Group 13 (Boron family) — anomaly of B, diagonal B-Si; BX₃ Lewis acidity; diborane B₂H₆ (3c-2e bond), borazine

3.2.1 · Chemistry › p-Block

1. Boron anomalous kyun hai? (KYA & KYUN)

KYUN har property follow karti hai:

Property	Reason	Consequence
Non-metallic, hard	Chhota size, high IE → ions ki tarah e⁻ nahi khoyega	B ek metalloid hai; baaki sab metals hain
Sirf covalent compounds	Pehle 3 IEs ka sum bahut bada hai	Koi simple $B^{3 +}$ ion exist nahi karta
Max covalency 4	D-orbitals nahi → octet expand nahi ho sakta	$B F_{4}^{-}$ exist karta hai, $B F_{6}^{3 -}$ nahi karta

2. Diagonal relationship: B ↔ Si (KYUN similar hain)

Shared properties (evidence):

Dono giant covalent / acidic oxides banate hain ( $B_{2} O_{3}$ , $S i O_{2}$ ) jo glassy/weakly acidic hote hain.
Dono volatile, hydrolysable halides banate hain ( $B C l_{3} + 3 H_{2} O \to H_{3} B O_{3} + 3 H C l$ ; $S i C l_{4} + 2 H_{2} O \to S i O_{2} + 4 H C l$ ).
Dono electron-precise/covalent hydrides banate hain jo flammable hote hain (boranes, silanes).
Dono borates / silicates banate hain chains aur rings ke saath.

3. BX₃ ki Lewis acidity (TREND DERIVE karo)

$B X_{3}$ mein Boron ke around sirf 6 electrons hote hain (ek empty $2 p_{z}$ orbital) → yeh ek lone pair accept karta hai → Lewis acid: $B F_{3} + : N H_{3} ⟶ F_{3} B \leftarrow N H_{3}$

Trend ki derivation (KAISE): B ka empty $2 p_{z}$ halogen lone pair se back-bonding ( $p π$ – $p π$ ) ke zariye fill ho sakta hai. Zyada strong back-bonding → B kam electron-deficient → kamzor Lewis acid.

F period 2 mein hai, B ke same size → $2 p$ – $2 p$ overlap excellent hai → strong back-donation → B sabse kam acidic.
F→Cl→Br→I jaane par, halogen ke p-orbitals bade aur zyada diffuse hote jaate hain → B ke chhote $2 p$ ke saath overlap kharab hota jaata hai → back-bonding kamzor hoti hai → B electron-hungry rehta hai → acidity badhti hai.

4. Diborane $B_{2} H_{6}$ — 3c–2e bond (STRUCTURE DERIVE karo)

Structure (electrons kaise distribute hote hain KAISE):

4 terminal B–H bonds → normal 2c–2e → $4 \times 2 = 8$ electrons use hote hain.
2 bridging B–H–B bonds → 3c–2e → $2 \times 2 = 4$ electrons use hote hain.
Total $= 8 + 4 = 12$ electrons ✔ (match karta hai!)

Figure — Group 13 (Boron family) — anomaly of B, diagonal B-Si; BX₃ Lewis acidity; diborane B₂H₆ (3c-2e bond), borazine

Preparation & key reactions: $3 L i A l H_{4} + 4 B F_{3} \to 2 B_{2} H_{6} + 3 L i F + 3 A l F_{3}$ $B_{2} H_{6} + 6 H_{2} O \to 2 H_{3} B O_{3} + 6 H_{2}$ $B_{2} H_{6} + 2 N M e_{3} \to 2 M e_{3} N \cdot B H_{3} (symmetric cleavage)$ $B_{2} H_{6} + 3 O_{2} \to B_{2} O_{3} + 3 H_{2} O (Δ H = - 2165 k J / m o l, very exothermic)$

5. Borazine $B_{3} N_{3} H_{6}$ — "inorganic benzene"

Flashcards

Free

B^{3 +}

ion kyun exist nahi karta?

Iske pehle teen ionisation enthalpies ka sum itna bada hai ki use lattice/hydration energy compensate nahi kar sakti → B sirf covalent bonds banata hai.

Boron ki maximum covalency kya hai aur kyun?

4; kyunki iske paas d-orbitals nahi hain aur yeh apna octet expand nahi kar sakta (e.g.

B F_{4}^{-}

exist karta hai,

B F_{6}^{3 -}

nahi).

Boron ki diagonal relationship batao.

B, Si se milta-julta hai (Al ke same group mein lekin diagonally placed) similar electronegativity aur charge/size ratio ki wajah se.

B–Si diagonal relationship ka ek evidence do.

Dono ke paas acidic covalent oxides (

B_{2} O_{3}

S i O_{2}

) hain aur dono volatile, easily hydrolysed halides banate hain.

Boron halides ki Lewis acidity ka order?

B F_{3} < B C l_{3} < B B r_{3} < B I_{3}

B F_{3}

sabse weak Lewis acid kyun hai?

F se strong

2 p

–

2 p

back-bonding B ke empty

2 p

orbital ko fill kar deti hai, electron deficiency reduce kar deti hai.

B_{2} H_{6}

mein kitne valence electrons hain?

12 (

2 \times 3 + 6 \times 1

B_{2} H_{6}

electron-deficient kyun hai?

8 normal 2c–2e B–H bonds ke liye 16 electrons chahiye hote lekin sirf 12 available hain.

B_{2} H_{6}

mein bonding describe karo.

4 terminal 2c–2e B–H bonds (8 e⁻) + 2 bridging 3c–2e B–H–B bonds (4 e⁻); koi direct B–B bond nahi.

3c–2e bond kya hota hai?

Teen atoms ke beech share hone wala ek electron pair (e.g. B–H–B bridge).

Diborane mein bridging vs terminal B–H bond length?

Bridging ≈ 133 pm (longer/weaker), terminal ≈ 119 pm.

Diborane hydrolysis ka product?

H_{3} B O_{3}

(boric acid) +

H_{2}

Borazine kya hai aur iska formula?

"Inorganic benzene",

B_{3} N_{3} H_{6}

; alternating B aur N ki planar ring.

Borazine benzene se zyada reactive kyun hai?

Iska

π

ring polar hai (

B^{δ +}

N^{δ -}

), isliye yeh HCl jaise reagents easily add kar leta hai.

Recall Feynman: ek 12-saal ke bache ko explain karo

Boron ek chhota sa atom hai jiske paas sirf teen "haath" (electrons) hain lekin chaar khali jebein hain haath milane ke liye. Toh yeh hamesha extra haath pakadne ki koshish karta hai — isi se yeh "Lewis acid" (haath pakadne wala) ban jaata hai. Jab do boron-hydrogen molecules milte hain, toh unke paas enough haath nahi hote, isliye do hydrogens do borons ke beech khade ho jaate hain aur teen atoms ek single handshake share karte hain — ek special "3-dost-ek-handshake" bond. Fluorine ke saath, F atom apne spare haath itni acche se share karta hai ki boron pakadna band kar deta hai — isi liye $B F_{3}$ sabse kam pakadne wala hai. Aur kyunki boron silicon (uska diagonal neighbour) se itna milta-julta hai, dono similar glassy oxides aur smelly halides banate hain.

Connections

p-Block Overview
Lewis Acids and Bases
Back-bonding (pπ–pπ)
Ionisation Enthalpy Trends
Boron Trihalides
Boric Acid H3BO3
Benzene Aromaticity (borazine se compare karo)
Group 14 — Silicon (diagonal partner)