# solvent

A solvent (also solvent or solvent , also menstruum ) is a substance that can dissolve or dilute gases , liquids or solids without causing chemical reactions between the dissolved substance and the solvent [ see also: solution (chemistry) ]. As a rule, liquids such as water and liquid organic substances are used to dissolve other substances. But solids can also dissolve other substances. For example, in the hydrogen tanks of fuel cell powered cars, gaseous hydrogen is dissolved in solid material (metal-organic framework compounds, MOFs for short ).

## "Solvent" or "Solvent"

Both terms have been used in literature for over 200 years. In research and laboratory area has solvents established in industrial and technical chemicals industry, however solvent . For example, the Römpp Lexikon Chemie speaks of solvents , while the TRGS (Technical Rules for Hazardous Substances) prefer solvents .

## Definition in everyday life

Probably the best known solvent is water. When it comes to paints, varnishes, adhesives, etc., however, the term “solvent” is used to refer to substances that can cause unpleasant smells, damage to health and the environment and explosive vapors. What is meant here are solvents in the sense of TRGS (Technical Rules for Hazardous Substances) 610, according to which only volatile organic solvents with a boiling point of up to 200 ° C are referred to as solvents.

The "high boilers", less volatile substances with boiling points above 200 ° C, are therefore not legally considered solvents. While classic solvents, due to their volatility, have completely evaporated a few hours to days after processing, the high boilers contained in some "solvent-free" products may still be released into the room air for months or years and are therefore sometimes even judged to be significantly more critical than Products with classic solvents.

Avoiding toxic and environmentally harmful substances is part of green chemistry .

## chemistry

Although the solvent does not take part in the chemical reaction itself, it is very important for chemical reactions. The effects of the solvent are different and depend on the reaction. By dissolving reactants in a solvent, reactions can be controlled thermally. Concentrations of substances that are dissolved in a solvent only apply to a certain temperature due to the 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 ). Some important procedures are named here as examples:

## Market economy aspects

The most important group of solvents are alcohols such as ethanol, n -butanol, isopropanol and methanol. In 2011, around 6.4 million tons of this were in demand worldwide. An above-average increase in consumption of more than 3% annually is expected for ethanol and ethers in the period 2011 to 2019. In addition to halogenated solvents, which are continuing their downward trend in Western Europe and North America, aromatics and pure hydrocarbons will continue to lose importance in the long term.

## Dissolving properties

The quantitative prediction of dissolving properties is difficult and often defies intuition . General rules can be drawn up, but they 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 are e.g. B .:

### Aprotic solvents

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

#### Aprotic-non-polar

Alkanes are non-polar because of the small difference in electronegativity between carbon and hydrogen . This makes all substances of these groups easily soluble in one another; they are very lipophilic (actually even more lipophilic than the very weakly polar, eponymous fats) and very hydrophobic (water-repellent). But not only water cannot dissolve, but also all other strongly polar substances, such as B. short-chain alcohols , hydrogen chloride or salts . In the liquid, the particles are only held together by van der Waals forces . This is why the boiling temperatures of this group of substances are significantly lower in comparison to the molecular size and mass than with permanent dipoles . Since protons can only be split off with the formation of carbanions with extremely strong bases, they are aprotic . The group of aprotic non-polar solvents also includes compounds such as carboxylic acid esters or ethers, which contain polar bonds but are not able to dissolve ionic compounds due to their low permittivity .

Representatives of this group are:

• halogenated hydrocarbons which are either completely non-polar (such as carbon tetrachloride) or despite the high electronegativity of the halogen in question, e.g. B. chlorine, are only slightly polar ( methylene chloride )
• A special subgroup of halogenated hydrocarbons are the perfluorinated hydrocarbons (e.g. hexafluorobenzene ), which are not only non-polar themselves, but also very difficult to polarize from the outside and therefore also tend to be poorly compatible with the other non-polar solvents.

#### Aprotically polar

However, if the molecule is substituted with strongly polar functional groups such as the carbonyl group , the nitro group or the nitrile group , then the molecule has a dipole moment , so that there is now an intermolecular electrostatic attraction of permanent dipoles to the still present (but much weaker) van der- Waals forces added. This results in a substantial increase in the boiling point and in many cases a deterioration in the miscibility with non-polar solvents and an improvement in the solubility of and in polar substances. Typical aprotic polar solvents have a permittivity greater than 15 and are able to solvate cations . Since the anions are hardly solvated ( naked anions ), they show a high S N 2 reactivity. Such solvents are eminently suitable for carrying out nucleophilic substitutions under mild conditions. This includes:

### 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 ), one speaks of a protic solvent. These contrast with the aprotic solvents .

The most important protic solvent is water , which (in simplified terms) dissociates into a proton and a hydroxide ion.

Other protic solvents are e.g. B. Alcohols and carboxylic acids . Here the proton is always split off at the OH group, since the electronegative oxygen can easily absorb the negative charge.

The degree 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 significant dissociation in a neutral medium. The release of these protons is only possible with very strong bases.

Polar protic solvents dissolve salts and polar compounds, while the solubility of non-polar compounds is low.

Protic solvents are:

### Polarity scales

Reichardt dye

A well-known scale for the polarity of a solvent is the E T (30) or E T N scale . It is derived from empirical spectroscopic measurements. The E T (30) value is defined as the transition energy of the longest-wave Vis / NIR absorption band in a solution with the negatively solvatochromic Reichardt dye (betaine 30) under normal conditions in kcal · mol −1 . The E T N value is the E T (30) value normalized to the polarity extremes tetramethylsilane (= 0) and water (= 1) .

### Table with solvents and their data

solvent Melting point
[° C]
Siedep.
[° C]
Flammp.
[° C]
Density
[g / cm 3 ]
at 20 ° C
Permittivity
at 25 ° C
Dipole moment
[· 10 −30 cm]
Refractive index
${\ displaystyle n _ {\ mathrm {D}} ^ {20}}$
${\ displaystyle E _ {\ tau} (30)}$
[kJ / mol]
Compressibility [
10 −6 / bar]
acetone −95.35 56.2 −19 0.7889 20.70 9.54 1.3588 176.4 126
Acetonitrile −45.7 81.6 13 0.7857 37.5 (20 ° C) 11.48 1.3442 192.3 115
aniline −6.3 184 76 1.0217 6.89 (20 ° C) 5.04 1.5863 185.2 -
Anisole −37.5 155.4 41 0.9961 4.33 4.17 1.5179 155.5 -
benzene 5.5 80.1 −8 0.87565 2.28 0.0 1.5011 142.2 95
Benzonitrile −13 190.7 70 1.0102 (15 ° C) 25.20 13.51 1.5289 175.6 -
Bromobenzene −30.8 156 51 1.4950 5.40 5.17 1.5597 156.8 -
1-butanol −89.8 117.3 34 0.8098 17.51 5.84 1.3993 209.8 -
tert -butyl methyl ether (MTBE) −108.6 55.3 −28 0.74 ? ? 1.3690 145.2 -
γ-butyrolactone −44 204-206 101 1.13 39.1 4.12 1,436 - -
Quinoline −15.6 238 101 1.0929 9.00 7.27 1.6268 164.7 -
Chlorobenzene −45.6 132 28 1.1058 5.62 5.14 1.5241 156.8 -
chloroform −63.5 61.7 - 1.4832 4.81 (20 ° C) 3.84 1.4459 163.4 100
Cyclohexane 6.5 80.7 4.5 0.7785 2.02 (20 ° C) 0.0 1.4266 130.4 118
Dibutyl ether −98 142.5 25th 0.764 4.34 (20 ° C) 3.9 1,399 187.6 -
Diethylene glycol −6.5 244.3 124 1.1197 (15 ° C) 7.71 7.71 1.4475 224.9 -
Diethyl ether −116.2 34.5 −40 0.7138 4.34 (20 ° C) 4.34 1.3526 144.6 -
Dimethylacetamide −20 165 66 0.9366 (25 ° C) 37.78 12.41 1.4380 182.7 -
Dimethylformamide −60.5 153 67 0.9487 37.0 12.88 1.4305 183.1 -
Dimethyl sulfoxide 18.4 189 88 1.1014 46.68 13.00 1.4770 188.1 -
1,4-dioxane 11.8 101 12 1.0337 2.21 1.5 1.4224 150.0 -
Glacial acetic acid 16.6 117.9 42 1.0492 6.15 (20 ° C) 5.60 1.3716 214.0 -
Acetic anhydride −73.1 139.5 49 1.0820 20.7 (19 ° C) 9.41 1.3900 183.5 -
Ethyl acetate −83.6 77.06 −2 0.9003 6.02 6.27 1.3723 159.3 104
Ethanol −114.5 78.3 18th 0.7893 24.55 5.77 1.3614 216.9 114
1,2-dichloroethane (ethylene dichloride) −35.3 83.5 13 1.2351 10.36 6.2 1.4448 175.1 -
Ethylene glycol −13 197 117 1.1088 37.7 7.61 1.4313 235.3 -
Ethylene glycol dimethyl ether −58 84 −6 0.8628 7.20 5.70 1.3796 159.7 -
Formamide 2.5 210.5 175 1.1334 111.0 (20 ° C) 11.24 1.4472 236.6 -
n -hexane −95 68 −20 0.6603 1.88 0.0 1.3748 129.2 150
n -heptane −91 98 −4 0.684 1.97 0.0 1.387 130.1 120
2-propanol (isopropyl alcohol) −89.5 82.3 16 0.7855 19.92 5.54 1.3776 203.1 100
Methanol −97.8 64.7 6.5 0.7914 32.70 5.67 1.3287 232.0 120
3-methyl-1-butanol (isoamyl alcohol) −117.2 130.5 42 0.8092 14.7 6.07 1.4053 196.5 -
2-methyl-2-propanol ( tert- butanol) 25.5 82.5 9 0.7887 12.47 5.54 1.3878 183.1 -
Methylene chloride (dichloromethane, DCM) −95.1 40 - 1.3266 8.93 5.17 1.4242 171.8 -
Methyl ethyl ketone (butanone) −86.3 79.6 −4 0.8054 18.51 (20 ° C) 9.21 1.3788 172.6 -
N -Methyl-2-pyrrolidone (NMP) −24 202 245 1.03 32.2 4.09 1.47 - -
N -methylformamide −3.8 183 111 1.011 (19 ° C) 182.4 12.88 1.4319 226.1 -
Nitrobenzene 5.76 210.8 81 1.2037 34.82 13.44 1.5562 175.6 -
Nitromethane −28.5 100.8 35 1.1371 35.87 (30 ° C) 11.88 1.3817 193.5 -
n -pentane −130 36 −49 0.6262 - - 1.358 129.7 -
Petroleum ether / light petrol - 25-80 -26 0.63-0.83 - - - - -
Piperidine −9 106 4th 0.8606 5.8 (20 ° C) 3.97 1.4530 148.4 -
Propanol −126.1 97.2 24 0.8035 20.33 5.54 1.3850 211.9 100
Propylene carbonate (4-methyl-1,3-dioxol-2-one) −48.8 241.7 130 1.2069 65.1 16.7 1.4209 195.6 -
Pyridine −42 115.5 23 0.9819 12.4 (21 ° C) 7.91 1.5095 168.0 -
Carbon disulfide −110.8 46.3 −30 1.2632 2.64 (20 ° C) 0.0 1.6319 136.3 -
Sulfolane 27 285 177 - 43.3 (30 ° C) 16.05 1.4840 183.9 -
Tetrachlorethylene −19 121 - 1.6227 2.30 0.0 1.5053 133.3 -
Carbon tetrachloride −23 76.5 - 1.5940 2.24 (20 ° C) 0.0 1.4601 135.9 110
Tetrahydrofuran −108.5 66 −22.5 0.8892 7.58 5.84 1.4070 156.3 -
toluene −95 110.6 7th 0.8669 2.38 1.43 1.4961 141.7 87
1,1,1-trichloroethane −30.4 74.1 - 1.3390 7.53 (20 ° C) 5.24 1.4379 151.3 -
Trichlorethylene −73 87 - 1.4642 3.42 (16 ° C) 2.7 1.4773 150.1 -
Triethylamine −114.7 89.3 −7 0.7275 2.42 2.90 1.4010 139.2 -
Triethylene glycol −5 278.3 166 1.1274 (15 ° C) 23.69 (20 ° C) 9.97 1.4531 223.6 -
Triethylene glycol dimethyl ether (triglyme) - 222 113 - 7.5 - 1.4233 161.3 -
water 0.0 100 - 0.9982 78.39 6.07 1.3330 263.8 46

### Table with alcoholic solvents and their evaporation rates

relative to acetic acid n- butyl ester (= 1)

solvent Siedep.
[° 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 alcohol 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

### Indifferent solvents

In polymer chemistry, an inert or neutral solvent is understood to mean a medium which

## literature

• C. Reichardt: Solvents and Solvent Effects in Organic Chemistry . Wiley-VCH Verlag, Weinheim 1979 (1st edition), 1988 (2nd edition), 2003 (3rd edition), 2010 (4th edition; with T. Welton).

Wiktionary: Solvents  - explanations of meanings, word origins, synonyms, translations
Commons : Solvent  - collection of images, videos, and audio files

## Individual evidence

1. Product group adhesives (lga.de) ( Memento from September 16, 2010 in the Internet Archive )
2. Entry on aprotic solvents. In: Römpp Online . Georg Thieme Verlag, accessed on 2014-06-03.
3. ^ Alan R. Katritzky, Dan C. Fara, Hongfang Yang, Kaido Tämm et al .: Quantitative Measures of Solvent Polarity , p. 183 Spectroscopic Measurements .
4. Karl Dimroth , Christian Reichardt , Theodor Siepmann, Ferdinand Bohlmann : About pyridinium- N -phenol-betaine and their use to characterize the polarity of solvents . In: Justus Liebig's Annals of Chemistry . tape 661 , no. 1 , February 18, 1963, p. 1-37 , doi : 10.1002 / jlac.19636610102 ( PDF ).
5. ^ AG Reichardt: E T (30) values ​​of the aliphatic, cycloaliphatic, aromatic ethers, thioethers and acetals and alkanes
6. Agilent Technologies : Table 9: Compressibility of solvents. (PDF; 5.1 MB) February 2009, archived from the original on July 31, 2013 ; Retrieved July 31, 2013 .
7. Nicholas P. Cheremisinoff: Industrial Solvents Handbook . 2nd Edition. Marcel Dekker, 2003, ISBN 0-8247-4033-5 , p. 6 .
8. ^ MD Lechner, K. Gehrke, EH Nordmeier, Makromolekulare Chemie, 4th edition, Basel 2010, p. 160.
9. H.-G. Elias, Makromoleküle Volume 1, 5th edition, Basel 1990, p. 797.