A chiral centre is a carbon atom bonded to four different atoms or groups of atoms. It is described as asymmetric, as the molecule has no plane of symmetry.

What are enantiomers ?

Enantiomers are non-superimposable mirror images of each other. They arise from a chiral centre and differ in how they interact with plane-polarised light and other chiral molecules.

How many optical isomers arise from one chiral centre?

One chiral centre gives rise to two optical isomers (a pair of enantiomers). The relationship is: number of stereoisomers = 2 n , where n is the number of chiral centres.

A molecule with two chiral centres has .......... pairs of optical isomers and a total of .......... stereoisomers.

A molecule with two chiral centres has two pairs of optical isomers and a total of four stereoisomers.

In what two ways do enantiomers differ from each other?

How they interact with plane-polarised light How they react with other chiral molecules (such as biological sensors)

True or False? Enantiomers are superimposable mirror images of each other.

False. Enantiomers are non-superimposable mirror images of each other. Just as a left hand cannot be placed on top of a right hand to match exactly, enantiomers cannot be superimposed.

What drawing technique should be used to represent optical isomers?

Draw mirror images using wedge and dashed bonds to show the three-dimensional arrangement of groups around the chiral centre.

True or False? A molecule with three chiral centres has six stereoisomers.

False. Using the formula 2 n , a molecule with three chiral centres has 2 3 = eight stereoisomers.

Define racemic mixture

A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer. It is optically inactive because the two enantiomers cancel out each other's rotation of plane-polarised light.

How do the two enantiomers in a pair differ in their effect on plane-polarised light?

One enantiomer rotates plane-polarised light clockwise (+) and the other rotates it anticlockwise (-) by the same number of degrees.

A racemic mixture is .......... because the equal amounts of each enantiomer cancel out each other's effect on .......... light.

A racemic mixture is optically inactive because the equal amounts of each enantiomer cancel out each other's effect on plane-polarised light.

What does the term enantiopure mean?

Enantiopure describes a compound that contains only one enantiomer. It is obtained by separating the two enantiomers of a racemic mixture.

Why are many pharmaceutical drugs sold as racemic mixtures rather than as pure enantiomers?

Separating enantiomers is very expensive and time-consuming . For many drugs, the cost is not considered worthwhile, even though only one enantiomer may be pharmacologically active.

True or False? The two enantiomers of carvone have different smells.

True. One enantiomer of carvone smells of spearmint and the other smells of caraway . This is because optical isomers interact with biological sensors (such as olfactory receptors) in different ways.

What is the general rule about the physical properties of enantiomers?

Enantiomers have identical physical properties , with one exception: they rotate plane-polarised light in opposite directions.

True or False? Ibuprofen is sold as an enantiopure compound.

False. Ibuprofen is sold as a racemic mixture , even though only one enantiomer is pharmacologically active. The cost of separating the enantiomers is not considered worthwhile.

Why does an S N 1 reaction produce a racemic mixture?

The first step forms a trigonal planar carbocation. The nucleophile can attack from either side of the plane, producing equal amounts of each enantiomer and therefore a racemic mixture .

Why does an S N 2 reaction produce an enantiopure product (if starting from one enantiomer)?

In S N 2, the nucleophile attacks from the opposite side of the leaving group only, causing an inversion of configuration . One enantiomer in gives one enantiomer out.

What is meant by inversion of configuration ?

Inversion of configuration is the complete reversal of the spatial arrangement around a chiral centre during an S N 2 reaction. It is often compared to an umbrella turning inside out.

An S N 1 mechanism is a .......... -step reaction, while S N 2 is a .......... -step reaction.

An S N 1 mechanism is a two -step reaction, while S N 2 is a one -step reaction.

What type of carbocation intermediate is formed in the first step of an S N 1 reaction, and why does this lead to a racemic product?

A trigonal planar tertiary carbocation forms. Because it is planar, the nucleophile has equal probability of attacking from either face, giving a racemic mixture of products.

True or False? An S N 2 reaction starting with an enantiopure reactant always produces a racemic mixture.

False. S N 2 involves backside attack only, leading to inversion of configuration . Starting with one enantiopure enantiomer gives an enantiopure product of opposite configuration.

How can the mechanism of a nucleophilic substitution reaction be determined from the optical activity of its products?

If the product is a racemic mixture , an S N 1 mechanism is indicated. If the product is enantiopure (with inverted configuration), an S N 2 mechanism is indicated.

In S N 2, the nucleophile attacks from the .......... side of the C-X bond, causing the groups around the carbon to ..........

In S N 2, the nucleophile attacks from the opposite side of the C-X bond, causing the groups around the carbon to invert .

International A Level (IAL)ChemistryEdexcelFlashcards4: Rates, Equilibria & Further Organic ChemistryOrganic Chemistry: Chirality

Organic Chemistry: Chirality (Edexcel International A Level (IAL) Chemistry): Flashcards

Exam code: YCH11

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FrontChirality in Optical Isomers
Define chiral centre

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Cards in this collection (24)

Define chiral centre
A chiral centre is a carbon atom bonded to four different atoms or groups of atoms. It is described as asymmetric, as the molecule has no plane of symmetry.
What are enantiomers?
Enantiomers are non-superimposable mirror images of each other. They arise from a chiral centre and differ in how they interact with plane-polarised light and other chiral molecules.
How many optical isomers arise from one chiral centre?
One chiral centre gives rise to two optical isomers (a pair of enantiomers). The relationship is: number of stereoisomers = 2ⁿ, where n is the number of chiral centres.
A molecule with two chiral centres has .......... pairs of optical isomers and a total of .......... stereoisomers.
A molecule with two chiral centres has two pairs of optical isomers and a total of four stereoisomers.
In what two ways do enantiomers differ from each other?
1. How they interact with plane-polarised light
2. How they react with other chiral molecules (such as biological sensors)
True or False?
Enantiomers are superimposable mirror images of each other.
False.
Enantiomers are non-superimposable mirror images of each other. Just as a left hand cannot be placed on top of a right hand to match exactly, enantiomers cannot be superimposed.
What drawing technique should be used to represent optical isomers?
Draw mirror images using wedge and dashed bonds to show the three-dimensional arrangement of groups around the chiral centre.
True or False?
A molecule with three chiral centres has six stereoisomers.
False.
Using the formula 2ⁿ, a molecule with three chiral centres has 2³ = eight stereoisomers.
Define racemic mixture
A racemic mixture (or racemate) is a mixture containing equal amounts of each enantiomer. It is optically inactive because the two enantiomers cancel out each other's rotation of plane-polarised light.
How do the two enantiomers in a pair differ in their effect on plane-polarised light?
One enantiomer rotates plane-polarised light clockwise (+) and the other rotates it anticlockwise (-) by the same number of degrees.
A racemic mixture is .......... because the equal amounts of each enantiomer cancel out each other's effect on .......... light.
A racemic mixture is optically inactive because the equal amounts of each enantiomer cancel out each other's effect on plane-polarised light.
What does the term enantiopure mean?
Enantiopure describes a compound that contains only one enantiomer. It is obtained by separating the two enantiomers of a racemic mixture.
Why are many pharmaceutical drugs sold as racemic mixtures rather than as pure enantiomers?
Separating enantiomers is very expensive and time-consuming. For many drugs, the cost is not considered worthwhile, even though only one enantiomer may be pharmacologically active.
True or False?
The two enantiomers of carvone have different smells.
True.
One enantiomer of carvone smells of spearmint and the other smells of caraway. This is because optical isomers interact with biological sensors (such as olfactory receptors) in different ways.
What is the general rule about the physical properties of enantiomers?
Enantiomers have identical physical properties, with one exception: they rotate plane-polarised light in opposite directions.
True or False?
Ibuprofen is sold as an enantiopure compound.
False.
Ibuprofen is sold as a racemic mixture, even though only one enantiomer is pharmacologically active. The cost of separating the enantiomers is not considered worthwhile.
Why does an S_N1 reaction produce a racemic mixture?
The first step forms a trigonal planar carbocation. The nucleophile can attack from either side of the plane, producing equal amounts of each enantiomer and therefore a racemic mixture.
Why does an S_N2 reaction produce an enantiopure product (if starting from one enantiomer)?
In S_N2, the nucleophile attacks from the opposite side of the leaving group only, causing an inversion of configuration. One enantiomer in gives one enantiomer out.
What is meant by inversion of configuration?
Inversion of configuration is the complete reversal of the spatial arrangement around a chiral centre during an S_N2 reaction. It is often compared to an umbrella turning inside out.
An S_N1 mechanism is a ..........-step reaction, while S_N2 is a ..........-step reaction.
An S_N1 mechanism is a two-step reaction, while S_N2 is a one-step reaction.
What type of carbocation intermediate is formed in the first step of an S_N1 reaction, and why does this lead to a racemic product?
A trigonal planar tertiary carbocation forms. Because it is planar, the nucleophile has equal probability of attacking from either face, giving a racemic mixture of products.
True or False?
An S_N2 reaction starting with an enantiopure reactant always produces a racemic mixture.
False.
S_N2 involves backside attack only, leading to inversion of configuration. Starting with one enantiopure enantiomer gives an enantiopure product of opposite configuration.
How can the mechanism of a nucleophilic substitution reaction be determined from the optical activity of its products?
If the product is a racemic mixture, an S_N1 mechanism is indicated. If the product is enantiopure (with inverted configuration), an S_N2 mechanism is indicated.
In S_N2, the nucleophile attacks from the .......... side of the C-X bond, causing the groups around the carbon to ..........
In S_N2, the nucleophile attacks from the opposite side of the C-X bond, causing the groups around the carbon to invert.

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