Borazine analogy with benzene

The gas-phase chemistry of borazine B3N3H6 (147) and the conjugate N-protonated acid B3N3H7+ indicates analogies with benzene,189 although the aromatic stabilization energy of neutral borazine is only 30% that of benzene, and the reactivities of benzene and borazine are not similar (Scheme 62). Comparable conclusions were reached when HOMA and Iq aromaticity indices were used.190a... 

IF the product were a monomer, its structure could be drawn as Cl2PsN, which is analogous to organic nitriles. R—C=N. For this reason the original names used for these compounds were phosphonitriles, phosphonitrilic chloride, etc. However, the products are actually either cyclic or linear polymers of general formula [NPC ,] . Thus, by analogy with benzene, borazine. etc., these compounds hove become known as phosphazenes. The rn jor product of the reaction in Eq. 16.47 and the easiest to... 

The synthesis of borazine (Fig. 1) was first described in 1926 by Stock and Pohland.12 More recently, Wideman and Sneddon reported interesting three one-step synthetic procedures using various starting compounds including 2,4,6-trichloroborazine, metal borohydrides, and ammonia-borane.13 The B N bond is isoelectronic with the C C bond, which explains why borazine is often presented as the inorganic analog of benzene, that is, borazine has almost the same colligative properties as benzene. 

The grouping B—N is isoelectronic with C—C. In 1926 Alfred Stock discovered borazine (HBNH)j, which is isoelectronic with benzene and which has a similar planar ring structure. The analogy between B—N and C—C has been fruitfully extended, particularly by E. Wiberg and H. Noth in Munich (Table 3.9). Commercial applications, apart from boron nitride itself, are very limited, on account of the ready hydrolysis of most of the compounds which contain 3-coordinate boron. 

Borazine is a colourless liquid (mp 215 K, bp 328 K) with an aromatic odour and physical properties that resemble those of benzene. The B N distances in the planar B3N3 ring are equal (144 pm) and close to those in the layered form of BN (Table 13.2). This is consistent with a degree of delocalization of the N lone pairs arotmd the ring as represented in 13.28. Structure 13.27 gives one resonance form of borazine, analogous to a Kekule structure for benzene. Despite the... 

As the BN unit is isoelectronic with a C2 fragment, replacement of the latter by the former in various organic compounds leads to azaborane structural analogs. Some examples are shown below. Planar borazine (or borazole) B3N3H6 is stabilized by n -delocalization, but it is much more reactive than benzene in view of the partial positive and negative charges on the N and B atoms, respectively. 

With intermediate steric requirements, specifically if R = t-Bu and R = i-Pr or sec-Bu, thermal oligomerization forms trimers that are not typical borazines. These trimers are bicyclic rings with structures analogous to Dewar benzene . They are commonly referred to as Dewar borazines. 

Borazine, (BH)3(NH)3, is an analog of CgHg, benzene. It can be prepared from the reaction of diborane with ammonia, with hydrogen as another product or from lithium borohy-dride and ammonium chloride, with lithium chloride and hydrogen as the other products, (a) Write balanced chemical equations for the production of borazine using both synthetic methods, (b) Draw the Lewis dot structure of borazine. 

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