Trimethyl boron

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

The formation of a fourth bond by boron would strengthen the bonds (all bond orbitals becoming tetrahedral) and stabilize the molecule we can thus understand the ability of boron trimethyl to add ammonia to... 

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... 

For example, Figure 2.3 shows plots of the a constants of X vs. log p/T of aliphatic carboxylic acids (XCOaH) and vs. log k for the dehydration of acetaldehyde hydrate by XC02H. Deviations from Equations 2.18 and 2.19 occurwhen the rate of reaction or position of equilibrium becomes dependent on steric factors. For example, Taft studied the enthalpies of dissociation, A Hd, of the addition compounds formed between boron trimethyl and amines (X1X2X3N) and found that when the amine is ammonia or a straight-chain primary amine the dissociation conforms to Equation 2.20, in which 2 ° is the sum of the a values for the... 

The heat of combustion of boron trimethyl has been determined by Long and Norrish3i9 who obtained A//= -705 4 kcal for combustion of BMc3 (1) to B2O3 (amorphous). If we take the value of Prosen et for l Hf B2 z) obtain AHf BM e g) = -32... 

The first organic compound of boron was isolated by Frankland in 1862, by the interaction of zinc methyl and triethyl borate. This method and the use of zinc alkyls with boron trichloride was continued until the discovery of the Grignard reagent. In 1900 boron tri-methyl was prepared by the action of magnesium methyl iodide on trimethyl borate, and in 1921 a number of boron trialkyls were obtained by the interaction of magnesium alkyl halides and boron trifluoride. Of the derivatives of the type RgB, boron trimethyl is a gas, spontaneously inflammable in air boron triethyl, a spontaneously inflammable liquid whilst the higher members of the series are liquids, readily oxidised when exposed to air. The metl yL fpd ethyl derivatives combine with ammonia to foriii IliS type I BOH only... 

Boron trimethyl, McsB.— This compound was first obtained by the interaction of zinc method and triethyl borate. It has more recently been prepared by the interaction of magnesium methyl iodide and trimethvi borate. ... 

The gas is soluble with difficulty in water, but very easily soluble in alcohol and ether. It combines directly with ammoiua, yielding hmon trimethylammine, BMcg.NH, M.pt. 56° C., B.pt. 110° C., which crystallises from ether in many-sided, highly refractive, volatile crystals. The compound has the vapour pressures 1-0 mm. at 0° C., 1 8 mm. at 5 6° C., 2 mm. at 10° C., 3 2 mm. at 15-2° C., 11-0 mm. at 31-2° C, and 16-5 mm. at 36 9° C. The vapour density, measured between 25-0° C. and 70-0° C., shows that about 10 per cent, of undissoeiated ammine can exist in the gaseous state at 25° to 30° C., but molecular weight determinations in benzene solution by the cryoscopic method show that the ammine is practically undissociated. The benzene solution is stable in air, smells faintly of boron trimethyl, but not of ammonia. 

Treatment of boron trimethyl with potassium hydroxide or an ammoniacal solution with alcoholic potassium hydroxide yields a resinous mass, to which Frankland ascribed the composition BMc3.KOH ... 

In Table 1, the heats of addition reactions are shown for four series of compounds. The first series (a) is for the reactions between boron trimethyl and either ammonia or the methylamines, and shows the steady rise in — /III values, along the series ammonia, methyl-amine, dimethylamine with a fall on passing from dimethylamine to trimethylamine. 

On the other hand, for the addition of quinuclidine to boron trimethyl we have — AH 19 94 kcal/mole, which is the highest heat observed for any tertiary amine. In the quinuclidine compound... 

Results obtained with trie thy laluminium generally resemble those obtained with boron trimethyl, but the former acid is less severe in its steric demands . ... 

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