Isomerism — structural (chain, position, functional, metamerism, tautomerism) and stereo (geometrical - cis-trans - E-Z,

4.1.5 · Chemistry › General Organic Chemistry (GOC)

Isomers kyun exist karte hain?

Carbon 4 strong covalent bonds banata hai aur chains/branches aaram se form karta hai. Toh ek formula jaise $C_{4} H_{10}$ ek se zyada valid tarike se assemble ho sakta hai — aur har alag tarika ek real, alag-kiya-ja-sakne-wala compound hai (alag b.p., reactivity, smell...).

Badi division:

Structural (constitutional) isomers → connectivity mein fark (kaun sa atom kis se bonded hai).
Stereoisomers → same connectivity, sirf 3D arrangement mein fark.

Figure — Isomerism — structural (chain, position, functional, metamerism, tautomerism) and stereo (geometrical - cis-trans - E-Z,

1. Structural Isomerism

(a) Chain isomerism

Carbon skeleton mein fark hota hai (straight vs branched).

$C_{4} H_{10}$ : n-butane vs isobutane (2-methylpropane).

Yeh step kyun? H count fix rakhte hain aur sirf C-backbone rearrange karte hain.

(b) Position isomerism

Same skeleton + same functional group, lekin group alag position par hota hai.

$C_{3} H_{7} O H$ : propan-1-ol vs propan-2-ol.
$C_{4} H_{8}$ : but-1-ene vs but-2-ene.

(c) Functional isomerism

Same formula, alag functional group.

$C_{2} H_{6} O$ : ethanol ( $C H_{3} C H_{2} O H$ ) vs dimethyl ether ( $C H_{3} O C H_{3}$ ).
$C_{3} H_{6} O$ : propanal (aldehyde) vs propanone (ketone).

(d) Metamerism

Same functional group, lekin divalent atom (O, S, N) ke dono taraf alag alkyl groups hote hain.

$C_{4} H_{10} O$ ethers: $C H_{3} - O - C_{3} H_{7}$ vs $C_{2} H_{5} - O - C_{2} H_{5}$ .

Yeh step kyun? Total carbons central O ke across alag-alag split hote hain.

(e) Tautomerism — ek dynamic structural isomerism

Do isomers rapid equilibrium mein hote hain, ek proton (H) aur ek π bond ke movement se interconvert karte hain.

2. Stereoisomerism

Same connectivity → space mein arrangement alag hoti hai.

(a) Geometrical (cis–trans / E–Z)

Conditions: restricted rotation (C=C ya ring) AUR har doubly-bonded carbon par do alag groups hone chahiye.

cis: same/similar groups same side par. trans: opposite sides par.
E/Z CIP priority use karke rigorous version hai (higher atomic number = higher priority):
- Z (zusammen = together): do high-priority groups same side par.
- E (entgegen = opposite): high-priority groups opposite sides par.

(b) Optical isomerism (R/S, chirality)

Enantiomers plane-polarized light ko equally lekin opposite direction mein rotate karte hain ((+)/(−)). Ek racemic mixture (50:50) optically inactive hota hai (rotations cancel ho jaate hain).

R/S assign karna (CIP):

4 groups ko priority se rank karo (atomic number; pehle difference par bahar jaao).
Lowest priority group ko apne se door point karo.
1→2→3 trace karo: clockwise = R (rectus), anticlockwise = S (sinister).

Stereoisomers count karna: $n$ chiral centres ke liye (koi symmetry nahi), max $= 2^{n}$ . Ek meso compound mein chiral centres hote hain lekin ek internal mirror plane hoti hai → overall achiral (jaise meso-tartaric acid).

(c) Conformational isomerism

Ethane: staggered (low energy, dihedral 60°) vs eclipsed (high energy, 0°). Energy gap ≈ 12.5 kJ/mol (torsional strain).
Butane: anti (most stable) > gauche > eclipsed > fully eclipsed (syn).

Common mistake Classic traps ko steel-man karna

"cis–trans always = E–Z." Sahi lagta hai kyunki but-2-ene match karta hai. Fix: cis/trans aankhon se similarity use karta hai; E/Z CIP priority use karta hai, aur yeh flip ho sakte hain (ek high-priority group "cis" side par bhi ho sakta hai). "Har C jo 4 bonds banaata hai wo chiral hai." Sahi lagta hai (4 bonds!). Fix: chaaroon groups alag hone chahiye. $C H_{2} C l B r$ mein do H hain → chiral nahi. "Tautomers aur resonance same hain." Sahi lagta hai (dono electrons move karte hain). Fix: tautomers real, alag molecules hain jisme atoms (H) actually move karte hain; resonance structures same molecule hain, sirf electrons "draw" alag tarike se hote hain, koi atom nahi hilta. "Zyada chiral centres ⇒ hamesha $2^{n}$ isomers." Fix: symmetry (meso) count ko $2^{n}$ se kam kar deti hai.

Recall Feynman: 12-saal ke bachche ko explain karo

Socho tumhare paas same set ke Lego pieces hain. Tum ek lambi seedhi deewar bana sakte ho ya bump wali deewar — same pieces, alag shapes: yahi structural isomerism hai. Ab same shape do baar banao lekin apna left-hand build mirror mein flip karo — tumhaare do haath same lagte hain lekin tum left glove right haath par nahi pehen sakte: yahi optical isomerism hai (enantiomers). Aur ek tautomer ek fidget toy jaisa hai jahan ek chhota H atom baar baar aage-peechhe karta rehta hai, toh molecule do forms ke beech flicker karta hai.

Active Recall

Isomers definition

Same molecular formula, alag structure ya spatial arrangement (alag properties).

Two main classes of isomerism

Structural (constitutional) aur stereoisomerism.

Chain isomerism

Same formula, alag carbon-skeleton (straight vs branched), jaise n-butane vs isobutane.

Position isomerism

Same skeleton aur group, group alag position par, jaise propan-1-ol vs propan-2-ol.

Functional isomerism

Same formula, alag functional group, jaise ethanol vs dimethyl ether.

Metamerism

Same functional group lekin divalent atom (ethers/amines) ke around alag alkyl groups.

Tautomerism

Isomers ke beech H aur π bond shift ke through rapid equilibrium, jaise keto–enol.

Condition for keto–enol tautomerism

Ek α-hydrogen ka hona zaroori hai.

Resonance vs tautomerism

Resonance = same molecule, sirf electrons; tautomers = alag molecules, atoms (H) move karte hain.

Condition for geometrical isomerism

Restricted rotation (C=C ya ring) AUR har carbon par do alag groups.

Z vs E

Z = do higher CIP-priority groups same side par; E = opposite sides par.

CIP priority rule

Higher atomic number = higher priority; pehle difference par bahar jaake compare karo.

Chiral centre

Carbon jo chaar alag groups se bonded ho; mirror image non-superimposable.

Enantiomers

Non-superimposable mirror images; plane-polarized light ko equally lekin opposite direction mein rotate karte hain.

Racemic mixture

50:50 enantiomers; net optical rotation zero (inactive).

Meso compound

Chiral centres hain lekin internal mirror plane hai → overall achiral.

Max stereoisomers for n stereocentres

2^n (kam agar meso/symmetry ho).

How to assign R/S

Lowest priority away; 1→2→3 clockwise = R, anticlockwise = S.

Conformational isomers

Single (σ) bond ke around rotation se interconvert hote hain; separable nahi hote.

Most stable conformer of ethane

Staggered (dihedral 60°), minimal torsional strain.

Most stable conformer of butane

Anti (do methyls 180° apart).

Connections

Hybridization and Bonding — kyun C=C π bond rotation block karta hai.
CIP Priority Rules — E/Z aur R/S ka backbone.
Resonance and Mesomeric Effect — tautomerism se contrast.
Acidity of Alpha-Hydrogens — keto–enol equilibrium drive karta hai.
Optical Activity and Polarimetry — enantiomers measure karna.
Newman Projections — conformers visualize karna.