Tautomerism - Practice Questions & MCQ

Tautomers are isomers of a compound which differ only in the position of H⁺ and electrons. The carbon skeleton of the compound is unchanged. A reaction which involves simple proton transfer in an intramolecular fashion is called tautomerism.

In the above molecule, the  has changed its position from carbon to oxygen atom with the rearrangement of the double bond. This can be visualised in the following manner hypothetically:

(1) Breaking of C-H bond and generation of Carbanion at  

(2) Resonance of the carbanion with the conjugated keto group

(3) Attachment of the proton at the O atom carrying negative charge

Can you write the above steps in diagrammatic fashion in order to get a better understanding?

It is to be noted that the keto form is preferred over the enol form in most of the cases. This is because of high bond energy of the C=O. However, the enol form is more stable under cases where the product is aromatic (Phenol) or in the case of 1,3- dicarbonyl compounds which is stabilised due to intramolecular H-Bonding.

 : Aromaticity 

: Intramolecular H Bonding

Some other cases where Tautomerism is seen

• Enamine -Imine

• Nitroso - oxime

The decreasing -I effect or increasing +I effect order is as follows:
–NH₃⁺ > –NO₂ > –SO₂R > –CN > –SO₃H > –CHO > –CO > –COOH > -F > –COCl > -CONH₂ > –Cl > –Br > –I > –OR > -OH > -NR₂ > –NH₂ > –C₆H₅ > –CH=CH₂ > –H

In hyperconjugation, more is the number of -carbons, more is the number of hyperconjugated structures and thus more is the stability. Thus, stability follows the given order:
3^o carbocation > 2^o carbocation > 1^o carbocation

Optical Isomerism
It is the type of isomerism in which the compounds possessing same molecular formula differ in their direction of bringing the optical rotation or the extent of bringing the optical rotation.
Plane Polarized Light
The light from an ordinary source is composed of waves vibrating in many different planes perpendicular to the plane of its propagation. When such a light is passed through a nicol prism, the light coming through is found to be vibrating in only one plane. Such a light is called as plane polarized light and here nicol prism is called Polaroid.

Optical Activity or Optical Rotation
The optically active substances rotate the plane polarized light clockwise or anti clockwise that is, dextro and laevo rotation, respectively at an angle . This rotation is called optical rotation. Here, in both the rotations value of  is the same but with opposite signs, that is, same in magnitude but opposite in direction.

Optical rotation is measured by polarometer in terms of specific rotation.


Here the = specific rotation
 (observed) = observed value of rotation
l = length of solution
c = concentration of solution.

Specific Rotation Depends Upon

• Nature of solute and solvent.
• Wavelength of light during the experiment.
• Temperature during the experiment.
• Length and concentration of solution.

Reason for Optical Activity or Optical Isomerism

• The compound must be asymmetric or dissymmetric with a non superimposable mirror image. For example, a hand in mirror, and ambulance in mirror.
• The asymmetric molecule does not have plane or central or axial axis of symmetry.

Plane of Symmetry: For it, minimum two stercogenic centers are needed. Here, the molecule can be divided into two equal halves.

• It is an optically inactive form due to mutual cancellation (or) internal compensation.
  
  The cis and trans forms of 1,4 -dimethyleyclohexane are diastercomers of each other. Both compound are achiral.
  

Diastereomers are stereoisomers that are not related as object and mirror image are not enantiomers. Unlike enantiomers which are mirror images of each other and non-sumperimposable, diastereomers are not mirror images of each other and non-superimposable. Diastereomers can have different physical properties and reactivity. They have different metting points and bolling points and different densities. They have two or more stereocenters.
It is easy to mistake between diasteromers and enantiomers. For example, we have four steroisomers of 3 -bromo- 2 -butand. The four possible combination are SS, RR, SR and RS. One of the molecule is the enantiomer of its mirror image molecule and diasteromer of each of the other two molecule (SS is enantiomer of RR and diasteromer of RS and SR). SS's mirror image is RR and they are not superimposable, so they are enantiomers. RS and SR are not mirror image of SS and are not superimposable to each other, so they are diasteromers.

Meso compounds are achiral compounds that has multiple chiral centers. It is superimposed on its mirror image and is optically inactive despite its stereocenters.

Introduction
In general, a meso compound should contain two or more identical substituted stereocenters. Also, it has an internal symmetry plane that divides the compound ir half. These two halves reflect each other by the internal mirror. The stereochemistry of stereocenters should" "cancel out". What it means here is that when we have an intermal plane that splits the compound into two symmetrical sides, the stereochemistry of both left and right side should be opposite to each other, and therefore, result in optically inactive. cyclic compounds may also be meso.

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

1. Look for an internal plane, or internal mirror, that lies in between the compound.
2. The stereochemistry (e.g. R or S) is very crucial in determining whether it is a meso compound or not. As mentioned above, a meso compound is optically inactive, so their stereochemistry should cancel out. For instance, R cancels S out in a meso compound with two stereocenters.
   
   

To name the enantiomers of a compound unambiguously, their names must include the "handedness" of the molecule. The method for this is formally known as R/S nomenclature.

Introduction

The method of unambiguously assigning the handedness of molecules was originated by three chemists: R.S. Cahn, C. Ingold, and V. Prelog and, as such, is also often called the Cahn-Ingold-Prelog rules. In addition to the Cahn-Ingold system, there are two ways of experimentally determining the absolute configuration of an enantiomer:

1. X-ray diffraction analysis. Note that there is no correlation between the sign of rotation and the structure of a particular enantiomer.
2. Chemical correlation with a molecule whose structure has already been determined via X-ray diffraction.

However, for non-laboratory purposes, it is beneficial to focus on the R/S system. The sign of optical rotation, although different for the two enantiomers of a chiral molecule,at the same temperature, cannot be used to establish the absolute configuration of an enantiomer; this is because the sign of optical rotation for a particular enantiomer may change when the temperature changes.

Stereocenters are labeled R or S

The "right hand" and "left hand" nomenclature is used to name the enantiomers of a chiral compound. The stereocenters are labeled as R or S.

Consider the first picture: a curved arrow is drawn from the highest priority (1) substituent to the lowest priority (4) substituent. If the arrow points in a counterclockwise direction (left when leaving the 12 o' clock position), the configuration at stereocenter is considered S ("Sinister" → Latin= "left"). If, however, the arrow points clockwise,(Right when leaving the 12 o' clock position) then the stereocenter is labeled R ("Rectus" → Latin= "right"). The R or S is then added as a prefix, in parenthesis, to the name of the enantiomer of interest.