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Write preparation of diborane and give physical and chemical properties of it. |
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Answer» Solution :Preparation : The simplest boron hydride known, is diborane. It is prepared by treating boron trifluoride with `LiAlH_4` in diethyl ETHER. `4BF_3 + 3LiAlH_4 to 2B_2H_6+ 3LiF + 3AlF_3` A convenient laboratory method for the preparation of diborane involves the oxidation of sodium borohydride with iodine. `I_2 + 2NaBH_4 to B_2H_6 + 2NaI + H_2` Diborane is produced on an industrial scale by the reaction of `BF_3` with sodium hydride. `2BF_3 + 6NaH overset(450 K)to B_2H_6+6NaF` Physical properties : Diborane is a colourless, highly toxic gas with a b.p. of 180 K. Diborane catches fire spontaneously uponexposure to air. It burns in oxygen releasing an enormousamount of energy. Chemical properties : Boranes are readily hydrolyzed by water to give boric acid . `B_2H_(6(g)) + 6H_2O_((l)) to 2B(OH)_(3(aq)) + 6H_(2(g))` Diborane undergoes CLEAVAGE reactions with Lewis bases (L) to give borane adducts, `BH_3 xx L`. `B_2H_6+2NMe_3 to 2BH_3 . NMe_3` `B_2H_6+ 2CO to 2BH_3 . CO` Reaction of ammonia with diborane gives initially `B_2H_6 .2NH_3` which is FORMULATED as `[BH_2(NH_3)_2]^(+) [BH_4]^-`further heating gives borazine, `B_3N_3H_6` known as "inorganic benzene" in view of its ring structure with alternate BH and NH GROUPS. `2B_2H_6+6NH_3 to 2[BH_2(NH_3)_2]^(+)[BH_4]^(-)oversetDeltato 2B_3N_3H_6+12H_2`  The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four terminal B-H bonds are regular two centre-two electron bonds while the two bridge (B-H-B) bonds are different and can be DESCRIBED in terms of three centre-two electron bonds shown in fig. Boron also forms a series of hydridoborates, the most important one is the tetrahedral `[BH_4]^-`ion. Tetrahydridoborates of several metals are known. Lithium and sodium tetrahydridoborates, also known as borohydrides, are prepared by the reaction of metal hydrides with `B_2H_6` in diethyl ether. `2MH+B_2H_6 to 2M^(+) [BH_4]^(-)`where , [M=Li,Na] Both `LiBH_4` and `NaBH_4` are used as reducing agents in organic synthesis. They are useful starting materials for preparing other metal borohydrides. |
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