Types of Structural Isomers Organic Chemistry Tutorial

Key Concepts

• Structural isomers are also known as constitutional isomers.
• Molecules that are structural isomers have:

  ⚛ same molecular formula

  ⚛ different arrangement of atoms making up the structural formula

• There are 3 types of structural isomers:

  ⚛ chain isomers (also known as skeletal isomers)

  ⚛ position isomers (also known as regioisomers)

  ⚛ functional group isomers (also known as functional isomers)

• Chain isomers (skeletal isomers) have :

  ⚛ same molecular formula

  ⚛ same functional groups (if any) in the same location

  ⚛ different branching of their carbon chains

• Position isomers have :

  ⚛ same molecular formula

  ⚛ same arrangement of branches (side-chains) on their carbon chains

  ⚛ same functional group

  ⚛ functional group in different positions in the molecule

• Functional group isomers (functional isomers) have:

  ⚛ same molecular formula

  ⚛ different functional groups

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Chain Isomers (Skeletal Isomers)

Consider an alkane molecule with the molecular formula C₄H₁₀.
We know that each carbon atom will be covalently bonded to 4 other atoms and that each hydrogen atom will be covalently bonded to just 1 other atom.
So we could draw a possible structure for the molecule as shown below:

This molecule is called butane and it is referred to as a straight chain alkane because all the carbon atoms have bonded in a single line called the carbon skeleton.

There is another molecule with the molecular formula C₄H₁₀ that we could draw:

This molecule, 2-methylpropane, is also an alkane but only 3 of the carbon atoms are joined in a chain, while a fourth carbon atom has branched-off the long chain, indeed we call this CH₃ group a branch, or a side-chain, and the molecule 2-methylpropane is referred to as a branched chain alkane.

Butane and 2-methylpropane are said to be structural isomers because they have same the molecular formula but a different arrangement of the atoms making up the structural formula. But because the only difference in the structural formula of butane and 2-methylpropane is that butane is a straight chain molecule and 2-methylpropane contains a branch, or side-chain, we refer to these 2 molecules as chain isomers, or skeletal isomers.
They are called chain isomers because they have different numbers of carbon atoms in the longest carbon chain.
Since this longest carbon chain is also known as the molecule's skeleton, these the isomers are also known as skeletal isomers.

You can produce chain isomers (skeletal isomers) for other classes of molecules besides hydrocarbons.
For example, if we substitute an hydroxyl (OH) functional group for a hydrogen atom on the first carbon atom of butane we would have the alkanol known as butan-1-ol (C₄H₁₀O):

Butan-1-ol (1-butanol) is a straight chain alkanol, all 4 carbon atoms are linked to form a single chain. There are no branches (no side-chains).

But we could also draw a different structural formula for an alkan-1-ol with the same molecular formula, C₄H₁₀O, as shown below:

Note that both butan-1-ol and 2-methylpropan-1-ol have the same molecular formula, C₄H₁₀O, and the OH functional group is located on the same carbon atom (the first carbon atom) in both molecules, BUT, butane is a straight chain alkan-1-ol while 2-methylpropan-1-ol is a branched chain alkan-1-ol, so butan-1-ol and 2-methylpropan-1-ol are chain isomers (or skeletal isomers).

2 molecles are chain isomers if they have:

1. same molecular formula
2. same functional group (if any) located in the same position
3. different numbers of carbon atoms in each branch or side-chain (0, 1, 2, etc)

Position Isomers (Regioisomers)

Let's keep thinking about alkanols with the molecular formula C₄H₁₀O.
In the section above we draw a structural formula for butan-1-ol, a straight chain alkanol in which the OH functional group is located on the first carbon atom of the chain.
In the diagram below, we have numbered the carbon atoms 1, 2, 3 and 4:

If the OH functional group was positioned on carbon 2 instead of carbon 1, we would have a different structural isomer because, although the molecular formula remains the same, the structural formula will be different:

In terms of their structural formula, the only difference between these two molecules (butan-1-ol and butan-2-ol) is the position of the OH functional group along the chain of 4 carbon atoms, so these 2 molecules are referred to as position isomers.

Similarly we could draw 2 different position isomers for the branched chain alkanols with molecular formula C₄H₁₀O (2-methylpropan-1-ol and 2-methylpropan-2-ol) as shown below:

Molecules are referred to as position isomers if they have:

• same molecular formula
• same arrangement of branches (if any)
• same functional group
• functional group in a different position along the carbon chain

Functional Group Isomers (Functional Isomers)

Let's continue thinking about molecules with the molecular formula C₄H₁₀O.
So far we have drawn 4 molecules with the same molecular formula but different structural formula.
All 4 molecules have belonged to a class of compounds known as alcohols, molecules in which an OH functional group has substituted for an H atom in the saturated parent alkane.
But does the functional group have to be OH?
Couldn't we position an oxygen atom (O) between 2 carbon molecules?
Yes we could and this would produce a molecule belonging to a different class of compounds called ethers

For example, butan-1-ol and 1-ethoxyethane are functional group isomers because they have the same molecular formula (C₄H₁₀O) but different functional groups (hydroxyl, OH, functional group in butan-1-ol and the ether, -O-, functional group in 1-ethyoxyethane):

Functional group isomers have:

• same molecular formular
• different functional groups

Worked Example of Types of Structural Isomers

Problem: Draw structural isomers of C₂H₆O and classify them as either chain isomers, position isomers, or, functional group isomers.

Solution to the problem:

(Based on the StoPGoPS approach to problem solving in chemistry)

1. What is the question asking you to do?

   Draw and classify structural isomers of C₂H₆O

2. What data (information) have you been given in the question?

   Extract the data from the question:

   molecular formula: C₂H₆O

3. What is the relationship between what you know and what you need to find out?

   (1) Structural isomers have the same molecular formula but different structural formulae (arrangement of atoms)

   (2) Chain isomers: different branching, same functional group in same location

   (3) Position isomers: different position of same functional group, same branching

   (4) Functional group isomers: different functional groups

4. Draw isomers of C₂H₆O

   Only 2 carbon atoms so only straight chain isomers are possible:

   H |   H |   HO− C(1) − C(2) −H   | H   | H   ethan-1-ol

   H |   H |   H− C(1) − C(2) −OH   | H   | H   ethan-2-ol

   But these are not structural isomers!
   If you rotate ethan-2-ol 180° in space you would have the same structural formula as ethan-1-ol.
   Put another way, if you number the carbon atoms in "ethan-2-ol" from right to left (instead of left to right as shown), then the OH functional group is attached to carbon (1) and the name of the molecule is ethan-1-ol.
   There is only one way to draw a structural formula for an alkanol with the molecular formula C₂H₆O, and the molecule is called ethanol.

   But we can draw a structural formula in which the O atom covalently bonds to 2 carbon atoms:

   	H |		H |
   H−	C	−O−	C	−H
   	| H		| H
   methoxymethane (methyl ether) (dimethyl ether)

   Ethanol and methoxymethane are structural isomers because they have the same molecular formula (C₂H₆O) but different structural formula.

5. Classify the two isomers of C₂H₆O

   Ethanol contains the hydroxyl (-OH) functional group.

   Methoxymethane contains the ether (-O-) functional group.

   Since ethanol and methoxymethane are structual isomers that have different functional groups, they are classed as functional group isomers (or functional isomers).

6. Is your answer plausible?

   Have we answered the question?

   Yes, because we have :

   ⚛ drawn two structural isomers of C₂H₆O

   ⚛ classified them as functional group isomers

   Check that these 2 isomers are in fact functional group isomers:

   Isomer 1 (ethanol)	Isomer 2 (methoxymethane)
   H |   H |   HO− C − C −H   | H   | H	H |   H |   H− C −O− C −H   | H   | H

   Are the two molecules structural isomers?

   Yes, they have:

   ⚛ same molecular formula C₂H₆O

   ⚛ different arrangement of atoms in the structural formula

   Are the two molecules functional group isomers?

   Yes, they are structural isomers with different functional groups:

   ⚛ Isomer 1 (ethanol) has an -OH functional group

   ⚛ Isomer 2 (methoxymethane) has an -O- functional group

7. State your solution to the problem "draw and classify structural isomers of C₂H₆O":

   Functional Group Isomers of C2H6O
   H |   H |   HO− C − C −H   | H   | H	H |   H |   H− C −O− C −H   | H   | H