Take a look at your shoes. Stick them out in front of you and wave them around in the air. They may have a few scratches or flecks of mud on them, but apart from that, they are almost identical. They have exactly the same shape and design, but one is simply the reverse of the other. In fact, they are mirror images of each other. These shoes show something called optical isomerism.
- This article is about optical isomerism in chemistry.
- We’ll start by defining isomerism as a whole before looking specifically at optical isomerism.
- This will involve investigating chiral centres and enantiomers.
- We'll also look at the properties of enantiomers, including their role in drug design.
- After that, we'll discuss some examples of optical isomerism.
- You'll be able to practice identifying and drawing optical isomers.
- Finally, we'll end with uses of optical isomerism.
Isomerism definition
Before we explore exactly what optical isomerism is, let’s recap isomerism in general.
Isomers are molecules with the same molecular formula but different arrangements of atoms.
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.
A molecular formula shows the actual number of atoms in a molecule or compound. It is one of the simplest ways of representing a species. On the other hand, structural formulae take a molecule that bit further and show the unique arrangement of its atoms. Check out Organic Compounds for a review of different types of 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. Vaia 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 isomers are molecules that have the same structural and molecular formulae, but are non-superimposable mirror images of each other.
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. Vaia 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.
Enantiomers are pairs of molecules that exist in two optical isomer forms, which are non-superimposable mirror images of each other.
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. Vaia 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
Plane-polarised light is light that vibrates in just one direction, or plane, only.
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. Vaia 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.
At A-level, you don't need to know how to name enantiomers - just that the two names exist.
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. Vaia 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.
Enantiomers and drug design
Many common drugs are optical isomers. However, more often than not, only one of the enantiomers is useful. The second enantiomer might have no effect whatsoever, or it could be outright harmful to our health! This leads to a few challenges when it comes to drug synthesis.
As we know, optical isomers are produced in racemic mixtures, containing a 50:50 ratio of the two enantiomers. This means that we need to remove the second, perhaps harmful enantiomer before we can sell the drug. A compound containing just one enantiomer is known as enantiopure.
Even if the second enantiomer doesn't have any effect on the body, leaving it in results in a weaker drug that is only half as powerful. As a result, the dosage is higher, increasing production costs for the manufacturer.
However, separating enantiomers is a fiddly and often expensive process. One way of avoiding this is by selectively producing just one enantiomer, using a chiral catalyst. Catalysts are great because they can be used over and over again, and only a small amount is needed to facilitate a reaction.
An example of optical isomerism in drugs is the common painkiller Ibuprofen. Ibuprofen consists of two enantiomers, also known as (S+) and (R-) forms. Although both forms have identical boiling points, solubility, and other physical properties, the (R-) form has absolutely no effect on the body.
Ibuprofen. Only the (S+) enantiomer works as a painkiller in the body. Vaia Originals
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.
Remember, to show optical isomerism, a molecule must contain a chiral centre, also known as an asymmetric carbon. This is a carbon atom attached to four different groups.
Take a look at the following molecule.
1-chloroethanol. Vaia 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. Vaia 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. Vaia 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. Vaia Originals
Have a go at spotting the asymmetric carbon in the following molecule, butan-2-ol.
Butan-2-ol. Vaia 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. Vaia 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.
The nucleophile attacks the halogenoalkanes’s δ+ carbon atom at the same time as the C-X bond breaks.
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.
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.
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. Vaia 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.
First, the C-X bond breaks and the halogen leaves the halogenoalkane as a halide ion.
The remaining three bonding groups spread out as far apart as possible around the positive central carbon atom, forming a trigonal planar arrangement.
The nucleophile now attacks the positive central carbon atom. It can attack from either above or below the plane.
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.
The overall process produces a racemic mixture of two mirror-image, optically active enantiomers.
The mechanism for an SN1 nucleophilic substitution reaction. Vaia Originals
What would happen if you started an SN2 mechanism with a racemic mixture? You would actually still end up with a racemic mixture. Although each enantiomer reactant results in just one product, we start with two mirror image enantiomers, and so the reaction results in two molecules that are optical isomers of each other.
On the other hand, not all SN1 nucleophilic substitution reactions result in optical isomers. Your final products need to have a chiral centre, which you'll remember is a carbon atom joined to four different bonding groups. For example, when 2-chloro-2-methylpropane reacts in a nucleophilic substitution reaction with hydroxide ions, we end up with 2-methylpropan-2-ol. This molecule doesn’t have a chiral centre; you can see below that the central carbon is bonded to three -CH3 groups. As a result, we end up with just one product instead of two optical isomers.
SN1 nucleophilic substitution of 2-chloro-2-methylpropane doesn't produce optical isomers. Vaia Originals
Optical Isomerism - Key takeaways
- Isomers have the same molecular formula but different arrangements of atoms.
- Structural isomers have different structural formulae
- Stereoisomers have the same structural formula but different arrangements of atoms in space.
- Optical isomerism is a type of isomerism. It occurs when a molecule has a chiral centre, which is a carbon atom bonded to four different groups.
- Optical isomers are non-superimposable mirror images of each other.
- Optical isomers are known as enantiomers. They have identical chemical and physical properties apart from their effect on plane-polarised light, and their reactions with other chiral molecules.
- (+) enantiomers rotate plane-polarised light clockwise.
- (-) enantiomers rotate plane-polarised light anticlockwise.
- A racemic mixture, which is a 50:50 mixture of two different enantiomers, does not rotate plane-polarised light at all.
- Many modern drugs are optical isomers, which can lead to issues in drug design and production.
- Optical isomerism can be used to predict a reaction's mechanism.
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