Solvents & Water

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Solvents

A solvent is a substance that can dissolve or dilute gases, liquids or solids without causing chemical reactions between the solute and the dissolving substance. Usually, liquids such as water and liquid organic substances are used to dissolve other substances. However, solids can also dissolve other substances. For example, in hydrogen tanks of fuel cell-powered cars, gaseous hydrogen is dissolved in solid material (metal-organic framework compounds, or MOFs).

Chemistry

Although the solvent does not itself participate in the chemical reaction, it is very important for chemical reactions. The effects of the solvent vary and depend on the reaction. By dissolving reactants in a solvent, reactions become thermally controllable. Concentration data of substances dissolved in a solvent are valid only for a certain temperature because of temperature dependence.
The most important tasks of the solvent in chemical reactions are

● convective heat and mass transfer
● stabilization of transition states of the reaction
● dilution to avoid side reactions

Solvents also play an important role in the purification and processing of reaction mixtures (downstream process). Here are some examples of important processes:

● precipitation
● crystallization
● recrystallization
● extraction
● chromatography

Dissolution properties

Quantitative prediction of dissolution properties is difficult and often defies intuition. General rules can be established, but these can only be used as a rough guide.

Polar substances generally dissolve well in polar solvents (e.g. salts in water). Non-polar substances generally dissolve well in non-polar solvents (e.g., non-polar organic substances in benzene or ether).
Solvents are usually divided into classes according to their physical properties. Such classification criteria include:

● Boiling point
● permittivity
● flash point
● volatility
● viscosity
● Polarity
● CH acidity

Aprotic solvent

If a molecule does not have a functional group from which hydrogen atoms in the molecule can be split off as protons (dissociation), it is called an aprotic solvent. These are opposite to protic solvents.

Aprotic-unpolar

Alkanes are nonpolar because of the small difference in electronegativity between carbon and hydrogen. This makes all substances in these groups readily soluble in each other; they are very lipophilic (actually even more lipophilic than the very weakly polar, namesake fats) and very hydrophobic (water-repellent). However, not only water cannot dissolve them, but also all other strongly polar substances, such as short-chain alcohols, hydrogen chloride or salts. In the liquid, the particles are held together only by van der Waals forces. For this reason, the boiling temperatures for this group of substances are much lower than for permanent dipoles, compared to the size and mass of the molecule. Since the elimination of protons to form carbanions is only possible with extremely strong bases, they are aprotic. Also included in the group of aprotic-nonpolar solvents are compounds such as carboxylic acid esters or ethers, which contain polar bonds but are unable to dissolve ionic compounds due to their low permittivity.
Representatives of this group are:

● Alkanes (kerosenes)
● alkenes (olefins), alkynes
● benzene and other aromatics with aliphatic and aromatic substituents
● carboxylic acid esters
● ethers, e.g. diethyl ether
● completely symmetrical molecules such as tetramethylsilane or carbon tetrachloride
● carbon disulfide, at high pressure also carbon dioxide
● halogenated hydrocarbons that are either completely nonpolar (like carbon tetrachloride) or only slightly polar (methylene chloride) despite the high electronegativity of the halogen in question, e.g. chlorine.
● A special subgroup of halogenated hydrocarbons is formed by the perfluorinated hydrocarbons (e.g. hexafluorobenzene), which are not only nonpolar themselves, but also very poorly polarizable from the outside and therefore also tend to be poorly compatible with the other nonpolar solvents.

Aprotic-polar

However, if the molecule is substituted with strongly polar functional groups such as the carbonyl group, the nitro group or the nitrile group, the molecule exhibits a dipole moment, thus intermolecular electrostatic attraction of permanent dipoles is now added to the still existing (but much weaker) van der Waals forces. This results in a substantial increase in boiling point and, in many cases, a deterioration in miscibility with nonpolar solvents and an improvement in solubility of and in polar substances. Typical aprotic polar solvents have a permittivity above 15 and are capable of solvating cations. Since the anions are hardly solvated (bare anions), they exhibit high SN2 reactivity. Such solvents are excellent for performing nucleophilic substitutions under mild conditions. These include:

● ketones, e.g. acetone.
● lactones such as γ-butyrolactone
● lactams such as N-methyl-2-pyrrolidone
● nitriles such as acetonitrile
● nitro compounds such as nitromethane
● tertiary carboxylic acid amides such as dimethylformamide
● urea derivatives such as tetramethylurea or dimethylpropyleneurea (DMPU)
● sulfoxides such as dimethyl sulfoxide (DMSO)
● sulfones such as sulfolane
● carbonic acid esters such as dimethyl carbonate or ethylene carbonate

Protic solvents

As soon as a molecule has a functional group from which hydrogen atoms in the molecule can be split off as protons (dissociation), it is called a protic solvent. These are opposed to the aprotic solvents.
The most important protic solvent is water, which (simplified) dissociates into a proton and a hydroxide ion.

Other protic solvents are, for example, alcohols and carboxylic acids. Here, the splitting off of the proton always occurs at the OH group, since the electronegative oxygen can readily absorb the resulting negative charge.

The extent to which the respective solvent dissociates is determined by the acidity (according to the acid-base concept of Brønsted and Lowry). It should be noted that hydrogen atoms bonded to carbon can also be split off as protons (CH acidity), but the acidity of these compounds is usually too low to allow appreciable dissociation in neutral medium. The release of these protons is only possible by very strong bases.

Polar protic solvents dissolve salts and polar compounds, whereas the solubility of nonpolar compounds is low.

Protic solvents are:

● water, the most important solvent of all, especially in animate nature.
● methanol, ethanol and other short-chain alcohols (the larger the C-scaffold, the less pronounced the polar character; cholesterol, for example, is an alcohol, but still highly lipophilic)
● primary and secondary amines
● carboxylic acids (formic acid, acetic acid)
● primary and secondary amides such as formamide
● mineral acids (sulfuric acid, hydrogen halides or hydrohalic acids)
polarity scales

Reichardt dye

A well-known scale for the polarity of a solvent is the ET(30) or ETN scale. It is derived from empirical spectroscopic measurements. The ET(30) value is defined as the transition energy of the longest wavelength Vis/NIR absorption band in a solution containing the negative solvatochromic Reichardt dye (betaine 30) at normal conditions, in kcal-mol-1. The ETN value is the ET(30) value normalized to the polarity extrema of tetramethylsilane (=0) and water (=1).



Table with solvents and their data

Solvent Melting point
[°C]
Boiling point
[°C]
Flash point
[°C]
Acetone −95.35 56.2 −19
Acetonitrile −45.7 81.6 13
Aniline −6.3 184 76
Anisole −37.5 155.4 41
Benzene 5.5 80.1 −8
Benzonitrile −13 190.7 70
Brombenzene −30.8 156 51
1-Butanol −89.8 117.3 34
Tert-butyl methyl ether
(MTBE)
−108.6 55.3 −28
γ-Butyrolactone −44 204–206 101
Quinoline −15.6 238 101
Chlorbenzene −45.6 132 28
Chloroform −63.5 61.7
Cyclohexane 6.5 80.7 4.5
Dibutyl ether −98 142.5 25
Diethylen glycol −6.5 244.3 124
Diethyl ether −116.2 34.5 −40
Dimethylacetamide −20 165 66
Dimethylformamide −60.5 153 67
Dimethyl sulfoxide 18.4 189 88
1,4-Dioxane 11.8 101 12
Glacial acetic acid 16.6 117.9 42
Acetic anhydride −73.1 139.5 49
Ethyl acetate −83.6 77.06 −2
Ethanol −114.5 78.3 18
1,2-Dichloroethane
(Ethylendichloride)
−35.3 83.5 13
Ethylen glycol −13 197 117
Ethylen glycol dimethyl ether −58 84 −6
Formamide 2.5 210.5 175
n-Hexane −95 68 −20
n-Heptane −91 98 −4
2-Propanol
(Isopropyl alcohol)
−89.5 82.3 16
Methanol −97.8 64.7 6.5
3-Methyl-1-butanol
(Isoamyl alcohol)
−117.2 130.5 42
2-Methyl-2-propanol
(tert-Butanol)
25.5 82.5 9
Methylene chloride
(Dichloromethane, DCM)
−95.1 40
Methyl ethyl keton
(Butanone)
−86.3 79.6 −4
N-Methyl-2-pyrrolidone
(NMP)
−24 202 245
N-Methylformamide −3.8 183 111
Nitrobenzene 5.76 210.8 81
Nitromethane −28.5 100.8 35
n-Pentane −130 36 −49
Petrol ether 25–80 -26
Piperidine −9 106 4
Propanol −126.1 97.2 24
Propylene carbonate
(4-Methyl-1,3-dioxol-2-one)
−48.8 241.7 130
Pyridine −42 115.5 23
Carbon disulfide −110.8 46.3 −30
Sulfolane 27 285 177
Tetrachloro ethene −19 121
Carbon tetrachloride −23 76.5
Tetrahydrofuran
(THF)
−108.5 66 −22.5
Toluene −95 110.6 7
1,1,1-Trichloro ethane −30.4 74.1
Trichlorethene −73 87
Triethylamine −114.7 89.3 −7
Triethylene glycol −5 278.3 166
Triethylene glycol dimethyl ether
(Triglyme)
222 113
Water 0.0 100



























































































Solvent Density
[g/cm3] at 20 °C
Permittivity
at 25 °C
Dipole moment
[· 10−30 Cm]
Acetone 0.7889 20.70 9.54
Acetonitrile 0.7857 37.5 (20 °C) 11.48
Aniline 1.0217 6.89 (20 °C) 5.04
Anisole 0.9961 4.33 4.17
Benzene 0.87565 2.28 0.0
Benzonitrile 1.0102 (15 °C) 25.20 13.51
Brombenzene 1.4950 5.40 5.17
1-butanol 0.8098 17.51 5.84
tert-butyl methyl ether
(MTBE)
0.74 - -
γ-Butyrolactone 1.13 39.1 4.12
Quinoline 1.0929 9.00 7.27
Chlorbenzene 1.1058 5.62 5.14
Chloroform 1.4832 4.81 (20 °C) 3.84
Cyclohexane 0.7785 2.02 (20 °C) 0.0
Dibutyl ether 0.764 4.34 (20 °C) 3.9
Diethylen glycol 1.1197 (15 °C) 7.71 7.71
Diethyl ether 0.7138 4.34 (20 °C) 4.34
Dimethylacetamide 0.9366 (25 °C) 37.78 12.41
Dimethylformamide 0.9487 37.0 12.88
Dimethyl sulfoxide 1.1014 46.68 13.00
1,4-Dioxane 1.0337 2.21 1.5
Glacial acetic acid 1.0492 6.15 (20 °C) 5.60
Acetic anhydride 1.0820 20.7 (19 °C) 9.41
Ethyl acetate 0.9003 6.02 6.27
Ethanol 0.7893 24.55 5.77
1,2-Dichloroethane
(Ethylendichloride)
1.2351 10.36 6.2
Ethylen glycol 1.1088 37.7 7.61
Ethylen glycol dimethyl ether 0.8628 7.20 5.70
Formamide 1.1334 111.0 (20 °C) 11.24
n-Hexane 0.6603 1.88 0.0
n-Heptane 0.684 1.97 0.0
2-Propanol
(Isopropyl alcohol)
0.7855 19.92 5.54
Methanol 0.7914 32.70 5.67
3-Methyl-1-butanol
(Isoamyl alcohol)
0.8092 14.7 6.07
2-Methyl-2-propanol
(tert-Butanol)
0.7887 12.47 5.54
Methylene chloride
(Dichloromethane, DCM)
1.3266 8.93 5.17
Methyl ethyl keton
(Butanone)
0.8054 18.51 (20 °C) 9.21
N-Methyl-2-pyrrolidone
(NMP)
1.03 32.2 4.09
N-Methylformamide 1.011 (19 °C) 182.4 12.88
Nitrobenzene 1.2037 34.82 13.44
Nitromethane 1.1371 35.87 (30 °C) 11.88
n-Pentane 0.6262
Petrol ether 0.63–0.83
Piperidine 0.8606 5.8 (20 °C) 3.97
Propanol 0.8035 20.33 5.54
Propylene carbonate
(4-Methyl-1,3-dioxol-2-one)
1.2069 65.1 16.7
Pyridine 0.9819 12.4 (21 °C) 7.91
Carbon disulfide 1.2632 2.64 (20 °C) 0.0
Sulfolane 1.261 (25 °C) 43.3 (30 °C) 16.05
Tetrachloro ethene 1.6227 2.30 0.0
Carbon tetrachloride 1.5940 2.24 (20 °C) 0.0
Tetrahydrofuran
(THF)
0.8892 7.58 5.84
Toluene 0.8669 2.38 1.43
1,1,1-Trichloro ethane 1.3390 7.53 (20 °C) 5.24
Trichlorethene 1.4642 3.42 (16 °C) 2.7
Triethylamine 0.7275 2.42 2.90
Triethylene glycol 1.1274 (15 °C) 23.69 (20 °C) 9.97
Triethylene glycol dimethyl ether
(Triglyme)
0.98 7.5
Water 0.9982 78.39 6.07

Solvent
Refractive index
nD20
E τ ( 30 )
[kJ/mol]
Compressibility
[10−6 /bar]
Acetone 1.3588 176.4 126
Acetonitrile 1.3442 192.3 115
Aniline 1.5863 185.2 -
Anisole 1.5179 155.5 -
Benzene 1.5011 142.2 95
Benzonitrile 1.5289 175.6 -
Brombenzene 1.5597 156.8 -
1-butanol 1.3993 209.8 -
tert-butyl methyl ether
(MTBE)
1.3690 145.2 -
γ-Butyrolactone 1.436 -
Quinoline 1.6268 164.7 -
Chlorbenzene 1.5241 156.8 -
Chloroform 1.4459 163.4 100
Cyclohexane 1.4266 130.4 118
Dibutyl ether 1.399 187.6 -
Diethylen glycol 1.4475 224.9 -
Diethyl ether 1.3526 144.6 -
Dimethylacetamide 1.4380 182.7 -
Dimethylformamide 1.4305 183.1 -
Dimethyl sulfoxide 1.4770 188.1 -
1,4-Dioxane 1.4224 150.0 -
Glacial acetic acid 1.3716 214.0 -
Acetic anhydride 1.3900 183.5 -
Ethyl acetate 1.3723 159.3 104
Ethanol 1.3614 216.9 114
1,2-Dichloroethane
(Ethylendichloride)
1.4448 175.1 -
Ethylen glycol 1.4313 235.3 -
Ethylen glycol dimethyl ether 1.3796 159.7 -
Formamide 1.4472 236.6 -
n-Hexane 1.3748 129.2 150
n-Heptane 1.387 130.1 120
2-Propanol
(Isopropyl alcohol)
1.3776 203.1 100
Methanol 1.3287 232.0 120
3-Methyl-1-butanol
(Isoamyl alcohol)
1.4053 196.5 -
2-Methyl-2-propanol
(tert-Butanol)
1.3878 183.1 -
Methylene chloride
(Dichloromethane, DCM)
1.4242 171.8 -
Methyl ethyl keton
(Butanone)
1.3788 172.6 -
N-Methyl-2-pyrrolidone
(NMP)
1.47 -
N-Methylformamide 1.4319 226.1 -
Nitrobenzene 1.5562 175.6 -
Nitromethane 1.3817 193.5 -
n-Pentane 1.358 129.7 -
Petrol ether
Piperidine 1.4530 148.4 -
Propanol 1.3850 211.9 100
Propylene carbonate
(4-Methyl-1,3-dioxol-2-one)
1.4209 195.6 -
Pyridine 1.5095 168.0 -
Carbon disulfide 1.6319 136.3 -
Sulfolane 1.4840 183.9 -
Tetrachloro ethene 1.5053 133.3 -
Carbon tetrachloride 1.4601 135.9 110
Tetrahydrofuran
(THF)
1.4070 156.3 -
Toluene 1.4961 141.7 87
1,1,1-Trichloro ethane 1.4379 151.3 -
Trichlorethene 1.4773 150.1 -
Triethylamine 1.4010 139.2 -
Triethylene glycol 1.4531 223.6 -
Triethylene glycol dimethyl ether (Triglyme) 1.4233 161.3 -
Water 1.3330 263.8 46


Table of alcoholic solvents and their evaporation rates relative to acetic acid n-butyl ester (= 1)
Solvent
Boiling point
[°C]
Evaporation rate
Methanol 65 2.1
Ethanol 78 1.6
2-propanol 82 1.4
tert-butanol 83 0.95
tert-amyl alcohol 102 0.93
1-propanol 97 0.86
2-butanol 100 0.81
2-methyl-1-propanol 108 0.62
1-butanol 118 0.44
4-methyl-2-pentanol (MIBC) 132 0.3
1-Pentanol (Amyl alcohol) 137 0.2
Diacetone alcohole 166 0.14
2-Ethyl-1-butanol 146 0.11
Hexanol 148 0.096
Cyclohexanol 161 0.05
Tetrahydrofurfuryl alcohol 178 0.03
2-ethylhexanol 185 0.02
2-octanol 177 0.018
1-octanol 196 0.007
Benzyl alcohol 205 0.007
1-Decanol 231 0.001