The pentanone is an organic compound, belonging to the ketone family, whose chemical formula is C5H10O. It is still one of the simplest ketones, and consists of a colorless liquid with an aroma similar to acetone. From there on, with respect to the carbon number of their skeleton, ketones show position 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..
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..
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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..
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 is a methyl radical, CH3-, and to his right, a propyl radical. This 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 (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, CH3CHtwo-. This is why 3-pentanone is also called diethyl ketone (or dimethyl acetone).
The presence of the carbonyl group, both in 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 density of negative charge, δ-, seen in 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.
Probably symmetries do 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.
Colorless liquids with an aroma similar to acetone.
86.13 g / mol
0.81 g / mL (3-pentanone)
0.809 g / mL (2-pentanone)
-78 ºC (2-pentanone)
-39 ºC (3-pentanone)
102 ºC (for both isomers)
35 g / L (3-pentanone)
60 g / L (2-pentanone)
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..
12.78 ° C (3-pentanone)
10 ºC (2-pentanone)
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..
Although the two isomers of pentanone are physically and chemically very similar, their synthetic routes are independent of each other, so it is unlikely that both are obtained in the same synthesis.
Starting from the reagent 2-pentanol, a secondary alcohol, it can be oxidized to 2-pentanone using hypochlorous acid formed in situ, HClO, as an oxidizing agent. The oxidation would be:
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 can be synthesized by ketonic decarboxylation of propanoic acid, CH3CHtwoCOOH. This synthetic route produces symmetric ketones, and consists of a molecule of CH3COOH will lose the COOH group as COtwo, while the other its OH group in the form of HtwoOR:
2CH3CHtwoCOOH => CH3CHtwoCartwoCH3 + COtwo + HtwoOR
Another slightly more complex route is to directly react a mixture of ethylene, CHtwo= CHtwo, COtwo and Htwo to form 3-pentanone.
Unless the application is very rigorous, it is a specific synthesis, or it has also been shown that the two isomers do not produce 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.
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