A solvent dissolves a solute, resulting in a solutionEthyl acetate, a nail polish solvent.[1]
Asolvent (from theLatinsolvō, "loosen, untie, solve") is a substance that dissolves a solute, resulting in asolution. A solvent is usually a liquid but can also be a solid, a gas, or asupercritical fluid. Water is a solvent forpolar molecules, and the most common solvent used by living things; all the ions and proteins in acell are dissolved in water within the cell.
When one substance isdissolved into another, asolution is formed.[4] This is opposed to the situation when the compounds areinsoluble like sand in water. In a solution, all of the ingredients are uniformly distributed at a molecular level and no residue remains. A solvent-solute mixture consists of a singlephase with all solute molecules occurring assolvates (solvent-solutecomplexes), as opposed to separate continuous phases as in suspensions, emulsions and other types of non-solution mixtures. The ability of one compound to be dissolved in another is known as solubility; if this occurs in all proportions, it is calledmiscible.
In addition to mixing, the substances in a solution interact with each other at the molecular level. When something is dissolved, molecules of the solvent arrange aroundmolecules of the solute.Heat transfer is involved andentropy is increased making the solution morethermodynamically stable than the solute and solvent separately. This arrangement is mediated by the respective chemical properties of the solvent and solute, such ashydrogen bonding,dipole moment andpolarizability.[5] Solvation does not cause a chemical reaction or chemical configuration changes in the solute. However, solvation resembles acoordination complex formation reaction, often with considerable energetics (heat of solvation and entropy of solvation) and is thus far from a neutral process.
When one substance dissolves into another, a solution is formed. A solution is a homogeneous mixture consisting of a solute dissolved into a solvent. The solute is the substance that is being dissolved, while the solvent is the dissolving medium. Solutions can be formed with many different types and forms of solutes and solvents.
Solvents can be broadly classified into two categories:polar andnon-polar. A special case is elementalmercury, whose solutions are known asamalgams; also, othermetal solutions exist which are liquid at room temperature.
Generally, thedielectric constant of the solvent provides a rough measure of a solvent's polarity. The strong polarity of water is indicated by its high dielectric constant of 88 (at 0 °C).[6] Solvents with a dielectric constant of less than 15 are generally considered to be nonpolar.[7]
The dielectric constant measures the solvent's tendency to partly cancel the field strength of the electric field of acharged particle immersed in it. This reduction is then compared to thefield strength of the charged particle in a vacuum.[7] Heuristically, the dielectric constant of a solvent can be thought of as its ability to reduce the solute's effectiveinternal charge. Generally, the dielectric constant of a solvent is an acceptable predictor of the solvent's ability to dissolve commonionic compounds, such as salts.
Dielectric constants are not the only measure of polarity. Because solvents are used by chemists to carry out chemical reactions or observe chemical and biological phenomena, more specific measures of polarity are required. Most of these measures are sensitive to chemical structure.
TheGrunwald–Winstein mY scale measures polarity in terms of solvent influence on buildup of positive charge of a solute during a chemical reaction.
Kosower'sZ scale measures polarity in terms of the influence of the solvent onUV-absorption maxima of a salt, usuallypyridiniumiodide or the pyridiniumzwitterion.[8]
Donor number and donor acceptor scale measures polarity in terms of how a solvent interacts with specific substances, like a strongLewis acid or a strong Lewis base.[9]
TheHildebrand parameter is the square root ofcohesive energy density. It can be used with nonpolar compounds, but cannot accommodate complex chemistry.
Reichardt's dye, asolvatochromic dye that changes color in response to polarity, gives a scale ofET(30) values.ET is the transition energy between the ground state and the lowest excited state in kcal/mol, and (30) identifies the dye. Another, roughly correlated scale (ET(33)) can be defined withNile red.
Gregory's solvent ϸ parameter is a quantum chemically derived charge density parameter.[10] This parameter seems to reproduce many of the experimental solvent parameters (especially the donor and acceptor numbers) using this charge decomposition analysis approach, with an electrostatic basis. The ϸ parameter was originally developed to quantify and explain theHofmeister series by quantifying polyatomic ions and the monatomic ions in a united manner.
The polarity, dipole moment, polarizability andhydrogen bonding of a solvent determines what type ofcompounds it is able to dissolve and with what other solvents or liquid compounds it ismiscible. Generally, polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds best; hence "like dissolves like". Strongly polar compounds likesugars (e.g.sucrose) or ionic compounds, likeinorganicsalts (e.g.table salt) dissolve only in very polar solvents like water, while strongly non-polar compounds likeoils orwaxes dissolve only in very non-polar organic solvents likehexane. Similarly, water andhexane (orvinegar and vegetable oil) are notmiscible with each other and will quickly separate into two layers even after being shaken well.
Polarity can be separated to different contributions. For example, theKamlet-Taft parameters are dipolarity/polarizability (π*), hydrogen-bonding acidity (α) and hydrogen-bonding basicity (β). These can be calculated from the wavelength shifts of 3–6 different solvatochromic dyes in the solvent, usually includingReichardt's dye,nitroaniline anddiethylnitroaniline. Another option,Hansen solubility parameters, separates the cohesive energy density into dispersion, polar, and hydrogen bonding contributions.
Solvents with a dielectric constant (more accurately,relative static permittivity) greater than 15 (i.e. polar or polarizable) can be further divided intoprotic and aprotic. Protic solvents, such aswater, solvateanions (negatively charged solutes) strongly viahydrogen bonding.Polar aprotic solvents, such asacetone ordichloromethane, tend to have largedipole moments (separation of partial positive and partial negative charges within the same molecule) and solvate positively charged species via their negative dipole.[11] Inchemical reactions the use of polar protic solvents favors theSN1reaction mechanism, while polar aprotic solvents favor theSN2 reaction mechanism. These polar solvents are capable of forming hydrogen bonds with water to dissolve in water whereas non-polar solvents are not capable of strong hydrogen bonds.
The solvents are grouped intononpolar, polaraprotic, and polarprotic solvents, with each group ordered by increasing polarity. Theproperties of solvents which exceed those of water are bolded.
TheHansen solubility parameter (HSP) values[15][16][17] are based ondispersion bonds (δD),polar bonds (δP) andhydrogen bonds (δH). These contain information about the inter-molecular interactions with other solvents and also with polymers, pigments,nanoparticles, etc. This allows for rational formulations knowing, for example, that there is a good HSP match between a solvent and a polymer. Rational substitutions can also be made for "good" solvents (effective at dissolving the solute) that are "bad" (expensive or hazardous to health or the environment). The following table shows that the intuitions from "non-polar", "polar aprotic" and "polar protic" are put numerically – the "polar" molecules have higher levels of δP and the protic solvents have higher levels of δH. Because numerical values are used, comparisons can be made rationally by comparing numbers. For example, acetonitrile is much more polar than acetone but exhibits slightly less hydrogen bonding.
If, for environmental or other reasons, a solvent or solvent blend is required to replace another of equivalent solvency, the substitution can be made on the basis of the Hansen solubility parameters of each. The values for mixtures are taken as theweighted averages of the values for the neat solvents. This can be calculated bytrial-and-error, a spreadsheet of values, or HSP software.[15][16] A 1:1 mixture oftoluene and1,4 dioxane has δD, δP and δH values of 17.8, 1.6 and 5.5, comparable to those ofchloroform at 17.8, 3.1 and 5.7 respectively. Because of the health hazards associated with toluene itself, other mixtures of solvents may be found using a full HSP dataset.
The boiling point is an important property because it determines the speed of evaporation. Small amounts of low-boiling-point solvents likediethyl ether,dichloromethane, or acetone will evaporate in seconds at room temperature, while high-boiling-point solvents like water ordimethyl sulfoxide need higher temperatures, an air flow, or the application ofvacuum for fast evaporation.
Low boilers: boiling point below 100 °C (boiling point of water)
Most organic solvents have a lowerdensity than water, which means they are lighter than and will form a layer on top of water. Important exceptions are most of thehalogenated solvents likedichloromethane orchloroform will sink to the bottom of a container, leaving water as the top layer. This is crucial to remember whenpartitioning compounds between solvents and water in aseparatory funnel during chemical syntheses.
Often,specific gravity is cited in place of density. Specific gravity is defined as the density of the solvent divided by the density of water at the same temperature. As such, specific gravity is a unitless value. It readily communicates whether a water-insoluble solvent will float (SG < 1.0) or sink (SG > 1.0) when mixed with water.
Multicomponent solvents appeared after World War II in theUSSR, and continue to be used and produced in the post-Soviet states. These solvents may have one or more applications, but they are not universal preparations.
Most organic solvents areflammable or highly flammable, depending on theirvolatility. Exceptions are some chlorinated solvents likedichloromethane andchloroform. Mixtures of solvent vapors and air canexplode. Solvent vapors are heavier than air; they will sink to the bottom and can travel large distances nearly undiluted. Solvent vapors can also be found in supposedly empty drums and cans, posing aflash fire hazard; hence empty containers of volatile solvents should be stored open and upside down.
In addition some solvents, such as methanol, can burn with a very hot flame which can be nearly invisible under some lighting conditions.[23][24] This can delay or prevent the timely recognition of a dangerous fire, until flames spread to other materials.
Ethers likediethyl ether andtetrahydrofuran (THF) can form highly explosiveorganic peroxides upon exposure to oxygen and light. THF is normally more likely to form such peroxides than diethyl ether. One of the most susceptible solvents isdiisopropyl ether, but all ethers are considered to be potential peroxide sources.
The heteroatom (oxygen) stabilizes the formation of afree radical which is formed by the abstraction of ahydrogen atom by another free radical.[clarification needed] The carbon-centered free radical thus formed is able to react with an oxygen molecule to form a peroxide compound. The process of peroxide formation is greatly accelerated by exposure to even low levels of light, but can proceed slowly even in dark conditions.
Unless adesiccant is used which can destroy the peroxides, they will concentrate duringdistillation, due to their higherboiling point. When sufficient peroxides have formed, they can form acrystalline, shock-sensitive solidprecipitate at the mouth of a container or bottle. Minor mechanical disturbances, such as scraping the inside of a vessel, the dislodging of a deposit, or merely twisting the cap may provide sufficient energy for the peroxide todetonate or explode violently.
Peroxide formation is not a significant problem when fresh solvents are used up quickly; they are more of a problem in laboratories which may take years to finish a single bottle. Low-volume users should acquire only small amounts of peroxide-prone solvents, and dispose of old solvents on a regular periodic schedule.
To avoid explosive peroxide formation, ethers should be stored in an airtight container, away from light, because both light and air can encourage peroxide formation.[25]
Peroxides may be removed by washing with acidic iron(II) sulfate, filtering throughalumina, ordistilling fromsodium/benzophenone. Alumina degrades the peroxides but some could remain intact in it, therefore it must be disposed of properly.[26] The advantage of using sodium/benzophenone is thatmoisture and oxygen are removed as well.[27]
General health hazards associated with solvent exposure include toxicity to the nervous system, reproductive damage, liver and kidney damage, respiratory impairment, cancer, hearing loss,[28][29] anddermatitis.[30]
Chronic solvent exposures are often caused by the inhalation of solvent vapors, or the ingestion of diluted solvents, repeated over the course of an extended period.
Chronic exposure to organic solvents in the work environment can produce a range of adverse neuropsychiatric effects. For example, occupational exposure to organic solvents has been associated with higher numbers of painters suffering fromalcoholism.[35] Ethanol has asynergistic effect when taken in combination with many solvents; for instance, a combination oftoluene/benzene and ethanol causes greaternausea/vomiting than either substance alone.
A major pathway of induced health effects arises from spills or leaks of solvents, especiallychlorinated solvents, that reach the underlying soil. Since solvents readily migrate substantial distances, the creation of widespreadsoil contamination is not uncommon; this is particularly a health risk ifaquifers are affected.[37]Vapor intrusion can occur from sites with extensive subsurface solvent contamination.[38]
Solvents are often refluxed with an appropriatedesiccant prior to distillation to remove water. This may be performed prior to a chemical synthesis where water may interfere with the intended reaction
^Kosower, E.M. (1969) "An introduction to Physical Organic Chemistry" Wiley: New York, p. 293
^Gutmann V (1976). "Solvent effects on the reactivities of organometallic compounds".Coord. Chem. Rev.18 (2): 225.doi:10.1016/S0010-8545(00)82045-7.
^Gregory, Kasimir P.; Wanless, Erica J.; Webber, Grant B.; Craig, Vincent S. J.; Page, Alister J. (2024). "A first-principles alternative to empirical solvent parameters".Phys. Chem. Chem. Phys.26 (31):20750–20759.Bibcode:2024PCCP...2620750G.doi:10.1039/D4CP01975J.PMID38988220.
^Anderson JE, Magyarl MW, Siegl WO (1 July 1985). "Concerning the Luminosity of Methanol-Hydrocarbon Diffusion Flames".Combustion Science and Technology.43 (3–4):115–125.doi:10.1080/00102208508947000.ISSN0010-2202.
^Raitta C, Husman K, Tossavainen A (August 1976). "Lens changes in car painters exposed to a mixture of organic solvents".Albrecht von Graefes Archiv für Klinische und Experimentelle Ophthalmologie. Albrecht von Graefe's Archive for Clinical and Experimental Ophthalmology.200 (2):149–56.doi:10.1007/bf00414364.PMID1086605.S2CID31344706.
