资源简介

(共49张PPT)
Non-Aqueous Solvents
Why non-aqueous
Some reagents may react with H2O
Non-polar molecules are insoluble in water
eg:
dichloromethane, hexane, toluene and ethers such as diethyl ether, tetrahydrofuran, and diglyme
ionic liquids
Inorganic non-aqueous solvent
liquid ammonia, NH3
liquid sulfur dioxide, SO2
sulfuryl chloride, SO2Cl2
sulfuryl chloride fluoride, SO2ClF
phosphoryl chloride, POCl3
dinitrogen tetroxide, N2O4
antimony trichloride, SbCl3
bromine pentafluoride, BrF5
hydrogen fluoride, HF
pure sulphuric acid and other inorganic acids
non-aqueous solvents categories
protic solvents (e.g. HF, H2SO4, MeOH);
aprotic solvents (e.g. N2O4, BrF3);
coordinating solvents (e.g. MeCN, Et2O, Me2CO).
A protic solvent undergoes self-ionization to provide protons which are solvated, If it undergoes self-ionization.
an aprotic solvent does so without the formation of protons.
Limitations of non-aqueous solvents
many are highly reactive
quantitative data are scarce qualitative
ion-association (in solvents of relative permittivity lower than that of water) data are difficult to interpret
no integrated treatment of inorganic chemistry in non-aqueous solvents is yet possible
Relative Permittivity相对介电常数
dielectric constant,
the force is reduced by an amount that depends upon the relative permittivity of the material.
In a vacuum
Absolute permittivity of a material = o r
QP neutralizes part of the charge on each plate
Relative Permittivity
In a fluid with dielectric constant r, the force on Q2 is reduced by the factor 1/ r compared with the force in vacuum.
Since intermolecular forces are electrical, the dielectric constant r of a solvent affects equilibrium constants and reaction rate constants.
The deviation of r from 1 is due to two effects
the induced polarization and the orientation of the permanent dipole moments.
r increases as the molecular polarizability a increases
r increases as the molecular electric dipole moment m increases.
Using the Boltzmann distribution law to describe the orientations of the dipoles in the applied electric field
Nonaqueous acid-base chemistry
The acid-base reactions in non-aqueous solvents are typically described by means of the solvent-system definition, although the regular Br nsted-Lowry theory may be applied for the protic solvents, which possess a hydrogen atom that can dissociate.
solvent-system Acid-Base definition
acids are the compounds that increase the concentration of the solvonium (positive) ions
bases are the compounds that result in the increase of the solvate (negative) ions,
where solvonium and solvate are the ions found in the pure solvent in equilibrium with its neutral molecules
protic solvents autodissociation
2NH3 NH4+ (ammonium) + NH2 (amide)
3HF H2F+ + HF2- (hydrogen difluoride)
2H2SO4 H3SO4+ + HSO4-
aprotic solvents autodissociation
N2O4 NO+ (nitrosonium) + NO3 (nitrate)
2SbCl3 SbCl2+ (dichloroantimonium) + SbCl4- (tetrachloroantimonate)
POCl3 POCl2+ + POCl4-
Acid-Base Reaction
NaNH2 is a base and NH4Cl is an acid in liquid ammonia, they react, producing the salt and the solvent:
NaNH2 + NH4Cl → 2NH3 + NaCl
aprotic example,
NaNO3 + NOCl → N2O4 + NaCl
Limiting acids and limiting bases
The limiting acid in a given solvent is the solvonium ion, such as H3O+ (hydronium) ion in water. An acid which has more of a tendency to donate a hydrogen ion than the limiting acid will be a strong acid in the solvent considered, and will exist mostly or entirely in its dissociated form.
The limiting base in a given solvent is the solvate ion, such as OH (hydroxide) ion, in water. A base which has more affinity for protons than the limiting base cannot exist in solution, as it will react with the solvent.
Noble gas chemistry
The reactions of the compounds containing xenon are mostly conducted in hydrogen fluoride or bromine pentafluoride,
Sulphuric solvents are also used sometimes, in particular sulfuryl chloride fluoride for strong oxidants.
which dissolve readily both xenon difluorides and its multiple derivatives,
Extreme oxidants
Sulfuryl chloride fluoride is the solvent of choice for many reactions that deal with extreme oxidants.
For example, it can be used to generate and study free carbocations and arenium ions (cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution)
Super Acid, Magic Acid & Super Base
HOS(O)2F
SbF5 + HOS(O)2F
Li(n-C4H9)
The Hard/Soft Acid/Base (HSAB) Principle
HSAB is an extremely useful qualitative theory that enables predictions of what adducts will form in a complex mixture of potential Lewis acids and bases
Hard acids
Low electronegativity (c) of the acidic atom. A value in the range 0.7-1.6 is typical of hard acids;
Relatively small size;
Relatively high charge ( 3+).
(s,f blocks, left side of d block in higher OS's)
Hard bases
Very high c of the donor atom (in the range 3.4-4);
Relatively small size of the donor atom.
Soft bases
intermediate to high c (2.1-3.0)
large size, leading to polarizability
Soft acids
intermediate to high c (1.9-2.5);
large size;
low charge (1+, 2+)
Acids Bases
hard soft hard soft
Hydronium H+ Mercury CH3Hg+, Hg2+, Hg22+ Hydroxide OH- Hydride H-
Alkali metals Li+,Na+,K+ Platinum Pt2+ Alkoxide RO- Thiolate RS-
Titanium Ti4+ Palladium Pd2+ Halogens F-,Cl- Halogens I-
Chromium Cr3+,Cr6+ Silver Ag+ Ammonia NH3 Phosphine PR3
Boron trifluoride BF3 borane BH3 Carboxylate CH3COO- Thiocyanate SCN-
Carbocation R3C+ P-chloranil Carbonate CO32- carbon monoxide CO
bulk Metals M0 Hydrazine N2H4 Benzene C6H6
Gold Au+
Hard and soft acids and bases
In most acid-base interaction, the combination of HOMO-LUMO forms the NEW HOMO-LUMO of the products
A base has a pair of electrons in HOMO of suitable symmetry to interact with LUMO of the acid
HSAB Interaction
Hard acids tend to bind to hard bases.
Soft acids tend to bind to soft bases.
Fundamentals. The basic premise of Hard/Soft Acid/Base Theory is very simple: Hard acids prefer hard bases; soft acids prefer soft bases.
Hard acid–base interactions are predominantly electrostatic;
soft acid–base interactions are predominantly covalent
Estimation of HOMO & LUMO Energy
EHOMO = -I
ELUMO = -A
Absolute Hardness & Absolute Electronegativity
ILs are materials composed solely of anions and cations.
Molecular solvents are composed of neutral species benzene, propylene carbonate, water
Ionic Liquids
Cation Structures
Inorganic
Organic
BR
4
-
Sulfonate
-
O
-
(SO
2
R)
PR
6
-
Imide
-
N
-
(SO
2
R)
2
Methide
-
C
-
(SO
2
R)
3
Anion Structures
R = halide, perfluoroalkyl and other electron
withdrawing alkyl or aryl substituents
Ionic liquids:
Polarity (adjustable)
Very low vapor pressure
Liquid ranges (> 300 °C)
Solubility of gases & a wide range of compounds
Immiscibility with solvents
Hydrophilicity/lipophilicity (adjustable)
Acidity (adjustable)
Electrolyte Storage: 150oC for 2 hr
Ionic Liquid
1M TEABF4/PC
Supercritical Fluids
Beyond the Critical Point
S
L
G
Supercritical
Fluid
(SF)
P
T
Tc
pc
If T>Tc, gas can not be liquefied at any pressure
critical temperature of gas
Beyond the Critical Point
S
L
G
P
T
Tc
pc
G L
G
L
T<TT Tc
Supercritical Fluids Have Unusual Properties
1. Densities are variable (like a gas)
2. Good solvents (like a liquid)
3. Viscosities are low (like a gas)
Density of a SF
r, g/mL
0
1
p, atm
0
400
ideal gas law
prediction
actual
pc
n
V
=
P
RT
Other Applications of SFs
1. De-caffeination of coffee, tea
The old way:
acetone, benzene, ethanol, freons, pentane, hexane, CCl4, CHCl3, CH2Cl2
Other Applications of SFs
1. De-caffeination of coffee, tea
The modern way:
(a) extract caffeine using SF CO2
(b) contact CO2 with water
(c) discard water, recycle CO2
Comparisons of acidity and basicity between solvents
There exists a large corpus of data concerning acid strengths in aqueous solution (pKa values), and it is tempting to transfer this to other solvents. Such comparisons are, however, fraught with danger, as they only consider the effect of solvation on the stability of the hydrogen ion, while neglecting its effects on the stability of the other species involved in the equilibrium. Gas phase acidities (normally known as proton affinities) can be measured, and their relative order is often quite different from that of the aqueous acidities of the corresponding acids. Few quantitative studies on acidities in nonaqueous solvents have been carried out, although some qualitative data are available. It appears that most acids which have a pKa value of less than 9 in water are indeed strong acids in liquid ammonia. However, the hydroxide ion is often a much stronger base in nonaqueous solvents (e.g. liquid ammonia, DMSO) than in water.
It should be noted that pH values are at present undefined in aprotic solvents, as the definition of pH assumes presence of hydronium ions. In other solvents, the concentration of the respective solvonium/solvate ions should be used, such as pCl in POCl3.

展开更多......

收起↑