Furan; One of a class of heterocyclic aromatic compounds characterized by
five-membered ring structure consisting of four CH2 groups and one oxygen atom.
The simplest furan compound is furan itself; a clear, volatile and mildly toxic
liquid; melts at -86 C, boils at 32 C, insoluble in water, soluble in alcohol
and ether. In the absence of inhibitors, it may form peroxides and and accumulate peroxides
which may explode when subjected to heat or shock. It may discolor on exposure
to air. This material is hazardous when peroxide levels are concentrated by
distillation or evaporation. It can be stabilized with BHT. It can be obtained from wood oils. It is used as a solvent as well as
in the synthesis of furfural and other organic compounds. It is converted to
more important solvent, tetrahydrofuran by hydrogenation. Niitro-substituted
furan derivatives are used as biocides or fungicides to inhibit bacterial
growth. Sulfur-substituted furan derivatives are used as flavouring agents.
Furfural (Furfuraldehyde), a derivative of furan, is a viscous, colorless
liquid that has a pleasant aromatic odor; upon exposure to air it turns dark
brown or black; boils at about 160 C; soluble in ethanol, ether and somewhat in
water. It is commonly used as a solvent. Furfural is the aldehyde of pyromucic
acid; it has properties similar to those of benzaldehyde. It is prepared
commercially by dehydration of pentose sugars obtained from cornstalks and
corncobs, husks of oat and peanut, and other waste products. The major
application of furfural is being use as a feedstock for furfuryl alcohol. The
most commercial quantity of furfuryl alcohol is used in the production of thermosetting furan resin
and furan cement, strong adhesive, in which the furan ring is an integral part of
the polymer chain providing highly resistance to chemicals. Furfural is used as
a solvent for refining lubricating oils and butadiene extraction. It is used as a fungicide and
weed killer. It is used in
the production of tetrahydrofuran (THF), saturated form of furan. THF is one of the most
polar ethers. It is used as an important industrial solvent recognized for its
unique combination of useful properties. It is a colorless, volatile
cycloaliphatic (5-membered) ether with a characteristic odor; boiling point at
66 C; soluble in water and organic solvents. THF is unstable at room temperature
due to possibility of peroxide formation; stabilized sometimes with BHT. Its unhindered oxygen
atom carries two unshared pairs of electrons - a structure which favors the
formation of coordination complexes and the solvation of cations. THF is made
also by eliminating water from 1,4-butanediol. THF is used as an useful chemical
intermediate especially as a starting materials for the preparation of
nylon. |
Solvent is a substance, usually a liquid, that acts as a dissolving agent or
that is capable of dissolving another substance. In solutions of solids or gases
in a liquid, the liquid is the solvent. In all other homogeneous mixtures (i.e.,
liquids, solids, or gases dissolved in liquids; solids in solids; and gases in
gases), solvent is the component of the greatest amount. The minor proportion
substances are called solutes. The solvent offers several functions during a
chemical reaction. It solves not only the substance that reacts with another one
to produce a new set of substances (reactant) but also the compound that
supplies the molecule, ion, or free radical, which is considered as the
attacking species in a chemical reaction (reagent). The solvent is conductive to
collisions between the reactants and reagents to transform the reactants to new
products. The solvent also takes roll of temperature control, either to provide
the energy of the colliding particles for speedy reaction and to absorb heat in
exothermic reaction. The appropriate solvent should be selected based on the
inactivity in the reaction conditions, dissolving the reagents as well as
reactants, appropriate boiling point and easy removal at the end of the
reaction.
Polarity
The most common solvent is water. Other common
solvents which dissolve substances that are insoluble (or nearly insoluble) in
water are acetone, alcohol, formic acid, acetic acid, formamide. BTX, carbon
disulfide, diemthyl sulfoxide, carbon tetrachloride, chloroform, ether,
tetrahydrofuran, furfural, hexane and turpentine. They may be classified as
polar and non-polar. Polar solvents, like water, have molecules whose electric
charges are unequally distributed, leaving one end of each molecule more
positive than the other. Usually polar solvent has O-H bond of which water
(HOH), (CH3OH) and acetic acid (CH3COOH) are examples.
Propanol, butanol, formic acid, formamide are polar solvents. Dipolar solvents
which contain a C-O solid bond without O-H bond are acetone [(CH3)2C=O], ethyl
acetate (CH3COOCH2CH3), methyl ethyl ketone, acetonitrile, N,N-dimethylformamide
and diemthyl sulfoxide. Nonpolar solvents, like carbon tetrachloride (CCl4),
benzene (C6H6), and diethyl ether ( CH3CH2OCH2CH3), have molecules whose
electric charges are equally distributed and are not miscible with water. Hexane, tetrahydrofuran and methylene chloride are non-polar solvents. Polar
solvents are hydrophilic but non-polar solvents are lipophilic. Polar reactants
will dissolve in polar solvents. Non-polar solvents dissolve non-polar compounds
best. Oil and water don't mix but separate into two layers. There are three
measures of the polarity as "dipole moment", "dielectric constant" and
"miscibility with water". Though low dipole moments and small dielectric
constants indicates non-polar solvents, sharp boundaries between polar and
non-polar solvents are not available. The polarity reflects the balance between
a polar component (OH) and a non-polar hydrocarbon component, existing in the
same molecule. If hydrocarbon character increases relatively, the polarity
decreases. On an operational basis, solvents that are miscible with water are
polar.
Polar Protic and Dipolar Aprotic
Protic refers to a hydrogen atom
attached to an electronegative atom. Protic solvents can donate an H+ (proton)
since they contain dissociable H+, such as hydrogen attached to oxygen as in a
hydroxyl group, nitrogen as in a amine group. Examples are water, methanol,
ethanol, formic acid, hydrogen fluoride and ammonia. Aprotic solvents don't has
O-H bond but a C=O bond typically. Examples are acetone [(CH3)2C=O] and ethyl
acetate (CH3COOCH2CH3). Polar protic solvents are useful in SN1 reaction, while
polar aprotic solvents are SN2 reaction.
Solvents |
Boiling
point C |
Dipole Moment |
Dielectric Constant
|
Density
(g/ml)
|
Polar Protic |
Water |
100 |
1.85 |
80 |
0.998
|
Methanol |
68 |
1.70 |
33 |
0.791
|
Ethanol |
78 |
1.69 |
24.3 |
0.789
|
n-Propanol |
97 |
1.68 |
20.1 |
0.803
|
n-Butanol |
118 |
1.66 |
17.8 |
0.810
|
Formic acid |
100 |
1.41 |
58 |
1.21
|
Acetic acid |
118 |
1.74 |
6.15 |
1.049
|
Formamide |
210 |
3.73 |
109 |
1.134 |
Polar Aprotic |
Acetone |
56 |
2.88 |
20.7 |
0.786
|
Tetrahydrofuran |
66 |
1.63 |
7.52 |
0.886
|
Methyl ethyl ketone |
80 |
2.78 |
18.5 |
0.805
|
Ethyl acetate |
78 |
1.78 |
6.02 |
0.894
|
Acetonitrile |
81 |
3.92 |
36.6 |
0.786
|
N,N-Dimethylformamide |
153 |
3.82 |
38.3 |
0.944
|
Diemthyl sulfoxide |
189 |
3.96 |
47.2 |
1.092
|
Non-Polar |
Hexane |
69 |
- |
2.02 |
0.655
|
Benzene |
80 |
0 |
2.28 |
0.879
|
Diethyl ether |
35 |
1.15 |
4.34 |
0.713
|
Methylene chloride |
40 |
1.60 |
9.08 |
1.326
|
Carbon tetrachloride |
76 |
0 |
2.24 |
1.594 |
|