BUDS PUBLIC SCHOOL, DUBAI
12 th Grade chemistry
HALOALKANES AND HALOARENES
Topic : stereo isomerism
Stereoisomerism : Isomerism exhibited by two or more compounds with the same molecular formula and
structural formula but different spatial arrangement of atoms or groups in space
Optically active substance : Those substances which rotate the plane polarized light when passed
through their solutions
Laevorotatory :Those substances which rotate the plane polarized light towards left denoted by (-) or l
Dextrorotatory:Those substances which rotate the plane polarized light towards right denoted by (+) or d
Asymmetric molecule : If all the four substituent’s attached to carbon are different ,resulting molecule
will lack symmetry .such a molecule is asymmetric molecule. The carbon is asymmetric carbon or stereo
centre or chiral carbon
Chiral :An object which is non –superimposable on its mirror image is said to be chiral . The property of
being chiral is known as chirality .
Enantiomers :The stereo isomers which are non –super imposable mirror images .They rotate the plane
polarized light to the same extent but in opposite direction .
Racemic mixture :An equimolar mixture of pair of enantiomers is called racemic mixture . A racemic
mixture is optically inactive due to external compensation .The process of conversion of an enantiomer
into racemic mixture is racemization .
Resolution of racemic mixture : It is the process of separating a racemic mixture into its enantiomers
Me so compound :The compounds with two or more chiral carbons but possessing achiral molecular
structure because of having plane of symmetry are mesocompounds .A me so compound is optically
active due to internal compensation .
Diastereoisomers : The stereo isomers which are non super imposable non mirror images
Retention :If an optically active molecule the relative configuration of the atom /groups around a chiral
centre remains the same before and after the reaction , the reaction is said to proceed with retention of
configuration
Inversion :If the relative configuration of the atom /groups around a chiral centre in the product is
opposite to that in reactant , the reaction is said to proceed with inversion of configuration
If A is the only compound obtained ,the process is Retention of configuration
If B is the only compound obtained ,the process is Inversion of configuration.
If 50:50 mixture of the above two (A+B) is obtained then the process is called racemisation and the
product is inactive ,as one isomer will rotate light in opposite direction to another.
. Retention of configuration is the preservation of integrity of the spatial arrangement of bonds to an
assymetric centre during a chemical reaction .If during a chemical reaction ,no bond to the stereo centre is
broken will have the same general configuration of groups around the stereocenter as that of reactant .
Chloroform: It is stored in dark colored bottles because it gets oxidized in the presence of sunlight to
form a poisonous gas phosgene (COCl2). It is used as a solvent for fats, waxes ,resins, rubber etc.It is
used as an anesthetic agent in surgery and also as an flavoring agent in cough syrups because it is
poisonous due to phosgene
Iodoform: it is used as antiseptic for dressing wounds .and is used to distinguish between methyl ketones
from other ketones and alcohols .
Carbon tetra chloride (tetrachloromethane ): it is used in the manufacture of refrigerants and
propellants for aerosol cans .it causes dizziness ,lightheadedness ,nausea and vomiting which cause
permanent damage to nerve cells .when carbon tetrachloride is released into air ,it rises to the atmosphere
and depletes the ozone layer .
Freons CCl2F2 :It is manufactured from CCl4 by swarts reaction 3CCl4 + 2 SbF3 → 3CCl2F2 +2 SbCl3.It
is used extensively as propellants for aerosols and foams to spray out deodorants ,cleansers ,hairspray
,shaving cream .use of Freon is reduced because they are responsible for ozone layer depletion .
DDT: It is used as an insecticide and non biodegrabale.it results in pollution due to extreme stability .it is
prepared from chloral and chlorobenzene in presence of conc. sulphuric acid .
If there is no struggle, there is no progress. --Fredrick Douglass, 1857 You don't
learn from easy things. Only hard tasks help you grow intellectually.