Boron trifluoride-ammonia adduct

Dative bonding may also be found in molecular addition compounds (or adducts), such as boron trifluoride ammonia, BF3.NH3 (Figure 4.36). The boron atom in boron trifluoride has only six electrons in its outer shell and so can accept an additional two electrons to fill the shell and obey the octet rule. The nitrogen atom on the ammonia molecule donates its lone pair of electrons to form the dative bond between the nitrogen and the boron atom. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

The reactions of boron trifluoride adducts of ammonia,and primary, secondary, and tertiary amines with phosphorus pentachloride have been studied and in the first two cases acyclic phosphazenes were obtained. With the ammonia-adduct, a previously characterized phosphazene salt was obtained ... 

The significance of the Lewis concept is that it is much more general than other definitions. Lewis acid-base reactions include many reactions that do not involve Brpnsted acids. Consider, for example, the reaction between boron trifluoride (BF3) and ammonia to form an adduct compound (Figure 15.11) ... 

An SCF-MO treatment of addition reactions of ammonia or amines to boron trifluoride has satisfactorily rationalised their very low activation enthalpies. Exchange reactions of amine-boron and phosphine-boron adducts have been studied over a wide range of temperatures and in a variety of solvents. ... 

Figure 18.2 shows the reaction between boron trifluoride and ammonia to form a stable adduct. The donor orbital is a lone pair on the nitrogen atom of ammonia and the acceptor is the empty p orbital left over from the sp hybridization at boron in boron trifluoride. 

Note that BF3 neither provides hydrogen ions in solution nor is a proton donor as required by the Arrhenius and Bronsted-Lowry definitions, respectively. Similarly, NH3 neither provides hydroxide ions in solution nor acts as a proton acceptor. (There are other instances in which ammonia is a proton acceptor, but it does not play that role in this reaction.) Therefore, this is not an acid-base reaction under these more restricted definitions. Boron trifluoride is, however, an electron-pair acceptor and ammonia an electron-pair donor, so the reaction can be classified as acid-base under the Lewis definitions. (In a sense, the product of a reaction between a Lewis acid and a Lewis base could be called a Lewis salt. However, the more technical term for such a product is a Lewis adduct.)... 

Boron trifluoride has an empty orbital that can accept the electron pair from ammonia and form the product (the product of a Lewis acid-base reaction is sometimes called an adduct). The reaction just shown demonstrates an important property of Lewis acids ... 

First and foremost is the existence of isolable coordination compounds. By analogy with ammonium ions and the boron trifluoride adduct of ammonia (discovered by Gay-Lussac in 1809) it has been possible to isolate and characterize oxonium salts of alcohols, ethers, and ketones (49,176,314) and to prepare Lewis acid adducts of the ethers, sulfides, and phosphines (329), as well as many other compounds which fall under our definition of weak bases. 

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