Acids and bases are essential for life. Amino acids have both acidic and basic groups. DNA and RNA a — Analytical Chemistry Chemistry Question
Lewis acids and bases
Acids and bases are essential for life. Amino acids have both acidic and basic groups. DNA and RNA are nucleic acids that contain bases such as adenine, guanine, thymine, cytosine, and uracil. Thus, understanding acid–base chemistry is essential for understanding life. Oxygen was so named by Lavoisier because of its acid–forming nature; the acid–forming nature of oxygen is a manifestation of its high electronegativity. Lewis extended the definition of acids and bases, and electronegativity is again central in understanding Lewis acidity and basicity.
Describe the molecular structure of BX3. What is the hybridization of the boron orbitals?
Model Answer
Central B has sp2 hybridization and BX3 is triangular.
How does this hybridization change when the boron halide forms an adduct with a base such as pyridine (C5H5N)? Is the structural change around boron upon adduct formation more favorable when X is F or I? List BF3, BCl3, and BBr3 in the order of increasing Lewis acidity based on the above structural consideration.
Model Answer
When an adduct is formed with pyridine, the structure around the central boron becomes tetragonal sp3 hybrid type (tetrahedron) structure. This structural change will induce steric hindrance around boron which is more pronounced with larger groups (i.e., iodine atoms) and adduct formation is not preferred. Therefore, BF3 is predicted to show the greatest preference to form adduct. (BF3 is expected to show the strongest Lewis acidity)
Electronegativity is another important consideration in predicting Lewis acidity. List BF3, BCl3, and BBr3 in the order of increasing Lewis acidity, based only on the electronegativity of the halogen elements (inductive effect).
Model Answer
The more electronegative halogen is expected to remove effectively electron density from the central boron and increase acidity.
Lewis acidity: BF3 > BCl3 > BBr3
Is adduct formation between the boron halide (Lewis acid) and pyridine (Lewis base) exothermic or endothermic? Which Lewis acid will show the greatest enthalpy change upon adduct formation?
Although the gaseous state would be best for computing the relative strengths of the three boron halides under consideration, the liquid state of these materials could be used as a satisfactory reference state since the boron halides are relatively non–polar liquids or gases.
Model Answer
Like neutralization that occurs between HCl and NaOH, the reaction producing stable acid–base adduct is expected to be exothermic. The enthalpy change will be the largest for the strongest Lewis acid, BF3.
The enthalpy changes when mixing liquid boron halide with nitrobenzene, ∆H1, and when mixing the nitrobenzene–boron halide solution with pyridine also in nitrobenzene, ∆H2, are given below.
BX3(l) + C6H5NO2(l) → C6H5NO2⋅BX3(soln) ∆H1
C6H5NO2⋅BX3(soln) + C5H5N(soln) → C5H5N⋅BX3(soln) + C6H5NO2(soln) ∆H2
Calculate ∆H3 for the following reactions. Do they agree with your prediction in 11.4?
BX3(l) + C5H5N(soln.) → C5H5N⋅BX3(soln.)
Model Answer
∆H3 = ∆H1 + ∆H2
The actual order of acidity is opposite from prediction based on the electro–negativity of the halides.
Boron halides also show very different reactivity with water. BF3 forms stable addition compounds whereas BCl3 and BBr3 react violently with H2O at temperatures below 20°C. Predict the products, A, B, and C, for the following reactions:
BF3 + H2O → A
BCl3 (or BBr3) + 3 H2O → B + C
Model Answer
A = BF3·H2O
B = B(OH)3, C = 3 HX (3 HCl or 3 HBr)
Strong Lewis acids such as BCl3 and BBr3 can activate O–H bonds in H2O molecule to produce B(OH)3 by releasing HX. Dative π–bonding with lone pair electrons of O, which have a similar energy level, can stabilize B(OH)3 as explained in 11.7.
What kind of extra bond can be formed in BX3 between the central boron and one of its halides possessing lone pair electrons in order to fulfill the ‘octet rule’? Explain how this extra bond affects the Lewis acidity of BX3.
Model Answer
Empty pz–orbital in boron can accept a dative π–bond from the lone pair electrons of fluorine, which satisfies the ‘octet rule’ for boron and shortens the boron–fluorine bond distance.
Since resonance structures of this kind are not possible in the adduct compounds, effective resonance will reduce the tendency for pyridine adduct formation.
The ability to form dative π–bonding appears to decrease sharply in the heavier elements due to the energy differences between B and X. Resonance of this dative π–bonding should be of lesser importance in the chloride and least importance in the bromide. These resonance structures having dative π–bonding are sufficiently large so as to reverse the trend expected from the relative inductive effects and the steric effects from the adduct formations.