What is a chiral carbon (chiral centre) ?

A carbon atom bonded to four different atoms or groups . Compounds with a chiral centre can exist as two non-superimposable mirror images called enantiomers.

True or False? Enantiomers have identical melting points and boiling points.

True. Enantiomers have identical physical properties (melting point, boiling point, solubility) and identical chemical properties with achiral substances. They differ only in how they rotate plane-polarised light.

One enantiomer rotates plane-polarised light .......... , while the other rotates it by the same amount in the .......... direction. This property makes enantiomers .......... active.

One enantiomer rotates plane-polarised light clockwise , while the other rotates it by the same amount in the anticlockwise direction. This property makes enantiomers optically active.

What are enantiomers ?

Non-superimposable mirror images of a molecule containing a chiral centre. They are a type of stereoisomer that rotate plane-polarised light in opposite directions by equal amounts.

True or False? The (+) and (−) symbols used to distinguish enantiomers indicate the arrangement of atoms in space around the chiral centre.

False. The (+) and (−) symbols indicate the direction of rotation of plane-polarised light (clockwise or anticlockwise). They do not directly describe spatial arrangement — that is described by R/S or D/L nomenclature, which is not required for AQA.

Stereoisomers are molecules with the same .......... formula but different .......... in space. The two main types are .......... isomerism and .......... isomerism.

Stereoisomers are molecules with the same structural formula but different arrangement of atoms in space. The two main types are geometrical (E/Z) isomerism and optical isomerism.

How do enantiomers differ in their effect on plane-polarised light?

Each enantiomer rotates the plane of polarisation by the same magnitude but in opposite directions — one clockwise (+), one anticlockwise (−).

How do you identify a chiral centre in a molecule?

Look for a carbon atom bonded to four different atoms or groups . If two substituents are the same, the carbon cannot be chiral.

In stereochemical drawings, a .......... line represents a bond in the plane of the paper, a .......... line is a bond receding behind the paper, and a .......... wedge is a bond coming out of the paper.

In stereochemical drawings, a solid line represents a bond in the plane of the paper, a dotted line is a bond receding behind the paper, and a solid wedge is a bond coming out of the paper.

What symbol is used to mark a chiral centre in a displayed formula?

An asterisk (*) placed on the chiral carbon atom.

True or False? When drawing optical isomers, it is sufficient to draw both structures with the same arrangement of groups and simply label them as enantiomers.

False. You must use stereochemical conventions to show the mirror image relationship: one bond must come out of the paper and one must recede, with the groups arranged to show the non-superimposable mirror image. Simply labelling them is not enough.

Alanine, CH 3 CH(NH 2 )COOH, has a chiral centre on the .......... carbon. When drawing its optical isomers, you draw a vertical .......... line to represent a line of symmetry, then place the chiral carbon with four bonds in a .......... arrangement.

Alanine, CH 3 CH(NH 2 )COOH, has a chiral centre on the middle carbon. When drawing its optical isomers, you draw a vertical dotted line to represent a line of symmetry, then place the chiral carbon with four bonds in a tetrahedral arrangement.

True or False? A carbon bonded to two hydrogen atoms and two different alkyl groups is a chiral centre.

False. A chiral centre requires four different substituents. Two identical groups (here, two H atoms) mean the carbon is not chiral.

Enantiomers are a pair of optical isomers that are non-superimposable mirror images of each other. They arise when a molecule contains a chiral centre (a carbon bonded to four different groups) and have identical physical properties except for the direction in which they rotate plane-polarised light.

What is a racemic mixture (racemate) ?

A mixture containing equal amounts of both enantiomers of a chiral compound. It is optically inactive because the equal and opposite rotations of plane-polarised light cancel out.

True or False? A racemic mixture rotates plane-polarised light by a net amount of zero.

True. The two enantiomers rotate plane-polarised light in equal but opposite directions, so they cancel, giving an overall rotation of zero. The mixture is optically inactive.

If a mixture contains 80% of the (−) enantiomer and 20% of the (+) enantiomer, the mixture is .......... active and the plane of polarised light will be rotated .......... .

If a mixture contains 80% of the (−) enantiomer and 20% of the (+) enantiomer, the mixture is optically active and the plane of polarised light will be rotated anticlockwise .

What does enantiopure mean?

A sample that contains only one enantiomer of a chiral compound. Separating enantiomers from a racemic mixture to give an enantiopure compound is expensive and time-consuming.

Why are most chiral drugs sold as racemic mixtures rather than as single enantiomers?

Separating enantiomers is technically difficult and expensive. In many cases only one enantiomer is pharmacologically active, and the other enantiomer may be inactive or less active — making the cost of separation difficult to justify. However, this is not always the case: in some drugs (e.g. thalidomide) the other enantiomer is harmful, so the assumption that the other enantiomer is "harmless" cannot be generalised.

Thalidomide was sold as a .......... mixture. One enantiomer was an effective treatment for morning sickness; the other caused .......... . This is an example of why .......... of a racemic drug can be important.

Thalidomide was sold as a racemic mixture. One enantiomer was an effective treatment for morning sickness; the other caused birth defects . This is an example of why enantiopure production of a racemic drug can be important.

True or False? Ibuprofen is sold as an enantiopure compound because the other enantiomer is harmful.

False. Ibuprofen is sold as a racemic mixture . The cost and difficulty of separating the enantiomers is not considered worthwhile in this case.

A LevelChemistryAQAFlashcardsAdvanced Organic ChemistryOptical Isomerism

Optical Isomerism (AQA A Level Chemistry): Flashcards

Exam code: 7405

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FrontOptical Isomerism
What is a chiral carbon (chiral centre)?

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Cards in this collection (21)

What is a chiral carbon (chiral centre)?
A carbon atom bonded to four different atoms or groups. Compounds with a chiral centre can exist as two non-superimposable mirror images called enantiomers.
True or False?
Enantiomers have identical melting points and boiling points.
True.
Enantiomers have identical physical properties (melting point, boiling point, solubility) and identical chemical properties with achiral substances. They differ only in how they rotate plane-polarised light.
One enantiomer rotates plane-polarised light .......... , while the other rotates it by the same amount in the .......... direction. This property makes enantiomers .......... active.
One enantiomer rotates plane-polarised light clockwise, while the other rotates it by the same amount in the anticlockwise direction. This property makes enantiomers optically active.
What are enantiomers?
Non-superimposable mirror images of a molecule containing a chiral centre. They are a type of stereoisomer that rotate plane-polarised light in opposite directions by equal amounts.
True or False?
The (+) and (−) symbols used to distinguish enantiomers indicate the arrangement of atoms in space around the chiral centre.
False.
The (+) and (−) symbols indicate the direction of rotation of plane-polarised light (clockwise or anticlockwise). They do not directly describe spatial arrangement — that is described by R/S or D/L nomenclature, which is not required for AQA.
Stereoisomers are molecules with the same .......... formula but different .......... in space. The two main types are .......... isomerism and .......... isomerism.
Stereoisomers are molecules with the same structural formula but different arrangement of atoms in space. The two main types are geometrical (E/Z) isomerism and optical isomerism.
How do enantiomers differ in their effect on plane-polarised light?
Each enantiomer rotates the plane of polarisation by the same magnitude but in opposite directions — one clockwise (+), one anticlockwise (−).
How do you identify a chiral centre in a molecule?
Look for a carbon atom bonded to four different atoms or groups. If two substituents are the same, the carbon cannot be chiral.
In stereochemical drawings, a .......... line represents a bond in the plane of the paper, a .......... line is a bond receding behind the paper, and a .......... wedge is a bond coming out of the paper.
In stereochemical drawings, a solid line represents a bond in the plane of the paper, a dotted line is a bond receding behind the paper, and a solid wedge is a bond coming out of the paper.
What symbol is used to mark a chiral centre in a displayed formula?
An asterisk (*) placed on the chiral carbon atom.
True or False?
When drawing optical isomers, it is sufficient to draw both structures with the same arrangement of groups and simply label them as enantiomers.
False.
You must use stereochemical conventions to show the mirror image relationship: one bond must come out of the paper and one must recede, with the groups arranged to show the non-superimposable mirror image. Simply labelling them is not enough.
Alanine, CH₃CH(NH₂)COOH, has a chiral centre on the .......... carbon. When drawing its optical isomers, you draw a vertical .......... line to represent a line of symmetry, then place the chiral carbon with four bonds in a .......... arrangement.
Alanine, CH₃CH(NH₂)COOH, has a chiral centre on the middle carbon. When drawing its optical isomers, you draw a vertical dotted line to represent a line of symmetry, then place the chiral carbon with four bonds in a tetrahedral arrangement.
True or False?
A carbon bonded to two hydrogen atoms and two different alkyl groups is a chiral centre.
False.
A chiral centre requires four different substituents. Two identical groups (here, two H atoms) mean the carbon is not chiral.
Enantiomers
Enantiomers are a pair of optical isomers that are non-superimposable mirror images of each other. They arise when a molecule contains a chiral centre (a carbon bonded to four different groups) and have identical physical properties except for the direction in which they rotate plane-polarised light.
What is a racemic mixture (racemate)?
A mixture containing equal amounts of both enantiomers of a chiral compound. It is optically inactive because the equal and opposite rotations of plane-polarised light cancel out.
True or False?
A racemic mixture rotates plane-polarised light by a net amount of zero.
True.
The two enantiomers rotate plane-polarised light in equal but opposite directions, so they cancel, giving an overall rotation of zero. The mixture is optically inactive.
If a mixture contains 80% of the (−) enantiomer and 20% of the (+) enantiomer, the mixture is .......... active and the plane of polarised light will be rotated .......... .
If a mixture contains 80% of the (−) enantiomer and 20% of the (+) enantiomer, the mixture is optically active and the plane of polarised light will be rotated anticlockwise.
What does enantiopure mean?
A sample that contains only one enantiomer of a chiral compound. Separating enantiomers from a racemic mixture to give an enantiopure compound is expensive and time-consuming.
Why are most chiral drugs sold as racemic mixtures rather than as single enantiomers?
Separating enantiomers is technically difficult and expensive. In many cases only one enantiomer is pharmacologically active, and the other enantiomer may be inactive or less active — making the cost of separation difficult to justify. However, this is not always the case: in some drugs (e.g. thalidomide) the other enantiomer is harmful, so the assumption that the other enantiomer is "harmless" cannot be generalised.
Thalidomide was sold as a .......... mixture. One enantiomer was an effective treatment for morning sickness; the other caused .......... . This is an example of why .......... of a racemic drug can be important.
Thalidomide was sold as a racemic mixture. One enantiomer was an effective treatment for morning sickness; the other caused birth defects. This is an example of why enantiopure production of a racemic drug can be important.
True or False?
Ibuprofen is sold as an enantiopure compound because the other enantiomer is harmful.
False.
Ibuprofen is sold as a racemic mixture. The cost and difficulty of separating the enantiomers is not considered worthwhile in this case.

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