally abbreviated as shown at the bottom of the graphic. The molecular formulae of cycloalkanes fits the expression C_nH_2n, but they are not alkenes, since they do not have a C=C bond. They do not behave as unsaturated compounds. Cyclohexane is frequently used in teaching laboratories to illustrate the inertness of saturated hydrocarbons compared to unsaturated hydrocarbons.