Tetravalency of carbon; hybridization recap (sp, sp², sp³)

4.1.1 · Chemistry › General Organic Chemistry (GOC)

1. Tetravalency: WHY carbon 4 bonds banata hai

Puzzle KYA hai? Ground-state carbon ( $Z = 6$ ) ka configuration hai: $1 s^{2} 2 s^{2} 2 p_{x}^{1} 2 p_{y}^{1} 2 p_{z}^{0}$ Sirf do unpaired electrons ( $2 p_{x}^{1}, 2 p_{y}^{1}$ ) → iska matlab yeh divalent hona chahiye (jaise :CH₂, ek carbene mein hota hai). Phir bhi methane CH₄ hai.

Puzzle KAISE solve hota hai? (First principles se derivation)

Promotion (excitation): Ek $2 s$ electron ko empty $2 p_{z}$ mein promote kiya jaata hai: $1 s^{2} 2 s^{1} 2 p_{x}^{1} 2 p_{y}^{1} 2 p_{z}^{1}$ Ab chaar unpaired electrons hain → chaar bonds possible hain.
Energy cost vs payoff (WHY yeh worth it hai): Promotion mein energy lagti hai ( $\approx 400$ kJ/mol, $2 s \to 2 p$ move karne ke liye). Lekin do extra C–H bonds banane se ( $\approx 2 \times 414$ kJ/mol) energy release hoti hai. Net release ≫ cost, isliye molecule kaafi zyada stable hai. Nature thoda sa bill bharta hai, ek bada refund paane ke liye.
Naya problem: Ab hamare paas ek $s$ orbital + teen $p$ orbitals hain — lekin inki shapes aur energies alag-alag hain. Phir bhi CH₄ mein chaaon C–H bonds identical hain (same length 109 pm, same energy). Toh orbitals ko mix karna hi padega. Wahi mixing hybridization hai.

2. Hybridization: WHAT hai yeh aur HOW kaam karta hai

Rules (har ek ke peeche WHY):

Conservation of orbitals: $n$ atomic orbitals andar → $n$ hybrid orbitals baahir. (Orbitals bana ya mita nahi sakte.)
Sirf central atom hybridize karta hai, similar energy ke valence orbitals use karke.
Zyada $s$ -character → orbital nucleus ke paas aur tight → shorter, stronger bond, bada bond angle, us carbon ki zyada electronegativity.

Formula Steric number → hybridization

$Steric Number (SN) = (no. of σ bonds) + (no. of lone pairs on central atom)$

SN	Hybridization	Geometry	Angle
2	$s p$	linear	$18 0^{\circ}$
3	$s p^{2}$	trigonal planar	$12 0^{\circ}$
4	$s p^{3}$	tetrahedral	$109. 5^{\circ}$

SN = σ + lone pairs kyon (π bonds nahi)? π bonds unhybridized p orbitals se bante hain aur ek existing σ framework ke upar hote hain — inhe apna hybrid orbital nahi chahiye.

Figure — Tetravalency of carbon; hybridization recap (sp, sp², sp³)

3. Teen carbon hybridizations

sp³ (4 hybrids)

Ek $s$ + teen $p$ → chaar $s p^{3}$ orbitals.
25% s-character, angle $109. 5^{\circ}$ , tetrahedral.
Example: CH₄, ethane C–C, diamond. Koi leftover p orbitals nahi → sirf σ bonds → sirf single bonds.

sp² (3 hybrids)

Ek $s$ + do $p$ → teen $s p^{2}$ orbitals (planar, $12 0^{\circ}$ ); ek unhybridized p perpendicular bacha rehta hai.
33% s-character.
Woh bacha hua p ek π bond banata hai → ek double bond hota hai.
Example: ethene C₂H₄, benzene, carbonyl C=O.

sp (2 hybrids)

Ek $s$ + ek $p$ → do $s p$ orbitals (linear, $18 0^{\circ}$ ); do unhybridized p bache rehte hain.
50% s-character.
Do p orbitals → do π bonds → ek triple bond (ya do cumulated doubles, jaise allene C=C=C mein).
Example: ethyne C₂H₂, CO₂, –C≡N.

4. Worked examples

5. Common mistakes (Steel-man + fix)

6. Active recall flashcards

Ground-state carbon divalent kyon hona chahiye?

Sirf do unpaired electrons (

2 p_{x}^{1}, 2 p_{y}^{1}

) hain;

2 s

filled hai.

Iske bawajood carbon tetravalent kyon hai?

2 s

electron ka

2 p_{z}

mein promotion chaar unpaired electrons deta hai; energy do extra bonds se wapas milti hai.

Hybridization define karo.

Ek hi atom ke similar energy ke atomic orbitals ki mixing, jisse equal number mein equivalent hybrid orbitals milte hain.

Steric number ka formula?

SN = (σ bonds ki sankhya) + (central atom par lone pairs).

π bonds SN mein kyon count nahi hote?

Yeh unhybridized p orbitals se bante hain, σ framework ke upar hote hain.

SN 2, 3, 4 se kaunsi hybridizations aur angles milte hain?

2→sp (180°), 3→sp² (120°), 4→sp³ (109.5°).

sp, sp², sp³ mein s-character ka order?

sp (50%) > sp² (33%) > sp³ (25%).

Zyada s-character ka bond length aur angle par kya asar?

Shorter, stronger bonds aur bade bond angles; zyada electronegative carbon.

Ethyne mein carbon ki hybridization kyon sp hai?

Har C ke paas 2 σ bonds + 0 lone pairs hain (triple bond = 1σ + 2π).

CO₂ mein C ki hybridization?

sp (O ko 2 σ bonds, C par koi lone pair nahi).

sp, sp², sp³ mein kitne unhybridized p orbitals?

Kramasah 2, 1, 0.

Terminal alkyne H acidic kyon hai?

sp carbon (50% s-character) C–H electrons ko tightly pakadta hai, conjugate base anion ko stabilize karta hai.

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho carbon ke paas doosre atoms ko pakadne ke liye chaar haath hain. Lekin pehle lagta hai ki sirf do haath free hain aur do aapas mein chipke hain. Ek choti si push (promotion) se woh unhe free kar leta hai toh sab chaar haath khul jaate hain. Phir carbon apne haatho ko shuffle karta hai taaki sab same length ke hon aur equally spread out — wahi shuffling hybridization hai. Agar carbon apne haath pyramid ki tarah 4 directions mein phailaata hai, toh woh $s p^{3}$ hai. Flat triangle mein, $s p^{2}$ (aur ek extra haath double-grip ke liye rakh leta hai). Seedhi line mein, $s p$ (do extra haath triple-grip ke liye). Jitna seedha spread, utni mazboot aur chhoti grip.

Connections

Sigma and Pi bonds — orbital overlap
VSEPR theory and molecular geometry
Inductive effect and electronegativity of hybrid carbons
Acidity of terminal alkynes
Resonance and delocalization (sp² systems)
Bond length and bond strength trends
Aromaticity and benzene (all-sp² ring)