The Boron Halogenides

The observed bond lengths in Cl2 (1.988 A), (2.284 A), and I (2.667 A) are therefore shorter than the lengths expected for pure single bonds by 0.082 A, 0.094 A, and 0.059 A. These shortenings correspond (Sec. 7-7) to between 18 percent and 33 percent of doubie-bond character. 

A similar treatment of bond lengths indicates somewhat smaller amounts of double-bond character, 5 to 20 percent, for S—S, Se—Se, P—P. and As—As bonds. 

The observed bond lengths for bonds between unlike atotns (not of the first row) agree with those calculated from the average for bonds between like atoms, with the electronegativity correction. For example, the values 2.20 A for P—P and 1.98 A for Cl—Cl lead, with the electronegativity correction —0.054 A, to 2.036 A for P—Cl, in agreement with the experimental value 2.043 0.003 A for PC1S. We conclude that these bonds have about the same amount of doublebond character as those between like atoms. 

In the boron trimethyl molecule the boron atom is surrounded by three pairs of valence electrons, which are involved in the formation of single covalent bonds to the three carbon atoms of the methyl groups. An electron-diffraction study10 has shown the molecule to be planar (except for the hydrogen atoms), as would be expected for sp1 hybrid orbitals. The —C distance is 1.56 i 0.02 A, which agrees reasonably well with the value 1.54 A calculated, with the electronegativity correction, by the use of 0.81 A for the boron single-bond radius.11 

Bonds with Partial Double-Bond Character 

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Finally, an important form of boron nitride should be mentioned, pyrolytic boron nitride. It is manufactured by reacting ammonia and a boron halogenide at about 2000°C and depositing the BN vapor on a graphite substrate or mandrel. The characteristic feature of pyrolytic boron nitride is the high degree of crystal orientation with the hexagonal basal plane parallel to the mold surface and the c-direction perpendicular to the substrate. 

The inductively coupled plasma mass spectrometry (ICP-MS) measurement confirmed the absence of copper in both the products obtained with halogenides and with the boronic acids. The catalyst was recovered and dried after washing with ethanol followed by acetone before reuse. This was recycled up to eight runs with a marginal linear loss of activity of the catalyst due to some loss during handling in each step. 

In another one-pot procedure, a Suzuki coupling and a Wittig olefination were realized back to back. A great variety of aryl- and heteroaryl halogenides and aldehyde-functionalized aryl- and heteroaryl-boronic acids were used along with ethoxycarbonylmethylenephosphorane (Scheme 95/1). In another variation, 2-bro-mothiophene, (4-formylphenyl)boronic acid and three different phosphoranes were used (Scheme The biaryl/biheteroaryl/arylheteroaryl acrylic or other... 

Alkylation describes the reaction where an active hydrogen is replaced with an alkyl group, for example, a methyl group. Most polar groups can be alkylated, and many reagents can be used, for example, alkyl halogenides, diazo alkanes, and N,N -dimethylformamide dialkyl acetal. An example of an alkylation is the formation of methyl esters of fatty acids by using methanolic BF3, where boron trifluoride (BF3) acts as a catalyst. 

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