Meso Compound | How to identify them?

This blog will provide you general definition of meso compounds, methods to identify them, some examples and their properties. and also, We will be able to identify a meso compound by R and S system. This blog also contains some characteristics of meso compounds. Let’s start by having a look at their Introductio.

Introduction

Generally, a meso compound should have contain two or more (not less than two) chiral centers (stereocenters). Meso compounds also have internal plane of symmetry which divides the compound in two halves. These two halves will be mirror images and they both can reflect each other. They are super-imposable. Cyclic compounds may also be meso.

Meso compounds are also called mesomers. And their net rotation is zero.

Definition

It is a compound that has more than one chiral carbon and that is super imposable on it’s mirror image. They are also optically inactive.

A meso compound is an achiral compound that contains tetrahedral stereogenic centers.

Recognition

If A is a meso compound, it should have two or more stereocenters, an internal plane and the stereochemistry should be R and S.

  • Have a look for an internal plane that lie in between the compound.
  • The Stereochemistry (for example: R and S) is deciding whatever it is a meso compound or not. Stereochemistry of meso compounds should cancel out because we know that meso compound is optically inactive. For a moment, R cancels S out in a meso compound with two chiral centers.
trans-1,2-dichloro-1,2-ethanediol

This structure contains 2 chiral centers. One is R and the other is S. First Chiral center is R and Second is S. But, If we take mirror image of this structure (as shown in above diagram) then both chiral centers changes from R to S and Vice versa. Also Chlorine replaces Hydrogen and Hydrogen replaces Chlorine. And if we cut this structure from the center then we will get two equal identical halves.

(meso)-2,3-dibromobutane

This structure also have two chiral centers. One is S and other is R. By putting a mirror in front of this structure we get a mirror image. S chiral center will be changed to R and R will become S because of mirror image. (by taking mirror image Bromine interchanges it’s position with Ch3 group). And if we cut it from internal plane into two halves then we get equal two parts and they will be same.

Note: In mirror image of a compound, Dash bond will be changed to Wedge and Wedge bond will be changed to Dash bond.

Identifying a meso compound through R and S

We can identify a meso compound by R and S system.

  1. In the structure of very first compound, substituents are arranged symmetrically and stereocenters are opposite to each other. We can cut it into two equal halves. So, this is a meso compound.
  2. In the structure of second compound, substituents are arranged symmetrically and stereocenters are opposite. We can cut it inti two equal halves. So, this is also a meso compound.
  3. In the structure of third compound, substituents are arranged symmetrically but stereocenters are the same so this is not a meso compound.

Types of meso compound

Meso compounds can be in many different forms such as butane, pentane, heptane and even cyclobutane.

It is not necessary for a compound to be called a meso by having only two stereocenters. Meso compound can have more than two stereocenters but not less than two.

Other examples of meso compounds

Tartaric Acid

Tartaric acid is one of the best example of meso compounds.

As we can see the mirror images of tartaric acid. There are two type of tartaric acid. One is +ve tartaric acid and other is -ve tartaric acid. There are two Chiral centers in tartaric acid (R and S).

2,3-dihydroxybutane (2,3-butanediol)

2,3-butanediol is also an example of meso compound.

example of meso compound

If we have a look at first structure, this structure is called as (+)-2,S,3S-Butanediol because rotation is in clockwise and if we look at second structure, it is called as (-)-2,S,3S-Butanediol because rotation is in anti-clockwise. And the Last structure is a Meso structure of 2,3-Butanediol.

Properties of meso compound

  • Meso compounds have an internal mirror plane, or a plane of symmetry.
  • They have at least two chiral centers.
  • They can have more than two chiral carbons but not less than two.
  • Meso compounds are symmetric.
  • Mirror image of a meso compound will be same.
  • We can cut a meso compounds into two equal halves.
  • When polarized light passes through an achiral compound, no net rotation of polarized occurs, so achiral compounds are optically inactive.

Conclusion:

Meso compounds are those compounds that have more than two chiral carbons and they are super-imposable on their mirror images like your hands on opposite sides of your body. They have the plane of symmetry and we can cut them into two equal halves they can be identified by R and S system. Tartaric acid is the best and simplest example of meso compounds.

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