Naming Organic Functional Groups

Functional Groups and Nomenclature
Dr. MJ Patterson

A functional group in an organic molecule is an atom or a group of atoms that replaces a hydrogen atom in a hydrocarbon.  These replacement groups are much more reactive than the hydrogen atom that was replaced.  They give the molecule its functionality, or its reactivity.

You need to memorize the names and structural formulas of the following functional groups.  The R stands for the hydrocarbon that makes up the rest of the molecule.
 
 

Name

Structure

alcohol

R - OH

acid

aldehyde

ketone

amine

Heteroatoms

Any atom other than C or H in an organic molecule is called a heteroatom.

The three most common heteroatoms are O, N and X, where X stands for any of the halogens (F, Cl, Br, I).  O will form 2 bonds, N will form 3 bonds, and X will form one bond.

Naming Alcohols

Alcohols are named just like alkanes, but with an -ol ending.  You also need to put a number in front of the name, separated with a dash, to indicate which carbon atom the -OH group is attached to.

Example 1:
Name the following compounds.
 

a.    

b.

c.

d.

Solution 1:

  1. It contains only C and H with all single bonds => alkane.  It contains 5 C atoms => pentane
  2. It is pentane with one H on the #1 C replaced by -OH => 1-pentanol
    (Since the -OH functional group bonds to the rest of the molecule through the oxygen atom, you will frequently see the alcohol functional group written backwards HO- to indicate the attachment through the O atom.)
  3. It is pentane with the H on the #2 C replaced by -OH => 2-pentanol
  4. It is pentane with the H on the #3 C replaced by -OH => 3-pentanol

Naming Aldehydes

An aldehyde is an organic molecule that has an oxygen atom doubly bonded to the terminal carbon of the backbone carbon chain.  An alcohol is named with the -al ending.  Since the C=O must be on the terminal #1 carbon atom, the position of the C=O is not specified in the name.

Example 2:
Name the following molecules.
 

a.    

b.

c.

Solution 2:

  1. There are 2 C's with a C=O on the end => ethanal
  2. There is 1 C with a C=O => methanal
    (The common name for this compound is formaldehyde.)
  3. There are 5 C's with a C=O on the end => pentanal

Naming Ketones

Ketones are very similar to aldehydes.  The only difference is that the C=O in a ketone is in the middle of a chain - not on the terminal carbon.  To name a ketone, use the -one ending and specify the position of the C=O with a number at the beginning of the name.

Example 3:
Name the following compounds.
 

a.    

b.

c.

Solution 3:

  1. a.    3 C ketone => propanone (Why do you not need to specify the position of the C=O?  If it were on either of the other 2 C atoms, it would be an aldehyde.)
    (The common name for propanone is acetone.)
  2. b.    4 C ketone => butanone (Why do you not need to specify the position of the C=O?)
  3. 5 C ketone with the C=O on the #2 carbon => 2-pentanone