Chemistry Term

8/4/2019 Chemistry Term

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CHEMISTRY TERM

PAPER

(CHE 101)

OPTICAL ISOMERISM INCOORDINATION COMPOUNDS

Submitted By:- Submitted To:-

Jagwant Dr. Ashish Kumar

Roll No.:-RB1901B45 CHEMISTRY

Lecturer

Section:-

Group-2


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Acknowledgement

This is a humble effort to express our sincere gratitude towards those who have

guided and helped us to complete this project.

A project reported is major milestone during the study period of a student. We

could have faced many problems but our teachers kind response to our needs

and requirement, their patient approach and their positive criticism helped us

in making our project. Very warm thanks to our project-in-charge Dr Ashish

Kumar with her support and constant encouragement and LPU LIBRARY

it was not very easy without whose support to finish our project .


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TABLE OF CONTENTS

1. Introduction

2. History

3. Isomerism in coordination compounds

4. Conditions for optical isomerism

5. Formation of optical isomers

6. Priority for enatiomereric carbon compounds

7. Properties of optical isomerism

8. Chiral and achiral molecules

9. Examples of optical isomers

10.Polarization of light

11.Optical isomerism in amino acids

12.Refrences


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INTRODUCTION:-

Coordination Compounds :- Those compound which contain metal ion and appended groups,

which are Lewis bases and can be monoatomic or polyatomic, neutral or anionic, are called

coordination compounds.

Ligand:- Lewis base bonded to a metal ion in a coordination complex, are called ligands.

Ligands which have one point of attachment are called monodentate ligands.

Ligands which have two (or more) coordinated, do not atoms are called bidentate (orpolydentate) liagnds

Coordination number:- Number of ligands bonded to metal ion.

Coordination geometry (octahedral, tetrahedral, square planar):-Geometrical arrangements of ligands arround a metal ion or ways in which ligands can be

bonded to a metal ion.

Isomers: When two or more compounds having the same molecular formula, buthave a different arrangement of the atoms in space are called isomers . Thatexcludes any different arrangements which are simply due to the molecule rotatingas a whole, or rotating about particular bonds.

Structural isomer:- compounds having same composition, but different atom connectivity arecalled structural.

Stereo isomer:- compounds having same atom connectivity, but different spatial arrangement

of atoms are called streoisomers.

Optical isomers are two compounds which contain the same number and kinds of atoms, and

bonds and different spatial arrangements of the atoms in space, but have non-super imposable

mirror images. Each non-super imposable mirror image structure is called an enantiomer.

Molecules or ions which exist as optical isomers are called chiral. Optical isomers are the part of

stereoisomers.

Chiral: A chiral molecule is amolecules type that have not an internal plane of symmetry and

has a non-superimposable mirror image. The cause of chirality in molecules is due to the

presence of an asymmetric carbon atom. Optical isomers are also called chiral.

HISTORY:-


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Le Bel was the first scientist ,who describe the concept of optical isomerisataion. He described

that a molecule in which four different atoms or groups were attached to a carbon atom would

exist in two forms, mirror images that could not be superimposed. One of that form would bedissymmetric and thus optically active. He also described correctly that other elements also

would give rise to optically active compounds.

The term optical activity is derived from the interaction of chiral materials with polarized light.

A solution of the ()-form of an optical isomer rotates the plane of polarization of a beam ofplane polarized light in a counterclockwise direction, in a clockwise direction for the (+) optical

isomer. The polarization property was first predicted by Jean-Baptiste Biot in 1815, and he

gained good importance in the sugar industry, analytical chemistry, and pharmaceuticals.

The first resolution of optical isomers was observed by Werner and King for thecomplexes cis-[CoX(NH3)(en)2]2+, where X=Cl- or Br-.

Isomerism in Coordination Compounds:-

Two or more different compounds having the same

formula are called isomers. Two main types ofisomerism are known among coordination

compounds. Each part have further subparts.

1. Stereoisomerism.

a) Geometrical isomerismb) Optical isomerism

2. Structural Isomerism.

a) Coordination isomerism

b) Ionization isomerism

c) Hydrate isomerismd) Linkage isomerism

Stereoisomers Stereoisomers have the same atoms, same sets of bonds, but

differ in the relative orientation of these bonds.

Geometric isomers are possible for both square planar and

octahedral complexes, but not tetrahedral. Optical isomers are possible for both tetrahedral and

octahedral complexes, but not square planar.

examples of stereoisomerism complex of Co3+. salts of [CoCl2(en)2]+,

It is cis- and trans- geometric isomers.

Optical Isomers

Optical isomers are related as nonsuperimposable mirror images and differ in

the direction with which they rotate planepolarised light.

These isomers are referred to as enantiomers

or enantiomorphs of each other and theirnon-superimposable structures are described


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as being asymmetric.

method to denote the absolute configuration of optical isomers:-

Methods are R or S, or or C and A The IUPAC rules :- for octahedral complexes helixes are defined and

then designated Lambda (left-handed) and Delta (right-handed)

For tetrahedral complexes R and S is usedsimilar method to tetrahedral Carbon species.

Conditions for optical isomerism:-

Optical isomers can occur when there is an asymmetric carbon atom. An asymmetric carbon

atom is one which is bonded to four different groups. The groups can be something complex, orsomething simple like a hydrogen or chlorine atom.

Remember these two conditions:-


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there must be four groups bonded to carbon atom , and

all the bonded groups must be different.

Formation of optical isomers:-

Priority for enatiomereric carbon compounds:-

Priorities are assigned for mononuclear coordination systems based on the standard sequence

rules developed for enantiomeric carbon compounds by Cahn, Ingold and Prelog (CIP rules).

By using the coordinating atom these rules arrange the ligands into a priority order such that thehighest atomic number gives the highest priority number.

Example:-

The hypothetical complex [Co Cl Br I NH3 NO2 SCN]2-

Here CIP rules would assign the I- as 6, Br as 5, Cl as 4, SCN as 3, NO2 as 2 and NH3 as 1.


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The reference axis for an octahedral centre is that axis which contain the ligating atom of CIPpriority 1 and the trans ligating atom of lowest possible priority (highest numerical value). The

atoms in the coordination plane perpendicular to the reference axis are viewed from the ligand

having that highest priority (CIP priority 1) and the clockwise and anticlockwise sequences ofpriority numbers are compared. The structure is assigned the symbol C or A, according to

whether the clockwise (C) or anticlockwise (A) sequence is lower at the first point of difference.

In the example shown above this would be C.

The two optical isomers of [Co(en)3]3+ have identical chemical properties and just denoting their

absolute configuration does not give any information regarding the direction in which they rotateplane-polarised light. We can determine it from measurement and then the isomers are further

distinguished by using the prefixes laevo ((-) or l) and dextro ((+) or d) depending on whether

they rotate left or right. The use of l- and d- is not recommended.


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left-handed isomer and right-handed isomer

For tetrahedral complexes, R and S would be used in a similar method to tetrahedral Carbon

species and although it is predicted that tetrahedral complexes with 4 different ligands should beable to give rise to optical isomers, generally tetrahedral complexes are too labile and can not be

isolated.

Properties of optical isomerism:-

Substances which show optical isomerism exist as two isomers known as

enantiomers.

Enantiomers solution which rotates the plane of polarisation in a

clockwise direction. That type of enantiomer is called the (+)form.

For example, one optical isomers (enantiomers) of the amino acid

alanine is known as (+)alanine.

Enantiomers solution which rotates the plane of polarisation inan anti-clockwise direction. That type of enantiomer is known as

the (-) form. One enantiomer of alanine is known as or (-)alanine.

If the solutions are equally concentrated the amount of rotation

caused by the two isomers is exactly the same - but in oppositedirections.

If we make the optically active substances in the lab, they will


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occur as a 50/50 mixture of the two enantiomers. This is called a

racemic mixture orracemate. It does not effect the plane

polarised light.

Chiral and achiral molecules:-

The essential difference between the two examples we have looked at lies

in the symmetry of the molecules.

If there are two same groups attached to the central carbon atom, themolecule has a plane of symmetry. If you imagine slicing through the

molecule, the left-hand side is an exact reflection of the right-hand side.

Where there are four groups attached, there is no symmetry anywhere inthe molecule.

A molecule which has no plane of symmetry is described as chiral. The

carbon atom with the four different groups attached which causes this lackof symmetry is described as a chiral centre or as an asymmetric carbon

atom.

The molecule on the left above (with a plane of symmetry) is described as

achiral.

Only chiral molecules have optical isomers.

The relationship between the enantiomers

One of the enantiomers is simply a non-superimposable mirror imageof the other one.

In other words, if one isomer looked in a mirror, what it would see is the

other one. The two isomers (the original one and its mirror image) have adifferent spatial arrangement, and these two isomers can't be


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superimposed on each other.

If an achiral molecule (one with a plane of symmetry) looked in a mirror,you would always find that by rotating the image in space, you could

make the two look identical. It would be possible to superimpose the

original molecule and its mirror image.

examples of optical isomers

Butan-2-ol

The asymmetric carbon atom in a compound which is attached with four

different groups is often shown by a star.

It's extremely important to draw the isomers correctly. We have to draw

one of them using standard bond notation to show the 3-dimensionalarrangement around the asymmetric carbon atom. Then the mirror image

will be.

.

We can draw the four groups in any order around the central carbon. As

long as your mirror image is drawn accurately, you will automatically

have drawn the two isomers.


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2-hydroxypropanoic acid (lactic acid)

Here the chiral centre is shown by a star which is attached to four

different groups.

The two enantiomers are:

2-aminopropanoic acid (alanine)

The naturally-occurring amino acids.

The two enantiomers are:

Only one of these two isomers occurs naturally: the (+) form.


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P olarisation of light:-

By passing ordinary light through a polarizing filterPlane-polarized light is obtained.

In a polarimeter the plane-polarized light is passedthough a chiral solution and the polarization plane

measured with an analyzing filter. If the plane rotates the light to the right then it would be dextrorotatory. If the plane rotates the light to the left then it would be levorotatory.

Equal amounts of each are racemic.

Examples of optical active and non active


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Optical isomerism in amino acid:-

Optical isomerism is a very important feature of the structure of amino acids . It applies to allamino acids except glycine.

The number-two carbon atom is in one

direction it is bonded to an amino group. It is

bonded to a carboxylic group , in anotherdirection. It is also bonded to a hydrogen

atom and an alkyl group or some other kind

of group. In the case of glycine where -R is a-H, that number two carbon atom is bonded

to four different groups. A carbon atom

which is attached to four different groups isknown as an asymmetric carbon atom or

sometimes it is also called chiral carbonatom. The importance of this depends onsome structural properties .


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We can notice that two structures are not identical to each other by trying to

superimpose one structure on another and get all of the same colored units tobe in the identical places. we can see it is not possible.

The two structures are different. These two structures are isomers of one

another. Therefore they are called optical isomers of one another because

they have optical properties that are different from one another.

Two enantiomers of a generic amino acid:-

(S)-Alanine (left) and (R)-alanine (right) in zwitterionic form ,this is at

neutral pH.

Refrences:-

Websites:-

(1) http://www.reference.com/browse/Chirality_(chemistry)

(2) http://wwwchem.uwimona.edu.jm/courses/IC10Kiso.html

(3) http://www.tutornext.com/isomerism-coordination-compounds/2467

(4) http://www.chemguide.co.uk/basicorg/isomerism/optical.html#top

(5) http://dl.clackamas.cc.or.us/ch106-05/optical.htm#top

(6) http://www.wisegeek.com/what-are-optical-isomers.htm
http://en.wikipedia.org/wiki/Amino_acidhttp://www.reference.com/browse/Chirality_(chemistry)http://wwwchem.uwimona.edu.jm/courses/IC10Kiso.htmlhttp://www.tutornext.com/isomerism-coordination-compounds/2467http://www.chemguide.co.uk/basicorg/isomerism/optical.html#tophttp://dl.clackamas.cc.or.us/ch106-05/optical.htm#tophttp://www.wisegeek.com/what-are-optical-isomers.htmhttp://en.wikipedia.org/wiki/File:Betaine-Alanine.pnghttp://en.wikipedia.org/wiki/File:Betaine-Alanine.pnghttp://en.wikipedia.org/wiki/Amino_acidhttp://www.reference.com/browse/Chirality_(chemistry)http://wwwchem.uwimona.edu.jm/courses/IC10Kiso.htmlhttp://www.tutornext.com/isomerism-coordination-compounds/2467http://www.chemguide.co.uk/basicorg/isomerism/optical.html#tophttp://dl.clackamas.cc.or.us/ch106-05/optical.htm#tophttp://www.wisegeek.com/what-are-optical-isomers.htm


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Books:-

(1) INORGANIC CHEMISTRY PRINCIPLES OF STRUCTURES AND

REACTIVITY

FOURTH EDITION

By

James E. Huheey

Ellen A. Keiter

Richard L. Keiter

Okhil K. Medhi

(2) ADVANCED INORGANIC CHEMISTRY

VOLUME 2

By

Satya Prakash

G.D. Tuli

S.K.Basu

R.D.Madan

(3)SHRIVE & ATKINS INORGANIC CHEMISTRY

FOURTH EDITION

By

Atkins

Oveerton

Rourke

Weller


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