Introduction to Functional Groups in Organic Chemistry Tutorial

Key Concepts

• Functional groups are the site of reactivity in an organic compound (carbon compound).
• The presence of a functional group gives the molecule its characteristic properties (physical properties and chemical properties).
• A functional group can also be referred to as a characteristic group.
• The functional groups (characteristic groups) you are likely to meet will be due to:

  (i) unsaturation
  (the functional group is considered to be the double bond, C=C, or the triple bond, C≡C)

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

  (iii) oxygen atoms

  (iv) nitrogen atoms

• The names of some typical functional groups are⁽¹⁾:

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

• Organic molecules (carbon molecules) with the same functional group belong to the same class of organic compounds.
• Organic compounds are grouped into classes on the basis of which functional group is present⁽²⁾:

  Class	Functional Group	Description
  Haloalkanes(3) (halogenoalkanes, 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.
  Nitriles	C≡N	A nitrogen atom is covalently triple bonded to a carbon atom at the end of a chain. N replaces 3 H atoms on a terminal carbon atom.

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

  prefix	name of parent hydrocarbon
  halo	alkane

  (see Naming Haloalkanes)

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

  name of parent hydrocarbon

  suffix

  Examples of suffixes are:

  Class	Functional Group	Suffix
  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
  Nitriles	C≡N	nitrile

• Organic molecules that belong to the same class of organic compounds have similar properties and undergo similar chemical reactions.

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Identifying a Functional Group (Characteristic Group) and Class

Because the properties of an organic molecule (carbon molecule) depend on its functional group, it is important to be able identify the functional group that is present in the molecule.

Because organic molecules with the same functional group have similar properties and undergo similar chemical chemical reactions, it is important to be able to identify the class of organic compounds that the organic molecule belongs to.

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

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

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 (halogenoalkanes)

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:

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 (halogenoalkanes)

Step 3: Look for a nitrogen atom

Single covalent bond between nitrogen atom and carbon atom(s): this is the functional group for the class of compounds known as amines

Triple covalent bond between nitrogen atom and carbon atom: this is the functional group for the class of compounds known as nitriles

Step 4: Look for oxygen atoms:

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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 which has the structure:

The parent hydrocarbon for each molecule below is butane.

The functional group is shown in red in each molecule in the table below.

Did you notice that:

(i) halogen atoms modify the prefix of the name of the parent hydrocarbon (_butane).

(ii) all other functional groups modify the suffix of the name of the parent hydrocarbon (but_).⁽⁵⁾