Boron and its Compounds — Revision Notes

⚡ 30-Second Revision

Boron (B): — Group 13, non-metal/metalloid, electron-deficient.
Electronic Configuration: — $[He]2s^22p^1$.
Anomalous Behavior: — Small size, high IE, no d-orbitals $ightarrow$ covalent, Lewis acidic, max covalency 4.
Boron Trihalides ($BX_3$): — Planar, $sp^2$. Lewis acidity order: $BI_3 > BBr_3 > BCl_3 > BF_3$ (due to backbonding).
Borax ($Na_2B_4O_7 cdot 10H_2O$): — Contains $[B_4O_5(OH)_4]^{2-}$ unit. Alkaline solution. Borax bead test: $Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3$.
Boric Acid ($H_3BO_3$ or $B(OH)_3$): — Weak monobasic Lewis acid (accepts $OH^-$ from $H_2O$).

$B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+$.

Diborane ($B_2H_6$): — Electron-deficient. Contains two 3-center 2-electron (B-H-B) 'banana bonds' and four 2-center 2-electron (B-H) terminal bonds. $sp^3$ hybridized boron.
Preparation of $B_2H_6$ (Lab): — $4BF_3 + 3LiAlH_4 \xrightarrow{ether} 2B_2H_6 + 3LiF + 3AlF_3$.
Boron Nitride (BN): — h-BN (graphite-like, lubricant), c-BN (diamond-like, abrasive).

2-Minute Revision

Boron, the first element of Group 13, is a non-metal exhibiting unique 'anomalous behavior' due to its small size, high ionization energy, and absence of d-orbitals. This leads to its characteristic 'electron deficiency' and strong tendency to form covalent compounds, acting as a Lewis acid.

Its maximum covalency is restricted to four. Key compounds include boron trihalides ($BX_3$), which are Lewis acids, with $BI_3$ being the strongest due to poor backbonding. Borax ($Na_2B_4O_7 cdot 10H_2O$) is a crucial mineral, forming a glassy bead of sodium metaborate and boron trioxide upon heating, used in the borax bead test.

Boric acid ($H_3BO_3$) is a weak monobasic Lewis acid, accepting $OH^-$ from water. Diborane ($B_2H_6$) is an electron-deficient hydride featuring unique 3-center 2-electron (B-H-B) 'banana bonds' and is prepared in the lab from $BF_3$ and $LiAlH_4$.

Boron also forms hard materials like boron nitride. Remember its diagonal relationship with silicon.

5-Minute Revision

Boron (B), atomic number 5, is the unique non-metallic member of Group 13. Its small size, high ionization enthalpy, and lack of d-orbitals explain its 'anomalous behavior' compared to its metallic congeners.

This behavior manifests as a strong tendency to form covalent, electron-deficient compounds, making them potent Lewis acids. For instance, boron trihalides ($BX_3$) are Lewis acids, but their strength follows the order $BI_3 > BBr_3 > BCl_3 > BF_3$, a reversal of electronegativity, due to the varying extent of $ppi-ppi$ backbonding from halogen to boron.

Strong backbonding in $BF_3$ reduces boron's electron deficiency most effectively.

Borax ($Na_2B_4O_7 cdot 10H_2O$) is a key mineral. Its aqueous solution is alkaline due to hydrolysis. The 'borax bead test' involves heating borax to form a glassy bead of sodium metaborate ($NaBO_2$) and boron trioxide ($B_2O_3$), which then reacts with metal oxides to give colored metaborates.

Boric acid ($H_3BO_3$ or $B(OH)_3$) is a weak monobasic acid, but crucially, it's a Lewis acid. It accepts an $OH^-$ ion from water, releasing $H^+$ from the water molecule: $B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+$. Heating boric acid yields metaboric acid ($HBO_2$), then tetraboric acid ($H_2B_4O_7$), and finally boron trioxide ($B_2O_3$).

Diborane ($B_2H_6$) is a highly important electron-deficient compound. Its structure features two 3-center 2-electron (3c-2e) B-H-B 'banana bonds' and four terminal 2-center 2-electron (2c-2e) B-H bonds.

The boron atoms are $sp^3$ hybridized. It is prepared in the lab by reacting $BF_3$ with $LiAlH_4$ in ether. Diborane is spontaneously flammable and hydrolyzes to boric acid. Boron also forms extremely hard materials like boron nitride (BN), existing in hexagonal (h-BN, lubricant) and cubic (c-BN, abrasive) forms.

Remember boron's diagonal relationship with silicon, sharing similarities in covalent character and acidic oxides.

Prelims Revision Notes

Boron (B): Atomic No. 5, Group 13. Non-metal/Metalloid. Electronic config: $[He]2s^22p^1$.

Anomalous Behavior:

Smallest size, highest IE in Group 13.
Absence of d-orbitals $ightarrow$ Max covalency = 4 (e.g., $[BF_4]^-$).
Forms covalent compounds, not ionic.
Electron-deficient compounds $ightarrow$ Strong Lewis acids.
Diagonal relationship with Silicon (similar electronegativity, covalent nature, acidic oxides).

Boron Trihalides ($BX_3$):

Planar triangular geometry, $sp^2$ hybridized boron.
Lewis acidity order: $BI_3 > BBr_3 > BCl_3 > BF_3$.
Reason: $ppi-ppi$ backbonding from halogen to boron. Strongest in $BF_3$ (effective $2p-2p$ overlap), weakest in $BI_3$ (poor $2p-5p$ overlap). Stronger backbonding reduces electron deficiency, making it a weaker Lewis acid.
Hydrolysis: $BX_3 + 3H_2O \rightarrow H_3BO_3 + 3HX$ (except $BF_3$ forms adducts).

Borax ($Na_2B_4O_7 cdot 10H_2O$):

Correct formula: $Na_2[B_4O_5(OH)_4] cdot 8H_2O$.
Aqueous solution is alkaline due to hydrolysis: $Na_2B_4O_7 + 7H_2O \rightleftharpoons 2NaOH + 4H_3BO_3$.
Borax Bead Test:

1. Heating: $Na_2B_4O_7 cdot 10H_2O \xrightarrow{\Delta} Na_2B_4O_7 \xrightarrow{\Delta} 2NaBO_2 + B_2O_3$ (transparent glassy bead). 2. Reaction with metal oxides: $MO + B_2O_3 \rightarrow M(BO_2)_2$ (colored metaborates).

Boric Acid ($H_3BO_3$ or $B(OH)_3$):

White crystalline solid, soapy touch.
Weak monobasic Lewis acid (not Brønsted-Lowry acid).
Mechanism: Accepts $OH^-$ from water: $B(OH)_3 + H_2O \rightleftharpoons [B(OH)_4]^- + H^+$.
Effect of Heat:

* $H_3BO_3 \xrightarrow{373K} HBO_2$ (Metaboric acid) * $HBO_2 \xrightarrow{413K} H_2B_4O_7$ (Tetraboric acid) * $H_2B_4O_7 \xrightarrow{red\ heat} B_2O_3$ (Boron trioxide)

Diborane ($B_2H_6$):

Colorless, toxic gas, spontaneously flammable.
Preparation (Lab): — $4BF_3 + 3LiAlH_4 \xrightarrow{ether} 2B_2H_6 + 3LiF + 3AlF_3$.
Structure: — Electron-deficient. Two $sp^3$ hybridized boron atoms.

* Four terminal B-H bonds (2c-2e). * Two bridging B-H-B bonds (3-center 2-electron or 'banana bonds').

Reactions:

* Hydrolysis: $B_2H_6 + 6H_2O \rightarrow 2H_3BO_3 + 6H_2$. * With $NH_3$: $B_2H_6 + 2NH_3 \rightarrow [BH_2(NH_3)_2]^+ [BH_4]^-$ (at low temp); $(BN)_x$ (at high temp).

Boron Nitride (BN):

h-BN (hexagonal): Graphite-like, soft, lubricant.
c-BN (cubic): Diamond-like, extremely hard, abrasive.

Vyyuha Quick Recall

Banana Bonds Bring Borane Brilliant!

Borax Bead Brings Bright Blue (for Copper).

Boric Acid Loves OH- (Lewis Acid).