Properties Of Diborane Assignment Help | Properties Of Diborane Homework Help

Properties of Diborane

(1)    Physical State.

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

(2)    Thermal Stability.

 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.

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.

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

(7)    Action with Ammonia.

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

(a) B2H→  B2H6, 2NH3 or [ (NH3)2 BH2]+[BH4] -
             (low temp.)    Diammoniate of diborane

(b) B2H →  (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 :

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 → 2BH  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

(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
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
(i)    On reflexing it with anhydrous carboxylic acid yields the alkane corresponding to the initial alkene.

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

                                 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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