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 with each other and the basic properties of these functional groups and you will
quickly master organic chemistry.
Aliphatic compounds are organic compounds that contain carbon and hydrogen atoms connected in straight chains, branched chain and rings (but NOT aromatic rings). 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.
| Alkane | Number of carbon atoms | Molecular Formula | Structure |
|---|---|---|---|
| methane | 1 | CH4 |  |
| ethane | 2 | C2H6 |  |
| propane | 3 | C3H8 |  |
| butane | 4 | C4H10 |  |
The alkane family is probably one of the first homologous series you learned about in chemistry, however there are many more; some of these are shown below:
| Homologous series | Name | Functional group | general formula |
|---|---|---|---|
| alkenes | suffix -ene |  |
CnH2n |
The first 3 members of the alkene homologous series are shown below:
The alkene are all unsaturated hydrocarbons which means that they contain a carbon carbon double bond (C=C), which is the reactive group or functional group present in all alkene molecules. The alkenes 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 pent-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. This is shown in the image opposite.
The halogenalkanes are another homologous series you may have met before. They are formed by replacing one or more of the hydrogen atoms in 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 simple halogenalkane molecules are shown below:
If more than one halogen is present then the positions and names are listed alphabetically, recall that when naming compounds in the names if you insert numbers into the name then use commas between numbers and hyphens are used between numbers and letters e.g.
The alcohols are a family of organic compounds which all contain the hydroxyl functional group (R-OH). The first 3 members of this homologous series are shown in the image below:
| Homologous series | Name | Functional group | general formula |
|---|---|---|---|
| alcohols | suffix -ol prefix -hydroxy |
C-OH | CnH2n+1 OH |
Alcohols are named as being derived from alkanes. To name an alcohol:
Carboxylic acids are a family of weak acids which contain the carboxyl 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:
Carboxylic acids all contain the carboxyl functional group (-COOH), they are named as being derived from alkanes, to name a carboxylic acid simply find the longest carbon chain containing the carboxyl functional group and simply replace the -e ending from the corresponding alkane with -oic for the carboxylic acid.
For naming purposes the carboxyl group is a high priority group and the carbon atom in the carboxyl group will always be carbon atom number 1 in the longest carbon chain in the molecule e.g. two examples of substituted carboxylic acid molecules are shown below.
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 amines depending
on how many of the hydrogen atoms in 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 in 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 similarly 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 molecule 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.
You may also see a slightly different naming systems for more complex amines and substituted secondary and tertiary amines which have different alkyl substituents attached; 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 attached to it; that is it is a tertiary amine. To name it you
simply pick the largest alkyl group which in this case is the propyl group (C3H7); this will form 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, where the N indicates they are attached directly to the nitrogen atom. 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 second example opposite there are two methyl groups attached to the nitrogen atom and an ethyl group. The ethyl group having the longest carbon chain will form the root name. The molecule can be named as:
Aldehydes are another family of organic molecules that 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:
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 contain the functional group R2CO (RCOR) and they are named by replacing the -e of the corresponding alkane with -one. The longest carbon 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:
Nitriles contain the functional group R-CN. Simple nitriles are named by 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.
Ethers contain the functional group R-O-R. Simple ethers are named by identifying the two alkyl groups and then adding the word ether e.g. the image below shows the structure of 2 simple ether molecules.
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 take precedence over amines, the table opposite lists the order of priority when it comes to ranking functional groups.
So in this particular 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. Recall that in a carboxylic acid
the carbon atom present in the carboxyl functional group is numbered as carbon atom number 1. So in this molecule the
alcohol -OH group is
attached to carbon atom number 2. Now the longest chain of carbon atoms containing the carboxylic acid
functional group is 3 carbon atoms long, so
the carboxlyic acid will be propanoic acid with the alcohol substituent on
carbon number 2. The
prefix -hydroxy to identify the
alcohol group (R-OH) if it is named as a low priority group in a molecule (see the section above on alcohols). 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.
This molecule has two functional groups present:
The molecule shown opposite contains two functional groups; an alcohol hydroxyl group and an alkene (C=C) unsaturated group. From the table of priority above we can see that the alkenes group is a low priority group, indeed alkenes 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 carbon chain in the molecule is four carbon atoms so the molecule will be named as a derivative of butanol. The molecule opposite will be called but-3-en-1-ol