How to find structural isomers

Written by john brennan
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How to find structural isomers
Carbon chains as seen in this molecular model often have complex structures. (Jupiterimages/BananaStock/Getty Images)

Organic chemistry is the study of carbon compounds. Since carbon can form an incredible variety of chains and rings, the structure of carbon compounds is often complex. Isomers are different versions of the same molecule that have the same number of atoms but different arrangements of those atoms or different structures. Structural isomers are a subclass of isomers that have different connectivity, for example when groups of atoms are inserted in different places. Drawing structural isomers is a common assignment in organic chemistry classes; this kind of assignment assumes you already know the basic concepts from general chemistry.

Skill level:
Moderately Challenging

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Things you need

  • Pencil
  • Paper
  • Molecular formula

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  1. 1

    Write out the molecular formula for the compound you need to draw. If you're working on this problem for a class assignment, you'll be given the molecular formula or the name of the compound in advance.

  2. 2

    Determine whether the compound in question is a cyclic or ring compound, an aromatic compound or an open chain compound. If you have the name of the compound, determine what type it is from the name; cyclohexane, for example, is a ring compound with six carbons, while hexane is an open chain compound with six carbons. Alternatively, the assignment may give you certain options you can draw -- cyclic compounds or open chain compounds that have a particular molecular formula.

  3. 3

    Determine whether the compound is an alkane, an alkene or an alkyne. Alkanes have only single-bonded carbons; alkenes have at least one double-bond and alkynes have at least one triple-bond. If you have the name of the compound determine its type from the name; alternatively, the assignment may give you specific options to choose from.

    In general, open chain alkanes have the formula C x H2 x +2 -- in other words, there will be 2 x + 2 as many hydrogens as there are carbons. If there are six carbons, for example, there will be fourteen hydrogens. Each double bond reduces the number of hydrogens by two, so a hydrocarbon with one double bond and six carbons would have 12 hydrogens. A triple bond will reduce the number of hydrogens by two again.

  4. 4

    Draw different structures that fulfil the criteria defined in the assignment. Start with the simplest structure first -- for example, if you need to draw isomers of "C6H14," start by drawing a simple unbranched six-carbon chain. Next, take parts of the chain and rearrange them.

    Take a carbon from one end of the chain and bond it to one of the carbons in the middle of the chain instead, or take an atom like a bromine atom that's attached to one part of the chain and reattach it elsewhere.

  5. 5

    Check each structure drawn to ensure it isn't just a twisted version of the original molecule. Remember that atoms can rotate around a single bond, so two carbons that are joined together by a single bond can rotate. Also remember that the molecule doesn't need to be in any particular orientation -- it could easily be flipped upside down.

    Try flipping each structure you draw in your mind to see whether it can be superimposed on any of the other structures you've already drawn. If it can, it's not really another isomer. Also try mentally rotating atoms in the chain around one of the double bonds that holds them together; if rotation around any of the bonds causes the structure to become identical to another one already drawn, the structure is a false isomer.

Tips and warnings

  • Many students find molecular model kits are extremely helpful in working on these kinds of problems. Many science supply stores or college bookstores sell molecular model kits; alternatively, you can make your own out of foam balls and toothpicks. Most professors, however, will not allow you to use a molecular model kit on the exam, so while the model is helpful in learning how to think about isomers it probably won't help you on a test.

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