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CHEMQUEST STUDY CENTRE 

ACCORDING TO THE SYLLABI

OF CALCUTTA UNIVERCITY

FIRST YEAR

2012

STERIOCHEMISTRY 

MODERN TERMINOLOGY 

CONTACTs

Mridul Ahamed Dr. Bellal Ali

Mob: 9830440993 Mob: 9830440886

E-Mail: chemquest2011@gmail.com 

Co-operated by   AL-HILAL MISSION 

(An Educational, Cultural & Social Welfare Organization)

Kadambagachhi ,Barasat, Kolkata-7000125

1 8 9 - P A R K C I R C U S ( 7 P O I N T , N E A R A R S A L A N ) K O L K A T A - 1 7

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Three-Dimensional Representations

There are two main categories of molecules, those that are in a straight-chain, or 

acyclic, form, and those that are in a ring, or cyclic, orientation.

The following pages will give information on the different structural notations, so thatmore complex molecules can be compared in later sections.

  Straight-Chain Structures 1. Wedge-Dash Notation 2. Newman Projections 3. Sawhorse Projections 4. Fischer Projections 

Three-Dimensional Representations: Straight-ChainStructures

HBr 

H

HCH3

Br COOH

CH2OH

HOH

HOH

OHH

CH3

CH3

CH3

Br 

Br  H

CH3

H CH3Cl

 

There are four main types of representations for straight-chain molecules, as shown above. From

left to right, these structures are the Wedge-Dash, the Newman Projection, the Fisher Projection,

and the Sawhorse Projection… 

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Flying-Wedge or Wedge-Dash projection

The Flying-Wedge projection is the most common three-dimensional representation of athree dimensional molecule on a two dimensional surface (paper). This kind of representation is usually done for molecules containing chiral centre. In thisrepresentation, the ordinary lines represent bonds in the plane of the paper. A solidWedge ( ) represents a bond above the plane of the paper and a dashed wedge ( )or a broken line ( ) represents a bond below the plane of the paper. The Flying-Wedgeprojection formula of (R)- Lactic acid , for example, can be shown as follows…..  

CH3

CH3 H

OH

bonds in the planeof the paper 

bond bellow the plane of the paper 

bond above the plane of the paper 

(R) - Lactic acid 

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Newman Projections 

Newman Projections are used mainly for determining conformational relationships. Recall thatconformers are molecules that can be converted into one another by a rotation around a singlebond. Newman Projections are also useful when studying a reaction involving prochiral molecules thathave a double bond, in which the addition of a new group creates a new stereocenter.

In this notation, you are actually viewing a molecule by looking down a particular carbon-carbon bond.The front carbon of this bond is represented by a dot, and the back carbon is represented by a largecircle. The three remaining bonds are drawn as sticks coming off the dot (or circle), separated by oneanother by 120 degrees. A Newman Projection can be drawn such that the groups on the front carbonare staggered (60 degrees apart) or eclipsed (directly overlapping) with the groups on the back carbon.Below are two Newman Projections of ethane, C2H6. The structure on the left is staggered, and the

structure on the right is eclipsed. These are the simplest Newman Projections because they have onlytwo carbons, and all of the groups on both carbons are identical.

H

H HH

H

Eclipsed

H

 

H H

HH

Staggered

H

H

  Adding more carbons makes Newman Projections more complicated. For example, Newman Projections can be made for butane, such that

it’s eclipsed , gauche, and anti conformations can be seen. (Recall that these threeforms of butane are conformational isomers of one another.) In this case, the front dotrepresents the second carbon in the butane chain, and the back circle represents thethird carbon in the butane chain. The Newman Projection condenses the bondbetween these two carbons.

CH3

CH3

H

HH

H

CH3

HH

CH3

CH3

CH3H

H

H H

HH

Eclipsed Gauche Staggered

 

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Sawhorse Projections 

Sawhorse Projections are very similar to Newman Projections, but are used more often because thecarbon-carbon bond that is compressed in a Newman Projection is fully drawn out in a SawhorseProjection. When properly laid-out, Sawhorse Projections are useful for determining enantiomeric or diasteromeric relationships between two molecules, because the mirror image or superimposibilityrelationships are clearer.

Like with Newman Projections, a Sawhorse Projection is a view of a molecule down a particular carbon-carbon bond, and groups connected to both the front and back carbons are drawn using sticks at 120degree angles. Sawhorse Projections can also be drawn so that the groups on the front carbonare staggered (60 degrees apart) or eclipsed (directly overlapping) with the groups on the back carbon.Below are two Sawhorse Projections of ethane. The structure on the left is staggered, and the structureon the right is eclipsed. These are the simplest Sawhorse Projections because they have only twocarbons, and all of the groups on the front and back carbons are identical.

H

H

H

HH

H

HH

H

HH

H

  Adding more carbons makes Sawhorse Projections slightly more complicated. As withNewman Projections, Sawhorse Projections can be made for butane, such thatit’s eclipsed , gauche, and anti conformations can be seen. (Recall that these threeforms of butane are conformational isomers of one another.) Notice that is much easier to determine the number of carbons in the longest chain using SawhorseProjections than it is for Newman Projections because the carbon-carbon bondbetween the second and third carbons is drawn out.

H H

CH3

H H

CH3

CH3

CH3

HH

H H

CH3

H H

H

CH3H

Eclipsed Gauch Staggered 

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Fischer Projections 

The Fischer projection, devised by Hermann Emil Fischer  in 1891, is a two-dimensional representationof a three-dimensional organic molecule by projection. Fischer projections were originally proposed for 

the depiction of carbohydrates and used by chemists, particularly in organic chemistry and biochemistry. The use of Fischer projections in non-carbohydrates are discouraged, as such drawings are ambiguous

when confused with other types of drawing. 

CH3

CH3

H OH

OH H

OH H

 

Fischer Projections are used often in drawing sugars and hydrocarbons, because the carbon backbone

is drawn as a straight vertical line, making them very easy to draw. When properly laid-out, Fischer 

Projections are useful for determining enantiomeric or diasteromeric relationships between two

molecules, because the mirror image relationship is very clear.

In a Fischer Projection, each place where the horizontal and vertical lines cross represents a carbon.The vertical lines are actually oriented away from you (similar to dashes in the Wedge-Dash Notation) and

the horizontal lines are oriented toward you (similar to wedges in the Wedge-Dash Notation).

Cl

OH

CH3

Br 

CH3

ClBr 

OH

??

 just copy this on ur page ......

u will get the fischer projection

OH

CH3

Br Cl

that u see....