F3B:O(C2H5)2 BF4- B(C6H5)4-
Amorphous boron is more reactive, if white hot, and is attacked by NH3 to form boron nitride which is isomorphous with graphite.
The "anhydrous" borates involve the ions: BO33-, B3O63-, (BO2)nn-, and larger aggregates.
The hydrated ones feature: BO3 units which are planar and BO4 unitsd which are tetrahedral and formally have a -ve charge on the boron. The charge on the ion is equal to th number of these latter units. The structures without BO4 units hydrate readily.
B-O-H + H-O-B B-O-B + H2O
B-O-H + O-H- B-O- + H2O
It is a very strong Lewis acid:
BF3 + :F- BF4-Unlike the others, BF3 is only partly hydrolysed:
BF3 + :L F3B:L
4BF3 + 6H2O 3H3O+ + 3BF4- + B(OH)3
BF4- + H2O BF3(OH) + HF
(BCl3 + 3H2O B(OH)3 + 3HCl)
In synthetic organic chemistry it is used as follows:
The conversion of ethers or alcohols with acids to esters e.g:
H+ + RCOOH [RCOOH2]+
[RCOOH2]+ + BF3 [RCO}+ + F3B:OH2
[RCO]+ + R'OH RCOOR' + H+
Friedel-Crafts alkylations and acylations:
RX + BF3 R+ + BF3X-
R+ + PhH PhR + H+
H+ + BF3X- BF3 + HX
Fluoroboric acid: "HBF4" is sold as a 40% solution in water. It is a strong acid and source of BF4- ions useful for crystallizations where a coordinating anion is to be avoided.
BCl3 + BBr3 BCl2Br + BBr2Cl
The exchange, presumably through a bridged intermediate, is very facile, so pure mixed compounds cannot be obtained.
BCl3 + 3C2H5OH B(OC2H5)3 + 3HCl
BCl3 + 3NH(C2H5)2 B(N(C2H5)2)3 + 3HCl
This will happen with any solvent with an exchangeable H.
Figure 12-4 shows some of their structures as perspective drawings. Note that the lines are intended to clarify the shape of the molecule and do not necessarily represent 2e- - 2-centre bonds.
Note also the nomenclature - the prefix gives the number of boron atoms and the number in parentheses the number of hydrogen atoms, e.g. pentaborane(9) is B5H9.
3NaBH4 + 4BF3 2B2H6 + 3NaBF4
2NaBH4 + I2 B2H6 + 2NaI + H2
BF3 + 6NaH B2H6 + 6NaFThe last is the main industrial method. The higher boranes are made by thermolysis of diborane under various conditions.
Each hydrogen must have one bond ending at it and each boron must have a total of 4 bonds ending at it. If an atom is in the middle of a three-centre bond, the curved line passing through it counts as only one bond.
Thus, for example, B6 has one normal bond to a terminal hydrogen and one normal bond to B2, plus it is at the end of two three-centre bonds through the bridging hydrogens for a total of four bonds i.e eight electrons.
B2 has one normal bond from B6, one normal bond from its terminal hydrogen, is at the end of a "closed" three-centre bond from B1 and B2, and it is in the centre of an "open" three-centre bond from B5 to B7. This is also equivalent to four bonds.
In some cases, more than one "resonance" (canonical) structure can be formulated to account for the observed molecular shape.
B2H6 + 3O2 B2O3 + 3H2O
B2H6 + 3H2O B(OH)3 + 6H2or alcohols:
B2H6 + 3HOR B(OR)3 + 6H2
B2H6 + HCl B2H5Cl + H2
B2H6 + 6Cl2 2BCl3 + 6HCl
B2H6 + N(CH3)3 2H3BN(CH3)3Unsymmetrical:
B2H6 + 2NH3 [H2B(NH3)2]+[BH4]-
2B2H6 + 2Na NaBH4 + NaB3H8
B2H6 + NaBH4 NaB3H8 + H2
5B2H6 + 2NaBH4 Na2B12H12
B5H9 + NaH NaB5H8 + H2The pyramidal B5H9 loses one of its four bridging hydrogens and the resulting ion is "fluxional", that is, the location of the missing bridge is not stationary, and all the atoms in the base of the pyramid (four borons, four terminal hydrogens and three bridging hydrogens) appear equivalent on the time scale of nmr experiments which might otherwise have distinguished them.
Attack by electrophiles can lead to substitution at the apex of the pyramid:
B5H9 + I2 B5H8I(apical) + HI
B10H14 + OH- B10H13- + H2O
or converted to terminal hydrogens by reducing agents:
B10H14 + 2Na Na2B10H14In this reaction, the product has two bridging hydrogens between B1 and B5 and B7 and B8.
Other nucleophiles will add at B6 and B9 with loss of two bridging hydrogens. Again, the two that are left bridge between B1 and B5 and B7 and B8:
B10H14 + 2CH3CN 6,9-(CH3CN)2B10H12 + H2Electrophiles substitute terminal hydrogens at the bottom of the "basket" in the 1 and 3 or 2 and 4 positions, e.g.:
B10H14 + I2 2,4-I2B10H12 + 2HIThere are two reactions that lead to a closed cage:
B10H14 + 2Et3NBH3 [Et3NH]+2[B12H12]2-and
(SEt2)2B10H14 + HCCH B10C2H12 + H2 + 2SEt2
Skip the chemistry of these species.
NaBH4 is stable in dry air and alkaline aqueous solution. (It will react with water initially but the reaction stops as the concentration of the hydrolysis product, sodium borate, builds up.)
LiBH4 is similar to NaBH4 but more sensitive to water.
Al(BH4)3 is liquid which explodes with air or water. It probably has pairs of hydrogen bridges like diborane.
Zr(BH4)4 is a molecular solid with three bridging hydrogens connecting each boron to the zirconium.
H3NRCl + LiBH4 H2RN:BH3 + LiCl + H2
The molecules are flat, and rotation about the BN bond is restricted, therefore the left-hand structure must be is a significant contributor. It is perhaps not correct to represent them as canonical structures since the geometries would be so different. The cleanest synthetic route is, for example:
(CH3)2NH + BCl3 (CH3)2HN:BCl3
(CH3)2HN:BCl3 (CH3)2N:BCl2 + HCl (on heating)
(CH3)2N:BCl3 + 2RMgBr (CH3)2N:BR2 + 2MgClBr
Unlike benzene, borazine undergoes addition reactions:
Notice where the Hd+ and the Cld- end up: This illustrates how unrealistic the formal charges on the boron and nitrogen atoms really are!
Like benzene, borazine can form p-complexes with transition metals: