Organic chemistry ( Kolhapur)

Unit 02 Stereochemistry of Organic chemistry)

Multiple choice questions

1) A compound in which one carbon atom is attached to four different groups or atoms is called.A. Geometrical Isomerism.B. Cis-trans Isomerism.C. Optical isomerism.D. Ring-chain Isomerism.

Answer is ‘C’2) Optical Rotation is measured by

A. Colorimeter.B. PolarimeterC. Photometer.D. All of the Above

Answer is ‘B’

Answer is ‘C’

4)What is ‘R notation in nomenclature system?

A. For clockwise configuration.B. For resonance .C. For Anticlockwise configuration .D. For trans isomers .

Answer is ‘A’

5) Enantiomers are

A. Compounds which are not mirror images of each other.

B. Compounds with same structural arrangements in space.

C. Compounds which share the same molecular and have some connectivity.

D. None of the above.

Answer is ‘C’

6) Chiral centre is

A. Carbon with four different functional groups.

B.Carbon with the same functional groups attached.

C. Carbon attached at the centre of the molecule.

D. Carbon with a multiple bond.

Answer is ‘A’

Answer the following ( 2mks)each.

Q: What is Stereoisomerism?

Steroisomerism is the isomerism which arises on account of the different arrangement of atoms or groups in space in a molecule.

These different arrangements are known as steroisomers. There are two types of Stereoisomerism namely Geometrical isomerism and optical isomerism.

Q: What is Optical activity?

Q What is Polarimeter?

Q Write the names of the Following Stereoisomers

(2S,3R)3-bromo-2-butanol L- Glyceraldehyde D(-) tartaric acid

Q: Draw the steroisomers of Erythrose and Threose

Q: Explain the chirality in latic acid.

The number of optical isomers can be theoretically derived from a molecule containing one or more chiral centres.

Chiral centre, in which the carbon atom is bonded to the four different groups is called as chiral centre or Asymmetric carbon atom.

Latic acid is the best example of the chiral centre.

Q: What is Racemic Modification?

When an equimolecular quantities of two enentiomr of a chiral molecules are mixed together or formed in a reaction , the resultant mixture is called as a ‘racemic modification’ or ‘ racemic mixture’.

They are optically inactive.

The rotation caused by a molecule of one isomer is cancelled by an equal and opposite rotation cause by a molecule of its enantiomers.

Q: What is R/S Nomeclature?

R/S nomenclature is also called as CIP nomenclature According to this system, the configuration of a molecule is specified by either ‘R’ or ‘S’ Where ‘R’ is for clockwise configuration and ‘S’ is for anticlockwise configuration. This system is applied for the molecule having two chiral centers.

Example:

Answer the Following (5mks) each

Q: Explain the isomerism of compounds having carbon-carbon double bond.

Geometrical isomerism normally arises because of the spatial geometry of atoms or groups associated with a double bond.

Carbon atoms forming a part of the double bond system and the various atoms or groups attached to these two carbons remain in the same plane.

The lateral overlap of the two ‘p’ atomic orbitals to give a π bond, completely prevents the rotation of the groups attached to the two carbons.

The net result is free rotation about a double bond is restricted as it involves a high energy.

In cis- isomer , the two methyl groups are located close enough to cause steric crowding, while In trans- isomer the two methyl groups are well separated from each other, hence there is no

possibility of steric crowding. From this picture we can expect the cis- isomer to be less stable and more energetic than the

trans- isomer.

Q Explain the isomerism in compounds having carbon nitrogen bond.

In oximes both the ‘C’ and ‘N’ atom are sp2 hybridized thus, C double N contains one σ bond and one π bond.

The third sp2 – orbital of nitrogen contains a lone pair of electrons. Sice the oxime molecule is coplanar about the double bond, it exhibit geometrical isomerism due to

restricted rotation about ‘C ‘double bond ‘N’. For Example in aldoxine when ‘H’ atom and the ‘OH’ group lie on the same side of the double bond known

as ‘syn isomer’ and when the ‘H’ atom and the ‘OH’ group lie on the either side is known as ‘anti isomer’.

Q: Explain the Isomerism in alicyclic compounds.

As the double bond in the olefinic compounds, the ring formation in cyclic compounds inhibits the free rotation of the carbon atoms.

Alicyclic compounds are non- aromatic ring molecule. When two substituents atoms or groups are bend in the same direction, the molecule is called

as cic isomer.

When the substituents atoms or groups are bend in the opposite direction or across the plane of the carbon – carbon double bond it is called as trans-isomer.

The hindrance to rotation about a linkage between the atoms as joined in a ring is not free to rotate around a sigma bond.

For example 1,2 dimethyl cyclohexane exist in two geometrical isomeric forms.

Q: Explain the geometrical isomerism in maleic acid and fumaric acid.

Configuration of geometrical isomers can be determined by the method of cyclisation. Wislicenus suggested a principle which states that intramolecular reactions are more likely to

occur the closer together the reacting groups are in the molecule. As per this principle ring are formed due to intramolecular reaction between the groups in a

molecule. For example if we consider maleic acid and fumaric acid. Maleic acid readily forms a cyclic anhydride when heated, while fumaric acid does not form an

anhydride of its own. But fumaric acid when heated strongly forms maleic anhydride.

Since maleic acid cyclises readily it is probably the cis-isomer, while the fumaric acid after prolonged heating forms a maleic acid anhydrides is a trans- isomer.

Configuration of the two acid may be tested by hydrolyzing maleic anhydride in the cold, here only maleic acid is obtained.

Q: Explain the geometrical isomerism and causes of geometrical isomerism

Q: Explain the Configuration of Aldoximes.

Aldoximes exist in two forms of isomers ie ( syn and anti isomers). The two isomers resemble each other in many ways, but differ in the behavior of their acetyl

derivatives towards sodium carbonate solution. For example :

Benzaldoximes are determined by first converting them to the corresponding acetyl derivative (Ac2O), in presence of an acid and then treatment with acqueous solution of Na2CO3 .

In such reaction if the original oxime is regenerated the oxime is designated as ‘E’ or ‘syn’ isomer.

Q: Write a Short note on E/Z nomenclature system

Example:

Answer the Following ( 8mks)

Q: Show the geometrical isomerism in aldoximes and ketoximes and explain the configuration of ketoximes .

Geometrical isomerism in aldoximes and ketoximes arise due to restricted rotation around ‘C’ double bond ‘N’.

In aldoximes the isomers is named ‘syn’ if the hydrogen and hydroxyl group are on the same side of C– N bond.

I f these groups are on the opposite side of C-N bond then the isomer is ‘anti’ isomer.

Example.

The configuration of ketoximes is determined by means of Beckmann rearrangement. Beckmann rearrangement is an acid catalyzed transformation. According to this mechanism , the acidic reagent reacts with the oxime and forms the amide. Example.

The amide produced is an isomer of oxime.

- Electron withdrawing substituents located on the migrating group strongly retard the rearrangement, while the electron donating groups accelerate the rearrangement.

Q: Explain the elements of Symmetry in optical isomerism.

Q: What are Enantiomers and diasteroisomers?

Enantiomers are a pair of configuration isomers. Enantiomers are compounds that share the same molecular formula and the same

connectivity’s, but are chiral. They are non super imposable mirror images.

Example:

Enantiomers have similar properties and show different behavior only in the presence of other chiral substance.

On the other hand diasteroisomers are stereoisomers which are not mirror images of each other.

Thus Structure I, II, III and IV represent a pair of diasteroisomers.

Physical properties of diasteroisomers are

Answer the Following ( 10mks)

Q: Explain the Optical isomerism in tartaric acid and 2,3- dihydroxybutanoic acid .(10)

Optical isomers of compounds having two dissimilar chiral centers. The best example is the tartaric acid.