- 2 Pentaborane Bshg Is An Important Precursor For Higher Boranes It Is Prepared By The Thermal Degradation Of Even Lo 1 (213.73 KiB) Viewed 10 times
As well as being useful precursors for cluster synthesis, lower
boranes are important as Lewis acids and in organometallic
catalysis. e. Explain why tris(isopropyl)borane, iPr3B, is a
stronger Lewis acid than tris(dimethylamino)borane, (Me2N)3B. [2
marks] f. Explain why heating phenylboronic acid in the absence of
water leads to the formation of triphenylboroxime. [2 marks] g.
Explain why heating phenylboronic acid in the presence of water
leads to the formation of benzene. [2 marks] h. The reaction of
BF3, two equivalents of pyridine, and [Et3Si][BArF ] (where [BArF ]
- = [B(C6F5)4] - , a weakly coordinating anion) results in the
formation of a new boron-containing compound, A, whose positive
mass spectrum shows a molecular ion peak at m/z=207. The 11B{1H}
NMR for A shows two environments, one singlet and one triplet,
whilst the 1H NMR shows a single pyridine environment. What is A?
Justify your answer
2. Pentaborane, BsHg, is an important precursor for higher boranes. It is prepared by the thermal degradation of even lower boranes, as shown below: Na[B3H8] + HCI Na[B3H4Cl] + H2 Step 1 Δ Na[B3H,CI] BGH, + NaCl + H2 Step 2 a. Provide a balanced equation for step 2. [1 mark] b. Using Wade's Rules, predict the structure of BsHy. [2 marks] C. B3H, can be reduced by elemental sodium to give the salt Na2B3H9 – what happens to the cage in this process and why? [3 marks] d. Reaction of Na2BsHwith silicochloroform (HSiCl) at high temperature results in the formation of a new cluster, NaSiBsH. Using Wade's rules, predict the structure of the cluster. How many environments will it possess in the 'Hand 11B{'H} NMR spectra? [4 marks] As well as being useful precursors for cluster synthesis, lower boranes are important as Lewis acids and in organometallic catalysis. e. Explain why tris(isopropyl)borane, 'Pr3B, is a stronger Lewis acid than tris(dimethylamino)borane, (Me2N)3B. [2 marks] f. Explain why heating phenylboronic acid in the absence of water leads to the formation of triphenylboroxime. [2 marks] g. Explain why heating phenylboronic acid in the presence of water leads to the formation of benzene. [2 marks] h. The reaction of BF3, two equivalents of pyridine, and [Et3Si][BAr*] (where [BAr+] = [B(C6Fs)4] , a weakly coordinating anion) results in the formation of a new boron-containing compound, A, whose positive mass spectrum shows a molecular ion peak at m/z=207. The 11B{"H} NMR for A shows two environments, one singlet and one triplet, whilst the 'H NMR shows a single pyridine environment. What is A? Justify your answer. [4 marks]