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Introduction to Functional Groups

Key Concepts

  • Functional groups are the site of reactivity in an organic (carbon) compound.

  • The presence of a functional group gives the molecule its characteristic properties.

  • A functional group can also be referred to as a characteristic group.

  • The functional (characteristic) groups you are likely to meet will be due to:

    (i) unsaturation (the functional group is considered to be the double or triple bond)

    (ii) halogen atoms (Group 17 elements in the Periodic Table)

    (iii) oxygen atoms

    (iv) nitrogen atoms

  • The names of some typical functional groups are1:

    formula F Cl Br I -OH C=O
    O
    ||
     
    C-OH
    H
    |
     
    N-H
    name fluoro chloro bromo iodo hydroxyl
    (hydroxy)
    carbonyl carboxyl amine
    (amino)

  • Compounds can be grouped into classes on the basis of which functional group is present2:

    ClassFunctional GroupDescription
    Haloalkanes3
    (alkyl halides)
    X X = F, Cl, Br or I covalently bonded to a carbon atom.
    X replaces 1 H atom in a parent alkane molecule.

    Alcohols OH O atom covalently bonded to a hydrogen atom and to a carbon atom.
    OH group replaces 1 H atom in the parent hydrocarbon molecule.

    Aldehydes C=O O is covalently double bonded to the first carbon atom in the chain.
    O replaces 2 H atoms on the terminal carbon atom.

    Ketones C=O O is covalently double bonded to any carbon atom that is NOT at the end of a chain.
    O replaces 2 H atoms on a non-terminal carbon atom.

    Carboxylic Acids
    O
    ||
     
    C-OH
    An oxygen atom is covalently double bonded to a carbon atom at the end of a chain that is also bonded to an oxygen atom which is itself bonded to a hydrogen atom.
    O replaces 2 H atoms on a terminal carbon atom, and OH replaces 1 H atom on the same terminal carbon atom.

    Amines
    H
    |
     
    N-H
    A nitrogen atom is covalently bonded to a carbon atom.
    N replaces 1 H atom on a carbon atom. The nitrogen atom can then make 2 more bonds, either with H atoms as shown here, or with other C atoms.

  • The presence of a halogen functional group modifies the prefix of the parent hydrocarbon:

    prefixname of parent hydrocarbon
    haloalkane

  • The presence of the other functional groups above modifies the suffix of the parent hydrocarbon:

    name of parent hydrocarbonsuffix

    Examples of suffixes are:

    ClassFunctional GroupSuffix
    Alkenes C=C ene

    Alkynes C≡C yne

    Alcohols OH ol

    Aldehydes C=O al

    Ketones C=O one

    Carboxylic Acids
    O
    ||
     
    C-OH
    oic acid

    Amines
    H
    |
     
    N-H
    amine

Identifying a Functional (Characteristic) Group and Class

If you have been given the name of an organic compound:

Step 1: Break the name of the compound up into three sections:

prefixname of parent hydrocarbonsuffix

Step 2: Does the prefix include the name of any halogen elements (fluoro, chloro, bromo, iodo)?
If the answer is yes, you have located halogen functional groups, and the compound belongs to the class of compounds known as haloalkanes
If not, the molecule does not include halogen functional groups.

Step 3: Next, look at the suffix. Inspection will tell you which of the following functional groups are present and therefore which class it belongs to:

suffixeneynealoneolic acidamine
functional group presentC=CC≡CC=O
(terminal C atom)
C=O
(non-terminal C atom)
OHCOOH
(terminal C atom)
NH2
classalkenealkynealdehydeketoneOHcarboxylic acidamine

If you have been given the structure of an organic compound:

Step 1: Look for double and/or triple bonds between carbon atoms
    C=C is the functional group for the class of compounds known as alkenes
    C≡C is the functional group for the class of compounds known as alkynes

Step 2: Look for halogen atoms that could be replacing hydrogen atoms along the carbon chain
    halogen atoms are the functional groups for the class of compounds known as haloalkanes

Step 3: Look for a nitrogen atom
    nitrogen atom bonded to 1 or more carbon atoms is the functional group for the class of compounds known as amines

Step 4: Look for oxygen atoms:

Is the oxygen atom double bonded to a carbon atom?
 
Yes
Is there also an OH group on the same C atom?
  No
Is the oxygen atom also bonded to a H atom?
     
Yes
functional group is COOH
class is carboxylic acid
  No
Is the O atom bonded to a terminal carbon atom?
    Yes
functional group is OH
class is alcohol
         
    Yes
functional group is C=O
class is aldehyde
  No
functional group is C=O
class is ketone
   

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Examples

Each example below is based on a carbon chain containing 4 carbon atoms so each example can be thought of as a modified butane molecule:

butane   H- H
|
C
|
H
- H
|
C
|
H
- H
|
C
|
H
- H
|
C
|
H
-H

Class Structural Formula Functional Group Preferred IUPAC Name
(functional class name)4
Haloalkane
(alkyl halide)
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - Cl
  |
H
  |
H
  |
H
  |
H
 
- Cl
(halogen)
1-chlorobutane
(butyl chloride)

Alkene
  H
|
      H
|
  H
|
 
  C = C - C - C -H
  |
H
  |
H
  |
H
  |
H
 
C=C
(double bond)
but-1-ene
1-butene

Alkyne
          H
|
  H
|
 
  H - C C - C - C -H
          |
H
  |
H
 
C ≡ C
(triple bond)
but-1-yne
1-butyne

Aldehyde
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C = O
  |
H
  |
H
  |
H
     
- CO
(carbonyl group)
butanal
(butyraldehyde)

Ketone
  H
|
  H
|
  O
||
  H
|
 
H - C - C - C - C - H
  |
H
  |
H
      |
H
 
- CO
(carbonyl group)
butan-2-one
2-butanone
(ethyl methyl ketone)

Alcohol
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - OH
  |
H
  |
H
  |
H
  |
H
 
- OH
(hydroxyl group)
butan-1-ol
1-butanol
(butyl alcohol)

Carboxylic acid
  H
|
  H
|
  H
|
  O
||
 
H - C - C - C - C - OH
  |
H
  |
H
  |
H
     
- COOH
(carboxyl group)
butanoic acid
(butyric acid)

Amine
  H
|
  H
|
  H
|
  H
|
  H
|
H - C - C - C - C - N
  |
H
  |
H
  |
H
  |
H
  |
H
- NH2
(amine group)
butan-1-amine
(butylamine)


What would you like to do now?

1There are other functional groups that you might meet including:
    esters (R-CO-O-R')
    amides (R-CO-NH2)
    acid anhydrides (R-CO-O-CO-R')
    ethers (R-O-R')

2It is possible for more than one functional group to be present, but for this introduction to functional groups we will assume only one functional group will be present on a molecule.

3It is possible for halogen atoms to substitute for hydrogen atoms on other parent hydrocarbons such as alkenes and alkynes, but because we are restricting this discussion to the case when only one functional group is present if a halogen atom is present then the parent hydrocarbon must be saturated, that is, an alkane.

4There is more than one way to name organic compounds. In general, the IUPAC preferred name is either a substitutive name or a traditional (trivial) name, but there are exceptions (as in the cases of esters, anhydrides and polymers).
For a discussion of the general principles of substitutive IUPAC nomenclature go to Introduction to Naming Organic Compounds

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