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 .
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.
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 .:
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 .
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:
- Alkanes (paraffins)
- Alkenes (olefins), alkynes
- Benzene and other aromatics with aliphatic and aromatic substituents
- Carboxylic acid ester
- Ethers , e.g. B. diethyl ether
- completely symmetrically built molecules such as tetramethylsilane or carbon tetrachloride
- Carbon disulfide , at high pressure also carbon dioxide
- 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.
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:
- Ketones , e.g. B. acetone
- Lactones such as γ-butyrolactone
- Lactams such as N -methyl-2-pyrrolidone
- Nitriles such as acetonitrile
- Nitro compounds such as nitromethane
- tertiary carboxamides such as dimethylformamide
- Urea derivatives such as tetramethyl urea or dimethyl propylene urea (DMPU)
- Sulfoxides such as dimethyl sulfoxide (DMSO)
- Sulfones such as sulfolane
- Carbonic acid esters such as dimethyl carbonate or ethylene carbonate
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 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:
- Water , the most important solvent of all, especially in living nature
- Methanol , ethanol and other alcohols (the larger the carbon structure, 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)
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
[g / cm 3 ]
at 20 ° C
at 25 ° C
[· 10 −30 cm]
[kJ / mol]
10 −6 / bar]
|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||-|
|Benzonitrile||−13||190.7||70||1.0102 (15 ° C)||25.20||13.51||1.5289||175.6||-|
|tert -butyl methyl ether (MTBE)||−108.6||55.3||−28||0.74||?||?||1.3690||145.2||-|
|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||-|
|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||-|
|1,2-dichloroethane (ethylene dichloride)||−35.3||83.5||13||1.2351||10.36||6.2||1.4448||175.1||-|
|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||-|
|2-propanol (isopropyl alcohol)||−89.5||82.3||16||0.7855||19.92||5.54||1.3776||203.1||100|
|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||-|
|Nitromethane||−28.5||100.8||35||1.1371||35.87 (30 ° C)||11.88||1.3817||193.5||-|
|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||-|
|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||-|
|Carbon tetrachloride||−23||76.5||-||1.5940||2.24 (20 ° C)||0.0||1.4601||135.9||110|
|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||-|
|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||-|
Table with alcoholic solvents and their evaporation rates
relative to acetic acid n- butyl ester (= 1)
|tert -amyl alcohol||102||0.93|
|1-pentanol (amyl alcohol)||137||0.2|
In polymer chemistry, an inert or neutral solvent is understood to mean a medium which
- Termination and transfer reactions of polymerizations and thus also the rate and degree of polymerization little or no influence.
- has the same solution properties for all domains of a block copolymer (the opposite is a selective solvent).
- 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).
- Properties of solvents (Engl.)
- TRGS (baua.de) .
- Product group adhesives (lga.de) ( Memento from September 16, 2010 in the Internet Archive )
- Ceresana solvent market study .
- Entry on aprotic solvents. In: Römpp Online . Georg Thieme Verlag, accessed on 2014-06-03.
- Alan R. Katritzky, Dan C. Fara, Hongfang Yang, Kaido Tämm et al .: Quantitative Measures of Solvent Polarity , p. 183 Spectroscopic Measurements .
- 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 ).
- AG Reichardt: E T (30) values of the aliphatic, cycloaliphatic, aromatic ethers, thioethers and acetals and alkanes
- 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 .
- Nicholas P. Cheremisinoff: Industrial Solvents Handbook . 2nd Edition. Marcel Dekker, 2003, ISBN 0-8247-4033-5 , p. 6 .
- MD Lechner, K. Gehrke, EH Nordmeier, Makromolekulare Chemie, 4th edition, Basel 2010, p. 160.
- H.-G. Elias, Makromoleküle Volume 1, 5th edition, Basel 1990, p. 797.