functional groups

The importance of functional groups

One of the thrills in organic chemistry is the skill and enjoyment from figuring out how to build a new molecule from simple starting reagents. To be able to build large molecules from smaller ones you need to be able to predict how different reactants will interact with each other and to do this you need to be able to identify functional groups within molecules.

A functional group is the reactive part of a molecule which gives the molecule its distinctive chemical and physical properties. Learn how different functional groups interact and the basic properties of functional groups and you will quickly master organic chemistry.

Aliphatic compounds consist of straight and branched chain molecules. In year 12 almost all the molecules you will study in organic chemistry will be aliphatic, you may meet some cyclic molecules which consist of closed rings of atoms, but these rings are unlikely to be based on benzene. Compounds which contain benzene rings are often described as being aromatic and the chemistry of these aromatic molecules is considered mainly in A2 chemistry. There are probably hundreds of thousands or even millions of organic compounds and to help us identify them they are placed in families which form a homologous series of compounds. A homologous series is a series of compounds which contain the same functional group and the molecular formula of all the compounds in any one particular homologous serie can be represented by a general formula.

As an example consider the a homologous series you have met before, the alkanes. The alkanes form a homologous series of saturated hydrocarbons with the general formula CnH2n+2. All alkanes contain the functional group C-H. The first four alkanes are shown below.

Alkane Number of carbon atoms Molecular Formula Structure
methane 1 CH4
ethane 2 C2H6
propane 3 C3H8
butane 4 C4H10

More functional groups

The alkanes are probably the first homologous series you learned about in chemistry, however there are many more, some of these are shown below:

The alkenes

Homologous series Name Functional group general formula
alkenes suffix -ene CnH2n

The first 3 members of the alkene homologous series are shown below:

the first three alkenes

The alkene are all unsaturated hydrocarbons which means that they contain a carbon carbon double bond, C=C. They are named by simply adding the suffix -ene to the end of the name of corresponding longest chain of carbon atoms that contain the C=C functional group, that is the root name of the compound. You may also see numbers in the name of an alkene molecule, for example in the alkene but-2-ene the number 2 in the name is being used to tell you where in the longest carbon chain the C=C functional group is to be found.

The halogenalkanes

The halogenalkanes are another homologous series you may have met before. They are formed by replacing one or more of the hydrogen atoms on an alkane by a halogen.

Homologous series Name Functional group general formula
halogenalkane prefix - halo.
The halogen/halo can be fluoro, chloro, bromo, iodo
C-F, C-Cl, C-Br, C-I
or simply C-X
CnH2n+1X

Halogenalkanes are named in a similar way to the alkanes:

Some examples of halogenalkane molecules are shown below:

If more than one halogen is present then the positions and names are listed alphabetically, recall that between numbers we use commas and hyphens are used between numbers and letters in the names of compounds e.g.

The alcohols

Homologous series Name Functional group general formula
alcohols suffix -ol
prefix -hydroxy
C-OH CnH2n+1 OH

The first 3 members of the alcohol homologous series are shown below :

3d models of the first three alcohols

Alcohols are named as being derived from alkanes. To name an alcohol:

some examples are shown below:

The carboxylic acids

Carboxylic acids are a family of weak acids which contain the carboxylate functional group (-COOH).

Homologous series Name Functional group general formula
carboxylic acids suffix -oic COOH CnH2n+1 COOH

The first 3 members of the carboxylic acid homologous series are shown below:

3d Models of the first three carboxylic acids

Carboxylic acids all contain the carboxylate functional group (-COOH), they are named as being derived from alkanes. To name a carboxylic acid simply find the longest carbon chain containing the carboxylate functional group and simply replace the -e from the corresponding alkane with -oic for the acid.

For naming purposes the carboxylate group is a high priority group and the carbon atom in the carboxylate group will always be carbon atom number 1 in the longest carbon chain in the molecule, e.g. Two examples of substituted carboxylic acids are shown below.

Amines

ammonia molecule

Amines are derivatives of ammonia (NH3), shown opposite. Amines are formed by simply replacing one, two or even all three of the hydrogen atoms on a molecule of ammonia. Amines are classified as primary, secondary or tertiary depending on how many of the hydrogen atoms on an ammonia molecule have been replaced.

Homologous series Name Functional group
amines suffix -amine
prefix -amino
R-NH2

Primary amines are formed when one hydrogen atom on an ammonia molecule is replaced by an alkyl group. Naming amines is also straightforward. The primary amine, methyamine is shown below. It is simply named by placing the suffix- amine after the name of the alkyl group. Secondary amines are similarily formed by replacing two hydrogen atoms on an ammonia molecule by two alkyl groups. Diethylamine is a secondary amime, shown below. In this example both the alkyl groups are the same but this need not be the case. The tertiary amine trimethylamine is also shown below. Here all the hydrogen atoms on the ammonia have been replaced by three methyl groups, but as before the three groups could all be different, they do not need to be the same alkyl groups.

examples of substituted amines

Naming substituted secondary and tertiary amines.

You may also see a slightly different naming systems for substituted secondary and tertiary amines which have different alkyl substituents for example, in the molecule opposite the largest alkyl group is chosen as the parent name of the amine and the other substituents are named as N-substituents e.g.
In the example opposite the nitrogen atom has 3 different substituents, that is it is a tertiary amine. To name it you simply pick the largest alkyl group, in this case the propyl group (C3H7), this will be the parent or root name of the amine. The other two groups, the methyl substituents are attached directly to the nitrogen atom and are named as N-methyl substituents. Since there are two methyl groups the name will include dimethyl. This means the full name of this molecule will be N,N-dimethylpropylamine.

In the example opposite there are two methyl groups attached to the nitrogen and an ethyl group. The ethyl group being the longest will be the root name. The molecule can be named as:

  1. ethyldimethylamine
  2. or
  3. N,N-dimethylethylamine

Aldehydes

Aldehydes contain the functional group (-RCHO). This consists of a carbonyl group (-C=O) bonded to an R (alkyl or aryl) group and a hydrogen atom. The -RCHO carbon is always numbered as carbon atom number 1 in the carbon chain in an aldehyde molecule.

Homologous series Name Functional group general formula
aldehydes suffix -al RCHO CnH2nO

The first 3 members of the aldehyde homologous series are shown below:

3d models of the first three aldehydes

Aldehydes are named by simply replacing the -e on the longest alkyl chain that contains the CHO functional group with the suffix -al. The carbon atom in the CHO functional group is always numbered as carbon atom number 1 when we come to naming aldehydes, e.g. study the example below which shows a substituted aldehyde molecule.

Ketones

Ketones contain the functional group R2CO (RCOR) and they are named by replacing the -e of the corresponding alkane with -one. The longest chain selected in any molecule to be named as a ketone must contain the ketone functional group. Ketones are named in such a way as to ensure that the carbonyl carbon has the lowest possible number e.g.

Homologous series Name Functional group general formula
ketone suffix -one,
prefix -oxo
RCOR CnH2nO

Ketones have a structure which is similar in many ways to that found in aldehyde, the only difference is that the carbonyl carbon is bonded to two alkyl groups in a ketone whereas in an aldehyde it is bonded to an alkyl group and a hydrogen atom. The first 2 members of the ketone homologous series are shown below:

3d models of the first two ketones

Nitriles

Nitriles contain the functional group R-CN. Simple nitriles are named by simply adding the suffix -nitrile to the alkane root name.

Homologous series Name Functional group
nitrile suffix -nitrile
prefix - cyano
RCN

The image below shows 2 simple nitriles.

models of ethanenitrile and propanenitrile

Ethers

Ethers contain the functional group R-O-R. Simple ethers are named by simply identifying the two alkyl groups and then adding the word ether e.g. The image below shows the structure of 2 simple ether molecules.

3d models of ethers

Naming molecules which contain more than 1 functional group

Consider the molecule shown opposite. This molecule has

Order of priority
carboxylic acid
ester
nitrile
aldehyde
ketone
alcohol
amine
alkene
halogen

So how do we name this molecule? Is it named as a carboxylic acid or as an alcohol. To name it properly we just follow some simple rules. The functional groups present in a molecule can be ranked in order of priority for naming purposes. For example carboxylic acids take precedence over nitriles which take precedence over aldehydes and ketones which take precedence over alcohols which takes precedence over amines (see table opposite).
So in this molecule the carboxylic acid functional group will take priority over the alcohol, this means that the molecule will be named as a carboxylic acid with an alcohol substituent attached. Re-call that in a carboxylic acid the carbon atom in the carboxylate group is numbered as carbon atom number 1. So in this molecule the alcohol -OH group is attached to carbon atom number 2. The longest chain of carbon atoms containing the acid functional group is 3 carbon atoms long, so the acid will be propanoic acid with the alcohol substituent on carbon number 2. We will use the prefix -hydroxy to identify the alcohol group. If the molecule was to be named as an alcohol, then we would use the suffix -ol, however when it is present in a molecule with a higher priority ranking group then we use the prefix -hydroxy. So this molecule will be named as 2-hydroxypropanoic acid.

Example 2- What is the name of the molecule shown opposite?

This molecule has 2 functional groups:

The carboxylic acid group takes priority over the ketone, so the molecule will be named as a carboxylic acid with a ketone group as a substituent. The longest carbon chain containing the acid group is 5 carbon atoms long, so the root name will be pentanoic acid.
The carbonyl group (ketone) will be treated as a substituent on carbon atom number 4. The prefix oxo is used to identify a ketone with low priority. So the molecule will be called 4-oxopentanoic acid.


Example 3- What is the name of the molecule below?

If a molecule contains 2 functional groups and one of them is an alkene, C=C then from our table of priority above we can see that the alkenes group is a low priority group, indeed they are treated as substituents on the carbon chain. In the example opposite the alcohol functional group will take priority over the alkene group, this means that the molecule will be named as an alcohol using the suffix -ol. The location of the C=C group is noted by simply listing the carbon atom it is attached to, in this case it is carbon atom number 3 and using the prefix -en. The longest chain in the molecule is four carbon atoms, so the molecule will be named as a derivative of butane. The molecule opposite will be called but-3-en-1-ol

Key Points

Practice questions

Check your understanding on functional groups and naming compounds.

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