Constitutional isomers, also called structural isomers, are molecules that have the same molecular formula but different structure and different names.
Constitutional Isomers of Alkanes
Alkanes are hydrocarbons with all single bonds. Alkanes have the formula CnH2n+2.
Steps for drawing constitutional isomers of alkanes.
1. Start by drawing the longest chain of carbons.
2. Shorten the chain by one carbon and add it to the chain in all possible positions as a methyl group, creating new molecules. Please note, whenever adding the extra carbon you can not add it to the ends of the chain because that will make the same molecule you drew before. The best way to avoid drawing the same molecules is by naming the molecules you drew.
3. Shorten the chain by one more carbon and add the two extra carbons in all possible positions.
4. Continue until you do not create any new molecules.
Constitutional Isomers of C4H10
Constitutional Isomers of C5H12
Constitutional Isomers of C6H14
Constitutional Isomers Of Alkanes with Halogens
Steps for drawing constitutional isomers of alkanes with halogens (Group 17 elements such as Cl, Br, I , F).
1. Start by drawing the longest chain of carbons. Add the halogen on all possible positions and name the molecules to avoid duplicates.
2. Shorten the chain by one carbon and add it to the chain in all possible positions as a methyl group, creating new molecules. Add halogen to all the possible positions in these molecules, avoiding duplicates.
3. Shorten the chain by one more carbon and add the two extra carbons in all possible positions.
4. Continue until you do not create any new molecules.
Constitutional Isomers of C4H9Br
Constitutional Isomers Of Alkanes With an Oxygen
Follow the same steps as before but remember than an oxygen wants to have 2 bonds. It will either be attached as an OH (alcohol) or be in between two carbons as an ether (C-O-C).
Constitutional Isomers of C4H10O
Constitutional Isomers With Double Bonds And Rings
You can use molecular formula to calculate degrees of unsaturation which will tell you how many double bonds or rings the molecule has.
One degree of unsaturation means the molecule has either a ring or a double bond. Two degrees of unsaturation means the molecule has either two double bonds or two rings or a double bond and a ring or a triple bond.
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