CHEMCA
EXAM MASTER FORMULA SHEET
Isomerism in Organic Compounds
1. Structural (Constitutional) Isomerism
Compounds having the same molecular formula but different connectivities of atoms.
Difference in the nature of the main carbon skeleton (straight vs branched).
e.g., n-Pentane & Isopentane
Difference in the position of a functional group, multiple bond, or substituent on the same chain.
e.g., Propan-1-ol & Propan-2-ol
Difference in the nature of the functional group. (Alcohol/Ether, Aldehyde/Ketone, Acid/Ester).
e.g., Ethanol & Methoxymethane
Unequal distribution of alkyl groups attached to a polyvalent functional group ($-O-, -S-, -NH-, -CO-$).
e.g., Diethyl ether & Methyl propyl ether
One isomer has an open-chain structure and the other has a ring structure.
e.g., Propene & Cyclopropane
If two compounds show multiple types of isomerism, prioritize naming them in this order:
A special functional isomerism existing in dynamic equilibrium due to the migration of an active proton (1,3-shift).
Enol content $\uparrow$ with extended conjugation, aromaticity, or intramolecular H-bonding (e.g., Acetylacetone).
2. Geometrical Isomerism (GI)
Occurs due to Restricted Rotation around double bonds ($C=C$, $C=N$, $N=N$) or within Cyclic structures.
Cis / Trans System
Used when at least one identical group is present on both doubly bonded atoms.
• Cis: Identical groups on the same side.
• Trans: Identical groups on opposite sides.
E / Z System (CIP Rules)
Assign priority based on Atomic Number.
• Z (Zusammen): Higher priority groups on the same side (like Cis).
• E (Entgegen): Higher priority groups on opposite sides.
Syn / Anti (Oximes & Azo)
Aldoximes:
• Syn: $-H$ and $-OH$ on same side.
• Anti: $-H$ and $-OH$ on opposite sides.
Physical Properties Comparison
| Property | Cis Isomer | Trans Isomer |
|---|---|---|
| Dipole Moment ($\mu$) | Usually Higher | Usually Zero / Lower |
| Boiling Point | Higher (due to $\mu$) | Lower |
| Melting Point | Lower | Higher (Better packing) |
| Stability | Lower (Steric hindrance) | Higher |
For a polyene with $n$ double bonds showing GI:
Unsymmetrical Ends
Symmetrical Ends
($p=n/2$ if even, $p=(n+1)/2$ if odd)
3. Optical Isomerism
Optical Rotation: Experimental property. Indicates direction of rotation of plane-polarized light. $d(+)$ is dextrorotatory, $l(-)$ is levorotatory.
Relative Config: Used mainly for carbs/amino acids. Based on Glyceraldehyde. 'D' has $-OH$ on the right of the bottom chiral carbon.
Absolute Config: CIP priority. View down lowest priority group bond. 1$\to$2$\to$3 Clockwise = R (Rectus). Anticlockwise = S (Sinister).
$l$ = path length in dm, $c$ = concentration in g/mL
Measures excess of one enantiomer over racemic mixture.
| Term | Definition | Characteristics |
|---|---|---|
| Enantiomers | Non-superimposable mirror images of each other. | Identical physical properties (except direction of optical rotation). |
| Diastereomers | Stereoisomers that are NOT mirror images. | Different physical properties (M.P., B.P., solubility). Easy to separate. |
| Meso Compound | Molecule with multiple chiral centers but possesses a Plane of Symmetry (POS) or Center of Symmetry (COS). | Optically Inactive due to Internal Compensation. |
| Racemic Mixture | Equimolar (1:1) mixture of two enantiomers. | Optically Inactive due to External Compensation. |
4. Conformational Isomerism
Isomers formed by free rotation around $C-C$ single bonds. Energy barrier is very low ($\approx 1-20$ kcal/mol).
Repulsion between bond-pair electrons on adjacent carbons (highest in eclipsed form).
Van der Waals repulsion between bulky groups that are forced too close together.
Stability Orders (Lowest Energy to Highest)
Anti ($\theta=180^\circ$) > Gauche ($\theta=60^\circ$) > Partially Eclipsed ($\theta=120^\circ$) > Fully Eclipsed ($\theta=0^\circ$)
*Gauche Effect anomaly: In $O-CH_2-CH_2-F$ or Ethylene Glycol, Gauche form becomes more stable than Anti due to intramolecular H-bonding or strong dipole-dipole attraction.*
Chair > Twist Boat > Boat > Half Chair
Chair form has practically zero angle strain and zero torsional strain (all bonds staggered). Bulky groups strongly prefer the Equatorial position to avoid 1,3-diaxial steric interactions.
5. Calculation Formulas for Optical Isomers
| Molecule Nature | Optically Active Pair ($a$) | Meso Forms ($m$) | Total Stereoisomers ($a+m$) |
|---|---|---|---|
| Unsymmetrical ends ($n$ distinct chiral centers) |
\[ 2^n \] | \[ 0 \] | \[ 2^n \] |
| Symmetrical ends ($n$ is Even) |
\[ 2^{n-1} \] | \[ 2^{(n/2)-1} \] | \[ 2^{n-1} + 2^{(n/2)-1} \] |
| Symmetrical ends ($n$ is Odd) |
\[ 2^{n-1} - 2^{(n-1)/2} \] | \[ 2^{(n-1)/2} \] | \[ 2^{n-1} \] |
🎯 Complete Chemistry Revision Toolkit
Score higher by combining Formula Sheets, Flashcards and the Mistake Bank. These three resources help you revise faster, remember longer and avoid common exam mistakes for JEE Main, JEE Advanced, NEET and CBSE Board.
💡 Best Revision Strategy: Start with Formula Sheets for quick revision, reinforce concepts using Flashcards, and finish with the Mistake Bank to avoid repeating errors in the examination.
No comments:
Post a Comment