Boron trifluoride Lewis structure

An electrostatic potential map of boron trifluoride is shown. Is BF3 likely to be a nucleophile or an electrophile Draw a Lewis structure for BF3, and explain your answer. 

The pharmaceutical interest in the tricyclic structure of dibenz[6,/]oxepins with various side chains in position 10(11) stimulated a search for a convenient method for the introduction of functional groups into this position. It has been shown that nucleophilic attack at the carbonyl group in the 10-position of the dibenzoxepin structure renders the system susceptible to water elimination. Formally, the hydroxy group in the enol form is replaced by nucleophiles such as amines or thiols. The Lewis acids boron trifluoride-diethyl ether complex and titanium(IV) chloride have been used as catalysts. 

The stereoselectivity of any particular reaction depends on the details of the structure of the transition state. The structures of several enone-Lewis acid complexes have been determined by X-ray crystallography.11 The site of complexation is the carbonyl oxygen, which maintains a trigonal geometry, but with somewhat expanded angles (130-140°). The Lewis acid is normally anti to the larger carbonyl substituent. Boron trifluoride... 

Epoxides can be isomerized to carbonyl compounds by Lewis acids.104 105 Boron trifluoride is frequently used as the reagent. Carbocation intermediates appear to be involved, and the structure and stereochemistry of the product are determined by the factors which govern substituent migration in the carbocation. Clean, high-yield reactions can be expected only where structural or conformational factors promote a selective rearrangement. 

All Br0nsted-Lowry acids are Lewis acids, but in practice, the term Lewis acid is generally reserved for Lewis acids that don t also fit the Bronsted-Lowry definition. The best way to spot a Lewis acid-base pair is to draw a Lewis dot structure of the reacting substances, noting the presence of lone pairs of electrons. (We introduce Lewis structures in Chapter 5.) For example, consider the reaction between ammonia (NH3) and boron trifluoride (BFj) ... 

At first glance, neither the reactants nor the product appears to be an acid or base, but the reactants are revealed as a Lewis acid-base pair when drawn as Lewis dot structures as in Figure 16-1. Ammonia donates its lone pair of electrons to the bond with boron trifluoride, making ammonia the Lewis base and boron trifluoride the Lewis acid. 

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

A boron trifluoride molecule, BF3, has the Lewis structure shown in (5). There are three bonding pairs attached to the central atom and no lone pairs. According to the VSEPR model, as illustrated in Fig. 3.4, the three bonding pairs, and the fluorine atoms they link, lie at the corners of an equilateral triangle. Such a structure is trigonal planar, and all three F—B—F angles are 120° (6). 

Lewis acids and bases, because of their complexity, shall be examined briefly. Consider the structure of ammonia with its free pair of electrons. If the free pair of electrons were to make a bond with boron trifluoride, which substance is labeled as an acid and which one is the base Because the boron accepted a pair of electrons it is considered to be the Lewis acid. Ammonia is the substance that donated the electron pair and is classified as the Lewis base. (See Figure 9.3.)... 

The LE model is a simple but very successful model, and the rules we have used for Lewis structures apply to most molecules. To implement this model we have relied heavily on the octet rule. So far we have treated molecules for which this rule is easily applied. However, inevitably, cases arise where the importance of an octet of electrons is called into question. Boron, for example, tends to form compounds in which the boron atom has fewer than eight electrons around it—it does not have a complete octet. Boron trifluoride (BF3), a gas at normal temperatures and pressures, reacts very energetically with molecules such as water and ammonia that have available lone pairs. The violent reactivity of BF3 with electron-rich molecules occurs because the boron atom is electron-deficient. Boron trifluoride has 24 valence electrons. The Lewis structure that seems most consistent with the properties of BF3 is... 

A Lewis acid e.g. aluminium tert-butoxide, boron trifluoride, neutral alumina) is considered to coordinate with both the 17-OH and 20-keto functions, bringing them into a cfs-relationship and effectively locking the side chain in one conformation. Product formation is then determined by the relative ease of migration of the Cps)--C(i ) and C(i6>-C(i7) bonds towards the electron-deficient C<20). The structures of the resulting ketones show that the C i3)-C(i ) bond migrates in the i7j -hydroxy compound (i), and the C(i6)-"C(i7) bond in the i7a-hydroxy isomer (2). The reason for this difference has been the subject of much speculation, and is still not clear. The factors which have been considered [202] as affecting the stability of respective transition states include ... 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

Reetz and co-workers determined the structure of the benzaldehyde-boron trifluoride adduct by X-ray crystallography in 1986 (Fig. 1-1) [8J, which shows that the Lewis acid is placed 1.59 A from the carbonyl oxygen, along the direction of the oxygen lone pair and anti to the phenyl substituent. 

Methytation of alcohols. Although alcohols alone are inert to diazomethane, they can be methylated efficiently by diazomethane under catalysis by boron trifluoride etherate or fluoroboric acid. A mineral acid cannot be used for catalysis because it itself reacts with the reagent, but a Lewis acid performs the function ascribed above to an organic acid. In investigating the structure of estriol D-glucosiduronic acid,... 

In contrast to the low yield when hydrogen chloride is employed, an 88% yield of 2,4,6-triphenyl- 1,3,5-triazine (7) is obtained when chlorosulfonic acid is used as catalyst in a molecular ratio of 3 1 (CiSOjH/PhCN) at 0-5 C C and a reaction time of 12 to 24 hours.174 Trifluo-romethanesulfonic acid as a catalyst and solvent trimerizes benzonitrile at 91 °C in a yield of 66%.175 Lewis acids alone, such as aluminum, zinc, iron or titanium chlorides, phosphorus pentachloride, and boron trifluoride, have a considerably lower catalytic activity than the corresponding mixtures of Lewis acid with various promotors, such as mineral acids, organic acids and water. These differences are attributed to a change in the structure of the active complexes with the aryl cyanides. 

Addition compounds of 1,3,5-triazine with several Lewis acids can also be formed in which one or two molecules of the Lewis acid are bound to one molecule of 1,3,5-triazine. The structures of the obtained adducts 4 depend on the nature of the metal cation examples of Lewis acids that can be used in this reaction are aluminum trichloride, titanium(IV) chloride, tin(IV) chloride and antimony(V) chloride.26 In inert solvents, boron trifluoride and 1,3,5-triazine produce a stable adduct with the appearance of fine needles.28... 

As shown in Fig. 8, two possibilities are conceivable for the addition step of the imine to the lanthanoid-phosphite complex Ha. To determine whether structure V or VI seems to be the more reasonable in the addition reaction of a dimethyl phosphite with the C=N double bond of cyclic imines, several hydrophos-phonylation experiments using different types of phosphites were carried out in the presence or absence of the Lewis acid boron trifluoride. As it was shown that a high level of Lewis acid activation of the imine is required independently from the nucleophilicity of the phosphorus nucleophile which was used, a transition state of type VI appeared to exist as the dominant transition state structure. 

---