It is a colourless gas (b.p.,-92.6⁰ and m.p.,-164.9⁰C) with a foul smell and is extremely toxic.

 It is stable only at low temperatures. On heating at temperatures above 100⁰C in sealed vessels, it decomposes to form a number of higher hydrides.

       heat
B₃H₆   →  B₄H₁₀, B₅H₉, B₅H₁₁, B₆H₁₀ etc.

(3)    Action of Oxygen.

 It burns in oxygen liberating a lot of energy. It is spontaneously flammable.

B₂H₆ + 3O₂ → B₂O₃ + 3H₂O;               ?H = -2165 KJ mol^-1

Therefore. B2H₆ is used as a rocket fuel.

(4)    Hydrolysis.

It is immediately hydrolyzed by water to give hydrogen and therefore acts as a reducing agent.

B₂H₆ + 6H₂O → 2B(OH)₃ + 6H₂
It reacts with methanol in a similar reaction

B₂H₆ + 6MeOH → 2B(OMe)₃ + 6H₂

(5)    Action with Halogens.

It reacts with halogens under different conditions.

          explosive
B₂H₆ + CI₂  →  B₂H₅CI          + HCI
          at 25⁰C        Chlorodiborane

          only rapid at
B₂H₆ + Br₂  →  B₂H₅Br           + HBr
               100⁰C          bromodiborane

(6)    Action with Halogens Acids (HX).

Diborane reacts with halogen acids to yield halodborane.

B₂H₆ + HCI → B₂H₅CI + H₂
  B₂H₆ + HI  →  B₂H₅I +  H₂
                       Iododiborane

(7)    Action with Ammonia.

It reacts with ammonia to give different products under different reaction conditions.

(a) B₂H₆  →  B₂H₆, 2NH₃ or [ (NH₃)₂ BH₂]+[BH₄] ^-
             (low temp.)    Diammoniate of diborane

(b) B₂H₆  →  (BN)_x
       (high temp.)    boron nitride

Boron nitride, BN is a white crystalline solid. It is a giant molecule with a graphite-like structure in which boron and nitrogen atoms altemate in the rings giving structure with B-N bond length of 145 pm. The bonds are formed by sp² hybrid orbitals on boron and nitrogen atoms; the remaining electrons are used for π- bond formation. The layers are arranged in a manner so that boron atoms in one later are immediately over the nitrogen atoms in an adjacent layer.

(c) 3B₂H₆ + 6NH₃  →  B₃N₃H₆         + 12H₂
                          250⁰C  Borazole or borazine
                                        (' Inorganic benzene')

(8)    Action with Alkalies.

 Diborane dissolves in strong alkaline solutions (KOH) giving metaborates and H₂ gas is evolved.

B₂H₆ + 2KOH + 2H₂O  →  2KBO₂ + 6H₂
                                            Potassium metaborate

(9)    Action with Metals.

Diborane reacts slowly (over a few days) with electro-positive metals such as Na, K Ca or their amalgams. The reaction becomes fast in ethers (used as solvent).

2 B₂H₆ + 2Na →  NaBH₄ + Na B₃H₈

B₃H₈^- was the first polyborate anion prepared by this reaction. This anion is now prepared by the following reaction :

                    diglyme
B₂H₆ + NaBH₄   →  NaB₃H₈ + 2H₂
                          100⁰ C

(10)    Action with Amines.

It reacts with tertiary amines to form amine-borane complex.

B₂H₆(g) + 2R₃N(g) →  2BH₃ - NR₃(l)

These complexes can serve as a source of diborane in chemical reactions since thy can be stored or shipped safely.

(11)    Action with Hydrides.

It reacts with sodium hydride to form a complex hydride.

2NaH + B₂H₆ →  2Na+ [BH₄]^-
                           sodium borohydride

(12)    Action with CO.

 It combines with carbon monoxide at 100⁰C and 20 atmospheric pressure to form borne carbonyl. BH₃CO. This is the only example known where CO forms a complex with a non transition element.

B₂H₆ + 2CO →  2BH₃CO

(13)    Action with Boron halides.

 It reacts with BCI₃ or BBr₃ to give B₂H₅ CI or B₂H₅Br.

BCI₃ + B₂H₆ →  B₂H₅CI + BHCI₂
BBr3 + B₂H₆ →  B₂H₅Br + BHBr₂

(14)    Action with Ethers and Thioethers.

 It combines with ether or throatier to form addition compounds.

    B₂H₆ + 2(CH₃)₂O → 2BH₃  ←  O(CH₃)₂
B₂H₆ + 2(C₂H₅)2S →  2BH₃  ← S(C₂H₅)₂

(15)    Action with Pyridine.

It combines with pyridine to form a salt.

B₂H₆ + 2C₅H₅N →  2H₃B - NC₅H₅
              Pyridine

(16)    Action with Trimethylaluminium.

It reacts with trimethylaluminium to form aluminum borohydride.

AI(CH₃)₃ + 2B₂H₆ →  B(CH₃)₃ + A(BH₄)₃

(17)    Action with Alcohols.

Diborane reacts with alcohols to form alkoxides

B₂H₆ + 6CH₃OH → 2B(OCH₃)₃ + 6H₂
B₂H₆ + 4CH₃OH →  2BH(OCH₃)₂ + 4H₂

(18)    Action with Tetralkyllead

B₂H₆ + (CH₃)₄ Pb →  B₂H₅(CH₃) + (CH₃)3PbH

(19)    Action with Dialkylphosphines

2B₂H₆ + 4 Me₂PH →  Cyelo-(Me₂PBH₂)₄ + 4H₂
                                                  Tetramer
1.5B₂H₆ + 3Me₂PH →  Cyelo-(Me₂PBH₂)₃ + 3H₂                                                   Trimer

(20)    Addition to alkenes (Hydroboration Reaction).

Diborane adds to alkenes and alkynes in ether solvents at room temperature to form alkyboranes. This reaction is known as HYDROBORATION. This reaction leads to the formation of organoboranes which yield to a variety of synthetic organic chemistry. Hydroboration is regiopecific, the boron showing preferential attachment to the least substituted carbon atom (anti-Markonikov)

3RCH = CH₂ + 1 B₂H₆ →  B(CH₂CH₂R)₃
                           2
(i)    On reflexing it with anhydrous carboxylic acid yields the alkane corresponding to the initial alkene.

                   RCOOH
B(CH₂CH₂R)₃   →  3RCH₂CH₃
               Alkane
(ii)    Oxidative hydrolysis with alkaline hydrogen peroxide gives the corresponding primary alcohol.

                  NaOH/H₂O₂
B(CH₂CH₂R)  →  3RCH₂CH₂OH
                                 Primary alcohol

(21)    Diborane is an electrophilic reducing agent and preferentially attacks a molecule at a position of high electron density (borohdrides. BH₄^-, which is nuclephilic reagent). Reductions can occur with or without bond breaking. For example, alkenes and alkynes are reduced to alkanes, > C = 0 is reduced to > CH₂ and nitrites are reduced to amines. For example,

RCHO  →  RCH₂OH and RC = N → RCH₂NH₂.

The above reaction form (10) to (17) clearly demonstrates that diborane is an electron deficient molecule.

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