What is an Asymmetric Carbon? - Definition, Identification & Examples

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  • 0:04 All About Symmetry
  • 0:52 Definition of…
  • 1:14 Identification
  • 2:21 Examples
  • 3:27 Lesson Summary
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Lesson Transcript
Instructor: Korry Barnes

Korry has a Ph.D. in organic chemistry and teaches college chemistry courses.

The goal of this lesson will be to learn how to identify an asymmetric carbon by discussing their definition and carefully studying some specific examples of compounds that contain asymmetric carbons.

All About Symmetry

When you hear the word symmetry, what's the first thing that comes to mind? The idea of symmetry is that if we cut the object of interest in half, we end up with two equal halves. Did you know that the human body has an element of symmetry to it? If you were to imagine cutting yourself in half length-wise, you would end up with two equal halves of yourself. Another way of stating this would be that your body contains a mirror plane running longitudinally.

Did you know that sometimes organic molecules can have elements of symmetry, too? A molecule is symmetrical if you can divide it in half some way and end up with two halves that are the same. In this lesson, we're actually going to be focusing on the opposite of symmetry, a concept called asymmetry. Specifically, we're going to be learning about what's called an asymmetric carbon atom within an organic compound.

Definition of Asymmetric Carbon

An asymmetric carbon atom is defined as a carbon within an organic compound that contains four different atoms or groups of atoms (substituents) bonded to it. As an example, consider a carbon atom that's bonded to an -OH group, a hydrogen, a fluorine, and a bromine. Notice how the carbon is bonded to four different substituents, making it an asymmetric carbon.


An asymmetric carbon atom is bonded to four different substituents
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Identification

So how we can identify an asymmetric carbon within an organic compound? We need to emphasize again that the carbon must have four different substituents bonded to it. This means that carbons with double and triple bonds can never be labeled as asymmetric carbons because they only have three and two total bonds, respectively.

What about the compound 2-chloroethanol? Do you think this compound would have any asymmetric carbon atoms? It actually would not, because both carbon atoms contain two hydrogen atoms each. Since both of the carbons contain two of the same substituents, they cannot be classified as asymmetric carbons.


2-Chloroethanol does not contain any asymmetric carbon atoms
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What about 1-chloro-2-methylpropane? Do you think this compound contains any asymmetric carbon atoms? Look carefully at each individual carbon. While at first glance, it may seem there are asymmetric carbons, this molecule actually does not have any either. The carbon bonded to the chlorine is also bonded to two hydrogen atoms, making it not an asymmetric carbon. The carbon bonded to the two methyl groups is also not asymmetrical since the two methyl groups are identical, so that carbon doesn't have four different substituents either.


1-Chloro-2-methylpropane does not contain any asymmetric carbon atoms
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