polypropylene show satisfactory resistance. The grease used in the equipment should be fluorocarbon based, as boron trifluoride reacts with the hydrocarbon-based ones. Reactions Unlike the aluminium and gallium trihalides, the boron trihalides are all monomeric. They undergo rapid halide exchange reactions: BF3 + BCl3 → BF2Cl + BCl2F Because of the facility of this exchange process, the mixed halides cannot be obtained in pure form. Boron trifluoride is a versatile Lewis acid that forms adducts with such Lewis bases as fluoride and ethers: CsF + BF3 → CsBF4 O(C2H5)2 + BF3 → BF3·O(C2H5)2 Tetrafluoroborate salts are commonly employed as non-coordinating anions. The adduct with diethyl ether, boron trifluoride diethyl etherate, or just boron trifluoride etherate, (BF3·O(Et)2) is a conveniently handled liquid and consequently is widely encountered as a laboratory source of BF3. Another common adduct is the adduct with dimethyl sulfide (BF3·S(Me)2), which can be handled as a neat liquid. Comparative Lewis acidity All three lighter boron trihalides, BX3 (X = F, Cl, Br) form stable adducts with common Lewis bases. Their relative Lewis acidities can be evaluated in terms of the relative exothermicities of the adduct-forming reaction. Such measurements have revealed the following sequence for the Lewis acidity: BF3 < BCl3 < BBr3 < BI3(strongest Lewis acid) This trend is commonly attributed to the degree of π-bonding in the planar boron trihalide that would be lost upon pyramidalization of the BX3 molecule. which follows this trend: BF3 > BCl3 > BBr3 < BI3 (most easily pyramidalized) The criteria for evaluating the relative strength of π-bonding are not clear, however. One suggestion is that the F atom is small compared to the larger Cl and Br atoms, and the lone pair electron in pz of F is readily and easily donated and overlapped to empty pz orbital of boron. As a result, the pi donation of F is greater than that of Cl or Br. In an alternative explanation, the low Lewis acidity for BF3 is attributed to the relative weakness of the bond in the adducts F3B−L. Hydrolysis Boron trifluoride reacts with water to give boric acid and fluoroboric acid. The reaction commences with the formation of the aquo adduct, H2O−BF3, which then loses HF that gives fluoroboric acid with boron trifluoride. 4 BF3 + 3 H2O → 3 HBF4 + B(OH)3 The heavier trihalides do not undergo analogous reactions, possibly due to the lower stability of the tetrahedral