Editing Solvent (section)

==Solvent classifications==
Solvents can be broadly classified into two categories: ''polar'' and ''non-polar''. A special case is elemental [[mercury (element)|mercury]], whose solutions are known as [[Amalgam (chemistry)|amalgams]]; also, other [[liquid metal|metal solutions]] exist which are liquid at room temperature.

Generally, the [[dielectric 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&nbsp;°C).<ref name="NIST water">{{cite journal| vauthors = Malmberg CG, Maryott AA |title=Dielectric Constant of Water from 0° to 100&nbsp;°C|journal=Journal of Research of the National Bureau of Standards|date=January 1956|volume=56|issue=1|page=1|doi=10.6028/jres.056.001|doi-access=free}}</ref> Solvents with a dielectric constant of less than 15 are generally considered to be nonpolar.<ref name=p177>Lowery and Richardson, p. 177.</ref>

The dielectric constant measures the solvent's tendency to partly cancel the field strength of the electric field of a [[ion|charged particle]] immersed in it. This reduction is then compared to the [[field strength]] of the charged particle in a vacuum.<ref name=p177/> Heuristically, the dielectric constant of a solvent can be thought of as its ability to reduce the solute's effective [[chemical polarity|internal charge]]. Generally, the dielectric constant of a solvent is an acceptable predictor of the solvent's ability to dissolve common [[ionic compound]]s, such as salts.

===Other polarity scales===
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.

The ''[[Grunwald–Winstein equation|Grunwald–Winstein]] m'''Y''' scale'' measures polarity in terms of solvent influence on buildup of positive charge of a solute during a chemical reaction.

''[[Edward Kosower|Kosower]]'s '''Z''' scale'' measures polarity in terms of the influence of the solvent on [[Ultraviolet|UV]]-absorption maxima of a salt, usually [[pyridinium]] [[iodide]] or the pyridinium [[zwitterion]].<ref>Kosower, E.M. (1969) "An introduction to Physical Organic Chemistry" Wiley: New York, p. 293</ref>

''Donor number and donor acceptor scale'' measures polarity in terms of how a solvent interacts with specific substances, like a strong [[Lewis acid]] or a strong Lewis base.<ref>{{Cite journal| vauthors = Gutmann V |journal = Coord. Chem. Rev.|year = 1976|volume = 18|issue = 2|page = 225|doi = 10.1016/S0010-8545(00)82045-7|title = Solvent effects on the reactivities of organometallic compounds}}</ref>

The ''[[Hildebrand parameter]]'' is the square root of '''cohesive energy density'''. It can be used with nonpolar compounds, but cannot accommodate complex chemistry.

Reichardt's dye, a [[solvatochromic]] dye that changes color in response to polarity, gives a scale of ''E<sub>T</sub>''(30) values. ''E<sub>T</sub>'' 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 (''E<sub>T</sub>''(33)) can be defined with [[Nile red]].

Gregory's solvent ϸ parameter is a quantum chemically derived charge density parameter.<ref>{{Cite journal |last=Gregory |first=Kasimir P. |last2=Wanless |first2=Erica J. |last3=Webber |first3=Grant B. |last4=Craig |first4=Vincent S. J. |last5=Page |first5=Alister J. |year=2024 |title=A first-principles alternative to empirical solvent parameters |journal=Phys. Chem. Chem. Phys. |volume=26 |issue=31 |pages=20750–20759 |bibcode=2024PCCP...2620750G |doi=10.1039/D4CP01975J |pmid=38988220}}</ref> 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 the [[Hofmeister series]] by quantifying polyatomic ions and the monatomic ions in a united manner.

{{anchor|LikeDissolvesLike}}The polarity, dipole moment, polarizability and [[hydrogen bonding]] of a solvent determines what type of [[Chemical compound|compounds]] it is able to dissolve and with what other solvents or liquid compounds it is [[miscible]]. Generally, polar solvents dissolve polar compounds best and non-polar solvents dissolve non-polar compounds best; hence "''like dissolves like''". Strongly polar compounds like [[sugar]]s (e.g. [[sucrose]]) or ionic compounds, like [[Inorganic chemistry|inorganic]] [[Salt (chemistry)|salt]]s (e.g. [[table salt]]) dissolve only in very polar solvents like water, while strongly non-polar compounds like [[oil]]s or [[wax]]es dissolve only in very non-polar organic solvents like [[hexane]]. Similarly, water and [[hexane]] (or [[vinegar]] and vegetable oil) are not [[miscible]] with each other and will quickly separate into two layers even after being shaken well.

Polarity can be separated to different contributions. For example, the '''Kamlet-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 including [[Reichardt's dye]], [[nitroaniline]] and [[diethylnitroaniline]]. Another option, [[Hansen solubility parameter]]s, separates the cohesive energy density into dispersion, polar, and hydrogen bonding contributions.

===Polar protic and polar aprotic===
Solvents with a dielectric constant (more accurately, [[relative static permittivity]]) greater than 15 (i.e. polar or polarizable) can be further divided into [[protic]] and aprotic. Protic solvents, such as [[water]], solvate [[anion]]s (negatively charged solutes) strongly via [[hydrogen bonding]]. [[Polar aprotic solvent]]s, such as [[acetone]] or [[dichloromethane]], tend to have large [[Molecular dipole moment|dipole moments]] (separation of partial positive and partial negative charges within the same molecule) and solvate positively charged species via their negative dipole.<ref>Lowery and Richardson, p. 183.</ref> In [[chemical reaction]]s the use of polar protic solvents favors the [[SN1 reaction|S<sub>N</sub>1]] [[reaction mechanism]], while polar aprotic solvents favor the [[SN2 reaction|S<sub>N</sub>2]] 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.