The importance of functional groups

More functional groups

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:

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:

Naming alkenes

The alkenes are all unsaturated hydrocarbons, which means that they contain at least one carbon–carbon double bond (C=C). This C=C bond is the reactive group, or functional group, present in all alkene molecules.

Alkenes are named by first identifying the longest carbon chain that contains the C=C functional group. The name of this parent chain forms the root of the compound and the suffix -ene is added to show the presence of a carbon–carbon double bond.

In many cases the position of the C=C bond must also be specified. This is done by numbering the carbon atoms in the longest carbon chain so that the double bond is given the lowest possible number. The number used in the name tells you where the C=C functional group starts along the chain.

For example in the alkene pent-2-ene (shown opposite) the number 2 shows that the carbon–carbon double bond begins at the second carbon atom in the five-carbon chain. This method ensures that different alkene molecules can be named clearly and unambiguously.

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 in an alkane by a halogen.

Halogenalkanes are named in a similar way to the alkanes:

• As with naming the alkanes select the longest chain of carbon atoms you can find, note that it does not need to contain the halogen atom. Halogen atoms are low priority groups and are ranked very low. This means that if other functional groups are present in a molecule then they will take priority over the halogen; so for example if an organic molecule contains the two functional groups; let's say a C=C and a halogen do you name the molecule as an alkene or a halogenalkane; well the alkene group is considered a higher priority group than the halogen and so the molecule would be named as a halogen substituted alkene. The order of priority of the various functional groups is listed in the table opposite.
• Number the parent chain beginning at the end nearest the first substituent, whether it is a halogen or an alkyl group.
• If more than one halogen is present then number each using the prefixes di, tri, tetra etc.
• If different halogens are present then number each according to its position on the longest parent chain and list all substituents in alphabetical order. It is also worth considering the fact that the halogens are a very low priority group when it comes to naming compounds.
• If two different ways of numbering the carbon chain give the same set of locants; the locant is simply the numbered carbon atom in the chain to which the substituent is attached; then alphabetical order is used as a tie-breaker. List the substituents in alphabetical order (ignoring di-, tri-, tetra-), then compare their locants in that order; the correct numbering is the one in which the first alphabetical substituent has the lower locant. You will need to apply this rule to correctly name the molecule in the halogenalkane naming activity in the self check activity below.

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 if you insert numbers into the name then use commas between numbers and hyphens are used between numbers and letters e.g.

Self-Check

Use the information you have learned so far to name the halogenalkane below; especially those in the 5 bullet points above; simply click the button below to start.

Study the halogenalkane shown below and answer each step carefully, exactly as you would in an exam.

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Step 1 – Identify the parent chain

What is the name of the longest continuous carbon chain?

Show answer

The longest chain contains four carbon atoms, so the parent alkane is butane.

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Step 2 – Identify the substituents

Which halogen atoms are present in this molecule?

Show answer

Chlorine, bromine and iodine atoms are present.

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Step 3 – Number the carbon chain

From which end should the chain be numbered?

• A) From the left-hand end
• B) From the right-hand end
• C) From the end that gives the lowest set of numbers

Show answer

The chain is numbered to give the substituents the lowest possible set of locants or numbers.

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Step 4 – Write the name

Write the full IUPAC name, listing substituents in alphabetical order.

Show answer

2-bromo-3,4-dichloro-1-iodobut-2-ene

Exam tip: use commas between numbers, hyphens between numbers and words, and list substituents alphabetically (ignoring di-, tri-).

The alcohols

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:

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

• Select the longest carbon chain containing the hydroxyl functional group (C-OH), then simply replace the -e ending of the alkane with -ol for the alcohol.
• Number any substituents beginning at the end nearest the hydroxyl functional group.
• As usual all substituents are listed alphabetically.

Alcohols are a fairly low-priority functional group, hey are fairly close to the bottom in the ranking table above. If the hydroxyl (–OH) functional group is the highest-priority group present, then the molecule is named as an alcohol and its name simply ends in –ol. However, if a higher-priority functional group is also present, the hydroxy group (–OH) group is treated as a substituent and is identified using the prefix –hydroxy instead. This is explored in more detail later in this page.

Self-check: Naming simple alcohols

For each molecule below, identify the –OH group, choose the longest carbon chain containing –OH, number the chain correctly, then name the molecule.

Show answer

Correct name: propan-1-ol

• Longest chain has 3 carbon atoms → propan-
• –OH group on carbon 1 → -1-ol

Show answer

Correct name: propan-2-ol

• Longest chain has 3 carbon atoms → propan-
• –OH group on carbon 2 → -2-ol

Show answer

Correct name: 1-bromo-1-chloropropan-2-ol

• –OH has priority over halogens → name as an alcohol (suffix -ol)
• Longest chain containing –OH has 3 carbons → propan-
• Number chain to give –OH the lowest locant → -2-ol
• Both halogens on carbon 1
• List halogen substituents alphabetically: bromo then chloro

The carboxylic acids

Carboxylic acids are a family of weak acids which contain the carboxyl functional group (-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

Amines can be thought of as 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 with an alkyl group such as a methyl (-CH₃), ethyl (-C₂H₅) or a propyl (-C₃H₇) groups. Amines are classified as primary, secondary or tertiary amines depending on how many of the hydrogen atoms in an ammonia molecule have been replaced. In a primary amine one hydrogen atom in an ammonia molecule has been replaced by an alkyl group; while in a secondary amine two hydrogen atoms have been replaced by alkyl groups while in a tertiary amine all three hydrogen atoms in an ammonia molecule have been replaced by alkyl groups.

Naming amines can be a bit of a nightmare at times as there are several naming systems in use but luckily most exam boards seem to prefer the traditional naming system I have outlined below. Simple amine molecules are named by simply named by adding the suffix -amine to the name of the alkyl or aryl group; for example the molecules below can be thought of as simply an ammonia molecule where one or more of the hydrogen atoms has been replaced by an alkyl group.

N-substituted amines

If more than one alkyl group is attached to the nitrogen atom in an amine molecule then the largest alkyl group is chosen as the parent name and the other alkyl group or groups are assigned as a N-alkyl group, for example in each of the molecules shown below the largest alkyl chain is a propyl chain (C₃H₇), so this will be the parent name of the amine. The other substituent(s) are labelled as N-alkyl substituents to indicate that they are attached to the nitrogen atom:

You may come across another method used to name amines whereby instead of using the suffix -amine to name the compounds instead the prefix -amino is used; the image below gives several examples of this naming system. However most exam boards seem to prefer the traditional naming system above so I would use this unless your teacher tells you otherwise.

So for example simply use the prefix amino in front of the name of the longest alkyl chain present in the molecules below. The first two examples are primary amines while the third example is a secondary amine. Here use the same rule, identify the longest alkyl chain which will be the parent name and other substituents will be labelled as N-substituents.

Aldehydes

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.

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

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.

Ketones have a structure which is similar in many ways to that found in aldehydes, 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

Nitriles contain the functional group R-CN. Simple nitriles are named by adding the suffix -nitrile to the alkane root name. If the molecule containing the nitrile functional group also contains a functional group of higher priority then the prefix cyano is used to indicate the presence of the nitrile group.

The image below shows 2 simple nitrile molecules.

Ethers

Ethers contain the functional group R-O-R; where R is an alkyl group. To name a simple ether identify the longest carbon chain in any of the attached alkyl groups and use this as the parent alkane. The smaller alkyl group with the shorter carbon chain attached to the oxygen atom is then named as an alkoxy substituent for example:

• methane → methoxy (-OCH₃)
• ethane →ethoxy (-OC₂H₅)
• propane → propoxy (-OC₃H₇)

The alkoxy group is written before the name of the parent alkane, so for example an ether with a methoxy group attached to an ethane chain would be named methoxyethane; this is shown in the image below:

Note

An alkoxy substituent is simply a carbon chain that is attached to an oxygen atom. It is named in the same way as an alkyl group, but with the ending -yl replaced by -oxy. For example a methyl group (-CH₃) becomes a methoxy group (-OCH₃, and an ethyl group (-C₂H₅) becomes an ethoxy group (-OC₂H₅. When naming an ether, the alkoxy group is simply treated as a substituent on the longer carbon chain and is written before the name of the parent alkane.

You may also see simple ethers named using the older, common naming system where the two alkyl groups are listed alphabetically followed by the word ether, for example methylethyl ether. However the systematic name methoxyethane is the one you should use in your exams.

Naming molecules which contain more than 1 functional group

Consider the molecule shown opposite. This molecule has

• a carboxylic acid carboxyl functional group present, -COOH.
• and an alcohol hydroxyl (R-OH) functional group present.

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; we saw this earlier when naming substituted d halogenalkane molecules. 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 hydroxyl functional group, 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 hydroxyl group (-OH) 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 carboxylic acid will be propanoic acid with the alcohol substituent on carbon number 2.

Now the prefix -hydroxy is used 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.

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

This molecule has two functional groups present:

• A carboxylic acid, carboxyl group.
and
• A carbonyl ketone group.

The carbonyl group (ketone) will be treated as a substituent on carbon atom number 4. The prefix oxo is used to identify a ketone group with low priority. So the molecule will be called 4-oxopentanoic acid.

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

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 alkene group is a low priority group.

In the molecule shown opposite then the alcohol functional group, the hydroxyl group (-OH) 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 listing the carbon atom where the double bond starts, and using the infix -en- in the name. In this case the double bond starts at carbon atom number 3. The longest carbon chain in the molecule is four carbon atoms so the molecule will be named as a derivative of butan-1-ol. The molecule opposite will be called but-3-en-1-ol

Key Points

• Functional groups are the reactive parts of molecules which give them their distinctive chemical and physical properties. You should be able to identify common functional groups and name simple molecules which contain these groups.
• Many molecules have more than one functional group present. To name these molecules we rank the functional groups in order of priority and use the appropriate prefix or suffix to name these molecules.