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Introduction to Functional Groups in Organic Chemistry Tutorial

Key Concepts

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Identifying a Functional Group (Characteristic Group) and Class

  1. 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
    classalkenealkynealdehydeketonealcoholcarboxylic acidamine

  2. 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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Worked Example of Identifying Functional Groups and Classifying Organic Molecules

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

Butane is the parent hydrocarbon.

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)

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(1) There 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')

(2) It 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.

(3) It 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.

(4) There 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