Polar Protic and Aprotic Solvents

Core Concepts

In this article, you will learn about the characteristics of polar protic and aprotic solvents, along with examples and their uses.

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Solvents

Solvents are liquids that can dissolve a solute, another substance, to form a solution. The solvent particles must fully surround the solvent molecules, which evenly distribute throughout the solvent and resulting solution. Solvents can be polar and nonpolar.

Polarity

Nonpolar Solvents

Nonpolar solvents contain bonds between atoms that have similar electronegativities, like hydrogen and carbon. For example, propane has a symmetric structure containing covalent bonds between the hydrogen and carbon atoms with a dipole moment of approximately zero.

Examples of nonpolar solvents are benzene (C₆H₆), chloroform (CHCl₃), and hexane (C₆H₁₄).

Polar Solvents

Polar solvents contain bonds between atoms that have very different electronegativities, which causes a dipole moment. For example, water is a polar solvent because the bonds between oxygen and hydrogen are polar covalent, meaning the electrons are shared unevenly and are pulled closer to the more electronegative atom (oxygen). Since the electrons are attracted more to the oxygen on water molecules, there is a partial negative charge on the oxygen and partial positive charges on the hydrogens, causing a dipole moment (two poles). This makes the solvent molecules polar overall.

We can further divide polar solvents into two categories — protic and aprotic.

Polar Protic Solvents

Polar protic solvents are capable of hydrogen bonding and can donate a hydrogen bond. These solvents have either an N, O, or F atom directly bonded to a hydrogen atom.

Hydrogen Bonding

Hydrogen bonding is an intermolecular force between the hydrogen of one molecule and the oxygen, nitrogen, OR fluorine of another molecule. The hydrogen must be bonded to an N, O, or F atom on both of these molecules.

The difference in electronegativity between hydrogen and oxygen, nitrogen, or flourine causes the covalent bond between them to be polarized. The hydrogen atom then has a partial positive charge, and can interact with other atoms that have a partial negative charge in neighboring molecules.

Take methanol (CH₃OH) for example.

Methanol is a polar molecule because of the partial negative charge on the oxygen atom and the partial positive charges on the rest of the molecule. Since the highly electronegative atom, oxygen, is bonded to a hydrogen atom, methanol is a polar protic solvent. Methanol can donate this proton for hydrogen bonding.

Polar Protic Solvents and SN1 Reactions

Examples of protic solvents include isopropyl alcohol (CH₃CH(OH)CH₃), water (H₂O), and ethanol (CH₃CH₂OH).

In our example, we will use tert-Butyl bromide and the polar protic solvent water.

tert-butyl bromide example — Tert-Butyl bromide

The bond between carbon and bromide (which is the leaving group) breaks in the SN1 reaction, and the protic solvent can help stabilize the carbocation and the anion (bromine atom) that result. This is due to hydrogen bonding and the partial positive/negative charges on the polar protic solvent.

carbocation and polar protic water example — Oxygens of polar protic water molecules interacting with carbocation

The leaving group, which is the bromine anion, can be stabilized by the partial positive charge of the water molecule.

bromide ion and polar protic water example — Hydrogens of polar protic water molecules interacting with leaving group

The resulting carbocation is stabilized by the partial negative charge of the water molecule. These are the ionic species that form and are stabilized when the bromine detaches from the carbon.

Additionally, the polar protic solvent can stabilize the transition state with hydrogen bonding, which lowers the activation energy and increases the rate of the reaction.

tert-butyl bromide transition state water polar protic example — Polar protic water molecules interacting with transition state

Moreover, polar protic solvents typically have a high boiling point, which can be useful for driving chemical reactions.

Polar Aprotic Solvents and SN2 Reactions

Polar aprotic solvents have dipole moments, but they do not have available protons to donate for hydrogen bonding. These solvents may have hydrogens in their structure, but they will not serve as proton donors. Unlike protic solvents, aprotic solvents will lack the amine and hydroxyl functional groups with the O-H or N-H necessary for hydrogen bonding.

Take acetone (C₃H₆O) for example.

Acetone is polar because there is a partial negative charge on the oxygen and a partial positive charge on the rest of the molecule. This is an aprotic solvent because the highly electronegative atom, oxygen, is not bonded to a hydrogen atom. The hydrogen atoms are bonded to the carbon instead, which is a strong, equal sharing of atoms. This means that acetone will not donate any protons, making it an aprotic solvent.

Examples and Uses of Polar Aprotic Solvents

Examples of polar aprotic solvents include acetonitrile (CH₃CN), ethyl acetate (C₄H₈O₂), and sulfolane ((CH₂)₄SO₂).

Polar aprotic solvents tend to host SN2 reactions. In SN2 reactions, the nucleophile attacks the electrophile while the leaving group is still attached; the leaving group then breaks away. In this example, we will use an alkyl halide (electrophile) and KF. Our polar aprotic solvent will be acetone. Due to its polarity, the solvent will dissolve the salt, KF. The partial negative charge on the oxygen atom of the acetone molecule will interact with and solvate the K⁺.

acetone and potassium ions example — Oxygens of polar aprotic acetone molecules interacting with K_⁺ ions

The F^– can then attack the electrophile and continue the SN2 reaction.

flouride ions and alkyl halide example — Anionic nucleophile F^– attacking electrophile

An important point to understand is that polar protic solvents tend to use the nucleophile in a reaction to create hydrogen bonds. The solvent molecules will surround the nucleophile and stabilize it, but they will hinder the nucleophile’s interaction with the electrophile, which hinders the reaction. Therefore, polar aprotic solvents will host the SN2 reactions that depend heavily on the nucleophile’s concentration because these solvents will not solvate the nucleophile.

Typically, polar aprotic solvents have a lower boiling point than protic solvents, due to the lack of hydrogen bonding.

Practice Problems

Hint: draw out the Lewis Structures for the molecules in Q1 and Q2 to determine the answer.

Is acetic acid (CH₃COOH) a nonpolar, polar protic, or polar aprotic solvent?
Is dichloromethane (CH₂Cl₂) a nonpolar, polar protic, or polar aprotic solvent?

Identify the type of reaction (SN1 or SN2) occurring in the image above and which solvent (polar protic or polar aprotic) would host the reaction better.

Identify the type of reaction (SN1 or SN2) occurring in the image above and which solvent (polar protic or polar aprotic) would host the reaction better.

Practice Problems Solutions

Polar protic
Polar aprotic
SN2; polar aprotic
SN1; polar protic