Define chiral centre .

A chiral centre is a carbon atom bonded to four different atoms or groups of atoms, giving rise to a pair of non-superimposable mirror image molecules (enantiomers).

How do the two enantiomers of a chiral molecule differ?

Enantiomers have identical physical and chemical properties but differ in the direction they rotate plane polarised light. One rotates it clockwise (+) and the other anticlockwise (−). They may also have different biological activities.

True or False? A racemic mixture rotates plane polarised light because one enantiomer is always present in a slightly greater proportion.

False. A racemic mixture contains equal amounts of both enantiomers. Their opposite rotations cancel out exactly, so the mixture is optically inactive and there is no net rotation of plane polarised light.

Define racemic mixture .

A racemic mixture is a mixture containing equal amounts of the two enantiomers of a chiral compound, making it optically inactive.

The (+) enantiomer rotates plane polarised light in the .......... direction, while the (−) enantiomer rotates it .......... .

The (+) enantiomer rotates plane polarised light in the clockwise direction, while the (−) enantiomer rotates it anticlockwise .

Why do enantiomers often have different biological activities?

Enzymes are chiral proteins with specific binding sites. Only the enantiomer with the correct shape fits the active site . The mirror image enantiomer will not bind as well, giving it a different or absent biological effect.

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

False. Enantiomers are non-superimposable mirror images. They cannot be placed on top of each other and be identical, just as a left hand cannot be superimposed on a right hand.

A solution containing 80% of the clockwise-rotating enantiomer and 20% of the anticlockwise-rotating enantiomer is said to be optically .......... and will rotate light .......... .

A solution containing 80% of the clockwise-rotating enantiomer and 20% of the anticlockwise-rotating enantiomer is said to be optically active and will rotate light clockwise .

How can plane polarised light be used to distinguish between enantiomers?

Plane polarised light is passed through a solution of one enantiomer. The direction of rotation of the plane indicates which isomer is present: clockwise rotation identifies the (+) enantiomer and anticlockwise rotation identifies the (−) enantiomer . A racemic mixture gives no rotation.

Define chiral catalyst .

A chiral catalyst is a catalyst that directs a reaction to produce predominantly one enantiomer of a chiral product, enabling enantioselective synthesis.

Why is it important that drugs containing chiral centres are produced as single enantiomers rather than racemic mixtures?

The two enantiomers of a drug can have very different biological activities. One may be therapeutic while the other is inactive or even harmful . Using a single enantiomer increases effectiveness, reduces side effects and can halve the patient dosage.

True or False? Conventional organic synthesis of a chiral drug always produces a single enantiomer.

False. Conventional organic synthesis produces a racemic mixture containing equal amounts of both enantiomers. Chiral catalysts or enzymes are needed to produce a single enantiomer.

An organometallic .......... catalyst is used in the production of naproxen, directing synthesis towards the enantiomer that treats .......... .

An organometallic ruthenium catalyst is used in the production of naproxen, directing synthesis towards the enantiomer that treats arthritis .

Define stereoselectivity .

A stereoselectivity is the preference of a reaction to produce one stereoisomer (enantiomer) over the other, often due to the specific geometry of a catalyst or enzyme binding site.

What are two advantages of using enzymes instead of conventional synthetic routes for producing pure drug enantiomers?

Enzymes are highly stereoselective , producing only one enantiomer due to their specific active site geometry. Enzyme-based synthesis is a greener process , involving fewer steps than conventional synthetic routes.

True or False? Enzymes used in drug synthesis must be mixed into the final product because they cannot be separated from it.

False. Enzymes are fixed on inert supports, allowing reactants to pass over them. The enzymes remain on the support and do not need to be separated from the product.

Separating a racemic mixture into pure single enantiomers reduces patient .......... by half, because the pure enantiomer is more .......... than the mixture.

Separating a racemic mixture into pure single enantiomers reduces patient dosage by half, because the pure enantiomer is more potent than the mixture.

What is a disadvantage of using enzymes to produce pure enantiomers compared to using synthetic chiral catalysts?

Enzymes can be expensive to isolate from living organisms. This has driven research into synthetic enzymes as alternatives, though enzyme-based routes do offer advantages such as fewer steps and greener chemistry.

A LevelChemistryCambridge (CIE)Flashcards29. An Introduction to A Level Organic ChemistryIsomerism: Optical

Isomerism: Optical (Cambridge (CIE) A Level Chemistry): Flashcards

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FrontOptical Isomers
Define chiral centre.

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Cards in this collection (18)

Define chiral centre.
A chiral centre is a carbon atom bonded to four different atoms or groups of atoms, giving rise to a pair of non-superimposable mirror image molecules (enantiomers).
How do the two enantiomers of a chiral molecule differ?
Enantiomers have identical physical and chemical properties but differ in the direction they rotate plane polarised light. One rotates it clockwise (+) and the other anticlockwise (−). They may also have different biological activities.
True or False?
A racemic mixture rotates plane polarised light because one enantiomer is always present in a slightly greater proportion.
False.
A racemic mixture contains equal amounts of both enantiomers. Their opposite rotations cancel out exactly, so the mixture is optically inactive and there is no net rotation of plane polarised light.
Define racemic mixture.
A racemic mixture is a mixture containing equal amounts of the two enantiomers of a chiral compound, making it optically inactive.
The (+) enantiomer rotates plane polarised light in the .......... direction, while the (−) enantiomer rotates it .......... .
The (+) enantiomer rotates plane polarised light in the clockwise direction, while the (−) enantiomer rotates it anticlockwise.
Why do enantiomers often have different biological activities?
Enzymes are chiral proteins with specific binding sites. Only the enantiomer with the correct shape fits the active site. The mirror image enantiomer will not bind as well, giving it a different or absent biological effect.
True or False?
Enantiomers are superimposable mirror images of each other.
False.
Enantiomers are non-superimposable mirror images. They cannot be placed on top of each other and be identical, just as a left hand cannot be superimposed on a right hand.
A solution containing 80% of the clockwise-rotating enantiomer and 20% of the anticlockwise-rotating enantiomer is said to be optically .......... and will rotate light .......... .
A solution containing 80% of the clockwise-rotating enantiomer and 20% of the anticlockwise-rotating enantiomer is said to be optically active and will rotate light clockwise.
How can plane polarised light be used to distinguish between enantiomers?
Plane polarised light is passed through a solution of one enantiomer. The direction of rotation of the plane indicates which isomer is present: clockwise rotation identifies the (+) enantiomer and anticlockwise rotation identifies the (−) enantiomer. A racemic mixture gives no rotation.
Define chiral catalyst.
A chiral catalyst is a catalyst that directs a reaction to produce predominantly one enantiomer of a chiral product, enabling enantioselective synthesis.
Why is it important that drugs containing chiral centres are produced as single enantiomers rather than racemic mixtures?
The two enantiomers of a drug can have very different biological activities. One may be therapeutic while the other is inactive or even harmful. Using a single enantiomer increases effectiveness, reduces side effects and can halve the patient dosage.
True or False?
Conventional organic synthesis of a chiral drug always produces a single enantiomer.
False.
Conventional organic synthesis produces a racemic mixture containing equal amounts of both enantiomers. Chiral catalysts or enzymes are needed to produce a single enantiomer.
An organometallic .......... catalyst is used in the production of naproxen, directing synthesis towards the enantiomer that treats .......... .
An organometallic ruthenium catalyst is used in the production of naproxen, directing synthesis towards the enantiomer that treats arthritis.
Define stereoselectivity.
A stereoselectivity is the preference of a reaction to produce one stereoisomer (enantiomer) over the other, often due to the specific geometry of a catalyst or enzyme binding site.
What are two advantages of using enzymes instead of conventional synthetic routes for producing pure drug enantiomers?
1. Enzymes are highly stereoselective, producing only one enantiomer due to their specific active site geometry.
2. Enzyme-based synthesis is a greener process, involving fewer steps than conventional synthetic routes.
True or False?
Enzymes used in drug synthesis must be mixed into the final product because they cannot be separated from it.
False.
Enzymes are fixed on inert supports, allowing reactants to pass over them. The enzymes remain on the support and do not need to be separated from the product.
Separating a racemic mixture into pure single enantiomers reduces patient .......... by half, because the pure enantiomer is more .......... than the mixture.
Separating a racemic mixture into pure single enantiomers reduces patient dosage by half, because the pure enantiomer is more potent than the mixture.
What is a disadvantage of using enzymes to produce pure enantiomers compared to using synthetic chiral catalysts?
Enzymes can be expensive to isolate from living organisms. This has driven research into synthetic enzymes as alternatives, though enzyme-based routes do offer advantages such as fewer steps and greener chemistry.

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