Download Acids and bases

Chapter 9
Non-aqueous media
TOPICS
 Relative permittivity
 Acid--base behaviour in non-aqueous solvents
 Liquid ammonia
 Liquid hydrogen fluoride
 Sulfuric acid
 Fluorosulfonic acid
 Bromine trifluoride
 Dinitrogen tetraoxide
 Ionic liquids
 Supercritical fluids
Categories of non-aqueous solvents
• protic solvents (e.g. HF, H2SO4, MeOH);
• aprotic solvents (e.g. N2O4, BrF3);
• coordinating solvents (e.g. MeCN, Et2O, Me2CO).
[pyBu][AlCl4],
A protic solvent undergoes self-ionization to provide protons which are
solvated. If it undergoes self-ionization, an a protic solvent does so without the
formation of protons.
9.2 Relative permittivity (dielectric constant)
where 0 is the (absolute) permittivity of a vacuum (8.854 x 1012 Fm1),
e is the charge on the electron (1.602 x10  19C) and r is the separation
(in metres) between the point charges.
9.3 Energetics of ionic salt transfer from water to an organic solvent
9.4 Acid-base behaviour in non-aqueous solvents
Strengths of acids and bases
Strength of acid HX depends on relative proton donor abilities of HX and
H3O+
strength of a base, B, in aqueous solution depends upon the relative
proton accepting abilities of B and [OH]
Tabulated values of Ka (or Kb) generally refer to the ionizations of acids
in aqueous solution , thus when we consider HCl a strong acid in
aqueous medium.
Levelling and differentiating effects
 Non-aqueous solvents that are good proton acceptors (e.g.NH3) encourage
acids to ionize in them.
 In a basic solvent, all acids are strong.
 The solvent is said to exhibit a levelling effect on the acid, since the strength
of the dissolved acid cannot exceed that of the protonated solvent.
 In aqueous solution, no acidic species can exist that is a stronger acid than
H 3 O+ .
 Note, because of the common ion effect, if HCl is dissolved in acetic acid, the
extent of ionization is less than in water and HCl acts as a weak acid.
Hydrogen bromide and iodide behave similarly but the extent of ionization of
the three hydrogen halides varies along the series: HI > HBr > HCl.
This contrasts with the fact that all three compounds are classed as strong
acids (i.e. fully ionized) in aqueous solution.
Thus, acetic acid exerts a differentiating effect on the acidic behaviour of
HCl,
HBr and HI, whereas water does not.
‘Acids’ in acidic solvents
Effect of dissolving ‘acids’ in acidic non-aqueous solvents enables us to
realize that just because a compound is named an ‘acid’, it may not
behave as one in non-aqueous media.
The above reaction may be described as the sum of:
Formation of [NO2]+
is important in the
nitration of aromatic
compounds
Acids and bases: a solvent-oriented definition
Self-ionizing solvent: an acid is a substance that produces the cation
characteristic of the solvent, and a base is a substance that produces the
anion characteristic of the solvent.
Non-aqueous solvents that are self-ionizing may be classified as:
1) proton containing (NH3, HF, H2SO4, HOSO2F)
2) aprotic (BrF3, N2O4)
Dinitrogen tetroxide, N2O4, undergoes self-ionization.
In this solvent medium, nitrosyl salts behave as acids and metal nitrates
(MNO3) act as bases.
Non-ionizing non-aqueous solvents
This reaction requires the separation of
doubly charged ions, and on these grounds
alone, the establishment of this equilibrium
must be considered improbable.
Liquid SO2 is an inert solvent for both organic and inorganic compounds
•Good ionizing medium for compounds related to Ph3CCl (giving [Ph3C]+)
•Useful in synthesis of group 16 and 17 cations species (e.g. I3+ and I5+)
9.6 Liquid ammonia
•Relative permittivity of NH3 (25) is lower than that of water (79), thus the ability
of liquid ammonia to dissolve ionic compounds is less than that of H2O.
•Exceptions include [NH4]+ salts, iodides, and nitrates which are readily soluble
AgI, Ksp= 8.3 × 10-17, is sparingly soluble in water, but dissolves easily in liquid
NH3 (solubility = 206.8 g per 100 g of NH3)
•The Ag+ and I- interact strongly with the solvent, and Ag+ forms an ammine
complex.
Precipitation reactions in liquid ammonia
Solubility of AgCl is 0.29 g/100 g liquid NH3 compared with 1.91x10-4 g
per 100 g H2O and solubility of KCl is 0.04 g per 100 g NH3, compared
with 34.4 g per 100 g H2O.
Neutralization reactions in liquid ammonia
phenolphthalein
Influence of solvent on acids
Liquid NH3 is an ideal solvent for reactions requiring a strong base, since
the amide ion is strongly basic.
Ka = 1.01 x 10-1; a monoprotic acid in aqueous solution
a diprotic acid in liquid NH3
The levelling effect of NH3 means the strongest acid possible in this medium is
NH4+, thus solutions of ammonium halides may be used as acids:
Metals that form insoluble hydroxides under aqueous conditions, form
insoluble amides in liquid NH3, e.g. Zn(NH2)2. Just as Zn(OH)2 dissolves in
the presence of excess hydroxide ion (equation 9.24), Zn(NH2)2 reacts with
amide ion to form soluble salts containing anion 9.12(equation 9.25).
Solutions of s-block metals in liquid NH3
All Group 1 metals and group 2 metals Ca, Sr, and Ba dissolve in liquid NH3.
Dilute solutions of the metals are bright blue, color arising from a broad and
intense absorption in the IR region.
Dilute solutions occupy a volume greater than the sum of the metal plus
solvent, with electrons occupying cavities of radius 300-400 pm.
•Dilute solutions are paramagnetic and the magnetic response corresponds to
that of one free electron per metal atom.
•Molar conductivity initially decreases with increasing concentration, reaching
a minimum near 0.05 M. Conductivity increases at higher concentrations and
in saturated solutions the concentration is comparable with the solid metal.
•Saturated solutions are not blue and paramagnetic, but instead are bronze
and diamagnetic. They are essentially ‘metal-like’ and have been described as
expanded metals.
•The blue solutions of alkali metals in liquid NH3 decompose very slowly,
liberating H2 as the solvent is reduced.
•Used as a reducing agent in a variety of reactions
Ammonium salts (which are strong acids in liquid NH3) decompose immediately
Zintl ions
Early synthetic routes to Zintl ions involved reduction of Ge, Sn or Pb in
solutions of Na in liquid NH3.
The macrocyclic ligand
cryptand-222 (crypt-222)
Redox reactions in liquid NH3
Reduction potentials for the reversible reduction of metal ions to the
corresponding metal in aqueous solution and in liquid NH3 are listed in
Table 9.5. Note that the values follow the same general trend, but that the
oxidizing ability of each metal ion is solvent-dependent. Reduction
potentials for oxidizing systems cannot be obtained in liquid NH3 owing to
the ease with which the solvent is oxidized.
9.7 Liquid hydrogen fluoride
Physical properties
Hydrogen fluoride attacks silica glass thereby corroding glass reaction
vessels )‫(تآكل الزجاج‬,
and it is only relatively recently that HF has found applications as a nonaqueous solvent. It can be handled in polytetrafluoroethene (PTFE) containers,
or, if absolutely free of water, in Cu or Monel metal (a nickel alloy) equipment.
Liquid range: 190 to 292.5 K
Relative permittivity 84 at 273 K; 175 at 200 K
Liquid HF undergoes self-ionization: Kself = 2 x10-12 at 273 K
Large electronegativity difference between H (P = 2.2) and F (P = 4.0)
results in the presence of extensive intermolecular hydrogen bonding in the
liquid.
•Hydrogen bonded molecules (~7 molecules on average) in liquid phase
•Cyclic (HF)x species are present in the gas phase.
Acid- base behaviour in liquid HF
a species that produces [H2F]+ ions in liquid HF is an acid, and one that
produces [HF2] is a base
Few protic acids are able to exhibit acidic behaviour in liquid HF, on account of
the competition between HF and the solute as H+ donors. Perchloric acid and
fluorosulfonic acid do act as acids.
With SbF5, HF forms a superacid which is capable of protonating very weak
bases including hydrocarbons
9.8 Sulfuric acid and fluorosulfonic acid
Physical properties of H2SO4
The value of the equilibrium constant for the self-ionization process 9.46 is
notably large. In addition, other equilibria such as 9.47 are involved to a lesser
extent.
9.46
9.47
[H2S2O7]
Acid - base behaviour in liquid H2SO4
Physical properties of fluorosulfonic acid HSO3F
It has a relatively long liquid range and a high dielectric
constant.
9.9 Superacids
Superacids, capable of protonating even hydrocarbons, include mixtures of:
•HF and SbF5
•HSO3F and SbF5 (called magic acid)
The following figure shows the crystallographically determined structure of the
related adduct SbF5OSO(OH)CF3
In superacidic media, hydrocarbons act as bases, and this is an important route
to the formation of carbenium ions,
Carborane Superacids
Recently, a new class of superacids has been discovered: in contrast to wellestablished superacids, carbaborane (or carborane) acids possess chemically
inert, extremely weak, conjugate bases. Carbaboranes are molecular clusters
containing carbon and boron atoms, and the negative charge of a carbaborane
monoanion is delocalized over the cluster.
9.10 Bromine trifluoride (Good oxidizing and fluorinating agent)
Physical properties
Behaviour of fluoride salts and molecular fluorides in BrF3
Bromine trifluoride acts as a Lewis acid, readily accepting F. When dissolved
in BrF3, alkali metal fluorides, BaF2 and AgF combine with the solvent to give
salts containing the [BrF4] anion, e.g. K [BrF4], Ba[BrF4]2 and Ag[BrF4].
On the other hand, if the fluoride solute is a more powerful F acceptor than
BrF3, salts containing [BrF2]+ may be formed
Conductometric measurements on solutions containing [BrF2][SbF6] and
Ag[BrF4] ,or [BrF2]2[SnF6] and K[BrF4] exhibit minima at 1 : 1 and 1 : 2
molar ratios of reactants respectively. These data support the formulation of
neutralization reactions
Reactions in BrF3
Much of the chemistry studied in BrF3 media involves fluorination reactions, and
the preparation of highly fluorinated species.
Non-aqueous solvents that behave similarly to BrF3 in that they are good
oxidizing and fluorinating agents include ClF3, BrF5 and IF5.
9.11 Dinitrogen tetraoxide (Good oxidizing and nitrating agent)
Physical properties
The proposed self-ionization process for N2O4 is given in equation:
Reactions in N2O4
Electropositive metals such as Li and Na react in liquid N2O4 liberating NO
Fig. 9.7 [Zn(NO3)4]2-
9.12 Ionic liquids (also called molten or fused salts )
A eutectic is a mixture of two substances and is characterized by a sharp
melting point lower than that of either of the components; a eutectic behaves
as though it were a single substance.
For example, the melting point of NaCl is 1073 K, but is lowered if CaCl2 is
added as in the Downs process.
In the solid state, HgCl2 forms a molecular lattice, and layer structures are
adopted by HgBr2 (distorted CdI2 lattice) and HgI2.
An important group of molten salts with more convenient operating
temperatures contain the tetrachloroaluminate ion, [AlCl4]; an example is an
NaCl–Al2Cl6 mixture. The melting point of Al2Cl6 is 463 K (at 2.5 bar), and its
addition to NaCl (melting point, 1073 K) results in a 1:1 medium with a melting
point of 446 K.
alkylpyridinium ions,
dialkylimidazolium ions
9.13 Supercritical fluids
Properties of supercritical fluids and their uses as solvents
A supercritical fluid possesses solvent properties that resemble those of a
liquid, but also exhibits gas-like transport properties. Thus, not only can a
supercritical fluid dissolve solutes, but it is also miscible with ordinary gases
and can penetrate pores in solids.
Supercritical fluids exhibit :
•lower viscosities
•higher diffusion coefficients than liquids.
The density of a supercritical fluid increases
as the pressure increases, and as the
density increases, the solubility of a solute in
the supercritical fluid increases dramatically.
Table 9.11 lists the critical
temperatures and pressures of
selected compounds that are
used as supercritical fluids.
Box 9.4
Clean technology with
supercritical CO2 (scCO2)
Supercritical fluids as media for inorganic chemistry
A homoleptic complex is of the type [MLx]n+ where all the ligands are identical.
In a heteroleptic complex , the ligands attached to the metal ion are not all
identical.