Optical Isomerism

Isomerism definition

Before we explore exactly what optical isomerism is, let’s recap isomerism in general.

There are two categories of isomerism that we are going to look at today:

• Structural isomerism
• Stereoisomerism

Structural isomerism

Structural isomers are molecules with the same molecular formula but different structural formulae.

For example, propane has the molecular formula C₃H₈ but the structural formula CH₃CH₂CH₃. If we take a look at its bonding, we can see how the structural formula is derived: propane contains a carbon atom attached to three hydrogens (CH₃), joined to a carbon atom with two hydrogens (CH₂), joined to another carbon with three hydrogens (CH₃).

Propane, colour-coded so you can see how the molecule relates to its structural formula. StudySmarter Originals

Structural isomerism can be separated into three further categories.

• Chain isomers have different arrangements of carbon atoms in the skeleton of the molecule.
• Position isomers have their functional group attached to a different place on the carbon chain.
• Functional group isomers have different functional groups.

We won’t look at these isomers any further now, but if you want to explore them in more depth, check out Isomerism.

Stereoisomerism

Stereoisomers have the same molecular and structural formulae but different arrangements of atoms in space. Although they have the same functional groups and carbon chains, their bonding is arranged slightly differently.

Again, there are different types of stereoisomerism.

• Geometric isomers have different arrangements of atoms around a C=C double bond.
• Optical isomers are molecules that are non-superimposable mirror images of each other.

You can learn more about geometric isomers in Alkenes. However, you might not be familiar with optical isomers. Let’s take a look at them down below.

Optical isomerism definition

Optical isomerism sounds a lot more confusing than it actually is. As we mentioned earlier, your shoes show optical isomerism. So do gloves. They are mirror images of each other, but no matter how you rotate them, you will never be able to get one to match exactly on top of another. We can say that they are non-superimposable. Try it yourself to see. This is why you cannot wear a shoe on the wrong foot - it just doesn’t fit.

A pair of shoes - an example of optical isomers. StudySmarter Originals

Optical isomerism conditions

Optical isomerism is caused by molecules with a carbon atom joined to four different groups. We call this particular carbon atom a chiral centre or an asymmetric carbon. It is often indicated by an asterisk, *. The two different isomers are known as enantiomers.

Take bromochlorofluoromethane, for example. It contains one carbon atom attached to four different groups: a bromine atom, chlorine atom, fluorine atom, and hydrogen atom. As a result, the carbon atom is an example of a chiral centre. Try drawing this molecule out using different colour circles to represent the different groups. You should get two different arrangements of atoms around the central carbon atom. Like your shoes, these molecules are mirror images of each other. No matter which way you rotate the second molecule, it will never match exactly on top of the first molecule, so these molecules are non-superimposable. They are enantiomers.

Two enantiomers. These molecules are mirror images of each other. StudySmarter Originals

When optically active molecules are made in a chemical reaction, a 50:50 mixture of the two enantiomers is formed. This is known as a racemic mixture, or a racemate.

Properties of enantiomers

Enantiomers are almost identical. They contain the same atoms, the same carbon backbone, the same functional groups and the same angles between bonds. Therefore, they have the same chemical and physical properties. However, there are two exceptions:

• The effect of enantiomers on plane-polarised light.
• The reaction of enantiomers with other chiral molecules.

Plane-polarised light

Light normally vibrates in all directions perpendicular to its direction of travel, but if we pass it through a special filter called a polaroid, only vibrations in a certain direction are allowed through. We can measure the angle of the vibrations using a device called a polarimeter.

Light before and after being passed through a polaroid filter. It now only vibrates in one direction, in this case vertically. We say that it is plane-polarised. StudySmarter Originals

If you shine plane-polarised light through one enantiomer, the light will rotate in a certain direction. If you pass the light through the other enantiomer, the light will rotate in the opposite direction. We call the enantiomer that rotates light clockwise the (+) enantiomer, and the other molecule the (-) enantiomer.

As you now know, a racemic mixture contains an exactly 50:50 mixture of two optical isomers. Therefore, a racemic mixture doesn’t rotate plane-polarised light at all. This is because the clockwise rotation caused by the (+) enantiomer is completely cancelled out by the anticlockwise rotation caused by the (-) enantiomer.

The effect of enantiomers on plane-polarised light. The (+) enantiomer rotates light clockwise and the (-) enantiomer anticlockwise, whilst a racemic mixture has no effect. StudySmarter Originals

Reaction with chiral molecules

Enantiomers react with sensors in our bodies in different ways. This is because many of our proteins, enzymes and receptors also show chirality.

Reactions occur in the body when two molecules bind together. The molecules need to be a certain shape to ‘fit’ with one another. Because enantiomers have different 3D arrangements of atoms, only one will have the right shape to fit the receptor or enzyme. Enantiomers may be mirror images of each other, but they're not the same; it makes sense that only one of them will bind to a certain molecule.

For example, amino acids show optical isomerism. In fact, all natural amino acids are (-) enantiomers. The (+) forms are not found in nature because they are simply the wrong shape - they're not compatible with the rest of organic life.

Optical isomerism examples

Now we’ve learnt what optical isomerism is, we can practice spotting examples of it in molecules. We'll then learn how to draw the two enantiomers.

Take a look at the following molecule.

1-chloroethanol. StudySmarter Originals

The leftmost carbon atom, circled in red, is joined to four different groups. Therefore, it must be a chiral centre.

To draw the two different enantiomers of this molecule, first draw a carbon atom with four single bonds in a tetrahedral arrangement.

The bond arrangement of a tetrahedral molecule. StudySmarter Originals

Pick a group to attach to the top bond and join the rest of the groups to the other three bonds.

An enantiomer of 1-chloroethanol. StudySmarter Originals

Now, take your molecule and flip it along an imaginary vertical mirror line. Keep the same group at the top but reverse the bonding of the other three groups.

The second enantiomer of 1-chloroethanol. StudySmarter Originals

Have a go at spotting the asymmetric carbon in the following molecule, butan-2-ol.

Butan-2-ol. StudySmarter Originals

Counting from the left, carbon 2 has four different groups attached to it. It must therefore be an asymmetric carbon or chiral centre and show optical isomerism. Its two enantiomers are shown below.

The two enantiomers of butan-2-ol. StudySmarter Originals

Uses of optical isomerism

Finally, let's look at the uses of optical isomerism. In particular, optical isomerism gives us clues about the type of mechanism used by a reaction.

Consider Nucleophilic Substitution Reactions of Halogenoalkanes. You might know that primary halogenoalkanes react using an SN₂ mechanism. On the other hand, tertiary halogenoalkanes react using an SN₁ mechanism. The SN₂ mechanism produces just one product, whilst the SN₁ mechanism produces a mixture of two optically-active enantiomers. If the products of a particular nucleophilic substitution reaction are optical isomers, then we can predict that the reaction used an SN₁ mechanism.

Let’s look at both mechanisms in more detail.

SN2 mechanism

The number 2 in SN₂ tells us that the reaction’s initial step involves two species. In this case, it involves both the nucleophile and the reacting halogenoalkane. This mechanism results in just one product - we don't form optical isomers. Here’s what happens.

1. The nucleophile attacks the halogenoalkanes’s δ+ carbon atom at the same time as the C-X bond breaks.

2. The C-X bond, with its δ- halogen atom, repels the electron-rich nucleophile and means the nucleophile can only attack from the opposite side of the halogenoalkane.

3. As the C-X bond breaks and the C-Nu bond forms, the nucleophile repels the other bonding groups and causes the bonds to invert.

4. Overall, this reaction produces a molecule with one specific structure: an enantiopure substance. The bonds in this product are inverted compared to the starting molecule.

The mechanism for an SN2 nucleophilic substitution reaction. StudySmarter Originals

SN₁ mechanism

The number 1 in SN₁ tells us that this reaction’s initial step involves just one species: the halogenoalkane. If you start with an optically-active product, the SN₂ mechanism results in a racemic mixture of two enantiomers - in other words, we end up with optical isomerism. Here’s how the mechanism works.

1. First, the C-X bond breaks and the halogen leaves the halogenoalkane as a halide ion.

2. The remaining three bonding groups spread out as far apart as possible around the positive central carbon atom, forming a trigonal planar arrangement.

3. The nucleophile now attacks the positive central carbon atom. It can attack from either above or below the plane.

4. The C-Nu bond forms and repels the other bonding groups, pushing them into a tetrahedral arrangement. If the nucleophile attacked from the front, we get one particular arrangement of bonds, but if the nucleophile attacked from the back, we get a different arrangement.

5. The overall process produces a racemic mixture of two mirror-image, optically active enantiomers.

The mechanism for an SN1 nucleophilic substitution reaction. StudySmarter Originals