Hydrocarbons Class 11 Notes

Hydrocarbons Class 11 Notes



Meso Compound
A meso compound is a molecule with multiple stereocenters (al least two) that is superposable on its mirror image. All meso compounds have an internal plane of symmetry that bisects the molecule. Each half is a mirror image of the other half.
Meso compounds are achiral so it is optically inactive. Thus, meso compounds cannot be assigned with either dextrorotatory (+) or levorotatory (-) designations.
A meso compound is achiral so cannot have an enantiomer. When a molecule is superposable on its mirror image, then that molecule and the mirror image are merely the same molecule.
meso compound

Identify a Meso Compound
1. Find how many stereocenters in the molecule
A meso compound requires at least two stereocenters.
2. Find the molecule has an internal mirror plane
Molecule must have an internal mirror plane.
3. Ensure that both halves are mirror images of each other
The meso compound must be symmetric about the internal mirror plane. Each opposing stereocenter must have differing absolute configurations and the attachments on each stereocenter must be the same.

Enantiomers and Diastereoisomers
Enantiomers differ only by their absolute stereochemistry (R or S etc).
Diastereoisomers differ by their relative stereochemistry.
A molecule can only have one enantiomer but any number of diastereoisomers.
enantiomer and diastereomer
Optically active chiral molecules which are non-super imposable on its mirror images are called enantiomers and the phenomenon is known as enantiomerism. For a molecule to be an optical isomerism the molecule must have at least one asymmetric carbon atom.
We can easily understand the chirality by comparing our hands (left hand and right hand). Our left hand and right hand are the best example of non-superimposable mirror image of each other. Each hand is therefore considered as chiral.
The main Properties of enantiomers are given below-
The main properties of enantiomers are given below-
a. Enantiomers always exist in pair.
b. Enantiomers are non-super imposable on its mirror image.
c. Enantiomers have same physical properties (like boiling point, melting point, solubility, density, viscosity, refractive index etc.)and chemical properties in achiral environment.
d. Each enantiomers have opposite behavior with respect to plane polarized light, if one of them will rotate the plane polarized light towards right hand direction then definitely the other will rotate the plane polarized light towards left hand direction.
e. Each enantiomers shows the same chemical reactivity with achiral reagent; however they have different reactivity with chiral reagent.

Diastereomers are simply not enantiomers. They are compounds which have the same molecular formula and sequence of bonded elements but are not mirror image and nonsuperimposable. The must essential conditions for a molecule to be diastereomer there must be at least two asymmetric carbon.
The main properties of Diastereomers are given below-
a. Diastereomers have different physical properties such as melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations.
b. Diastereomers other than geometrical isomers may or maynot be optically active.
c. Diastereomers show similar, but not identical chemical properties. The rates of reactions of the two diastereomers with a given reagent provided tha reagent is not rapidly active.
d. Diastereomers can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc.

Meso compounds do not show optical activity because
a. they do not contain chiral carbon atoms
b. Bthey have non-super imposable mirror images
c. they contain plane of symmetry
d. they do not contain plane of symmetry

Among the following which one can have a meso form
a. CH3CH(OH)CH(Cl)C2H5