Isomerism

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Chirality and

Stereogenic Centers• What is chirality?

?The word "chirality" refers to the property of "handness". You right and left hands are very similar, yet

they are not identical. They are related to each other as mirror images, and as such they can not be

superimposed on top of each other. Molecules can also be chiral if they contain one or more chiral

centers.

• What is chiral centre?

For the purposes of introductory organic chemistry, a chiral center can be defined as an sp3

hybridizedcarbon which is bonded to four different groups. more contemporary term for "chiral center" !or

chiral carbon is stereogenic center !or stereocenter, and the terms will be used interchangeably. i.e.#f

all the four bonds of carbon are satisfied by four different atoms $ groups, it is chiral. %ere it should be

noted that isotopes are regarded as different atoms&groups chiral carbon is designated by an asteris'

!(.

)hirality, resulting from the presence of four different groups around a carbon results from an

asymmetry in the molecule. This can be seen by e*amining the molecules shown below+ when

two identical groups are on one carbon, there is an internal mirror plane passing through the

molecule+ when four groups are present, there is no internal mirror plane !no symmetry and

hence the carbon is chiral.

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Refer to this video for chirality:-

• Optical isomerism in 2-bromobutane:

The carbon which is bound to the bromine !carbon - is a stereogenic center+ it is sp3

hybridized and it is bonded to four different groups i.e. a hydrogen atom, a bromine atom, a

methyl group and an ethyl group.

ll of the other carbons in this molecule are not stereogenic centers, since they are all bonded to

at least two hydrogens.

-)hloro3bromobutane has two stereogenic centers/

)arbon - in this molecule is sp3 hybridized and is attached to four different

groups i.e. a hydrogen atom, a chlorine atom. a methyl group and a 0 )%1r)%3 group.

2i'ewise, carbon 3 is also sp3 hybridized and is attached to four different groups/

a hydrogen atom , a bromine atom, a methyl group, and a )%)l)% 3 group.

The remaining carbons are both sp3 hybridized, but each is attached to three hydrogens, and

neither is a stereogenic center.

• Optical isomerism in bromochloroiodomethane:

The structural formula of bromochloroiodomethane is

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The molecule has one chiral carbon as designated by star. o molecule is chiral. #t is non

superimposable on its mirror image.

ccording to 4an5t %off rule,

Total number of optical isomers should be = 2n; when n is number of chiral centre

The Fischer pro6ections of the two isomers are

• 7onsuper imposable

• )an rotate 882

• 9ptically active

Stereoisomers which are mirror - image of each other are called enantiomers or

enantiomorphs. (i) and (ii) are enantiomers. ll the physical and chemical

properties of enantiomers are same e!cept two"

:. They rotate 882 to the same e*tent but in opposite direction. 9ne which rotates 882 in

cloc'wise direction is called de*trorotatory !de*tro is 2atin word meaning thereby right and is

designated by d or !;. 9ne which rotates 882 in anticloc'wise direction is called laevo

rotatory !means towards left and designated by l or !0.

-. they react with optically active compounds with different rates.

Enantiomers, Diastereomers & Meso Compounds

#f an organic molecule contains more than one chiral carbons then the molecule may be chiral or

achiral depending whether it has element of symmetry or not.

Elements of symmetry: #f a molecule have either

a a plane of symmetry, and $ or

b centre of symmetry, and $ or

c nfold alternating a*is of symmetry

#f an ob6ect is superimposable on its mirror image+ it cannot rotate 882 and hence optically

inactive. #f an ob6ect can be cut e*actly into two e<ual halves so that half of its become mirror

image of other half., it has plane of symmetry.

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Centre of symmetry: #t is a point inside a molecule from which on travelling e<ual distance in

opposite directions one ta'es e<ual time

Thus, if an organic molecule contains more than one chiral carbons but also have any elements

of symmetry, it is superimposable on its mirror image, cannot rotate 882 and optically inactive.

#f the molecule have more than one chiral centres but not have any element of symmetry, it must

be chiral.

• What are enantiomers?

tereoisomers which are related to each other as mirror images are called enantiomers.

=nantiomers can contain any number of stereogenic centers, as long as each center is the e*act

mirror image of the corresponding center in the other molecule.

#f one or more of these centers differs in configuration, the two molecules are no longer mirror

images, but are totally different chemical compounds with differing physical and biological

properties. tereoisomers which are not enantiomers are called diastereomers.

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For a molecule with multiple chiral centers, the number of possible diastereomers is given

by Van't Hoff rule : x = 2n

>here * is the number of possible isomers and n is the number of stereogenic centers. Thus, for

molecules with two stereogenic centers there are four possible stereoisomers. For cholesterol,

with eight stereogenic centers, there are -?@ possible stereoisomers, etc.

third type of stereoisomer which must be considered is a meso compound. meso compound

contains at least two stereogenic centers, yet the molecule itself is not chiral. This is because

meso compounds contain an internal plane of symmetry+ the molecule can be split by an

imaginary mirror so that all atoms on one side of the mirror are the e*act reflection of the atoms

on the other side. This can be seen below for cis:,-dimethylcyclopentane+ there are two chiral

centers in the molecule since the two carbons labeled with the red asteris' are each bonded to

four different groups. mirror placed through the molecule, along the plane indicated by the

dashed line, will e*actly bisect the molecule with all groups e*actly reflected by their counterparts

on the other side of the "mirror".

• Stereoisomerism in 2,3-dibromopentane?

The structural formula of -,3dibromopentane is

The molecule contains two chiral carbons and hence according to 4an5t %off rule the total

number of optical isomers should be -n A -- A B and it is. The four optical isomers are.

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#,##,### and #4 are four stereoisomers of -,3dibromopentane.

# and ### are enantiomers.

### and #4 are also enantiomers

>hat is relation between # and ###+ or # and #4+ or ## and ###+ or ## and #4&

ll these pairs are diastereomers, stereoisomers which are not mirror image of each other are

called diastereomers.

Therefore,

# and ### are diastereomers

# and #4 are diastereomers

## and ### are diastereomers

## and #4 are diastereomers

• Stereoisomerism in Tartaric Acid

The #C8) name of tartaric acid is -,3 dihydro*y butandioic acid. The structural formula is

%99)()%9%()%9%)99%

The molecule contains two chiral carbon and the number of optical isomers should be - nA--AB+

but number of optical isomers reduces to 3 because one molecule has plane of symmetry.

### and #4 are same

Dotation of #4 by :EG yield ###.

# and ## are enantiomers

### is mesoform of tartaric acid.

meso compound is one which is optically inactive although have more than one chiral carbons.

Numer of !"tical #somers

7umber of possible optical isomers in compounds containing different no. of asymmetric atoms.

1. The molecule has no symmetry

The no. of d and l – forms a = 2n

n = no. of asymetric atoms

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The no. of meso l- forms m = 0

Total no. of optical isomers = a + m = 2n

2. The molecule has symmetry

The no. of d and l forms a = 2n -1

Meso forms m = 2n–1

Total = a + m

It is add a = 2n–1 , m = 2n-1!"2

• Difference beteen racemic mi!ture and meso compound

racemic mi*ture contains e<uimolar amounts of enantiomers. #t is optically inactive due to

e*ternal compensation. #t can be resolved into optically active forms. meso compound is

optically inactive due to internal compensation.

!"tically active com"ounds havin$ no chiral caron

The presence of chiral carbon is neither a necessary nor a sufficient condition for optical activity,

since optical activity may be present in molecules with no chiral atom and since molecules with

two or more chiral carbon atoms are superimposable on their mirror images and hence inactive.

i ny molecule containing an atom that has four bonds pointing to the corners of a tetrahedron will

be optically active if the four groups are different.

ii! #toms $ith pyramidal %ondin& mi&ht %e e'pected to &i(e rise to

optical acti(ity if the atom is connected to three different &roups,

since the unshared pair of electron is analo&ous to a fourth &roup.

Many attempts ha(e %een made to resol(e such compounds, %ut until

recently all failed %ecause of um%rella effect, also called

pyramidal in(ersion. The um%rella effect is rapid oscillation of the

unshared pair form one side of )* plane to the other.

iii! iphenyls, containin& four lar&e &roups in the ortho position

so that there is restricted rotation, are optically acti(e if the

rin&s are asymmetrical.

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If either or %oth rin&s are symmetrical, the molecule has

plane of symmetry and optically inacti(e.

#llenes, $ith e(en num%er of cumulati(e dou%le %onds are optically

acti(e if %oth sides are dissymmetric.

%"ecific Rotation

The specific rotation H I is an inherent physical property of an enantiomer, which varies with the

solvent used, temp !in G) and wavelength of the light used. #t is calculated from the observed

rotation a as follows.

H I A

>here l A length of tube in decimeters !dm

) A )oncentration in gram cm 03, for a solution density in gcm 03, for a pure lu<id

Isomerism

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