(a) +I (inductive) of alkyl groups — alkyl groups positive carbon ki taraf electron density push karte hain.
Toh order: 3°>2°>1°>CH3+.
(b) Hyperconjugation — C–H bonds jo adjacent hain (+carbon ke α par) apne σ-electrons ko empty p-orbital mein overlap karte hain.
(c) Resonance (the strongest) — agar empty p-orbital ek π bond ya lone pair ke saath conjugated hai, toh charge kai atoms par delocalise ho jaata hai.
allyl, benzyl>3° alkyl
Benzyl/allyl 3° ko bhi beat karte hain kyunki delocalisation > pure +I hota hai.
Yeh electron-deficient hai (apne octet se ek kam), isliye yeh cations jaisa hi trend follow karta hai:
3°>2°>1°>CH3∙;benzyl, allyl stabilised by resonance
Hyperconjugation aur resonance se stabilise hota hai, bilkul cations ki tarah.
How many electrons surround the carbon in a carbocation, and what is its geometry?
6 (sextet), empty p-orbital, sp² planar.
Give the alkyl-cation stability order and why.
3°>2°>1°>CH₃⁺ due to +I and hyperconjugation (more α C–H = more delocalisation of the deficiency).
Why can a benzyl cation be more stable than a 3° alkyl cation?
Resonance delocalises charge over the ring (4 positions), beating mere hyperconjugation/+I.
Why does an adjacent –OH stabilise a carbocation despite its –I effect?
Its lone pair donates by resonance (+M), forming a stable oxocarbenium; +M outweighs –I.
State the carbanion stability order and contrast it with carbocations.
CH₃⁻>1°>2°>3°, the reverse of cations, because alkyl +I destabilises a negative centre.
Why is an alkynide (sp) carbanion stable?
More s-character holds the lone pair closer to the nucleus (sp>sp²>sp³), lowering energy.
How many electrons does a free radical carbon have, and its stability order?
7 (one unpaired); 3°>2°>1°, same as cations (electron-deficient, stabilised by donors/resonance).
Difference between singlet and triplet carbene (electrons, geometry, reactivity)?
Singlet: paired electrons, bent ~103°, stereospecific. Triplet: two unpaired parallel electrons (diradical), bent ~133° (not linear), non-stereospecific.
Is the triplet always the carbene ground state?
No — only for parent CH₂. Multiplicity depends on substituents; π-donor/electron-withdrawing groups (e.g. NHCs, –OR) give singlet ground states.
How do singlet and triplet nitrenes differ in non-bonding electrons?
Singlet: two lone pairs (4e⁻), no unpaired electrons. Triplet: one lone pair (2e⁻) + two unpaired electrons (diradical).
What is a 1,2-hydride shift and why does it occur?
H migrates with its bonding pair from the β-carbon to the cationic carbon to form a more stable carbocation.
When does a methyl shift occur instead of a hydride shift?
When no adjacent H can give a more stable cation but migrating an alkyl group can (e.g. neopentyl → tert cation).
Recall Feynman: explain to a 12-year-old
Socho ek hot potato (positive charge) hai jo koi hold nahi karna chahta. Agar tum akele khade ho toh poori garmi tum feel karte ho (unstable methyl cation). Lekin agar doston ne tumhare around khade hokar haath badhaye (alkyl groups, lone pairs), toh garmi share ho jaati hai aur comfortable lagta hai — yahi stability hai. Kabhi kabhi potato ek jagah aage pass bhi kiya ja sakta hai aisi person ko jo ke aur bhi zyada doston ke saath surrounded ho — woh pass hi "1,2-shift" hai, aur potato (charge) hamesha wahan jaata hai jahan woh sabse comfortable ho. Carbanions iska opposite hain: ek extra electron jaise bahut zyada thand lagna ho, toh tumhe aisi doston ki zaroorat hai jo heat away pull karein (electron-withdrawers), na ki woh jo tumhe warm rakhein.
