CONSTITUTIONAL ISOMERS

Isomers with a differentorder of attachment ofthe atoms in their molecules

STEREOISOMERSIsomers with the same orderof attachment, but a differentconfiguration (3D arrangement)of groups on one or more of the atoms

ISOMERSDifferent compoundswith the same molecular formula

cis/trans ISOMERS

ENANTIOMERSStereoisomers whose molecules are non-superimposible mirrorimages of each other

DIASTEREOMERSStereoisomers whose molecules are not mirror images of each other

Each isomer could have stereoisomers

double bond or ring

both can applywith a ring

TYPES OF ISOMERISM(geometric)

Relationship of Constitutional and Stereoisomers

Relationships of Stereoisomers

FISCHER PROJECTIONS

في الكيمياء ) وخاصة (, يعنى والحيوية الكيمياء العضوية

مسقط فيشر تمثيل ثنائي األبعاد ثالثي األبعاد للجزيء العضوي

كمسقط. يتم رسم جميع الروابط كخطوط أفقية أو عمودية.

رأسيا, الكربونويتم تمثيل سلسلة

وتمثيل ذرات الكربون بمركز الخطوط المتقاطعة. كما يتم تمثيل ذرة

في األعلى. وقد C1الكربون األولى تم تقديم مسقط فيشر بواسطة

.إيميل فيشر

6

• , الروابط كل فيشر مسقط فىلو ) كما لألمام إسقاطها يتم األفقية

أو الصفحة من خارجة كانت , تدوير(. يمكن ال ذلك وعلى الشاشة

فيشر في 90°( × 2n+1)مسقط , حيث الشاشة أو الصفحة مستوىلبعضه متناسب الروابط إتجاه أن

, الجزيء محور يتغير ولذلك البعضمقابله إلى .الضوئي

CHO

HOCH2 HOH

OHH

CHO

CH2OHH

CH2OH

CHO

OH

EVOLUTION OF THEFISCHER PROJECTION

Substituents willstick out towardyou like prongs

Fischer Projection

Main chain bendsaway from you

“Sawhorse” Projection

Orient themain chainvertically withthe mostoxidized groupat the top.

ORIENTATION OFTHE MAIN CHAINAND THE SUBSTITUENTSIN A FISCHERPROJECTION

continuation of the main chain

CH3

OHH

OHH

OHH

CH3

OH

OH

OH

H

H

H

H HCH3 CH3

Cl Br

CH3

CH3

H

H

Cl

Br

Cl

CH3

Br

CH3

main chain in red

orient main chain vertically

convert toFischerProjection

rotate 90o

Br

Cl

CH3

H

H

DETERMINATION OF R / S CONFIGURATIONIN FISCHER PROJECTIONS

المطلقة الفراغية الهيئة تحديد (R وS ) فيشر مسقط خالل من

H

CH2OH

CHO

OH

PLACE THE PRIORITY=4 GROUP IN ONE OF THE VERTICALPOSITIONS, THEN LOOK AT THE OTHER THREE

H

OH

OHC CH2OH1

2

3

4

1

2 3

4

R

alternatively:

H

CH2OH

CHO

OH1

2

3

4 HOCH2 CHO

OH2

1

4

3

H

R

#4 at top position

#4 at bottom position

BOTH IN BACKSAME RESULT

H

CH2OH

CHO

OH 1

2

3

4

FOR THE MENTALLY AGILEWHY BOTHER INTERCHANGING? JUST REVERSE YOUR RESULT!

H comingtoward you

Same moleculeas on previousslide. S reverse R

Same resultas before.

CH3 C

CH2OH

CH3

CH2 CH CH

CH3

CH3

OH*

*

22 = 4 stereoisomers

R RR SS RS S

CH3

OH

CHCH3 CH3

***

23 = 8 stereoisomers

R R RR R SR S RS R R

R S SS R SS S RS S S

Link to: Drawing Enantiomers and Diastereomers Tutorial and Exercises

Tutorials Topic List Stereochemistry Practice Problems

Stereospecificityand

stereoselectivity

stereospecificity is the property

of a chemical reaction that

yields different stereoisomeric

reaction products from two

stereoisomeric reactants

depending on the reaction

conditions

stereoselectivity is the property of

a chemical reaction that yields an

unequal mixture of stereoisomers

from a single

reactant

مقارنــة بـين

stereoselectivity

describes a reaction where the mechanism allows for the formation of both products, but the formation of one of the products is

favored

Stereospecificity

describes a reaction that necessarily yields a given stereoisomer because of the

mechanism of the reaction

بين مقارنة

stereoselectivity

yields an unequal mixture of stereoisomers from a single reactant.

Stereospecificity

yields different stereoisomeric reaction products from two stereoisomeric

reactants depending on the reaction conditions

بين مقارنة

stereoselectivity

may be partial, where the formation of one stereoisomer is favoured over the other, or

it may be total where only one stereoisomer is formed

Stereospecificity

always yields 100% of a particular stereoisomer

بين مقارنة

stereoselectivity

An example of partial stereoselectivity is dehydrohalogenation of 2-iodo-butane

which yields 80% trans-2-butene and 20% cis-2-butene

Stereospecificity

For example, the addition of dibromocarbene to cis and trans-2-butene

CONFIGURATION

ABSOLUTE CONFIGURATION ( R / S )

CONFIGURATION

The three dimensional arrangement of the groups attached to an atom

Stereoisomers differ in the configuration at one ormore of their atoms.

2

CONFIGURATION - relates to the three dimensionalsense of attachment for groups attached to a chiralatom or group of atoms (i.e., attached to a stereocenter).

clockwise counterclockwise

(rectus) (sinister)

view with substituentof lowestpriority inback

1 2

4

3

C C

1

4

3

R S

Enantiomers are assigned a CONFIGURATIONusing the same priority rules we developed for E/Z stereoisomers.

1. Higher atomic number has higher priority.

2. If priority cannot be decided based on the first atom attached move to the next atom, following the path having the highest prioity atom.

3. Expand multiple bonds by replicating the atoms attached to each end of the bond.

SPECIFICATION OF CONFIGURATION

SEQUENCE RULECAHN-INGOLD-PRELOG

I

C

BrCl

F

I

C

ClBrF

1

2

3

4

R S

Bromochlorofluoroiodomethane

1

32

4

Enantiomers

How Many Stereoisomers Are Possible?

maximum number of stereoisomers = 2n,

where n = number of stereocenters(sterogenic carbons)

sometimes fewerthan this numberwill exist

IMPORTANCE OF STEREOCHEMISTRY

# AN ACHIRAL REAGENT REACTS IDENTICALLY WITH BOTH ENANTIOMERS.

# AN ACHIRAL SOCK FITS ON EITHER FOOT

# A CHIRAL REAGENT REACTS DIFFERENTLY WITH EACH ENANTIOMERS.

# A CHIRAL SHOE FITS ON ONLY ONE FOOT.

RECEPTORS ARE PROTEINS THAT BIND PARTICULAR MOLECULES.

BECAUSE A RECEPTOR WILL RECOGNISE ONLY ONE OF A PAIR OF ENANTIOMERS, ENANTIOMERS CAN HAVE DIFFERENT PHYSIOLOGICAL

PROPERTIES.

RECEPTORS LOCATED ON THE EXTERIOR OF NERVE CELLS IN THE NOSE ARE ABLE TO PERCIEVE AND DIFFERENTIATE THE ESTIMATED 10000 SMELLS TO WHICH THEY ARE EXPOSED.

(R-)-(-)CARVONE IS FOUND IN SPEARMINT OIL AND )S(-)+(- CARVONE IS FOUND IN

CARAWAY SEED OIL .

EACH HAS A DIFFERENT SMELL AS IT FITS INTO DIFFERENT RECEPTORS.

CH3O

C

CH2H3C

(R)-(-) CARVONE

CH3O

C

CH2H3C

(S)-(+) CARVONE

(S)-(-)-KETAMINE IS FOUR TIMES MORE POTENT THAN (R)-(-)-KETAMINE AND HAS LESS SIDE EFFECTS.

THE ACTIVE INGREDIENT OF IBUPROFEN IS THE (S)-(+)-ISOMER.

OCONHCH3

THE STEREOCHEMISTRY OF REACTIONS ARE VERY IMPORTANT.

A REGIOSELECTIVE REACTION FORMS MORE OF ONE CONSTITUTIONAL ISOMER THAN THE OTHER.

A B + C MORE ‘B’ IS FORMED THAN ‘C’, WHERE B AND C ARE CONSTITUTIONAL ISOMERS.

A STEREOSELECTIVE REACTION FORMS MORE OF ONE STEREOISOMER THAN THE OTHER.

A B + C MORE ‘B’ IS FORMED THAN ‘C’, WHERE B AND C ARE STEREOISOMERS.

IN A STEREOSPECIFIC REACTION, EACH STEREOISOMERIC REACTANT FORMS A DIFFERENT STEREOISOMERIC PRODUCT OR A DIFFERENT SET OF STEREOISOMERIC PRODUCTS.

A B

C D

‘A’ AND ‘C’ ARE STEREOISOMERS

‘B’ AND ‘D’ ARE STEREOISOMERS

A STEREOSPECIFIC REACTION IS ALSO STEREOSELECTIVE. A STEREOSELECTIVE REACTION IS NOT NECESSARILY STEREOSPECIFIC

ALL THE REACTIONS THAT OCCUR IN BIOLOGICAL SYSTEMS ARE CATALYSED BY ENZYMES.

AN ENZYME CATALYSED REACTION FORMS ONLY ONE STEREOISOMER BECAUSE AN ENZYME POSSESSES A CHIRAL BINDING SITE.

THEY ARE COMPLETELY STEREOSELECTIVE.

THE ENZYME FUMARASE, WHICH CATALYSES THE ADDITION OF WATER TO FUMARATE, FORMS ONLY )S(-MALATE.

COO-

-OOCH2COH

HC C

H

-OOC

COO-

H

+ H2Ofumarase

fumarate (S)-malate

ENZYME CATALYSIS IS STEREOSPECIFIC. AN ENZYME CATALYSES THE REACTION OF ONLY ONE STEREOISOMER.

FUMARASE CATALYSE THE ADDITION OF WATER TO FUMARATE BUT NOT TO MALEATE

C C

-OOC

H

COO-

H

+ H2Ofumarase

maleate

no reaction

AN ENZYME CAN DIFFERENTIATE STEREOISOMERS BECAUSE OF THE CHIRAL BINDING SITE