Boiling Points Of Given Compounds And The Factors Affecting Them

Write a logical, concise essay that scientifically answers the following:

What are the literature boiling points of the four compounds assigned to you. Thus order the boiling points in decreasing order (highest to lowest) of your compounds.

Using background theory on boiling points and the factors of molecular structure which affect boiling points, justify the order of boiling points for your set of molecules.

Your answer should include the relevant background theory and clear, logically presented reasoning.

Your compounds are 2-pentanone, butanoic acid, 2-methylpropanoic acid, 2-hexanone 

The Theory of Boiling Point of a Compound

A compound is identified by either its physical properties or chemical properties. Physical properties of a subsistence are those properties which can be measured or observed without changing the matter composition, such as the polarity, solubility, density, melting and boiling point, odor, color, texture, appearance, etc. In this essay, boiling points of inorganic given compounds are discussed. The boiling point of a substance is affected by the relative strength of the intermolecular forces, the length of carbon-carbon chain and polarity (OLABS, 2018). In this essay, it discusses the literature boiling points of the given four compounds which includes 2-pentanone, butanoic acid, 2-methylpropanoic acid, and 2-hexanone. Using theory of boiling point the four compounds are listed from the highest boiling point to lowest.

The process by which a liquid substances changes into gaseous state is called the boiling point of the substance. Consequently, the liquid changes into gaseous by a process of known as evaporation. Through evaporation process, the liquid substances transformed vapor escapes to the surrounding (Vogel, 2013). Liquid substance changes its state to gaseous state when heat is supplied, causing the temperature increase resulting to increase in kinetic energy in the molecule. The kinetic energy (KE) as a result increases the random motion of the molecules. Because of higher intensity, the supplied heat energy weakens the attraction force in liquid molecules which in turn becomes free and mobile thus changing its state. Environmental atmospheric pressure affects the boiling point of a given substance (Burrows, Holman, Parsons, Pilling, & Price, 2017).

Inorganic compounds have different boiling points depending on their physical structure such as the molecular weight, polarity etc. The compound usual boiling point indicate the volatility of the given compound. If the boiling point of a compound is low that means the volatile of that compound is high, and vice versa (Mirzaei, Leonardi, & Neri, 2016). In case, the boiling point of a substance is lower than the temperature of surrounding, then the substance normally exist as a gas at atmospheric pressure (OLABS, 2018). Furthermore, the compound appear as liquid or solid if the boiling point is higher (James, 2010). The general trends affecting the boiling point are:

The relative strength of intermolecular forces like Vander Waals dispersion, dipole-dipole interaction, hydrogen bonding, and ionic force do affect the boiling point of a compound. The functional group present in the compound does influence these intermolecular forces(Haynes, 2014). The Vander Waals dispersion is the weakest force of attraction while ionic forces are the strongest forces of attraction (Haynes, 2014).

The Length of carbon chain affect the boiling point in that as the bigger the length of carbon-carbon chain the higher the boiling point of that compound. The reason behind is because the force of attractions in the molecules do increase as the molecules become longer and has more electrons (burrows, et al, 2017).

The polarity of a given molecule affect the boiling point of that substance, the less the polarity the lower the boiling point. The functional group of a compound determines the polarity of that substance(Dalmaschio, et al., 2014).

Results and Discussion

Branching in molecules reduces the surface area thus reducing the forces of attraction between individual molecules, resulting to decrease the boiling point. As Van der Waals forces increases with increase in surface area since as the length of carbon chain increases, the compound increases surface area(Yaws, 2015). Thus, the power for individual molecules which draw to each other increases. More carbon-carbon chains in a molecule decreases the surface area henceforth reduces the power of force of attraction between each molecule resulting to a decrease in boiling point (Braude & Nachod, 2013).

Table 1 below shows the physical properties of the given four (4) compounds which includes 2-pentanone, 2-methylpropanoic acid, butanoic acid, and 2-hexanone, it indicates the boiling point of each compound from highest to lowest boiling point.

Table 1: Physical properties of the four (4) given compounds (PubChem, 2005-2018)

Comparing the boiling point of the two acids (Butanoic acid and 2-methylpropanoic acid). Though, they have the same Topological Polar Surface Area (37.3 A²), same element composition and both have strong hydro-gen bonds (Table 1), Butanoic acid has a high boiling point than 2-methylpropanoic acid. This can be illustrated by the carbon chain in functional group in these compounds, Butanoic acid has four carbon chain while 2-methylpropanoic acid has three (3) longest carbon chain,(as shown in figure 1) thus the longer the Carbon chain the higher the boiling point. Furtherer more, 2-methylpropanoic acid contain a branch, thus reducing the boiling point.  The two reasons explain why Butanoic acid has a higher BP than 2-methylpropanoic acid.

Figure 1: Structure of 2-methylpropanoic acid Butanoic acid

Now, comparing the acids and the ketones, the two acids has higher boiling point that the two ketones. Though, all are volatile and polar, the ketones has small surface area as compared to acids, thus the smaller the surface ares the lower the boiling point. Also, ketones are bonded by dipole-dipole forces and acids are bonded by strong hydrogen bonds. Hydogen bonds are stronger than the dipole-dipole bond, resulting acids to have higher boiling poits that ketones.

Both ketones (2-hexanone and 2-pentanone) have the same Topological Polar Surface Area (17.1 A²), same intermolecular forces and are both polar, their difference in boiling point is brought by the carbon chain in functional group and the branching similar to acids.

Conclusions

In this essay, the trends that affect the boiling point of a substance was done by comparing the boiling point of different compounds. It was discovered that the boiling point of a compound depends numerous factors such as the length of carbon-carbon chain in the functional group, strength of intermolecular forces, polarity and the branching of a compound.

References

Braude, E., & Nachod, F. (2013). Determination of organic structures by physical methods. Elsevier.

Burrows, A., Holman, J., Parsons, A., Pilling, G., & Price, G. (2017). Chemistry3: Introducing inorganic, organic and physical chemistry (2nd ed.). Oxford: Oxford University Press.

Dalmaschio, G., Malacarne, M., de Almeida, V., Pereira, T., Gomes, A., de Castro, E., . . . Romão, W. (2014). Characterization of polar compounds in a true boiling point distillation system using electrospray ionization FT-ICR mass spectrometry. Fuel, 115, 190-202.

Haynes, W. (2014). CRC handbook of chemistry and physics. CRC press.

James. (2010, Octomber 25). 3 Trends That Affect Boiling Points. Retrieved from Master Organic Chemistry: https://www.masterorganicchemistry.com/2010/10/25/3-trends-that-affect-boiling-points/

Mirzaei, A., Leonardi, S., & Neri, G. (2016). Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors. A review. Ceramics International, 42(14), pp. 15119-15141.

OLABS. (2018). Boiling Point of an Organic Compound. Retrieved from Olabs: https://amrita.olabs.edu.in/?brch=7&cnt=1&sim=111&sub=73

PubChem. (2005-2018). Data deposited in or computed by PubChem. Retrieved from PubChem.com: https://pubchem.ncbi.nlm.nih.gov

Vogel, A. (2013). A text-book of practical organic chemistry including qualitative organic analysis . London; New York; Toronto;: Longmans Green And Co.

Yaws, C. (2015). Yaws handbook of physical properties for hydrocarbons and chemicals. Elsevier Science.