Isomerism

Isomerism refers to the existence of compounds with the same molecular formula but different structural arrangements or spatial configurations. Isomers can exhibit distinct physical and chemical properties, even though they have the same number and types of atoms. 

1.0Types of Isomerism

Isomerism can be classified broadly into two types:

1. Structural Isomerism (also known as Constitutional Isomerism)
2. Stereoisomerism

Let's understand each type in detail:

Structural Isomerism  (Constitutional Isomerism)

Structural isomers have the same molecular formula but differ in how atoms are connected to each other in the molecule. There are several subtypes of structural isomerism:

(a) Chain Isomerism

In chain isomerism, the isomers differ in the arrangement of the carbon skeleton. The atoms are connected in a different chain or branching pattern.

• Example: Butane (C₄H₁₀) exists as two chain isomers:
• • n-butane (linear chain): CH₃–CH₂–CH₂–CH₃
  • isobutane (branched chain): (CH₃)₂CH–CH₃

(b) Position Isomerism

Position isomerism occurs when the position of a functional group or a substituent varies in the same carbon chain.

• Example: Propanol (C₃H₇OH) has two position isomers:
• • 1-propanol (OH group on the first carbon): CH₃–CH₂–CH₂–OH
  • 2-propanol (OH group on the second carbon): CH₃–CH(OH)–CH₃

(c) Functional Group Isomerism

Functional group isomers have the same molecular formula but different functional groups.

• Example: Ethanol (C₂H₆O) and dimethyl ether (C₂H₆O):
• • Ethanol: CH₃–CH₂–OH (alcohol functional group)
  • Dimethyl ether: CH₃–O–CH₃ (ether functional group)

(d) Metamerism

Metamers arise due to differences in the alkyl groups attached to the same functional group. This type of isomerism occurs mainly in compounds with divalent functional groups like ethers, ketones, or amides.

• Example:
• • Diethyl ether: C₂H₅–O–C₂H₅
  • Methyl propyl ether: CH₃–O–C₃H₇

(e) Tautomerism

Tautomerism is a special case of functional isomerism where isomers can interconvert, typically by the movement of a hydrogen atom and a shift in double bonds. It usually occurs between keto and enol forms of a compound.

• Example: Acetone (CH₃COCH₃) and its enol form:
• • Keto form: CH₃–CO–CH₃
  • Enol form: CH₂=CH–OH

2.0Stereoisomerism

Stereoisomers have the same structural formula but differ in the spatial arrangement of atoms. Stereoisomerism can be further divided into two main types:

Configurational Isomerism

It is a type of stereoisomerism where isomers have the same molecular formula but differ in the spatial arrangement of atoms or groups, and these arrangements cannot be interconverted without breaking covalent bonds.

Types of Configurational Isomerism

(a) Geometrical (Cis-Trans) Isomerism

Geometrical isomerism occurs due to restricted rotation around a double bond or in a ring structure, where atoms or groups of atoms are oriented differently in space. The two common forms are cis (same side) and trans (opposite side).

• Example: 2-butene (C₄H₈) can exist as:
• • Cis-2-butene: Both methyl (CH₃) groups on the same side of the double bond.
  • Trans-2-butene: Methyl (CH₃) groups on opposite sides of the double bond.

(b) Optical Isomerism

Optical isomers, also known as enantiomers, arise due to the presence of a chiral center (a carbon atom bonded to four different atoms or groups). These isomers rotate plane-polarized light in different directions, even though they have the same molecular structure. Optical isomerism is common in compounds like amino acids, sugars, and other biomolecules.

• Enantiomers: Non-superimposable mirror images that rotate light in opposite directions:
• • Dextrorotatory (d or +): Rotates light clockwise.
  • Levorotatory (l or -): Rotates light counterclockwise.
• Example: Lactic acid (C₃H₆O₃) has two enantiomers:
• • D-lactic acid: Rotates light to the right.
  • L-lactic acid: Rotates light to the left.

Nomenclature Systems of Geometrical Isomers with Configuration

Geometrical isomerism arises due to the restricted rotation around a double bond or a ring structure, where the relative spatial arrangement of atoms or groups leads to different isomers. These isomers are typically described using two main systems of nomenclature: Cis-Trans and E-Z notation. Both are important for describing the exact configuration of geometrical isomers.

1. Cis-Trans Nomenclature System

The Cis-Trans system is one of the simplest ways to denote the configuration of geometrical isomers, used mainly when there are two identical groups on a carbon-carbon double bond (C=C) or on a ring structure. It is based on the relative positioning of these identical groups:

• Cis Isomer: When two identical groups are on the same side of the double bond or ring.
• Trans Isomer: When two identical groups are on opposite sides of the double bond or ring.

Example: 2-Butene (C₄H₈)

• Cis-2-butene: Both methyl (CH₃) groups are on the same side of the C=C double bond.
• Trans-2-butene: The methyl groups are on opposite sides of the double bond.

Limitations of Cis-Trans Nomenclature:

The Cis-Trans system is only applicable when there are two identical groups on the double bond. When there are more than two substituents, this system becomes ambiguous. In such cases, the E-Z system is used.

2. E-Z Nomenclature System

The E-Z system (from the German Entgegen and Zusammen) is a more advanced method of naming geometrical isomers. This system is based on the Cahn-Ingold-Prelog priority rules, where the priority of substituents attached to the double bond is determined by atomic number.

• Z (Zusammen): The two higher priority groups are on the same side of the double bond.
• E (Entgegen): The two higher priority groups are on opposite sides of the double bond.

Steps for Determining E-Z Configuration:

• Identify the substituents attached to each carbon of the double bond.
• Assign priorities to the substituents based on atomic number (higher atomic number gets higher priority).
• Compare the position of the higher priority groups:

If the higher priority groups are on the same side → Z isomer.

If the higher priority groups are on opposite sides → E isomer.

Example 1: 2-Butene (C₄H₈)

• Z-2-butene: Methyl groups (higher priority than hydrogen) are on the same side of the double bond.
• E-2-butene: Methyl groups are on opposite sides of the double bond.

Example 2: 2-Bromo-2-butene (C₄H₇Br)

• Assign priorities based on atomic number. Bromine has a higher atomic number than hydrogen, so it gets higher priority.
• • Z-2-bromo-2-butene: The higher priority groups (Br and CH₃) are on the same side.
  • E-2-bromo-2-butene: The higher priority groups are on opposite sides.

Advantages of E-Z Nomenclature:

• The E-Z system can be applied to any set of substituents, no matter how many groups or how complex they are.
• It is unambiguous and provides a precise description of the spatial arrangement of substituents.

3.0Comparison of Cis-Trans and E-Z Systems

Cahn-Ingold-Prelog (CIP) Priority Rules for E-Z Nomenclature

The Cahn-Ingold-Prelog priority rules are used to assign priorities to the substituents attached to the double bond. The steps are as follows:

1. Atomic Number: The higher the atomic number of the atom directly attached to the double-bonded carbon, the higher the priority. For example, Br > Cl > F > O > N > C > H.
2. If the first atoms are the same, move to the next atoms along the chain until a point of difference is found.
3. Multiple bonds are treated as if they are multiple single bonds:

• A double bond to an atom is equivalent to having two single bonds to that atom.
• For example, a C=O group is treated as if the carbon is bonded to two oxygens.

Example: Assigning Priorities in 2-bromo-2-butene (CH₃–C(Br)=C(H)–CH₃):

Compare the atoms attached to each carbon of the double bond.

• On C₁: Br (atomic number 35) vs. H (atomic number 1).
• On C₂: CH₃ vs. H.

Bromine has higher priority than hydrogen, and the methyl group (CH₃) has higher priority than hydrogen.

Thus, for Z-2-bromo-2-butene, the high-priority groups (Br and CH₃) are on the same side. For E-2-bromo-2-butene, they are on opposite sides.

Conformational Isomerism 

It is also known as rotational isomerism occurs when molecules with the same molecular formula and connectivity differ by the rotation around a single sigma bond. These isomers are interconvertible by rotating the atoms or groups attached to the bond without breaking any bonds.

• Conformational isomers, or conformers, exist due to the freedom of rotation around single bonds (unlike double bonds, which restrict rotation).
• The most common examples are seen in alkanes and cycloalkanes, where rotation around C–C bonds generates different conformers.

Examples of Conformational Isomerism:

1. Ethane: The rotation around the C–C bond gives rise to staggered and eclipsed conformations. The staggered conformation is more stable due to minimal electron repulsion.

2. Cyclohexane: Exists in chair, boat, and twist-boat conformations. The chair conformation is the most stable due to minimized steric strain.

Different conformations of cyclohexane: Chair, Boat, and Twist-Boat conformations-