Geometrical Isomerism - Chemistry Class 11 - NEET PDF Download

What is Geometrical Isomerism?

Isomers that possess the same molecular and and the same connectivity of atoms but differ in the arrangement of atoms or groups in space due to restricted rotation are known as geometrical isomers. The phenomenon is called geometrical isomerism.

Geometrical Isomers

Conditions for Geometrical Isomerism

(I) Geometrical isomerism commonly arises when rotation about a bond is restricted, as in a double bond (e.g., , , - N = N - or in a ring structure. 

Restricted rotation fixes relative positions of substituents and allows distinct spatial arrangements to exist.

Examples showing restricted rotation:

(A)

(Restricted Rotation)

(B)

(Restricted Rotation)

(II) For a double bond to give geometrical isomers, each of the doubly bonded atoms must have two different groups attached (i.e., the two substituents on each end should be different).

Because the double bond or the ring does not permit free rotation at room temperature, two or more orientations become isolable; this is described as restricted, hindered or no rotation.

Compounds in which identical groups are attached in such a way that interchange does not produce a different arrangement are not geometrical isomers. For example:

Representative types of compounds that can show geometrical isomerism include substituted alkenes, azo compounds (-N=N-) and cyclic systems:

Examples of Geometrical Isomers

(I) Along

bond

(II) Along

(III) Along -N = N- bond

Geometrical Isomers of N₂H₂

and

and

(IV) Along bond of cycloalkane / cyclic systems

and

                    

(V) Along

 in ring structures

In small rings, the trans isomer of a cycloalkene is usually not stable because of angle and steric strain; therefore trans isomers are often absent in small rings. If the ring is sufficiently large, both cis and trans cycloalkenes can exist. The smallest trans-cycloalkene that can be isolated and stored is trans-cyclooctene.

Examples of Cis-Trans Cycloalkenes

Trans-Cyclohexene and Trans-Cyclooctene

Configurational Nomenclature in Geometrical Isomerism

For deciding the seniority of groups in E-Z configuration, the Cahn-Ingold-Prelog (CIP) sequence rules are applied.

1. Rule I: The group whose first atom has the higher atomic number is of higher priority. Typical order: I > Br > Cl > S > F > O > N > C > H.
2. Rule II: Between isotopes, the higher mass isotope has higher priority (for example, -T > -D > -H; and -C¹⁴H₃ > -C¹²H₃).
3. Rule III: If the first atoms of two groups are identical, consider the next atoms along the chains until a difference is found (compare atom-by-atom).
   e.g.(A) - CH₂Cl > - CH₂OH > - CH₂NH₂ > - CH₂ CH₃ > - CH₃
    
   (B)  >
4. Rule IV: Multiple bonds are treated by replacing the multiple-bonded atoms by an equivalent number of singly bonded atoms for comparison. 
   > C = Y as if it were  &  - C º Y as if it were
   
5. Rule V: Use the assigned priorities on each double-bonded atom to decide E (entgegen, opposite sides) or Z (zusammen, same side). The descriptor Z is used when the higher-priority groups are on the same side of the double bond; E when they are on opposite sides.

Some Examples of E-Z isomers

Number of Geometrical Isomers

The number of geometrical isomers is often estimated from the number of stereogenic units (n) present in the molecule. Here, n refers to the number of stereogenic double bonds capable of showing geometrical isomerism.

Physical Properties of Geometrical Isomers