Role of Intermolecular Forces on Compound Boiling Points

The Boiling Point of a compound is defined as the temperature at which a substance passes through the phase change from liquid to gas at a particular atmospheric pressure. There are several factors that affect the boiling point of a compound, these can range from simple components such as the total atomic mass of the compound to more complex features such as the strength of the intermolecular forces that are the forces that mediate forces between molecules. A key example of boiling point is the boiling point of Water (H₂O) being 100 degrees celsius, this is higher than the boiling point of Ethanol (C₂H₆O) being 78.15 degrees Celsius due to the intermolecular forces of Water, specifically hydrogen bonding, being stronger than the intermolecular forces within ethanol. Structure of a compound will also affect the boiling point of said compound. This is due to certain structural properties causing for stronger bonds between the atoms causing for more force to be required to cause a change in the molecular structure of the atom. In this essay the intermolecular forces of four compounds will be analysed and their effect on the boiling points of said compounds will be analysed. To begin we will discuss the types of intermolecular forces beginning with the most basic dispersion forces. Dispersion forces or van der waals forces as previously mentioned are the most basic intermolecular forces. Dispersion forces include attraction and repulsion between atoms and compounds.Usually compounds with only dispersion forces have relatively low boiling points and are unable to form significant bonds with other molecules causing them to break apart quite easily thus explaining why they have such low boiling points. The next intermolecular force is Dipole-Dipole forces, these forces also are relatively weak but not as weak as dispersion forces. While dispersion forces are the most basic interactions between molecules dipole dipole forces are more complex than that taking into account the polarity of different molecules, with dipole dipole forces between the attraction between the positive end of one molecule and the negative end of another molecule. So while simple they do take into account more factors than Dispersion forces. Finally there’s Hydrogen bonds, the strongest of the 3 intermolecular forces. Hydrogen bonds are the electromagnetic attraction created between a partially, positively charged Hydrogen atom and another nearby electronegative atom. The electromagnetic attraction proves difficult to be broken thus is why Hydrogen bonds are the strongest intermolecular force and why compounds with hydrogen bonds will therefore have the highest boiling point. There are some factors that affect the strength of the intermolecular forces but do not directly affect the boiling point. For example the shape of a compound can determine the strength of the intermolecular force due to the placement of certain molecules which can increase or decrease the strength of said intermolecular forces.

Detailed Below are the compounds 1-Pentanol, 3-Methyl-2-Butanone, 1-Hexanol and 2-Pentanol, this table displays their respective formulas, structures and boiling points, these values were sourced from Merck Index, 2019.

Table 1: Boiling Point and Structure of molecules

While the first two compounds have similar molecular formulas their structures and boiling points differ in one very simple and clear cut manner. Although they only differ with 2 hydrogen atoms the reason why Pentanol has a significantly higher boiling point is due to an alcohol having the ability to form hydrogen bonds which 3-Methyl-2-Butanone is unable to do only being able to form dipole dipole bonds. Due to the 3 alcohols all possessing hydrogen bonding they all possess boiling points greater than 100 degrees celsius however the difference in boiling points with 1-Pentanol and 2-Pentanol is noticeable. While the two atoms have the same formula 1-Pentanol has a substantially higher boiling point than that of 2-Pentanol. The reason for this is the shape of 2-Pentanol is less ideal for the intermolecular forces, in this case hydrogen bonds, of the molecule thus causing for the intermolecular forces to be slightly weakened which causes a decrease in the boiling point of 2-Pentanol. 1-Hexanol clearly has the highest boiling point and this is simply due to the fact that it has more intermolecular forces due to it being made up of more molecules which therefore causes it to have a higher boiling point than the other substances detailed, this along with the ideal shape of the molecule is why it has the highest boiling point of the four substances. The substantial difference between 1-Hexanol and 3-Methyl-2-Butanone, the highest and lowest boiling points respectively, are clear evidence as to the effects of intermolecular forces on the boiling points of compounds. With stronger attractions requiring more energy to seperate causing for a higher amount of energy to cause a phase change from liquid to gas.

In conclusion Intermolecular forces play a major role in the boiling points of compounds. This is due to the factors that involve the attraction of compounds forming together

Bibliography

• Rsc.org. (2019). Methyl Isopropyl Ketone | The Merck Index Online. [online] Available at: https://www.rsc.org/Merck-Index/monograph/m7432/methyl%20isopropyl%20ketone?q=authorize [Accessed 23 Aug. 2019].
• Rsc.org. (2019). 1-Pentanol | The Merck Index Online. [online] Available at: https://www.rsc.org/Merck-Index/monograph/m8500/pentanol?q=authorize
• Wessel, M. and Jurs, P. (1995). Prediction of Normal Boiling Points of Hydrocarbons from Molecular Structure. Journal of Chemical Information and Modeling, 35(1), pp.68-76.

         Rsc.org. (2019). 1-Hexanol | The Merck Index Online . [online] Available at: https://www.rsc.org/Merck-Index/monograph/m6004/hexanol?q=authorize

         Rsc.org. (2019). 2-Pentanol | The Merck Index Online . [online] Available at: https://www.rsc.org/Merck-Index/monograph/m8501/pentanol?q=authorize

• Pocius, A. (2012). Adhesion and Adhesives Technology. 3rd ed. Hanser Publishers, pp.88-90.