Principles of Stereochemistry MCQ Quiz in मराठी - Objective Question with Answer for Principles of Stereochemistry - मोफत PDF डाउनलोड करा

Concept:-

• An optically active compound, also known as a chiral compound, is a type of molecule that exhibits optical activity. Optical activity refers to the ability of a compound to rotate the plane of polarized light. This property arises due to the molecule's chiral nature, meaning it lacks a plane of symmetry, resulting in two non-superimposable mirror-image structures called enantiomers.
• Enantiomers have the same physical and chemical properties in an achiral environment but differ in their interaction with plane-polarized light. When polarized light passes through a solution of an optically active compound, the plane of polarization rotates either clockwise or counterclockwise. The amount of rotation is measured using a polarimeter and is given in degrees.
• ​According to van't Hoff, a carbon atom that is combined with four different univalent atoms or groups and whose affinities are directed toward the vertices of a tetrahedron is asymmetric. The term refers to the environment of the carbon atom at the center of the tetrahedron, rather than to the atom itself. Thus, asymmetry is not the property of any atom but refers to the space in which the atom has been placed. Tetrahedral geometry is also found to be in compounds of N, P, Si, and S.

• Presently, the term chirality rather than asymmetry are used and chiral center has the same meaning as the asymmetric center. In the case of third-row elements (N, P), a nonbonding pair of electrons may be counted as one of the four groups and many nitrogen and phosphorus-containing molecules also can have so called chiral center.

Asymmetric center: sp3 carbon with 4 different groups attached.
Optical activity: The ability to rotate the plane of plane-polarized light.
Chiral compound: A compound that is optically active (achiral compound will not rotate light).

Explanation:-

The given compound is :

These 6 are the optical centres for this molecules

Conclusion:-

So, The number of chiral centers present in the following molecule is 6

Concept: -

• Isomerism is the name given to a phenomenon in which more than one compounds have the same chemical formula but a different chemical structure. Compounds having the same molecular formula but different arrangements of atoms are called isomers.
• In isomerism, the chemical formula of compounds is the same but their arrangement is different.
• As the name suggests functional isomers are isomers that have the same chemical formula but the functional group present in the compound is different. 
• Structural isomers are divided into chain, positional, functional, metamerism, tautomerism, and ring chain isomers.
• Structural isomerism is commonly called constitutional isomerism.
  • In these isomers, functional groups and atoms in the molecules are connected in different ways.

• Stereoisomers: 
  • The compounds having the same molecular as well as the same structural formulae but differing in the relative arrangement of the atoms or groups in space are called stereoisomers.
  • This phenomenon is termed as stereoisomerism. 
  • Optically Active Stereoisomers: The stereoisomers that can rotate plane-polarized light are called as optically active whereas an optically inactive compound is not capable of optical rotation.
• Diastereomers:
  • Stereoisomers that are not mirror images of each other.
  • Diastereomers have the same configuration at least at one chiral centre and the opposite configuration at the remaining chiral centres.
• Enantiomers:
  • Stereoisomers are mirror images of each other.
  • Enantiomers have opposite configurations at all the chiral centres.

Explanation:-

• Let's draw a mirror between two structures given to check whether they are mirror images of each other or not -

• The mirror image of compound A is C, which does has any mirror image relationship with B
• Thus, compound A and B does not has any mirror image relationship.
• Now, let's check whether they are superimposable or not -

• If compound A is rotated in plane by 180^o, the generated compound is identical to compound B.
• Thus, both structures as identical.

Conclusion:-

• Hence, the two compounds given below are Identical.

Concept:-

Elimination reactions:

• The elimination reaction is an organic reaction in which two substituents are removed from a molecule to form a new product.
• The process takes place in the presence of acid, base, metal, and sometimes through heating.

Mechansim of elimination reaction:

1. E1 Mechansim:

• Depending on the leaving group, followed by ionization of the molecule results in the formation of a carbocation.
• Removal of the hydrogen atom through deprotonation in the presence of base to form the C=C bond.

2. E2 Mechansim:

• Simultaneous removal of the leaving group and hydrogen atom in presence of a base to form C=C bond.

Explanation:-

For the reaction of sodium iodide with dibromo cyclohexanes to give cyclohexane, the two bromine atoms must be in anti-orientation.

• The reaction pathway is shown below:

• In the reaction we can see that, the vinicial anti-compound contains the two Br atom that are anti to the each other and can undergo E2 elimination reaction with sodium iodide.

Conclusion:-

• Hence, option 3 is the correct option.

CONCEPT:

Conformation Stability in Pyranose Rings

• The stability of a conformation in six-membered rings like pyranose is governed by steric strain, torsional strain, and 1,3-diaxial interactions.
• Chair conformations are generally the most stable because they minimize steric hindrance and torsional strain.
• In the most stable conformation, bulky substituents like hydroxyl groups prefer to be in equatorial positions to reduce 1,3-diaxial interactions.

Explanation:-

Hydrogen bonding and no 1, 3-diaxial interaction

• For compound A (shown in the question), the most stable conformation corresponds to the chair form where the hydroxyl (OH) group occupies an equatorial position, reducing steric hindrance.
• In the given options, we identify the chair conformation where the hydroxyl group is equatorial and the methoxy group is in the axial position.
• This is represented in the second structure (from the top), which corresponds to the most stable conformation.

The correct option is the option 1 structure.

Concept:

In chemistry, a molecule or ion is called chiral if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality

Axial chirality: Axial chirality is a special case of chirality in which a molecule contains two pairs of chemical groups in a non-planar arrangement about an axis of chirality so that the molecule is not superposable on its mirror image.

 The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority than the far ones.

Planar chirality: This term is used for a chiral molecule lacking an asymmetric carbon atom, but possessing two non-coplanar rings that are each dissymmetric and which cannot easily rotate about the chemical bond connecting them: 2,2'-dimethylbiphenyl is perhaps the simplest example of this case. 

To assign the configuration of a planar chiral molecule, begin by selecting the pilot atom, which is the highest priority of the atoms that is not in the plane, but is directly attached to an atom in the plane.

Next, assign the priority of the three adjacent in-plane atoms, starting with the atom attached to the pilot atom as priority 1, and preferentially assigning in order of highest priority if there is a choice.

Then set the pilot atom to in front of the three atoms in question. If the three atoms reside in a clockwise direction when followed in order of priority, the molecule is assigned as R; when counterclockwise it is assigned as S.
 
Explanation: 

→ Structure I is having Axial chirality because the structure is starting from the ring ring present out of the plane and moving to the ring present behind the plane, i.e, moving anticlockwise, hence have M configuration.

→ Structure II is having Planar chirality, the configuration of a planar chiral molecule, begin by selecting the pilot atom, which is the highest priority of the atoms that is not in the plane, but is directly attached to an atom in the plane. The three atoms reside in a clockwise direction when followed in order of priority, the molecule is assigned as R

Conclusion:
The correct answer is I: M; II: R.

Explanation:- 

Specific rotation is a property found in chiral chemical compounds, which is remarked by the change in the angle of plane-polarized monochromatic light.

Calculation:-

Given,

Specific rotation of optically pure (R)-2-bromobutane = -112.00.

The specific rotation of the sample mixture = -82.88.

Using, ee = observed rotation / actual rotation x 100 ⇒

ee = -82.88 / -112.00 x 100 ⇒ ee = 74% (R).

Percentage of Racemic Mixture = 100 - R% = 100 - 74 % = 26%.⇒

26% = 13%(S)+13%(R).

Concept:

Stereoisomerism -

• This isomerism arises in compounds having the same chemical formula but different orientations of the atoms belonging to the molecule in three-dimensional space. 
• The compounds that exhibit stereoisomerism are often referred to as Stereoisomers. 
• This phenomenon can be further categorized into 2 subtypes. Both these subtypes are -
  • Optical Isomerism
  • Geometric Isomerism

Geometrical isomerism:

• Geometrical isomerism is shown in molecules that have a carbon-carbon double bond C = C.
• Geometrical isomerism is shown only when each carbon atom of the double bond is attached to two different atoms or groups.
• Compounds of the type abC = Cad, abC = Cab, and abC = Cde will show geometrical isomerism.
• The cause of geometrical isomerism is restricted rotation about a C = C bond.

Criteria to show optical isomerism-

• Absence of centre of Symmetry - when a line is drawn from the centre of a molecule towards the corner of each atom, it should not encounter similar atoms.
• Absence of plane of symmetry - A real or imaginary plane, vertical or horizontal when passed through a molecule, bisects it so that the one half of the molecule should not be the mirror image of the other half.
• The presence of a Chiral centre - An object or molecule which has no plane of symmetry and is not superimposable on its mirror image is said to be chiral or dissymmetric.

​Explanation:

• The analysis of all the given compounds is shown below:

​

Hence, the total number of compounds showing stereoisomerism is six.

Concept:

Diastereomers 

• Diastereomers are stereoisomers that are not mirror images of each other.  They have the same molecular formula and connectivity of atoms but differ in the spatial arrangement of those atoms.  

Explanation:

Therefore, the correct option is 4.

The correct answer is option 4.

Explanation:

Based on the nomenclature of the A, B, C and D. The correct relationships between the four structures are:

• A and D are Diastereomers.
• A and C are Enantiomers.
• B and D are Enantiomers.
• B and C are Diastereomers

Conclusion:

The correct statements regarding the four structures are Statements A, B, C and D.

Concept:

• The number of stereoisomers in an asymmetrical molecule is \(2^{n}\).
  • Where n is the number of asymmetric carbon atoms in the molecule. This is known as the Le Bel-van't Hoff rule.

Explanation:

This molecule has 5 asymmetric centers.

\(\text{The number of stereoisomer} = 2^n\)

\(\text{The number of stereoisomer} = 2^5\)

\(\text{The number of stereoisomer} = 32\)

The number of possible stereoisomers is 32.