Optical Isomerism Class 11 Notes | EduRev

Chemistry Class 11

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Class 11 : Optical Isomerism Class 11 Notes | EduRev

The document Optical Isomerism Class 11 Notes | EduRev is a part of the Class 11 Course Chemistry Class 11.
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optical isomerism 

Optical Isomerism Class 11 Notes | EduRev

If rotation of light is anti-clockwise
Optical Isomerism Class 11 Notes | EduRev
laevo rotatory (l-form)

If rotation of light is clockwise
Optical Isomerism Class 11 Notes | EduRev
dextro rotatory substance(d-form)

If there is no rotation of light then substance is called optically inactive.

Chiral Carbon: 

If all the four valencies of carbon are satisfied by four different atoms or four different group atom then carbon is known as chiral carbon.

e.g.

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

 

►  Optical Isomerism Class 11 Notes | EduRev (2 Chiral carbon)

 

► Chiral carbon <w<¡us ds fy, igys point of doubt check djuk pkfg,

Optical Isomerism Class 11 Notes | EduRev (no chiral carbon)

► For optically active isomer, the object and its mirror image must be non superimposable.

► If image and object are superimposable then they are not optically isomers to each other.

►To check, superimposable either of object or mirror image is rotated 180º along the mirror. After that mirror image is checked for superimposablility.

 

Optical Isomerism Class 11 Notes | EduRev

 

Optical Isomerism Class 11 Notes | EduRev
        Object                                    Mirror image

 

Optical Isomerism Class 11 Notes | EduRev After 180º rotation, mirror image and object are identical.(mirror image of meso compound will be identical)

► If the compound have only one chiral carbon then it will be certainly optically active.

 

Optical Isomerism Class 11 Notes | EduRev

Object and its non-superimposible mirror image are called enantiomers to each other

32. A and B are enantiomers to each other. If specific rotation of A is 20º and rotation of mix of A and B is -10º then find the percentage of A and B in the mixture? 

Sol. Let x mol A

(1-x) mol of B

total rotational of A = x × 20

total rotational of B = (1 - x)(-20)

Total rotation of the mixture = - 10

x × 20 + (1 - x) (-20) = - 10

20 x - 20+20x = - 10

40 x = 10 Optical Isomerism Class 11 Notes | EduRev x = Optical Isomerism Class 11 Notes | EduRev = 0.25

Optical Isomerism Class 11 Notes | EduRev % A = 25%

% B = 75%

Fisher Projection

By fisher projection three dimensional structure is converted into 2D.

33. Write the Fisher projection of CH3CH(OH)COOH  

Sol.

Optical Isomerism Class 11 Notes | EduRev

► Maximum carbon must be in vertical line.

► Place higher priority carbon containing functional group on top of vertical line.

34. Write Fisher projection of 

Optical Isomerism Class 11 Notes | EduRev

Sol. (i) Place higher priority carbon containing functional group on top of vertical line.

(ii) Arrange other group according to its clock wise or anti-clockwise position w.r.t. group on the top.

Optical Isomerism Class 11 Notes | EduRev

Wedge Dash Structure:

Optical Isomerism Class 11 Notes | EduRev

dsoy Fourth priority order dks Cross dj ldrs gSa] during checking of clock or anticlockwise.

► To draw Fisher projection of such structure.

(1) Dotted group is placed below.

(2) The group attached by dark line is placed on the top.

(3) Remaining group are placed according to their clock wise or anti-clockwise position w.r.t. group in dark line.

35. Draw the fisher projection of: 

Optical Isomerism Class 11 Notes | EduRev

Sol.

Optical Isomerism Class 11 Notes | EduRev * Optical Isomerism Class 11 Notes | EduRev

If fourth valency is not given then we assume it to be hydrogen.

R and S form

Optical Isomerism Class 11 Notes | EduRev R → Rectus → Right → Clock wise.

Optical Isomerism Class 11 Notes | EduRev S → Sinister → Left → Anti-clockwise.

Priority order

► Higher the atomic number of the element, greater will be its priority.

e.g.

Optical Isomerism Class 11 Notes | EduRev  Optical Isomerism Class 11 Notes | EduRev  Optical Isomerism Class 11 Notes | EduRev  Optical Isomerism Class 11 Notes | EduRev  Optical Isomerism Class 11 Notes | EduRev

► In case of double bond ( = ) or triple ( ≡ ) bond

 

-CH = CH2Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev -C º CH Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

-Optical Isomerism Class 11 Notes | EduRev- H Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

-COOH Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

 

(4)

Optical Isomerism Class 11 Notes | EduRev

1 → 2 → 3 If clock wise ⇒ R

1 → 2 → 3 If Anti clock wise ⇒ S

 

Optical Isomerism Class 11 Notes | EduRev

⇒ Mirror image of R is S

► If lowest priority element is in horizontal then exact order is obtained by reversing the form (i.e. it comes R then exact form will be S and vice versa )

e.g.

Optical Isomerism Class 11 Notes | EduRev

⇒ It comes as R but will be S form,

Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

If lowest priority order element is in dotted then

1 → 2 → 3 If clock wise ⇒ R

and if 1 → 2 → 3 If Anti clock wise ⇒ S

► If lowest priority element is not in dotted then we bring it in dotted by doing even number of inter changes (2,4 or 6) as by even no. of inter change, the configuration does not change.

► After odd number of changes the configuration changes (If R then goes into S and vice versa).

For Example:

Optical Isomerism Class 11 Notes | EduRev
(R)

After one change Optical Isomerism Class 11 Notes | EduRev
                                      (S)

After two interchanges Optical Isomerism Class 11 Notes | EduRev

(Again R)

Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

Ex.36 Draw the Fisher projection of 

Optical Isomerism Class 11 Notes | EduRev

Sol.

Optical Isomerism Class 11 Notes | EduRev

 All the carbons are placed in vertical.

► Groups attached to the chiral carbons are placed on any side. After that they are adjusted whether they are R or S form w.r.t chiral carbons.

 

Ex.37 Draw the Fisher projection of the following. 

Optical Isomerism Class 11 Notes | EduRev

Sol. Wedge Dash formula

Optical Isomerism Class 11 Notes | EduRev

Fisher projection

Racemic Mixture

It is an equimolar mixture of R and S or d and l. Racemic mixture is optically inactive.

► A compound is optically active due to:

(1) Absence of plane of symmetry (POS)

(2) Absence of centre of symmetry (COS)

Optical Isomerism Class 11 Notes | EduRev POS

(meso form )

► POS is an imaginary plane where if we place a mirror, mirror image will exactly overlap the other half.

► For meso form, there must be at least two identical chiral carbons.

Identical carbon ⇒ Chiral carbons having identical group attached.

► If compound has POS then it will be certainly optically inactive and will be called meso form.

► After finding two identical carbons. We assign them as R or S. If first part is R and other is S then they will rotate the light in opposite direction but to equal extent the compound will be optically inactive.

(meso form)

Optical Isomerism Class 11 Notes | EduRev

► If a compound have 'n' different chiral carbons then total no. of optically active isomers = 2n

No. of meso form = 0

e.g.

Optical Isomerism Class 11 Notes | EduRev

no. of different chiral carbon = 4

total optical isomer = 2n = 24 = 16

► There will be no meso as the compound does not have identical chiral carbon.

► If a compound has n identical chiral centre (symmetrical) ⇒ There must be symmetry from some where.

(i) If n is even

optical isomer (a) = 2n-1

mesoform (m) = 2n/2-1

total optical isomer = a + m

(ii) If n is odd

Optical Isomerism Class 11 Notes | EduRev

Total optical isomer =  Optical Isomerism Class 11 Notes | EduRev = 2n-1

 

► Optical Isomerism Class 11 Notes | EduRev

 

► When there is odd no. of identical carbon atom (i.e. symmetrical) then this compound will certainly contain pseudo chiral w.r.t. which compound be symmetrical (i.e. POS).

► Other meso compound of the above compound will form by changing the place Br and H around Pseudo Chiral carbon.

Optical Isomerism Class 11 Notes | EduRev

Total meso = 2

optical = Optical Isomerism Class 11 Notes | EduRev

= Optical Isomerism Class 11 Notes | EduRev

= 22 - 2

= 4 - 2 = 2

 

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

(Glucose)

Total no. of different chiral carbon = 4

Total optical isomer = 24 = 16

Optical Isomerism Class 11 Notes | EduRev

Total no. of even chiral = 4

a = Optical Isomerism Class 11 Notes | EduRev = Optical Isomerism Class 11 Notes | EduRev = 23 = 8

m = Optical Isomerism Class 11 Notes | EduRev = Optical Isomerism Class 11 Notes | EduRev = 22-1 = 21 = 2

Total isomers = 8 + 2 = 10

Diastereo Isomers

For single chiral centre, there is no diastereoisomer. The stereoisomer which are not related as object and mirror image. They may be optically active or optically inactive.

Optical Isomerism Class 11 Notes | EduRev                                       Optical Isomerism Class 11 Notes | EduRev

(Inactive)                                                  (active)

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev                               Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

► Fix one chiral carbon

After one inter-change

If (R, R) → (R, S)

For compound having 3 chiral carbon to get diastereoisomer, fix two chiral carbon and one interchange with left carbon or fix one chiral carbon and inter change with other two,

Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

(I)                                (II)

Total isomer = 23 = 8

Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

(III)                               (IV)

(I) and (III), I and (IV), (II) and (III), (II) and (IV) are diastereoisomers.

Ex.38

(I)    Optical Isomerism Class 11 Notes | EduRev                

(II)       Optical Isomerism Class 11 Notes | EduRev            

 (III)      Optical Isomerism Class 11 Notes | EduRev

                                                                                        

 (IV) Optical Isomerism Class 11 Notes | EduRev

 

What are the relation among the above compounds? 

Sol. I and II are identical
III and IV are identical

II and III are diastereo isomer
I and IV are diastereo isomer

Ex.39 Find total isomers obtained by dichlorination of cyctopentane? 

Sol.

Optical Isomerism Class 11 Notes | EduRev

Total isomers = 3 + 3 + 1 = 7

Optically isomers = 6, Optically active isomers = 4

 

Ex.40 Find the total isomers obtained by trichlorination of propane. 

Sol.

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

Total isomers = 6

optically isomers = 2

Ex.41 Find total isomers obtained by dichlorination of n-butane  

Sol.

Optical Isomerism Class 11 Notes | EduRev (1)

 

Optical Isomerism Class 11 Notes | EduRev (2)

 

Optical Isomerism Class 11 Notes | EduRev (2)

 

Optical Isomerism Class 11 Notes | EduRev (1)

 

Optical Isomerism Class 11 Notes | EduRev (1)

 

Optical Isomerism Class 11 Notes | EduRev (3) (2 optically 1 meso )

 

Total isomers = 10 (6 optically active + 1 meso + 3 structural)

Ex.42 How many stereoisomers of 1,2,3-cyclohexantriol 

Sol.

Optical Isomerism Class 11 Notes | EduRev No. of Chiral carbon = 3 (identical) symmetrical)

a = Optical Isomerism Class 11 Notes | EduRev = Optical Isomerism Class 11 Notes | EduRev = 4- 2

m = 2

total stereoisomers = 2 + 2 = 4

 

►  Optical Isomerism Class 11 Notes | EduRev
(Meso)

Optical Isomerism Class 11 Notes | EduRev

 

► Mesoform is optically inactive due to internal compensation and racemic mixture is optically inactive due to external compensation.

Ex.43 A and B are enantiomer of each other. Specific rotation of A is 20 º. Rotation of mixture of A and B = -5º what is the percentage of racemic part?
Sol. x mol A, 1-x mol B

x × 20 + ( 1- x ) (-20) = -5

20 x - 20 + 20x = - 5

40 x = 15 ⇒ x = 3/8 = 0.375

moles of A = 3/8

moles of B = Optical Isomerism Class 11 Notes | EduRev = Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev moles of A and Optical Isomerism Class 11 Notes | EduRev moles of B will form racemic mixture.

Enantiomer excess or optical purity = Optical Isomerism Class 11 Notes | EduRev - Optical Isomerism Class 11 Notes | EduRev = Optical Isomerism Class 11 Notes | EduRev

Ex.44

Optical Isomerism Class 11 Notes | EduRev

What is rotation of mixture?  

Sol. Rotation will be due to B only,

= 0.3 × (-20º)

= - 6º

► Chiral compound → optically active compound

Ex.45 Which of the following compound is Chiral (Optically active)?

(A)

Optical Isomerism Class 11 Notes | EduRev

 

(B)

Optical Isomerism Class 11 Notes | EduRev

(C) Both (D) None

Ans. (D)

Allene system

Optical Isomerism Class 11 Notes | EduRev

 

Optical Isomerism Class 11 Notes | EduRev

They are non-superimposable mirror image

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

(Optically active)

Optical Isomerism Class 11 Notes | EduRev

(Inactive)

Spiro Compounds

 

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

If no. of rings are even ⇒ optically active

If no. of rings are odd ⇒ Inactive

Optical Isomerism Class 11 Notes | EduRev

(Optically active)

Optical Isomerism Class 11 Notes | EduRev

(Optically Inactive)

Optical Isomerism Class 11 Notes | EduRev ⇒ This is the even no. of double bonds case.

(Optically active)

► For optical activity, the carbons at extreme position must have different group attached.

 

►  Optical Isomerism Class 11 Notes | EduRev

⇒ Planar compound

Always have POS, Optically inactive.

Biphenyls

 

Optical Isomerism Class 11 Notes | EduRev

If biphenyl contain bulky group at its ortho position (only) then due to repulsion the planarity of compound disappears and its mirror image is non superimposable.

► In the biphenyls none of the two rings must have symmetry.

 

Optical Isomerism Class 11 Notes | EduRev

(Optically inactive)

Optical Isomerism Class 11 Notes | EduRev (Optically active)

In 2º Amines.

Optical Isomerism Class 11 Notes | EduRev

Optically inactive due to formation of racemic mixture.

► Order of flipping in amines: 1º > 2º > 3º

D-Form :- For compound having one chiral carbon

(a) If OH is right side → D

(b) If OH is left side → L

(Note:  → All the carbon must be in vertical having highest O. N. Carbon on the top)

Optical Isomerism Class 11 Notes | EduRev
D-form                     

Optical Isomerism Class 11 Notes | EduRev
L-form

Conversion of Fisher Projection in Sawhorse Structure 

Optical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRevOptical Isomerism Class 11 Notes | EduRev

The above Fisher projection can also be written in Saw Horse form as follows

Optical Isomerism Class 11 Notes | EduRev

Conversion of Newmann Projection in to Sawhorse Projection 

Optical Isomerism Class 11 Notes | EduRev      

Optical Isomerism Class 11 Notes | EduRev

Optical Isomerism Class 11 Notes | EduRev

 

Optical Isomerism Class 11 Notes | EduRev

► If all the identical groups are same side or in exactly opposite direction to each other in Newmann projection then compound will be meso.

 

Optical Isomerism Class 11 Notes | EduRev

        (Meso)                                       (Meso)

 

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