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Chiral Centres and Chirality

Without excep...

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Without exception . Sn2 reaction at chiral carbon give 1 . Enantiomer of substrate 2. Diastereomeric mixture 2. Product of opposite polarity 4. Only 1 stereoisomer

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Q. Without exception,

S_{N^{2}}

reaction at chiral carbon gives:

A schematic analysis of the reaction of one enantiomer with racemic mixture is shown below

\begin{matrix} d & + & d a n d l & \to & (d) - (d) T h e (+) f o r m o f & A r a c e m i c m i x t u r e o f & + c h i r a l m o l e c u l e s & o t h e r m o l e c u l e s w i t h & (d) - (l) 50 % (d) a n d 50 % (l) \end{matrix}

The products (d-d) and (d-l) are clearly neither identical nor enantiomers (non-superimposable mirror images) as the mirror image of (d-d) is (l-l) not (d-l). They are diastereomers, 'stereoisomers that are not mirror images'.

The formation of diastereomers allows the separation of enantiomers (called resolution) which is not easy as enantiomers have identical physical properties. One general procedure for separating enantiomers is to allow them to react with naturally occurring chiral molecule to form a pair of diastereomers. These can be separated easily as they have different physical properties. if the original chemical reaction can be reversed, the enantiomers can be isolated.

Which of the following is an example of diastereomers?

Q. 4-bromo-2 pentene has a chiral carbon and can show enantiomerism. If a racemic mixture of 4-bromo-trans-2-pentene is treated with

B r_{2}

C C l_{4}

F e B r_{3}

, 2,3,4-tribromopentane is formed. How many stereoisomers would be produced in this reaction?

Q. Under the specified conditions, substrate

X

undergoes substitution and elimination reactions to give products

A - D

A

B

are stereoisomers, but not enantiomers.

C

D

are enantiomers. A is not an isomer of

C

. Which of the following could be the material

X

?

X H_{2} O - - \to Δ A + B + C + D

Column 1: Reaction

Column 2: Type of reactions

Column 3: Lab test for reactants

The only correct combination that gives two different stereoisomeric product is:

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