BUTENE: CHARACTERISTICS, CHEMICAL STRUCTURE AND USES - CHEMISTRY - 2025

The butene is the name given to a series of four isomers with chemical formula C ₄ H ₈. They are alkenes or olefins, that is, they have a C = C double bond in their structure. In addition, they are hydrocarbons, which can be found in oil fields or originated by thermal cracking and obtain products of lower molecular weight.

The four isomers react with oxygen to release heat and a yellow flame. Likewise, they can react with a wide spectrum of small molecules that add to their double bond.

Source: Ben Mills via Wikipedia

But what are the isomers of butene? The upper image represents the structure with white (hydrogens) and black (carbons) spheres for 1-Butene. 1-Butene is the simplest isomer of the C ₄ H ₈ hydrocarbon. Note that there are eight white spheres and four black spheres, which agrees with the chemical formula.

The other three isomers are cis and trans 2-Butene, and iso-Butene. All of them exhibit very similar chemical properties, although their structures cause variations in physical properties (melting and boiling points, densities, etc.). Also, their IR spectra have similar patterns of absorption bands.

Colloquially, 1-Butene is called butene, although 1-Butene refers only to a single isomer and not to a generic name. These four organic compounds are gases, but they can liquefy at high pressures or condense (and even crystallize) with a decrease in temperature.

They are a source of heat and energy, reagents for the synthesis of other organic compounds and, above all, necessary for the manufacture of artificial rubber after the synthesis of butadiene.

Butene characteristics

Molecular weight

56.106 g / mol. This weight is the same for all isomers of the formula C ₄ H ₈.

Physical aspects

It is a colorless and flammable gas (like the other isomers), and has a relatively aromatic odor.

Boiling point

The boiling points for the isomers of butene are as follows:

1-Butene: -6ºC

Cis-2-Butene: 3.7ºC

Trans-2-Butene: 0.96ºC

2-Methylpropene: -6.9ºC

Melting point

1-Butene: -185.3ºC

Cis-2-Butene: -138.9ºC

Trans-2-Butene: -105.5ºC

2-Methylpropene: -140.4ºC

Solubility

Butene is very insoluble in water due to its nonpolar nature. However, it dissolves perfectly in some alcohols, benzene, toluene, and ethers.

Density

0.577 at 25 ° C. Therefore, it is less dense than water and in a container it would be located above it.

Reactivity

Like any alkene, its double bond is susceptible to adding molecules or oxidizing. This makes butene and its isomers reactive. On the other hand, they are flammable substances, which is why they react with the oxygen in the air when overheated.

Chemical structure

The upper image represents the structure of 1-Butene. To its left you can see the location of the double bond between the first and second carbon. The molecule has a linear structure, although the region around the C = C bond is flat due to sp ² hybridization of these carbons.

If the 1-Butene molecule was rotated through an angle of 180º, it would have the same molecule with no apparent changes, therefore it lacks optical activity.

How would its molecules interact? The CH, C = C and CC bonds are apolar in nature, so none of them collaborate in the formation of a dipole moment. Consequently, the CH ₂ = CHCH ₂ CH ₃ molecules must interact via London scattering forces.

The right end of butene forms instantaneous dipoles, which at a short distance polarize the adjacent atoms of a neighboring molecule. For its part, the left end of the C = C bond interacts by superimposing the π clouds one on top of the other (like two wafers or sheets).

Because there are four carbon atoms making up the molecular backbone, their interactions are barely enough for the liquid phase to have a boiling point of -6ºC.

Constitutional and geometric isomers

Source: Gabriel Bolívar

1-Butene has the molecular formula C ₄ H ₈; However, other compounds may have the same proportion of C and H atoms in their structure.

How is it possible? If the structure of 1-Butene is closely observed, the substituents on the C = C carbons can be exchanged. This exchange produces other compounds from the same skeleton. Furthermore, the position of the double bond between C-1 and C-2 can move the C-2 and C-3 CH ₃ CH = CHCH ₃, 2-butene.

In 2-Butene the H atoms can be located on the same side of the double bond, which corresponds to the cis stereoisomer; or in an opposite spatial orientation, in the trans stereoisomer. They both constitute what are also known as geometric isomers. The same applies for the –CH ₃ groups.

Also, note that if the H atoms on one side were left in the CH ₃ CH = CHCH ₃ molecule and the CH ₃ groups on the other, a constitutional isomer would be obtained: CH ₂ = C (CH ₃) ₂, 2 -Methylpropene (also known as iso-butene).

These four compounds have the same formula C ₄ H ₈ but different structures. 1-Butene and 2-Methylpropene are constitutional isomers; and cis and trans-2-Butene, geometric isomers between them (and constitutional with respect to the rest).

Stability

Heat of combustion

From the image above, which of the four isomers represents the most stable structure? The answer can be found, for example, in the heats of combustion of each of them. When reacting with oxygen, the isomer with formula C ₄ H ₈ is transformed into CO ₂ releasing water and heat:

C ₄ H ₈ (g) + 6O ₂ (g) => 4CO ₂ (g) + 4H ₂ O (g)

Combustion is exothermic, so the more heat is released, the more unstable the hydrocarbon. Therefore, the one of the four isomers that releases the least heat when burning in air will be the most stable.

The heats of combustion for the four isomers are:

-1-Butene: 2717 kJ / mol

-cis-2-Butene: 2710 kJ / mol

-trans-2-Butene: 2707 kJ / mol

-2-Methylpropene: 2700 kJ / mol

Note that 2-Methylpropene is the isomer that gives off the least heat. While 1-Butene is the one that releases more heat, which translates into greater instability.

Steric and electronic effect

This difference in stability between the isomers can be deduced directly from the chemical structure. According to the alkenes, the one that has more R substituents acquires greater stabilization of its double bond. Thus, 1-Butene is the most unstable because it hardly has a substituent (-CH ₂ CH ₃); that is, it is monosubstituted (RHC = CH ₂).

The cis and trans isomers of 2-Butene differ in energy due to the Van der Wall stress caused by the steric effect. In the cis isomer, the two CH ₃ groups on the same side of the double bond repel each other, while in the trans isomer, they are far enough away from each other.

But why then is 2-Methylpropene the most stable isomer? Because the electronic effect intercedes.

In this case, although it is a disubstituted alkene, the two CH ₃ groups are on the same carbon; in geminal position relative to each other. These groups stabilize the carbon of the double bond by giving it part of its electron cloud (since it is relatively more acidic due to having sp ² hybridization).

Furthermore, in 2-Butene its two isomers only have 2º carbons; while 2-Methylpropene contains a 3rd carbon, with greater electronic stability.

Intermolecular forces

The stability of the four isomers follows a logical order, but the intermolecular forces do not. If their melting and boiling points are compared, it will be found that they do not obey the same order.

It would be expected that trans-2-Butene would present the highest intermolecular forces due to having greater surface contact between two molecules, unlike cis-2-Butene, whose skeleton draws a C. However, cis-2-Butene boils at higher temperature (3.7ºC), than the trans isomer (0.96ºC).

Similar boiling points for 1-Butene and 2-Methylpropene would be expected because structurally they are very similar. However, in the solid state the difference changes radically. 1-Butene melts at -185.3ºC, while 2-Methylpropene at -140.4ºC.

In addition, the cis-2-Butene isomer melts at -138.9ºC, at a temperature very close to 2-Methylpropenom, which may mean that in the solid they present an equally stable arrangement.

From these data it can be concluded that, despite knowing the most stable structures, they do not shed enough light on the knowledge of how intermolecular forces operate in the liquid; and even more, in the solid phase of these isomers.

Applications

-Butenes, given their heat of combustion, can simply be used as a source of heat or fuel. Thus, the 1-Butene flame would be expected to heat up more than that of the other isomers.

-Can be used as organic solvents.

-Serve as additives to raise the octane level of gasoline.

-Within organic synthesis, 1-Butene participates in the production of other compounds such as: butylene oxide, 2-glutanol, succinimide and tert-butylmecaptan (used to give cooking gas its characteristic smell). Likewise, butadiene (CH ₂ = CH-CH = CH ₂) can be obtained from butene isomers, from which artificial rubber is synthesized.

Beyond these syntheses, the diversity of the products will depend on which molecules are added to the double bond. For example, alkyl halides can be synthesized if they are reacted with halogens; alcohols, if they add water in an acid medium; and tert-butyl esters if they add low molecular weight alcohols (such as methanol).

References

1. Francis A. Carey. Organic Chemistry. Carboxylic acids. (sixth ed., pages 863-866). Mc Graw Hill.
2. Wikipedia. (2018). Butene Taken from: es.wikipedia.org
3. YPF. (July 2017). Buten us.. Taken from: ypf.com
4. William Reusch. (May 5, 2013). Addition Reactions of Alkenes. Recovered from: 2.chemistry.msu.edu
5. PubChem. (2018). 1-Butene. Recovered from: pubchem.ncbi.nlm.nih.gov