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Organic Chemistry

5.2 The Reason for Handedness in Molecules: Chirality

Organic Chemistry5.2 The Reason for Handedness in Molecules: Chirality

5.2 • The Reason for Handedness in Molecules: Chirality

A molecule that is not identical to its mirror image is said to be chiral (ky-ral, from the Greek cheir, meaning “hand”). You can’t take a chiral molecule and its enantiomer and place one on the other so that all atoms coincide.

How can you predict whether a given molecule is or is not chiral? A molecule is not chiral if it has a plane of symmetry. A plane of symmetry is a plane that cuts through the middle of a molecule (or any object) in such a way that one half of the molecule or object is a mirror image of the other half. A coffee mug, for example, has a plane of symmetry. If you were to cut the coffee mug in half from top to bottom, one half would be a mirror image of the other half. A hand, however, does not have a plane of symmetry. One “half” of a hand is not a mirror image of the other half (Figure 5.4).

Mirror shows a coffee mug with a symmetry line passing through the handle. To the right, mirror shows a hand with a symmetry line next to the middle finger.
Figure 5.4 The meaning of symmetry plane. (a) An object like the coffee mug has a symmetry plane cutting through it so that right and left halves are mirror images. (b) An object like a hand has no symmetry plane; the right “half” of a hand is not a mirror image of the left half.

A molecule that has a plane of symmetry in any conformation must be identical to its mirror image and must be nonchiral, or achiral. Thus, propanoic acid, CH3CH2CO2H, has a plane of symmetry when lined up as shown in Figure 5.5 and is achiral, while lactic acid, CH3CH(OH)CO2H, has no plane of symmetry in any conformation and is chiral.

Condensed formulas and structures along with ball-and-stick models of propanoic acid (achiral) and lactic acid (chiral). Propanoic acid has a symmetry plane. Lactic acid does not have a symmetry plane.
Figure 5.5 The achiral propanoic acid molecule versus the chiral lactic acid molecule. Propanoic acid has a plane of symmetry that makes one side of the molecule a mirror image of the other. Lactic acid has no such symmetry plane.

The most common, although not the only, cause of chirality in organic molecules is the presence of a tetrahedral carbon atom bonded to four different groups—for example, the central carbon atom in lactic acid. Such carbons are referred to as chirality centers, although other terms such as stereocenter, asymmetric center, and stereogenic center have also been used. Note that chirality is a property of the entire molecule, whereas a chirality center is the cause of chirality.

Detecting a chirality center in a complex molecule takes practice because it’s not always immediately apparent whether four different groups are bonded to a given carbon. The differences don’t necessarily appear right next to the chirality center. For example, 5-bromodecane is a chiral molecule because four different groups are bonded to C5, the chirality center (marked with an asterisk). A butyl substituent is similar to a pentyl substituent, but it isn’t identical. The difference isn’t apparent until looking four carbon atoms away from the chirality center, but there’s still a difference.

The condensed structure of 5-bromodecane (chiral). Substitutes on carbon 5 are H, Br, butyl, and pentyl and the chiral carbon is marked by an asterisk.

As other possible examples, look at methylcyclohexane and 2-methylcyclohexanone. Methylcyclohexane is achiral because no carbon atom in the molecule is bonded to four different groups. You can immediately eliminate all −CH2− carbons and the −CH3 carbon from consideration, but what about C1 on the ring? The C1 carbon atom is bonded to a −CH3 group, to an −H atom, and to C2 and C6 of the ring. Carbons 2 and 6 are equivalent, however, as are carbons 3 and 5. Thus, the C6–C5–C4 “substituent” is equivalent to the C2–C3–C4 substituent, and methylcyclohexane is achiral. Another way of reaching the same conclusion is to realize that methylcyclohexane has a symmetry plane, which passes through the methyl group and through C1 and C4 of the ring.

The situation is different for 2-methylcyclohexanone. 2-Methylcyclohexanone has no symmetry plane and is chiral because its C2 is bonded to four different groups: a −CH3 group, an −H atom, a −COCH2− ring bond (C1), and a −CH2CH2− ring bond (C3).

The ball-and-stick model and structures of methylcyclohexane (achiral) and 2-methylcyclohexanone (chiral). An asterisk denotes the chiral center and achiral molecule has a symmetry plane through C1, C4, and methyl carbon.

Several more examples of chiral molecules are shown below. Check for yourself that the labeled carbons are chirality centers. You might note that carbons in −CH2−, −CH3, C═OC═O, C═CC═C, and C≡CC≡C groups can’t be chirality centers. (Why not?)

The bond-line structures of carvone (spearmint oil) and nootkatone (grapefruit oil) with one and three carbons labeled with asterisks, respectively.

Worked Example 5.1

Drawing the Three-Dimensional Structure of a Chiral Molecule

Draw the structure of a chiral alcohol.


An alcohol is a compound that contains the −OH functional group. To make an alcohol chiral, we need to have four different groups bonded to a single carbon atom, say −H, −OH, −CH3, and −CH2CH3.


The structure of 2-butanol (chiral). C2 is labeled with an asterisk.
Problem 5-1

Which of the following objects are chiral?

(a) Soda can (b) Screwdriver (c) Screw (d) Shoe

Problem 5-2
Which of the following molecules are chiral? Identify the chirality center(s) in each.
The structure of coniine (poison hemlock), in which N is in first position. C2 is bonded to a propyl group.
The wedge-dash structure of menthol (flavoring agent).
The wedge-dash structure of dextromethorphan (cough suppressant).
Problem 5-3

Alanine, an amino acid found in proteins, is chiral. Draw the two enantiomers of alanine using the standard convention of solid, wedged, and dashed lines.

Alanine has a 3-carbon chain. C1 is a carboxylic acid group. C2 is bonded to an amino group.
Problem 5-4
Identify the chirality centers in the following molecules (gray = H, black = C, red = O, green = Cl, yellow-green = F):
The ball-and-stick model of threose (a sugar) where gray, black, and red spheres represent hydrogen, carbon, and oxygen, respectively.
The ball-and-stick model of enflurane (an anesthetic) where gray, black, green, yellow-green, and red spheres represent hydrogen, carbon, chlorine, fluorine, and oxygen, respectively.
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