Solvents come in various molecular forms with different polarities, whether they’re nonpolar, polar protic, or polar aprotic.
A solvent’s degree of polarity affects the types of solids that it can successfully dissolve. One of the most common solvents is acetone, which is used in various applications from paint thinners to nail polish remover.
Is acetone polar or nonpolar? Acetone molecules are polar because of the positive and negative charges formed by their carbonyl group. The molecules that compose acetone do have nonpolar covalent bonds within their overall structure, such as their carbon to hydrogen and carbon to carbon bonds. As a polar aprotic solvent, the polarity of acetone is only intermediate compared to polar protic solvents.
What is Acetone?
Acetone is a colorless ketone and organic solvent that is both volatile and flammable. It is water-soluble and miscible in ethanol, ether, benzene, ethanol, and methanol. Other names for acetone include propanone, dimethyl ketone, and beta-ketopropane (source).
The majority of acetone produced for industrial purposes is used as a solvent. Acetone is commonly used in nail polish remover, paint thinner, varnish, and some paints.
Acetone is a powerful degreaser used to clean objects made of metal and glass for example. This is particularly useful when prepping metal for welding, soldering, painting, or powder coating. (If you need help removing powder coating see my post “How to Remove Powder Coating“).
Even better is a 50/50 mix of acetone and transmission fluid, which is particularly useful for removing rust and penetrating the threads of rusted bolts and nuts.
Other less common but important applications for acetone include its uses in the production of plastics and other chemicals. It is converted to acetone cyanohydrin in order to make acrylic (methyl methacrylate), which is used to make things like plexiglass and acrylic paints.
It is also used to synthesize bisphenol A in the production of such polymers as epoxy resins.
Acetone is a naturally occurring chemical produced in plants and by the human body as a breakdown product of body fat. Additionally it is produced in trees and by forest fires and is present in volcanic gases.
Less natural industrial occurrences of acetone are more common, and acetone can be found in tobacco smoke and vehicle exhaust. Acetone has also attracted the interest of astrophysicists since its recent discovery in the Interstellar medium (source).
Acetone is present in urine and blood, and high levels of acetone can be toxic. Diabetics and alcoholics produce larger amounts of acetone (source).
High levels of ketone in the body can be produced by low carbohydrate diets like the popular ketogenic diets.
The medical condition where high levels of ketone are discovered in a person’s blood or urine is known as ketosis. A more serious condition known as Ketoacidosis is caused by the complete absence of insulin in diabetics, which leads to the overproduction of ketones.
The Molecular Structure of Acetone
The chemical composition of acetone is (CH3)2CO, which is based on two methyl groups (CH3) combined with a carbonyl group (C=O). The carbonyl group is composed of the central carbon atom attached to one oxygen atom by a polar covalent, double bond.
The central carbon atom is then attached to two more carbons through nonpolar covalent bonds. Each of the two outer carbon atoms of the methyl groups has three hydrogen atoms attached by nonpolar covalent bonds.
Covalent bonds are chemical bonds where electron pairs are shared between atoms. So long as their electronegativities are relatively close, the bond is covalent.
In contrast, ionic bonds occur between metal and nonmetal atoms when electrons are transferred. Covalent bonds can be either polar or nonpolar based largely on whether or not the electrons are equally or unequally shared.
The ability of an atom to attract electrons is called electronegativity. Polar bonds are formed when the difference in electronegativity between two atoms is greater than 0.4. When their difference in electronegativity is less than 0.4, the bond is nonpolar (source).
The higher the difference in electronegativity between the atoms, the more unequally the electrons are shared.
The shape of a molecule also helps determine its polarity. For example, carbon dioxide is a nonpolar molecule where the central carbon atom has an oxygen atom on both sides in a level, symmetrical, alignment.
Even though the bonds are polar bonds, the dipole moments are traveling in opposite directions canceling one another out. Acetone is a polar molecule that also has asymmetrical nonpolar covalent bonds between its carbon atoms which do not affect is polarity.
The Carbonyl Group: A Polar Covalent Bond of Oxygen and Carbon
The polarity of acetone is determined primarily by the relationship of the oxygen atom to the central carbon atom within the carbonyl group.
Oxygen has an electronegativity on the Pauling Scale of 3.44 compared to carbon’s 2.55. That’s a difference of 0.89, making the bond clearly polar.
This difference in electronegativity of over 0.4 causes the carbonyl group to have a bond dipole moment. It’s called a dipole bond because it has two poles, one positively charged and one negatively charged.
In this case, the oxygen atom is slightly negative and the carbon atom slightly positive. Intermolecular forces formed without hydrogen atoms are called dipole-dipole forces.
The overall polarity of a system of charges is measured in part by its electric dipole moment. The International System of Units (SI) measure for electric dipole moment is the coulomb-meter, while the debye (D), named after physicist Peter J. Debye, is another commonly used measure.
The bond dipole moments within a molecule are frequently shown in vector form to show its magnitude and direction.
A carbonyl group is composed of a double polar covalent bond between an oxygen atom and a carbon atom.
A single bond consists only of a sigma bond, the strongest type of bond, where the electrons directly overlap between the atoms. All of the bonds within acetone consist of at least one sigma bond.
Double covalent bonds, like the one formed in the carbonyl group, also have one sigma bond with the addition of one pi bond. The pi bond is not as strong since there is less overlap between the electrons.
Instead of being between the nuclei of the two atoms, like in sigma bonds, the pi bond is parallel to the bonded atoms. The addition of the pi bond also shortens the length of the overall bond to 121.3 picometers (pm), and shorter bonds are stronger bonds.
The Methyl Groups: Nonpolar Covalent Bonds of Hydrocarbons
Nonpolar molecules occur when either the electrons are equally shared in a diatomic (two-atom) structure or when the polar bonds are arranged more symmetrically in more complex molecules. Nonpolar molecules don’t have positive or negative polarity.
Carbon atoms can form four covalent bonds and can bond with other carbon atoms. In the case of the central carbon atom of acetone, it forms two single bonds with the adjacent carbon atoms and one double bond with the oxygen atom.
The carbonyl group with spaces for carbon variants to attach qualifies acetone as a ketone. The basic ketone structure is often written with Rs representing areas where carbon-containing structures like methyl groups can bond (source).
The bonds that connect acetone’s two methyl groups to the carbonyl group are nonpolar covalent bonds. A methyl group is an alkyl, which is an alkane that lacks one hydrogen atom.
The methyl group contains one carbon atom and three hydrogen atoms. Hydrocarbons are nonpolar because Hydrogen has an electronegativity of 2.20 while carbon’s is 2.55. The very small difference of 0.35 meets the necessary criteria.
The open space of the two methyl groups allows their carbon atoms to bond to the carbon atom of the carbonyl group. Carbon-to-carbon bonds are obviously nonpolar because there is no difference in their electronegativities.
Compounds made of nonpolar molecules have higher vapor pressure and form volatile compounds like gasoline. This is due to the fact that the bonds between the nonpolar hydrocarbon molecules are not as strong as more polar molecules.
Nonpolar molecules can form temporary dipoles due to London dispersed force. London dispersed force is the lowest intermolecular force that is caused by the relocation of electrons in an asymmetrical fashion around the nucleus of an atom (source).
The affected atom can then distort the polarity of adjacent atoms within the molecule. This can cause nonpolar molecules to condense into liquids at a low enough temperature or freeze into solids.
Properties of Polar Molecules
Polar molecules have stronger intermolecular bonds than nonpolar molecules. These intermolecular interactions can occur between polar molecules because of the differing charges at each end (source).
Polar protic molecules like water, formic acid, ethanol, and acetic acid have high boiling points and melting points compared to nonpolar molecules of comparable mass due to their stronger bonds.
Polar aprotic chemicals like acetone have intermediate polarity as well as boiling and melting points.
The boiling point of acetone is 132.89°F (56.05°C), and its melting point is -138.5°F (-94.7°C) compared with the nonpolar solvent benzene, which has a boiling point of 176°F (80.1°C) and a melting point of 41.95°F (5.53°C).
Acetone has a much lower molar mass (58.08 g·mol−1) than most nonpolar molecules like benzene (78.114 g·mol−1). Acetone also has nonpolar hydrocarbons in its structure, sharing some of the characteristics of nonpolar molecules.
Polar molecules with high boiling points and melting points also have low vapor pressure and a lower evaporation rate. They also have high surface tension, forming greater bonds with other polar molecules than with those of the air.
Since nonpolar molecules lack charges, polar molecules cannot form attractions with them. As a result, polar molecules can only be dissolved in polar solvents.
Is Acetone Polar Protic or Polar Aprotic?
Solvents are graded by their polarity and grouped as nonpolar, polar aprotic, or polar protic. Those molecules with the lowest polarity are nonpolar, followed by the higher-level polar aprotic and polar protic.
A polar protic solvent has a hydrogen atom attached to a diatomic nonmetal element such as oxygen, nitrogen, or fluorine. This includes ethanol (CH3CH2OH), water (H2O), ammonia (NH3), and hydrogen fluoride (HF) among others.
Acetone, on the other hand, is an aprotic solvent since its hydrogen atoms are not attached to oxygen, nitrogen, or fluorine.
Polar protic solvents have the highest polarity among solvents. While aprotic solvents are polar, their polarity is intermediate, being above nonpolar solvents but below polar protic solvents.
Polar protic solvents are acidic because they can donate hydrogen ions (H+) to reagents. Aprotic solvents are not acidic because, although they can accept hydrogen ions, they cannot donate hydrogen ions.
With a PH of 7, acetone is neither acidic nor a base. The polar protic solvent hydrogen fluoride is a super solvent but a weak acid on its own.
Both protic and aprotic solvents can dissolve salts, but the degree to which they can do so is affected by their dielectric constant.
Dielectric constant is a measure of a solvent’s ability to separate the molecules of a solute into ions. The dielectric constant of a molecule is a function of its dipole moment.
Why does Acetone have a Higher Dipole Moment than Ethanol?
The dipole moments of polar protic solvents like ethanol, water, and ammonia are 1.69 D, 1.85 D, and 1.42 D respectively, which is actually less than the dipole moments of some polar aprotic solvents like acetone.
While the electric dipole moment contributes to a molecule’s overall polarity, it is not the determining factor.
Acetone has a dielectric constant of 20.7 ε0 at 77°F (25°C) and a dipole moment of 2.88 D. A quick comparison with the polar protic solvent ethanol will show that acetone has a higher dipole moment than ethanol’s 1.69 D.
Ethanol does have a higher dielectric constant of 24.55, but the dielectric constants of some nonpolar solvents can be even higher than that of some polar protic solvents.
Ethanol still has a higher polarity than acetone, but acetone has a higher dipole moment because of its carbonyl group—the oxygen to carbon double bond (source).
The aforementioned hydrogen bonds with diatomic nonmetal elements are also much stronger than other dipole-dipole bonds. For example, the hydrogen to oxygen bond produces a difference in electronegativity of 1.24, that of hydrogen and fluorine 1.78, and nitrogen 0.84 (source).
The difference of electronegativity between hydrogen and fluorine is even higher than some ionic bonds, which is what makes hydrogen fluoride such a powerful solvent.
Both alcohols, like ethanol, and carboxylic acids, like acetic acid, contain at least one hydroxyl group (OH). Many common polar protic solvents contain these hydroxyl groups with high bond dipoles.
Acetone and Keto-Enol Tautomerism
Since acetone is a ketone, it is subject to a process known as keto-enol tautomerism. A tautomer is a structural isomer, meaning that it’s a structural arrangement within a subgroup that shares the same chemical formula.
In this case, acetone, (CH3)2CO, is one of the structural isomers (tautomers) of the chemical formula C3H6O with different bond connectivity.
Keto-enol tautomerism involves the ketone acetone, (CH3)2CO, and an enol, (CH3)C(OH)=(CH2), which is an alcohol. During the process, a proton (H+) is transferred from an oxygen atom to a carbon atom.
By donating hydrogen, the molecule is showing temporary protic properties. The double bond between the oxygen and central carbon atom (carbonyl) is swapped for a double bond between two carbon atoms (an alkene).
Diacetone Alcohol and Acetone Peroxide
Two molecules of acetone can form diacetone alcohol through the use of a catalyst. Diacetone alcohol is used in numerous applications as an intermediate for other compounds.
Acetone peroxide is formed by combining acetone and hydrogen peroxide. The peroxide created is used in high explosives.
With its headquarters in the United Kingdom, INEOS Phenol is the world’s largest producer of acetone at 1.2 million tons per year. INEOS Phenol has production centers in both Europe and America with its main supplier in Switzerland (source).
Shell Chemical, Dow Chemical, Westlake Chemical, Sunoco, and Domo Chemical are all involved in the expanding production of acetone thanks largely to the skincare industry.
The production of both Acetone and phenol is closely related with most acetone being produced today through the cumene process.
The cumene process takes the nonpolar solvent benzene and the hydrocarbon propylene to produce the hydrocarbon cumene through the process of alkylation. Cumene is then oxidized by the air to produce acetone and phenol (C6H5OH).
Acetone is a polar aprotic compound with an intermediate level of polarity due to its carbonyl group on the molecular level. Its use as a powerful degreaser, solvent, and in the formation of polymers has led to its large scale industrial production.
The industrial production of acetone now outpaces that of its occurrence in nature, causing some concern over its effects on the environment.
Acetone does contain nonpolar structures such as methyl groups within it, but these do not negate its polarity. These hydrocarbons contribute to its volatility, as acetone is highly flammable. In high enough doses, acetone is toxic, so it should always be handled with care.