Constitutional isomers

What isomers are

• Isomers have the same molecular formula but a different arrangement of atoms.
• They are different substances, with different melting points, boiling points and sometimes different reactions.
• Level 2 covers two kinds: constitutional isomers (this page) and geometric isomers (next page).

Constitutional isomers

• Constitutional isomers (also called structural isomers) have their atoms joined together in a different order.
• Same molecular formula, different structural formula.

There are two ways the order can differ, and questions use both:

Different carbon skeletons

• The chain can be straight or branched.
• $C_5H_{12}$ gives three compounds:
  • pentane — an unbranched chain of five carbons.
  • 2-methylbutane — a four-carbon chain with a methyl branch on carbon 2.
  • 2,2-dimethylpropane — a three-carbon chain with two methyl branches on carbon 2.
• All three have exactly 5 C and 12 H. They are different compounds — pentane boils at 36 °C, 2,2-dimethylpropane at 9.5 °C.

Different positions for the functional group

• The functional group can sit on a different carbon.
• $C_3H_8O$ gives two alcohols:
  • propan-1-ol — –OH on an end carbon (primary).
  • propan-2-ol — –OH on the middle carbon (secondary).
• These behave differently: propan-1-ol oxidises to a carboxylic acid, propan-2-ol to a ketone.

Finding all the isomers of a formula

Work through it systematically or you will miss one:

• Draw the longest straight chain first.
• Shorten the chain by one carbon and put the spare carbon on as a methyl branch — try every position on the chain.
• Shorten again and place two branches — again try every position.
• Move the functional group along each skeleton in turn.
• Check for duplicates: two drawings are the same compound if you can get from one to the other by rotating or flipping. Name each structure, and if two share a name they are the same compound.

For $C_4H_{10}$, this gives just two: butane and 2-methylpropane. There is no "3-methylpropane" — numbering from the other end makes it 2-methylpropane, the same molecule.