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Stereochemistry: Understanding Isomers and their Configurations, Study notes of Stereochemistry

Coppin State University (CSU)Stereochemistry

The concepts of stereochemistry, focusing on structural isomers and stereoisomers. It covers the differences between constitutional isomers and stereoisomers, the use of Cahn-Ingold-Prelog rules for determining substituent priority, and the complexity of multiple bonds. The document also discusses the concepts of chiral and achiral centers, enantiomers, and diastereomers, and provides examples for understanding these concepts.

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Stereochemistry

Structural or constitutional isomers...

Stereoisomers...

have the same molecular formula but different

connectivity (skeletal, positional, functional)

have the same connectivity but a different

arrangement of atoms in space

Recall:

C C

H3C

H H

CH3

C C

H3C

H CH3

2-butene

cis-2-butene trans-2-butene

Partial preview of the text

Download Stereochemistry: Understanding Isomers and their Configurations and more Study notes Stereochemistry in PDF only on Docsity!

Stereochemistry

Structural or constitutional isomers...

Stereoisomers...

 have the same molecular formula but different

connectivity (skeletal, positional, functional)

 have the same connectivity but a different

arrangement of atoms in space

Recall:

C C

H 3 C

H H

CH 3

C C

H 3 C

H CH 3

H

2-butene

cis -2-butene trans -2-butene

cis trans

H 3 C

H

CH 3

H

H 3 C

H

CH 3

1,2-dimethylcyclopropane

 Trans and Cis can also be used for disubstituted cycloalkanes (no

bond rotation):

C C

H 3 C

H CH 2 F

CH 2 Cl

C C

H 3 C

H CH 2 Cl

CH 2 F

 What if there are less than 2 H atoms?

 What if more than one bonded to the alkene is the same?

C

H OCH

3 OCH 3

H

OCH 3

CF 3

CH 2 SH

C C

CF 3

H CH 2 SH

H 3 C

Another level of complexity with multiple bonds:

C N C N

N 000

C 000

C

OH

O

C

O

O 000

O H

C 000

C C H vs.^ C

CH 3

How about these two?

F HO

Rotate about y-axis to place most atoms in the plane of the page. F HO

Rotate ~160º counterclockwise about z-axis

Y axis

X axis

Z axis

Stereochemistry at tetrahedral centres:

C

H

Cl CH 2 CH 3

CH 3 C

H

Cl H 3 CH 2 C

H 3 C

Are these molecules the same?

C

H

Cl CH 2 CH 3

CH 3

Rotate 180° around CH bond C

H

Cl CH 3

CH 2 CH 3

Configuration: the actual arrangement in space of the substituents around a given asymmetric atom that can be designated R or S. Also applies to E/Z.

Chiral or stereogenic centre: Any tetrahedral atom, frequently carbon, which has 4 different substituents. Also called stereocentre

Dextrotatory - rotates ppl in a clockwise direction.

Levorotatory - rotates ppl in counterclockwise direction

Solutions that rotate plane-polarized light are said to be "optically active".

C

H

HO

CH 2 CH 3

CH 3 C

H

OH

H 3 CH 2 C

H 3 C

(-)-2-Butanol (^) (+)-2-Butanol

The D/L Convention of Fischer

H

HO CH 2 OH

CHO

Absolute configuration: (^) The actual three-dimensional arrangement of groups around an asymmetric center.

All sugars with the same absolute configuration next to the CH 2 OH group are "D". Those with opposite absolute configuration are "L"

D-Glyceraldehyde

Note that this designation now has nothing to do with the direction the molecule rotates ppl.

How do we distinguish between different absolute configurations? How do we label them?

Historically…(bit still used today for certain classes of molecules)

C

CO 2 H

H 3 C H

Cl

C

CH 2 OH

H 3 C H

Cl

H 3 C

C C

HO

Br

H

CH 3

Cl

O

H Br

O

Try to assign these (a model kit is exceedingly helpful!):

Enantiomers : are molecules that are non-superimposable mirror-images of one another.

Cl CH 2 CH 3

CH 3 C

Cl H 3 CH 2 C

H 3 C

R S

 Enantiomers have identical physical properties in all respects (thus difficult to separate from each other), except they rotate plane polarized light in opposite directions (unrelated to absolute configuration). Such molecules are said to be optically active.

 Thus, a 50/50 mixture of two enantiomers (a racemic mixture ) will not rotate plane polarized light.

 A mixture of enantiomers that is optically active (i.e. not a 50/50 mxture) is said to be scalemic.

Can a molecule without any chiral centres be chiral?

C C C

Br Br H C C C H

BINAP

H 3 C

C

H

Br

H 3 C

Br

H CH

C

Br

H

CH 3

H

Br

Can a molecule with centres be achiral?

 Only if there is an even number of chiral centres.

Meso compounds have chiral centres, but are achiral (they have

an internal plane of symmetry)

N H 3 CH 2 C CH 3 Ph

Atoms other than carbon?

CH 3 CH 2 Br

H 3 CH 2 C CH 3

CH 2 CH 3

Br-

P H 3 CH 2 C CH 3 Ph

S H 3 CH 2 C Ph

H 3 CH 2 C CH 3

Ph H 3 CH 2 C N Ph CH 3

Inversion of Configuration

Molecules with two chiral centers...

H 3 C

C C

H CH 3

CH 3

C C

H H 3 C

H 3 C

C C

H CH 3

CH 3

C C

H H 3 C

Diastereomers : non-superimposable, non-mirror images

 Diastereomers have different physical properties and can easily be separated

from one another.

 A molecule can have a maximum of 2n possible stereoisomers (n = number of

chiral centres).

Stereochemistry: Understanding Isomers and their Configurations, Study notes of Stereochemistry

Related documents

Partial preview of the text

Download Stereochemistry: Understanding Isomers and their Configurations and more Study notes Stereochemistry in PDF only on Docsity!

Stereochemistry

Structural or constitutional isomers...

Stereoisomers...

 have the same molecular formula but different

connectivity (skeletal, positional, functional)

 have the same connectivity but a different

arrangement of atoms in space

Recall:

C C

H 3 C

H H

CH 3

C C

H 3 C

H CH 3

H

2-butene

cis -2-butene trans -2-butene

cis trans

H 3 C

H

CH 3

H

H 3 C

H

H

CH 3

1,2-dimethylcyclopropane

 Trans and Cis can also be used for disubstituted cycloalkanes (no

bond rotation):

C C

H 3 C

H CH 2 F

CH 2 Cl

C C

H 3 C

H CH 2 Cl

CH 2 F

 What if there are less than 2 H atoms?

 What if more than one bonded to the alkene is the same?

C

C

C

H OCH

H

H

OCH 3

Another level of complexity with multiple bonds:

C N C N

N 000

N 000

C 000

C 000

C

OH

O

C

O

O 000

O H

C 000

C C H vs.^ C

CH 3

CH 3

CH 3

C

H

CH 3 C

H

H 3 C

C

H

CH 3

H

CH 2 CH 3

C