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Optical Isomerism

Optical Isomerism

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Have you ever taken a close look at your shoes? If you wave them around, you might notice that they look almost identical, except for a few scratches or specks of mud. That's because one shoe is just a mirror image of the other. This is called optical isomerism in chemistry.

To understand optical isomerism, we first need to know what isomerism is. Isomerism is when two molecules have the same chemical formula, but different structures. Optical isomerism is a specific type of isomerism that occurs when a molecule has a chiral center, which means it has four different groups attached to it. The mirror image of this molecule is called an enantiomer.

Enantiomers have similar physical and chemical properties, but they interact differently with polarized light. This property makes them important in drug design, as the two enantiomers of a drug can have different effects on the body.

To better understand optical isomerism, we'll explore some examples and practice identifying and drawing optical isomers. This knowledge is useful in many fields of chemistry, from drug design to materials science.

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

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
A pair of shoes - an example of optical isomers

Optical isomerism conditions

Optical isomerism is a fascinating topic in chemistry, and it occurs when a molecule has a chiral center, which is a carbon atom joined to four different groups. This particular carbon atom is also known as an asymmetric carbon and is often indicated by an asterisk, *. Enantiomers are two different isomers that are non-superimposable mirror images of each other.

For instance, let's take a look at bromochlorofluoromethane. This molecule contains one carbon atom attached to four different groups: a bromine atom, a chlorine atom, a fluorine atom, and a hydrogen atom. As a result, the carbon atom in it is a chiral center. If you draw this molecule using different coloured circles to represent the different groups, you'll end up with two different arrangements of atoms around the central carbon atom. These molecules are mirror images of each other and cannot be superimposed. They are known as enantiomers, just like your shoes.

Understanding optical isomerism is essential in many fields of chemistry, from drug design to materials science. By practicing identifying and drawing optical isomers, you can gain a deeper understanding of this concept and how it plays a crucial role in our daily lives.

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

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.

plane-polarised
plane polarised

 

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 effect of enantiomers on plane-polarised light

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 play a significant role in drug design, as many common drugs are optical isomers. However, only one of the enantiomers is useful, and the other might be harmful or have no effect at all. This creates challenges in drug synthesis, as optical isomers are produced in racemic mixtures containing a 50:50 ratio of the two enantiomers. To sell the drug, we need to remove the second enantiomer, which is often a time-consuming and expensive process.

Producing an enantiopure compound, which contains just one enantiomer, is a solution to this problem. Leaving the second enantiomer in the drug results in a weaker drug, requiring a higher dosage and increasing production costs for the manufacturer. A chiral catalyst is a great way to selectively produce just one enantiomer. Catalysts can be used repeatedly, and only a small amount is required to facilitate a reaction. Separating enantiomers is often a fiddly and expensive process, and using a chiral catalyst can save time and money. Ibuprofen, a common painkiller, is an example of optical isomerism in drugs. It consists of two enantiomers, the (S+) and (R-) forms. Although both forms have identical physical properties, the (R-) form has no effect on the body. Understanding optical isomerism is crucial in drug design, as it can impact the effectiveness and safety of the medication.

Ibuprofen. Only the (S+) enantiomer works as a painkiller in the body
Ibuprofen. Only the (S+) enantiomer works as a painkiller in the body

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
1-chloroethanol

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
The bond arrangement of a tetrahedral molecule

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
An enantiomer of 1-chloroethanol

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
The second enantiomer of 1-chloroethanol

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

Butan-2-ol
Butan-2-ol

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
The two enantiomers of butan-2-ol

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
The mechanism for an SN2 nucleophilic substitution reaction

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
The mechanism for an SN1 nucleophilic substitution reaction

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
SN1 nucleophilic substitution of 2-chloro-2-methylpropane doesn't produce optical isomers

In summary, isomers are molecules with the same molecular formula but different arrangements of atoms. Structural isomers have different structural formulae, while stereoisomers have the same structural formula but different arrangements of atoms in space. Optical isomerism is a type of stereoisomerism that occurs when a molecule has a chiral centre. Optical isomers, also known as enantiomers, are non-superimposable mirror images of each other and 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, while (-) enantiomers rotate it anticlockwise. A racemic mixture, which is a 50:50 mixture of two enantiomers, does not rotate plane-polarised light. Optical isomerism is crucial in drug design, and many modern drugs are optical isomers. Optical isomerism can also be used to predict a reaction's mechanism.

Optical Isomerism

What is optical isomerism?

Optical  isomerism is a type of isomerism where molecules have the same molecular and structural formulae, but are non-superimposable mirror images of each other. An example is butan-2-ol. It has four different groups attached to its second carbon atom. This makes it a chiral centre and means it forms two optical isomers.

Which molecules show optical isomerism?

Molecules with a chiral centre show optical isomerism. A chiral centre is a carbon atom bonded to four different groups of atoms.

How do you check optical isomerism?

To check optical isomerism, shine plane-polarised light through the two different molecules. If they are enantiomers, they will rotate the light in opposite directions.

 How do you determine if a compound shows optical isomerism?

You can determine whether a compound shows optical isomerism by looking for a chiral centre. This is a carbon atom bonded to four different groups. You can also check by shining plane-polarised light through each of the isomers separately. Optical isomers will rotate the light in opposite directions.

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