Introduction
The study of solvent polarity and solvent interchangeability is essential in chemistry, as these factors greatly impact the behavior of solutes in solution, reaction mechanisms, and overall experimental results. Solvent polarity is a key property that affects how solvents interact with solutes, influencing characteristics like solubility, reactivity, and even the physical traits of the solvent itself. Solvents differ in their polarity, ability to form hydrogen bonds, and capacity to donate or accept protons, which determines their suitability for various chemical processes. This article explores solvation theory, the classification of solvents based on polarity and molecular structure, and examines how solvent interchangeability can improve reaction efficiency. Special attention is given to applications in clandestine synthesis, where careful solvent selection is crucial for effective extraction, crystallization, and product purification.
Solvation Theory
Solvation refers to the interaction between a solvent and dissolved particles. Both charged and neutral molecules engage significantly with the solvent, and the strength and characteristics of this interaction impact numerous properties of the solute, such as its dissolvability, reactivity, and color, as well as affecting the solvent’s properties like viscosity and density. When the attraction between solvent and solute particles exceeds the forces holding the solute together, the solvent molecules pull the solute particles apart and surround them. These solute particles then disperse from the solid into the solution. Ions, in particular, are enveloped by a surrounding layer of solvent. Solvation involves reorganizing solvent and solute molecules into complexes, involving bond formation, hydrogen bonding, and van der Waals interactions. When solvation occurs in water, it is termed hydration.
The solubility of solid compounds depends on a balance between lattice energy and solvation forces, including entropy effects due to structural changes in the solvent.
A sodium ion solvated by water molecules
Upon dissolution of substance A in solvent B, the destruction of intermolecular interactions of type A–A and B–B occurs and the occurrence of intermolecular interactions of type A–B. A substance is well soluble in a solvent if the forces of intermolecular interaction in a pure substance and a pure solvent have approximately the same order. On the contrary, if the intermolecular interaction in a pure substance is significantly stronger or weaker than the intermolecular interaction in a pure solvent, then the substance does not dissolve well in such a solvent. Briefly, this principle is formulated by the expression "like dissolves into like" (Latin: similia similibus solvuntur).
The following types of intermolecular interaction are distinguished:
- ionic (interaction between ions);
- ion-dipole (between ions and dipoles);
- directional (between permanent dipoles);
- induced (between permanent and induced dipoles);
- dispersive (between atomic dipoles);
- hydrogen bonds (between groups having X–H type polar bonds).
The intermolecular interaction in pure components A and B may be weaker than the interaction of type A–B in solution. In this case, during dissolution, the internal energy of the system decreases, and the dissolution process itself is exothermic. If the intermolecular interaction in pure components A and B is stronger than in solution, then the internal energy during dissolution increases due to the absorption of heat from the outside, that is, the dissolution is endothermic. Most dissolution processes are endothermic, and their course is facilitated by an increase in temperature.
Solvent Classifications
Solvents are classified according to their belonging to certain classes of chemicals:
- Organic solvents (aliphatic hydrocarbons, aromatic hydrocarbons, halocarbons, nitro compounds, alcohols, carboxylic acids, esters and esters, amides, nitriles, ketones, sulfoxides, etc.);
- Inorganic solvents (water, low-melting halides, oxohalides, nitrogen-containing solvents, SO2, HF, low-melting metals, salt melts, oxide melts).
The Dipole Moment and Solvent Polarity
Solvents are usually classified according to their dissolving ability into polar and nonpolar. However, since the polarity of the solvent cannot be expressed in specific physical quantities, attempts are being made to express the polarity through another physical property. One of these properties is the dipole moment of the solvent molecule.Charge separation in a water molecule (negative charge is shown in red, positive charge is shown in blue)
The dipole moment is the sum of the dipole moments of the individual bonds of the molecule, therefore symmetrical solvents (carbon tetrachloride, benzene, cyclohexane) have a zero dipole moment. Other aromatic solvents, as well as dioxane, have a low dipole moment. Less symmetrical molecules with polar bonds (alcohols, esters) have a higher dipole moment (1.6–1.9 D); glycols and ketones have an even higher dipole moment (2.3–2.9 D); nitropropane, DMFA and DMSO have a very high dipole moment (3.7–5.0 D). However, dioxane and DMSO have very different dipole moments, but similar solvent capacity, therefore, the magnitude of the dipole moment is not in all cases able to accurately reflect this property of the solvent.
Dipole moments of 1,3 and 1,2-dichlorobenzene
Differences in electron negativity cause electron density to be centered around one side of a molecule. This causes the molecule to become polarized with a partial negative and partial positive charge.
Polarity of common solvents
The Ability to Form Hydrogen Bonds
According to the strength of the hydrogen bonds formed, solvents are divided into three classes:
- solvents with weak hydrogen bonds (hydrocarbons, chlorinated hydrocarbons, nitro compounds, nitriles);
- solvents with moderately strong hydrogen bonds (ketones, esters, esters, aniline);
- solvents with strong hydrogen bonds (alcohols, carboxylic acids, amines, pyridine, glycols, water).
Quantitatively, the solvent's ability to form hydrogen bonds is estimated by the parameter γ. This parameter is obtained by dissolving deuteromethanol in the solvent under study and observing the shift of the oscillation band of the O–D bond in the infrared spectrum.
Solvents are also divided according to their role in the formation of a hydrogen bond:
Solvents are also divided according to their role in the formation of a hydrogen bond:
- proton donors (chloroform);
- proton acceptors (ketones, esters, esters, aromatic hydrocarbons);
- proton donors and acceptors (alcohols, carboxylic acids, primary and secondary amines, water), also called amphiproton solvents;
- non-hydrogen bonding (aliphatic hydrocarbons).
Hydrogen bonds in methanol
Accordingly, if only proton acceptors are present in the solution, a hydrogen bond cannot be formed. If there are also hydrogen bond donors in the solution, then such a bond is formed, which leads to an increase in solubility.
Protic and Aprotic Solvents
Among the solvents, some may be donors or acceptors of protons or electrons. According to this feature, there are four groups:- Protic solvents (water, alcohols, carboxylic acids, etc.) can act as proton donors and, as a rule, have a high dielectric constant (ε > 15);
- Donor solvents (esters) — act as donors of electronic pairs;
- Aprotic dipolar solvents (dimethylformamide, dimethyl sulfoxide, ketones, etc.) have a high dielectric constant, but are not proton donors and do not have donor—acceptor properties;
- Nonpolar solvents (hydrocarbons, carbon disulfide) have low dielectric permittivity and do not have donor—acceptor properties. Solvents from the last two specified groups are also called aprotic.
Applications In Cladestine Syntheses
Almost all psychoactive substance syntheses take solvents in depends on the reaction mechanism. For instance, 4-MMC takes such solvents as DCM, benzene, toluene, ethyl acetate and etc. Some substances, which is naturally produced as cocaine or THC, can be extracted by solvents such as kerosene and ethanol correspondingly. In addition, solvents are used for washing of ready made products and crystallization procedures. A solvent choice is very important during crystallization procedure because some solvents can make your product oily tar while one solvent can help get nice shaped crystals. Knowledge about solvent properties and their applications allow to handle laboratory procedures efficiently and faster.Solvents Interchangeability
As I said above, there are a lot of solvents, which are used in clandestine syntheses. Some of them have similar chemical properties and can substitute each other. For example some non-polar solvents as dioxane and diethyl ether. In amphetamine synthesis, such polar substance as amphetamine can be extracted by aprotic non-polar or weakly polar solvents from the aqueous reaction mixture such as petroleum ether, DCM, toluene, benzene by reason that they have similar dipole moments from 0 up to 1.14 and they are not soluble in water.
In addition, an another example is 4-MMC synthesis, which has vast variety of applicable solvents due to reaction specific. 2-Bromo-4'-methylpropiophenone methylamination allows to use NMP, DCM, benzene, toluene, xylene, ethyl acetate, and etc solvents. All of these solvents are aprotic and insoluble in water but has different polarity. For example benzene and NMP has 0 and 4.09 dipole moments correspondingly. Nevertheless, these solvents are suitable for the synthesis and dissolve 2-Bromo-4'-methylpropiophenone. These facts tell us that polarity of a solvent, in some cases, not a main factor in it's choosing.
Conclusion
In conclusion, solvent polarity and interchangeability are key factors that drive successful chemical reactions, extractions, and synthesis results. While solvent polarity is important, it is often balanced with other aspects such as hydrogen-bonding capacity, and proton-donating or -accepting properties to achieve desired reaction efficiency. The interchangeability of solvents provides flexibility in laboratory settings, enabling chemists to swap similar solvents based on reaction requirements or availability—particularly beneficial in complex syntheses. A detailed grasp of these properties helps chemists make strategic choices that streamline procedures, optimize reaction conditions, and ensure synthesis steps like crystallization or purification yield high-quality results.
Properties Charts of Common Solvents
Properties of common solvents
Sources