PENTANON: CHEMICAL STRUCTURE, PROPERTIES, SYNTHESIS, USES - CHEMISTRY - 2024

The pentanone is an organic compound belonging to the family of ketones, the chemical formula C ₅ H ₁₀ O. remains one of the most simple ketones, and is a colorless liquid with an aroma similar to that of acetone. From this point on, with respect to the carbon number of their skeleton, ketones show positional isomerism.

When talking about position isomerism, it refers to where the carbonyl group is, C = O. Thus, pentanone actually consists of two compounds with physical properties that differ slightly from each other: 2-pentanone and 3-pentanone (bottom image). Both are liquid, but their boiling points are the same, so they cannot be separated by distillation.

Molecules of the positional isomers for pentanone. Source: Jynto & Ben Mills via Wikipedia.

However, each of the two pentanones has its own synthesis route, so it is unlikely that there is a mixture of them; mixture that may be feasible to separate by freezing.

Their uses are especially restricted as organic solvents; alternatives to acetone in environments where it is used at a temperature higher than its low boiling point. Likewise, they are starting materials for other organic syntheses, since their C = O group can be reduced or undergo nucleophilic addition.

Chemical structure

Position isomers

In the image we have the molecular structures of the two positional isomers for pentanone, represented by a model of spheres and bars. The black spheres correspond to the carbon skeletons, the white ones to the hydrogen atoms, and the red ones to the oxygen atoms of the C = O group.

2-pentanone

The 2-pentanone (above) has the group C = O in the second carbon, which is equivalent to that if it were in the fourth; that is, 2-pentanone and 4-pentanone are equivalent, since it is enough to rotate them 180º so that their structures are the same.

To the left of C = O there is a methyl radical, CH ₃ -, and to its right, a propyl radical. That is why this ketone is also called (less commonly used) methyl propyl ketone (MPK). You can see the asymmetry of this structure at first glance.

3-pentanone

3-pentanone (bottom) has the group C = O on the third carbon. Note that this isomer is a larger version of acetone, and that it is also very symmetric; the two radicals on the sides of the C = O group are the same: two ethyl radicals, CH ₃ CH ₂ -. This is why 3-pentanone is also called diethyl ketone (or dimethyl acetone).

Intermolecular interactions

The presence of the carbonyl group, in both 2-pentanone and 3-pentanone, confers polarity to the molecule.

Therefore, both isomers of pentanone are capable of interacting with each other through dipole-dipole forces, in which the highest negative charge density, δ-, seen on an electrostatic potential map, will be oriented around the oxygen atom.

Likewise, the London dispersion forces intervene to a lesser degree between the carbonized and hydrogenated skeletons.

As pentanone has a larger molecular area and mass compared to acetone, this contribution adds to its dipole moment and allows it to have a higher boiling point (102 ºC> 56 ºC). In fact, the two isomers vaporize at 102 ° C; while freezing at different temperatures.

Crystals

Symmetries probably play an important role in crystallization. For example, the melting point of 2-pentanone is -78 ° C, while that of 3-pentanone, -39 ° C.

This means that 3-pentanone crystals are more stable against temperature; its molecules are more cohesive and require more heat to separate in the liquid phase.

If there were a mixture of both isomers, the 3-pentanone would be the first to crystallize, the 2-pentanone being last at a lower temperature. The more symmetrical 3-pentanone molecules orient themselves more easily than those of 2-pentanone: asymmetric and at the same time more dynamic.

Properties

Physical appearance

Colorless liquids with an aroma similar to acetone.

Molar mass

86.13 g / mol

Density

0.81 g / mL (3-pentanone)

0.809 g / mL (2-pentanone)

Melting point

-78 ºC (2-pentanone)

-39 ºC (3-pentanone)

Boiling point

102 ºC (for both isomers)

Water solubility

35 g / L (3-pentanone)

60 g / L (2-pentanone)

Vapor pressure

35 mmHg (3-pentanone)

270 mmHg (2-pentanone) at 20 ° C

These values ​​are not useful to directly compare the vapor pressures of both isomers, since the first one is reported without the temperature at which it was measured.

Flash point

12.78 ° C (3-pentanone)

10 ºC (2-pentanone)

Stability

The two isomers of pentanone are extremely flammable, so they should be kept as far away from any heat source as possible. The flash points indicate that a flame can ignite the vapors of both pentanones even at a temperature of 10ºC.

Synthesis

Although the two isomers of pentanone are physically and chemically very similar, their synthetic pathways are independent of each other, so it is unlikely that both are obtained in the same synthesis.

2-pentanone

Starting from the reagent 2-pentanol, a secondary alcohol, this can be oxidized to 2-pentanone using hypochlorous acid formed in situ, HClO, as an oxidizing agent. The oxidation would be:

Oxidation of 2-pentanol to 2-pentanone. Source: Gabriel Bolívar.

Simply the C-OH bond becomes C = O. There are other methods to synthesize 2-pentanone, but of all this is the easiest to understand.

3-pentanone

3-pentanone can be synthesized by ketonic decarboxylation of propanoic acid, CH ₃ CH ₂ COOH. This synthetic route produces symmetric ketones, and consists in that one molecule of CH ₃ COOH will lose the COOH group in the form of CO ₂, while the other its OH group in the form of H ₂ O:

2CH ₃ CH ₂ COOH => CH ₃ CH ₂ COCH ₂ CH ₃ + CO ₂ + H ₂ O

Another slightly more complex route is to directly react a mixture of ethylene, CH ₂ = CH ₂, CO ₂ and H ₂ to form 3-pentanone.

Applications

Unless the application is very rigorous, it is a specific synthesis, or it has also been shown that the two isomers do not give rise to the same results, it is irrelevant to choose between using 2-pentanone or 3-pentanone in any of their uses.

In other words, it "does not matter" if you want to use 2-pentanone or 3-pentanone as a solvent for paints or resins, or as a degreaser, since the most important criteria will be its price and availability. 3-pentanone is used in the automotive industry to study the behavior of gases within internal combustion engines.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
4. Wikipedia. (2019). 3-Pentanone. Recovered from: en.wikipedia.org
5. National Center for Biotechnology Information. (2019). 2-Pentanone. PubChem Database. CID = 7895. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Royal Society of Chemistry. (2019). 3-Pentanone. Recovered from: chemspider.com
7. Professor T. Nalli. (2013). Oxidation of 2-Pentanol and Distillation of the Product. Recovered from: course1.winona.edu