Geometrical Isomerism — Revision Notes

⚡ 30-Second Revision

Conditions for GI — Restricted rotation ($C=C$, cyclic) + two different groups on each relevant carbon.
Cis-Isomer — Identical/similar groups on the *same side*.
Trans-Isomer — Identical/similar groups on *opposite sides*.
E-Isomer (Entgegen) — Higher priority groups on *opposite sides* (CIP rules).
Z-Isomer (Zusammen) — Higher priority groups on *same side* (CIP rules).
CIP Rules — Priority by atomic number of directly attached atom. Higher atomic number = higher priority.
Stability — Trans > Cis (less steric hindrance).
Dipole Moment — Cis often polar (net dipole), Trans often non-polar (dipoles cancel).
Boiling Point — Cis often > Trans (due to polarity).
Melting Point — Trans often > Cis (better crystal packing).

2-Minute Revision

Geometrical isomerism, a type of stereoisomerism, arises from restricted rotation around a bond, typically a carbon-carbon double bond or within a cyclic structure. The crucial conditions are: (1) restricted rotation, and (2) each carbon involved in the restricted rotation must be bonded to two *different* groups. If these conditions are met, distinct isomers exist.

For simpler cases, the cis-trans system is used: 'cis' when identical/similar groups are on the same side of the double bond, and 'trans' when they are on opposite sides. For more complex cases, the E/Z system (based on Cahn-Ingold-Prelog priority rules) is employed. 'Z' (zusammen) means the two higher-priority groups are on the same side, while 'E' (entgegen) means they are on opposite sides. Priority is assigned based on the atomic number of the directly attached atom.

Key differences in properties: Trans isomers are generally more stable than cis due to reduced steric hindrance. Cis isomers often have a net dipole moment (polar), leading to higher boiling points, while trans isomers are often non-polar (dipoles cancel) and tend to have higher melting points due to better crystal packing. Always check the conditions and apply CIP rules carefully.

5-Minute Revision

Geometrical isomerism is a form of stereoisomerism where molecules share the same molecular formula and connectivity but differ in the spatial arrangement of atoms due to a 'lock' in the structure, preventing free rotation.

This 'lock' is typically a carbon-carbon double bond ($C=C$) or a rigid cyclic system. The second vital condition is that each carbon atom participating in the restricted rotation must be bonded to two *distinct* groups.

If either carbon has two identical groups, geometrical isomers cannot exist.

Two main nomenclature systems are used:

1
Cis-Trans System — Applicable when there are identical or similar groups. 'Cis' means these groups are on the same side of the double bond, while 'trans' means they are on opposite sides.

* Example: Cis-2-butene ($CH_3$ groups on same side) vs. Trans-2-butene ($CH_3$ groups on opposite sides).

1
E/Z System — Used for more complex alkenes, especially when all four groups on the double bond are different. It relies on the Cahn-Ingold-Prelog (CIP) priority rules:

* Step 1: Assign priority (1 for higher, 2 for lower) to the two groups on *each* carbon of the double bond based on the atomic number of the directly attached atom (higher atomic number = higher priority).

* Step 2: Compare the positions of the two higher-priority groups. If they are on the *same side*, it's the Z-isomer (Zusammen = together). If they are on *opposite sides*, it's the E-isomer (Entgegen = opposite).

* Mini-Example: For $C(Br)(CH_3)=C(Cl)(H)$: On left C, Br > $CH_3$. On right C, Cl > H. If Br and Cl are on the same side, it's Z. If Br and Cl are on opposite sides, it's E.

Stability and Physical Properties: Trans isomers are generally more stable than cis isomers due to reduced steric hindrance (bulky groups are further apart). This often translates to trans isomers having higher melting points (better crystal packing).

Cis isomers, however, often possess a net dipole moment (due to bond dipoles adding up vectorially), making them more polar and typically leading to higher boiling points compared to their non-polar (or less polar) trans counterparts.

Remember, geometrical isomers are distinct compounds with different physical and sometimes chemical properties.

Prelims Revision Notes

Geometrical Isomerism (GI) - NEET Revision Notes

1. Definition & Conditions:

Definition — A type of stereoisomerism where molecules have the same molecular formula and connectivity but differ in spatial arrangement due to restricted rotation.
Condition 1: Restricted Rotation — Primarily due to a $C=C$ double bond or a rigid cyclic structure. (No free rotation around $C=C$ due to $pi$-bond).
Condition 2: Different Groups — Each carbon of the double bond (or substituted carbon in a ring) must be attached to two *different* groups. If $R_1=R_2$ on one carbon, no GI.

* Example: But-2-ene ($CH_3-CH=CH-CH_3$) shows GI. Propene ($CH_3-CH=CH_2$) does not (terminal $CH_2$ has two H's).

2. Nomenclature Systems:

Cis-Trans System — For simpler cases with identical/similar groups.

* Cis: Identical/similar groups on the *same side* of the double bond. * Trans: Identical/similar groups on *opposite sides* of the double bond.

E/Z System (Cahn-Ingold-Prelog Rules) — For complex cases, especially when all four groups are different. Unambiguous.

* Step 1: Assign Priorities: For each $sp^2$ carbon, assign priority to its two attached groups based on atomic number of the directly attached atom (higher atomic number = higher priority). If first atoms are same, move to next atoms. * Step 2: Compare Positions: * Z (Zusammen): Higher priority groups on *same side*. * E (Entgegen): Higher priority groups on *opposite sides*.

3. Stability:

Trans-isomers are generally more stable than Cis-isomers.
Reason — Less steric hindrance in trans-isomers (bulky groups are further apart), leading to lower energy.

4. Physical Properties:

Dipole Moment

* Cis-isomers: Often have a net dipole moment (polar) because bond dipoles add up vectorially. * Trans-isomers: Often have zero or very small dipole moment (non-polar) because bond dipoles cancel out due to symmetry. * Example: Cis-1,2-dichloroethene is polar; Trans-1,2-dichloroethene is non-polar.

Boiling Point (BP)

* Cis-isomers often have higher BP than Trans-isomers. * Reason: Higher polarity of cis-isomers leads to stronger dipole-dipole intermolecular forces, requiring more energy to overcome.

Melting Point (MP)

* Trans-isomers often have higher MP than Cis-isomers. * Reason: Trans-isomers are generally more symmetrical, allowing for better packing in the crystal lattice, leading to stronger intermolecular forces in the solid state.

5. Number of Geometrical Isomers:

Unsymmetrical polyenes (different groups at ends) — with 'n' double bonds capable of GI: $2^n$ isomers.
Symmetrical polyenes (same groups at ends) — with 'n' double bonds capable of GI:

* If 'n' is even: $2^{n-1} + 2^{(n/2)-1}$ * If 'n' is odd: $2^{n-1}$

Quick Check for GI: Draw the double bond. For each carbon, check if the two groups attached are different. If yes for both, GI is possible.

Vyyuha Quick Recall

Cis Same Side, Trans Opposite Side. For E/Z, remember: Z is for Zusammen (together - higher priority groups on same side), E is for Entgegen (opposite - higher priority groups on opposite sides). Think of 'Z' as 'Zame' (same side) and 'E' as 'Enemies' (opposite sides).