Chirality and Stereoisomerism Lecture Notes

A set of flashcards designed to help review key concepts related to chirality, stereoisomerism, and separation techniques for stereoisomers.

What is a meso isomer?

A meso isomer has 2 or more asymmetric (chiral) carbons and a plane of symmetry; meso compounds are achiral.

How can you separate two diastereoisomers?

You can use a solvent in which one diastereoisomer is soluble and the other is less soluble; the less soluble isomer will crystallize and can be filtered.

Do diasteroisomers have the same physical properties?

No

How can you separate two enantiomers?

• Separation can be achieved using chiral chromatography where a racemic mixture is passed over a chiral column

• by making a salt with another chiral molecule (either acid or base)

• creating diastereoisomers which have different physical properties and can be more easily removed

Do enantiomers have the same physical properties?

Yes

Define a stereospecific reaction.

A stereospecific reaction is one where each stereoisomeric reactant produces a different stereoisomeric product or a different set of products.

What does the reaction mechanism indicate about stereochemistry?

The reaction mechanism means that the stereochemistry of the starting material determines the stereochemistry of the product, for example in SN2 reactions.

What characterizes a stereoselective reaction?

A stereoselective reaction causes the preferential formation of one stereoisomer over another, even if both types can be produced.

What does the reaction mechanism indicate about the formation of stereoisomers in SN1 reactions?

The reaction mechanism does not prevent the formation of two or more stereoisomers, but one does predominate, as seen in SN1 reactions.

What is a concerted mechanism?

In a concerted mechanism, bond making and breaking happen in the same step, and the stereochemistry is usually defined.

What characterizes a concerted mechanism?

stereochemistry is usually defined, either inverted or conserved.

What is a non-concerted mechanism?

In a non-concerted mechanism, bond breaking and making occur in different steps, allowing for intermediates that can change stereochemistry. Racemisation results if intermediate formed

Constitutional Isomerism:

Isomers whose atoms have a different connectivity

Stereoisomerism:

Isomers with the same connectivity but differ in arrangement of atoms in space

Enantiomers:

Stereoisomers that are non-superimposable mirror images of one another

Diastereoisomers:

Stereoisomers that are not mirror images of one another

Chiral Carbon:

stereocentre in a chiral molecule. A carbon with 4 different groups

attached to it.

Define chirality.

A chiral molecule/ion is one that is non-superimposable on its mirror image.

What is a racemic mixture?

A racemic mixture is a 1:1 mixture of enantiomers.

What is the equation for calculating enantiomeric excess?

% Enantiomeric excess = (observed specific rotation / specific rotation of pure sample) x 100.

Optically inactive:

an achiral molecule (or racemic mixture)

What is the relationship between 1 and 2 if they are enantiomers?

They are non-superimposable mirror images of one another.

What are the exceptions to the properties of enantiomers?

1. Rotation of plane polarized light; 2. Interaction with other chiral molecules.