isomerism q&a new

8/8/2019 Isomerism Q&A New

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1. What is meant by isomerism ? Mention its types.

Isomers are compounds having same molecular formula but differ in physical or

both physical and chemical properties. This phenomenon is called as isomerism.

Types of isomerism : 1. Structural isomerism 2. Stereois omerism

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2. What is meant by structural isomerism ?

The compounds with same molecular formula and different structural formulae are

called as structural isomers. This prop erty is called Structural isomerism.

This isomerism arises due to the difference in the arrangement of atoms in a molecule or

type of linkage and atoms which are linked to each other in the molecule. ( without

referring to space )

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3. What is meant by stereoisomerism ? Mention its types.

Some compounds have same molecular formula and structure. ut they have

difference in the arrangement of atoms in a molecule or type of linkage with reference to

each other in space. These compounds are called as stereoisomers. This property is

called as Stereoisomerism.

Types of stereoisomerism : 1. Geometrical isomeris m 2. ptical isomerism

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4. What is meant Cis Trans isomerism ? Give example.

1) Isomerism that arises out of difference in the spatial arrangement of atoms or groups

about doubly bonded atoms is called Geometrical isomerism.

2) This isomerism arises due to the hindrance to rotation about the C = C bond in such

molecules

3) Geometrical isomers are not mirror images of each other.

Geometrical isomerism is of two types. They are,

a. Cis Trans isomerism

b. E Z system or Cahn Ingold Prelog convention

a.Cis Trans isomerism :

The isomer in which similar groups lie on the same side is called Cis isomer.

The isomer in which similar groups lie in opposite direction is called Trans isomer.

Rotation about C=C is not possible at normal conditions and hence the se isomers are

isolable.

Example : 2 butene exists in two geometrical forms as follows.

Cis isomer

Two similar groups lie on t e

same side.

Hence it is cis - isomer.

Two similar groups lie on t e

opposite sides.

Hence it is trans - isomer

Trans isomer

H

CH3

H3CCH3

CH3

H

H

C

H

C

C

C

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5. What is meant by E,Z system ? Give example.

1) If no identical groups are found at the two carbon atoms with double bond as in

Br

I

Cl

C

F

C

the geometrical isomers cannot be designated in the form of cis and trans.

The geometrical isomers are designated in the form of E (similar to cis)

and Z (similar to trans).

2) In E Z system or Cahn-Ingold-Prelog convention of geometrical isomerism, the group

of highest priority on each carbon atom is determined first.

3) The priorities follow the order of decreasing atomic number of the atom directly bonded

to the carbon.

4) If the two higher priority gr oups are together on the same side of the double bonded

carbon atoms, the configuration is Z. If they are on opposite sides, the configuration is E.

Z = Zusammen german word means together

E = Entgegen german word means opposite

Example :

1 high

igh priorit

groups (1) are on

opposite sides.

ence E igh priorit

groups (1) are on the

same side. ence Z

low

1 high low

ZE

1 high

3 3

33

low1 high

3

low

3

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6. Label the following as E & Z isomers.

l

Solution :

high (1)

E Z

High p ioritygroups (1) areo

opposite sides. Hence E

H3

high (1)

low (2)

low (2)

high (1)

high (1) low (2)

low (2)

High prioritygroups (1) areonth

same side. Hence Z

H2OH

HH2OH

l H

r

C

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7. Identify cis & trans isomerism.

! "

! "

! "

! "

Solution :

Two s# $ # %

a&

g&oups

% #e on

'

he

sa $ e s # de (

Hence #'

# s c # s - # so $ e & (

s - so e

H3

Two s# $ # %

a&

g&oups

% #e on

'

he

oppos #'

e s # des (

Hence #'

# s'

& ans - # so $ e & (

T ans - so e

H3

H3

B

B

B

H3B

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7/27

8. Why cyclhexanol contains 90 % equatorial conformer and 10% axial conformer ?

(Or)

9. What is meant by 1:3 diaxial interaction ?

1) Mono substituted cyclohexane like cyclohexanol exists in the two chair forms as follows.

These two forms are interconvertible and exist in equilibrium.

1

3

3

)

0 )

0 )

1 ycl2 3 exa 4 2 l5

it 3 axial 6 0 ) 7I) Cycl8 9 exa @ 8 lA

it9

eqB

at8 C

ialD E F

GII)

H

2) In form ( I ) the group is axially oriented. In the other form ( II ) the group is

equatorially oriented.

3) The axial substitutent experiences steric interaction with the axial atoms present

at the third carbon atoms. This is called 1 : 3 diaxial interaction. This interaction

decreases the stability of the axial conformer.

4) This interaction is absent in the equatorial conformer. ence the energy of the axial

conformer is little higher than that of the equatorial conformer.

5) ence equatorial cyclohexanol is present to an extent of about 9 0% in the equilibrium

mixture and the axial con former is present only to 10%.

Note : In equatorial form, the repulsion between the groups is minimum.

ence equatorial is involved in intermolecular hydrogen bonding.

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10. What is meant by plane polarized light ?

1) Light is propagated in the form of waves.2) rdinary monochromatic light is supposed to consist of waves vibrating in all planes.3) If such a light is passed through a Nicol prism, the emergent light is found to consist of

waves vibrating in only one plane. Such a light is called p lane polarised light.

11. What is meant by optical activity? Give an example?

When the plane polarised light is passed through certain substances or solutions(such as lactic acid), the emergent light is found to be vibrating in a different plane.This is called optical rotation.

Such substances are called optically active substances.This property of such substances is called ptical activity.

If the rotation is towards the right side [clockwise] it is called dextrorotation and the

substance is said to be dextrorotatory. Dextrorotation is indicated by the symbol (+) or d.

If the rotation is towards the left side [Anticlockwise] it is called laevorotation and thesubstance is said to be laevo rotatory. Laevorotation is indicated by the symbol () or l.

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14. What are the conditions for a compound to be optically isomerism ?

1) If a carbon atom in a compound is attached with 4 different atoms or groups, is called

as chiral carbon atom. The molecule with one more chiral carbon atoms is called as

chiral molecule. This property is called as chirality. Chirality is the essential condition for

optical activity.

2) The structure of chiral molecule and its mirror image are not superimposible to eachother. This non superimposability is also the essential condition for optical activity.

Example :

CH3

OHH

COOH

CY

d,l- a of lacticacid

m

a a

oa

CH3

OH

C = c b iral carbon atom

H

COOH

Cc

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15. What are optical isomers ? Give examples.

Optical Isomers :

1) Chiral molecules can have two different configurations.

2) Each configuration stands for one optical isomer.

3) Thus dextrorotatory isomer has configuration which is the mirror image of the

laevorotatory isomer.4) Such optical isomers which differ only in the sign of (or direction of) optical rotation

are called enantiomers.

Example :

d - lactic acid l - lactic acid

d and l- lactic acid ard optical isomerto each otherie., enentiomer

H3

HH

H

*

mirror

H3

H

*e chiral carbon atom

H

H

*

d-lactic acid rotates plane polarized light in the clockwise direction.

l-lactic acid rotates plane polarized light in the anti -clokwise direction.

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16. What is racemic mixture? Give an example.

Racemic Mixture :

1) The mixture which has been obtained by mixing equal amounts of d-isomer and l-isomer

together is called racemic mixture. This process is called racemisation.

2) A racemic mixture becomes optically inactive. ecause, in this mixture rotation towards

clockwise direction by the dextro isomers is compensated by the rotation towards theanticlockwise direction by the laevo isomers.

3) The optical inactivity of a racemic mixture is said to be due to external compensation.

4) Any how an optically inactive racemic form can be separated into two active forms.

This process of separation is called resolution of the racemic mixture.

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17. Racemic mixture is optically inactive. Why?

1) The mixture which has been obtained by mixing equal amounts of d-isomer and l-isomer

together is called racemic mixture. This process is called racemisation.


clockwise direction by the dextro isomers is compensated by the rotation towards the

anticlockwise direction by the laevo isomers.


4) Racemic mixture can be separated into two optically active forms.

This process is called as resolution of racemic mixture.

Racemic mixture contains chiral molecules.

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5 Mark Questions:

1. What is meant Geometrical isomerism ? Give example s.

1) Isomerism that arises out of difference in the spatial arrangement of atoms or groups

about doubly bonded atoms is called Geometrical isomerism.

2) This isomerism arises due to the hindrance to rotation about the C=C bond in suchmolecules

3) Geometrical isomers are not mirror images of each other.

Geometrical isomerism is of two types. They are,

a. Cis Trans isomerism


a.Cis Trans isomerism :

The isomer in which similar groups lie on the same side is called Cis isomer.

The isomer in which similar groups lie in opposite direction is called Trans isomer.

Rotation about C=C is not possible at normal conditions and hence these isomers are

isolable.

Example : 2 butene exists in two geometrical forms as follows.

Cis isomer

Two similar groups lie on the

same side.

Hence it is cis - isomer.

Two similar groups lie on the

opposite sides.

Hence it is trans - isomer

Trans isomer

H

CH3

H3CCH3

CH3

H

H

C

H

C

C

C


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Br

I

Cl

C

F

C

the geometrical isomers cannot be designated in the form of cis and trans. T he

geometrical isomers are designated in the form of E (similar to cis)and Z (similar to trans).




to the carbon.

3) If the two higher priority groups are together on the same side of the double bonded

carbon atoms, the configuration is Z. If they a re on opposite sides, the configuration is E.



Example :

1 high

f igh p g iog ity gg oups h 1) a g e on

opposite sides. f ence Ef igh pg iog ity gg oups h 1) a g e on the

sa i e side. f ence Z

lo

1 high lo

ZE

1 high

3 p3

33

lo1 high

3

lo

3p

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2. Outline the procedure for determining E Z notation for geometrical isomers.

In hat ay this system is superior to cis trans nomenclature ?


Br

I

Cl

C

F

C

the geometrical isomers cannot be designated in the form of cis and trans. The

geometrical isomers are designated in the form of E (similar to cis)and Z (similar to trans).




to the carbon.

3) If the two higher priority groups are together on the same side of the double bonded

carbon atoms, the configuration is Z. If they are on opposite sides, the configuration is E.



Example :

1 high

q igh p r ior ity gr oups s 1) a r e on

opposite sides. q ence Eq igh pr ior ity g r oups s 1) a r e on th

sa t e side. q ence Z

lo

1 high lo

ZE

1 high

3 u3

33

lo1 high

3

lo

3u

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3. Outline the comparison bet een cis and trans forms.

1) The cis and trans isomers do not differ much in chemical properties.

2) They differin physical properties like boiling point, melting point, crystal structure,

solubility and refractive index.

3) ighly substituted olefin is more stable than less su bstituted olefin. Amongsubstituted olefins, trans olecin is more stable than cis olefin.

4) In the cis isomer, similar groups are very near to each other. ence, Vander Waals

repulsion and steric hindrance make th e molecule much unstable.

5) In the trans isomer, similar groups are diagonally opposite to each other. ence, there

is no such steric interaction. And therefore trans isomer is more stable.

6) The energy of the cis isomer is greater than that of trans isomer and hence the reactivity

of cis isomer is slightly higher than the trans isomer.

Interconversion of cis and trans forms :

1) At room temperature, cis and trans iso mers are stable and are not interconvertible.

2) Interconversion of cis into trans and trans into cis is possible at high temperature.

3) When heated, breaking of carbon carbon T - bond and its reformatin is responsible for

the interconversion.

Trans isomer

Cis isomer

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4. Outline the isomerism in 1,3 butadiene.

1) If the isomerism arises due to restricted rotation about C C bond and not due to C = C

bond is known as cisoid transoid isomerism or S-cis S-trans isomersm.

2) 1,3 butadiene [ C 2 = C C = C 2 ] exhibits this type of isomerism.

3) The S-cis and S-trans forms are similar to cis and trans respectively. ut, they are dueto restricted rotation along C C bond and not C = C bond.

4) These two forms do not differ very much in their energy and stability

CH2 CH

Cisoid Transoid

CH2

CH2

CH

CH

CH2CH

.

(S v Ciw ) (Sv x ry w )

Ci

i

r

i

5) The cisoid and transoid forms exist in equilibrium and they are inter convertible.

(S- Cis (S-Trans

Cisoid Transoid

6) S-trans is having 3 Kcal energy lower than S-cis form. ence, S- trans is more stable

than S-cis.

7) Cisoid and transoid forms differ in chemical reactivity and very often they give different

products in reactions.

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5. Explain about the conformations of the follo ing cycloalkanes:

a. cyclo propane b. cyclo butane c. cyclo pentane

1) The different forms which are formed due to rotation about C C single bond are called

conformers.

2) ree rotation about C C single bond is not always possible. The restriction in C C

sigle bond rotation depends on the nature and the size of atoms or groups bonded toeach carbon atom.

3) The different conformers differ slightly in energy.

4) Restriction to free rotation about C C single bond is much pronounced in cyclic

systems and hence the different forms are isolable or identifiable.

5) Thus Cyclopropane is flat .

Cyclobutane can form butterfly shape .

Cyclopentane can form an open envelope shape.

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6. Dra the different conformations of cyclohexane clearly indicating the

various C H bonds.

1) In 1890, sachse suggested that cyclohexane ring exists in two non planar forms.

i.e., boat and chair conformations.

2) The energy difference between these two forms was found to be only 6.5 K.Cal/mole

by Pitzer. The energy difference is small. ence interconversion can take place.

3) oth the boat form and chair form are free of angle strain called aeyers strain.

( i.e., the valence bond angle is 109 28)

Chair conformation :

There are two types of C bonds

Every pair of carbon atoms ( Two carbon atoms directly bonded to each other ) in chair form existsin skew from. ence the only destabilizing interaction is skew butane interaction.


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Boat conformation :

1) In boat form the eclipsing interaction between x and x destabilizes this from.

In addition to that, the flage pole hydrogen ( f,s ) are very close to each other leading to

steric strain.

2) oth these destabilizing interactions make boat from less stable than the chair form.

3)Though the chair form is stable, it is sufficiently flexible to turn itself upside down calledring flipping so that all bonds which are axial originally become equatorial and vice versa.

4)Thus, there exists an equilibrium between two chair forms with boat from as intermediate.

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7. Describe the conformations of cyclohexanol. Comment on their stability.

1) Mono substituted cyclohexane like cyclohexanol exists in the two chair forms as follows.

These two forms are interconvertible and exist in equilibrium.

1

3

3

H

OH

OH

Cy

l

l

it

i

l

OHI) Cy l

l

it j k t l i l m OHnII)

H

2) In form ( I ) the group is axially oriented. In the other form ( II ) the group is

equatorially oriented.

3) The axial substitutent experiences steric interaction with the axial atoms present

at the third carbon atoms. This is called 1 : 3 diaxial interaction. This interaction

decreases the stability of the axial conformer.

4) This interaction is absent in the equatorial conformer. ence the energy of the axial

conformer is little higher than that of the equatorial conformer.

5) ence equatorial cyclohexanol is present to an extent of about 90% in the equilibrium

mixture and the axial conformer is present only to 10%.

Note : In equatorial form, the repulsion between the groups is minimum.

ence equatorial is involved in intermolecular hydrogen bonding.

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8. Explain about the optical activity of lactic acid.

Optical Activity of Lactic Acid :

The structure of lactic acid can be illustrated as

C*= chiro l c o r n o t m

CH3

H OH

COOH

C*

L ctic cid

As lactic acid contains one chiral carbon atom, it is optically active. The structure of lactic acid

and its mirror image are non -superimposible. It can be illustrated as follows

d,l- r flact c acid

d - lactic acid l - lactic acid

d and l- lactic acid ar optical isomerto each otherie., enentiomer

CH

HH

COOH

C*

mirror

CH

OH

C* chiral car on atom

H

COOH

C*

d latic acid :

d lactic acid rotates the plane polarized light to clockwise ( right )direction. ence it is

called as dextro rotatory and is denoted as (+)lactic acid.

l lactic acid :

l lactic acid rotates the plane polarized light to anti -clockwise ( left ) direction. ence it is

called as laevo rotatory and is denoted as ( )lactic acid.


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Enantiomers :

The d,l pair of lactic acid are enantiomers. They have same magnitude but different sign

of optical rotation.

Racemic Mixture or dl mixture :

1) The mixture which has been obtained by mixing equal amounts of d-lactic acid and

l-lactic acid together is called racemic mixture. This process is called racemisation.


clockwise direction by the d lactic acid is compensated by the rotation towards the

anticlockwise direction by the l lactic acid.



This process is called resolution of racemic mixture.


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9. Explain about the optical activity of tartaric acid.

Optical Activity of Tartaric Acid :

The structure of tartaric acid can be illustrated as

tartariz

aci{

*

OO

*

O

O

OO

*=ch

ira

lcarb | }

atom

As tartaric acid contains two identical chiral carbon atoms, it is optically active.

Usually the number of isomers for the molecule with n different chiral centres is 2n.

As tartaric acid contains two identical chiral carbon atoms, the number of optical isomers is not

22

= 4, it is less than 4 i.e., 3

The structure of tartaric acid and its mirror image are non -superimposible. It can be illustrated as

follows

d,l-pai~

o

Ta~

a~

ic acid

C* C*

COOH

HO

COOH COOH

d and l-

a ~

a ~ ic acid a ~ e op

ical iso e ~

o each o

he ~ ie., enen

io e ~

C*C*

H

HH

HOH

OHHO

COOHmirror

C*= chi ~ al ca ~ bon a

o

d tartaric acid :

d tartaric acid rotates the plane polarized light to clockwise ( right )direction. ence it is

called as dextrorotatory and is denoted as (+) tartaric acid.

l tartaric acid :

l tartaric acid rotates the plane polarized light to anti -clockwise ( left ) direction . ence it is

called as laevorotatory and is denoted as ( ) tartaric acid.


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Enantiomers :

The d,l pair of tartaric acid are enantiomers. They have same magnitude but different

sign of optical rotation.

Racemic Mixture or dl mixture :

1) The mixture which has been obtained by mixing equal amounts of d-tartaric acid and

l-tartaric acid together is called racemic mixture. This process is called racemisation.

2) A racemic mixture becomes optically inactive. ecause, in this mixture rotati on towards

clockwise direction by the d tartaric acid is compensated by the rotation towards the

anticlockwise direction by the l tartaric acid.



This process is called resolution of racemic mixture.


Mesotartaric acid :

C*

COOH

C*= c iral carbon atom

C*

H

H

OH

OH

COOH

-------------------------------- lane ofsymmetry

Mesotartaricacid

1) Mesotartaric acid has two asymmetric carbon atoms.

2) There is a symmetric plane, which divides the molecule into two equal halves.

ence whole molecule becomes achiral.

The two halves are super imposable to each other.

3) ne half of the molecule (with one chiral carbon atom) is dextrorotatory and

the other is laevorotatory. oth are rotating the plane polarized light to the same

extent in opposite directions.

4) ence mesotartaric acid becomes optically inactive due to internal compensation .

Meso form is a single substance and not a mixture. ence separation into optically active

isomers is not possible.

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10. Distinguish: Enantiomers and Diastereomers.

Enantiomer Diastereomer

1. Enantiomers having the same magnitude butdifferent sign of optical rotation.

Differ in the magnitude of optical rotation

2. They have configuration with non -super Iimposable object mirror image relationship.

They are never mirror images.

3. Enantiomers are identical in all propertiesexcept the sign of optical rotation.

Diastereomers differ in all physical properties.

4. Separation of enantiomers is a tediousprocess.

Separation from the other pai rs of enantiomersis easy.

5. Example : d,l pair of lactic acid isenantiomer to each other.

Example : Meso tartaric acid.


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isomerism q&a new

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