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4.2: How to test for chirality

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    225779
  • You will often be asked to look at a chemical structure, and determine if it is chiral or not. Don’t be fooled by the fact that a structure may be drawn with wedged and dashed bonds!  In cases where you have only one carbon with four different substituents – thus one chiral carbon – then it is clearly a chiral molecule.  If you have more than one chiral center, then there are some useful ways to check quickly to see if a molecule is chiral:

    Tests for chirality

    • (a) (Most reliable) Check to see if the molecule has a pair of non-superimposable mirror image forms.
    • (b) (Nearly always works) Look for a mirror plane; if no mirror plane, the molecule is nearly always chiral
    • (c) Look for chiral centers – tetrahedral atoms (usually carbon) with four different groups attached. As long as these don’t mirror each other, the molecule is chiral.

    Method (a) will always give the correct answer, but it may be difficult to do quickly unless you have a model of both enantiomers to work with.

    The second method (b) is very useful as a quick test, and in this course you will never come across molecules where this test leads to a wrong answer.  However, you should be careful to ensure that you are looking at the most symmetrical form you can draw or make, otherwise you may be misled.

    Method (c) is quick and very helpful for almost every case you will see in this class.  As with (b), you will need to take care to try and make the molecule as symmetrical as possible.  If you have a meso form, the two chiral centers will be reflections of each other and the molecule will (overall) be achiral.

    There are some special cases:

    • Uncharged nitrogens with a lone pair are achiral, but an N+ with four different attached groups will be chiral.
    • When looking at cyclohexanes, you can treat them as flat, and look for symmetry.

     

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