The boiling point is the point at which a liquid will turn into a gas. This is realized when the forces of attraction in the liquid break due to the application of heat (Acton, 2012). When the forces of the attraction breaks and the losses, the forces formed will be very weak in which in most cases they form weak van der Waal forces of the gases (forming gaseous substances). The boiling points of a pure substance like for the compound given have a very sharp point and it is affected by a number of factors of the molecular structures. Some of these common factors are the weight, size, bonds and the molecular structure. A compound with a relatively higher molecular weight will have a relatively higher boiling point while those with relatively lower molecular weight will have a relatively lower boiling point (Hat, 2013).
This is because for a compound with a relatively higher molecular structure will require relatively higher heat energy to break the molecular structure. Another important factor is the forces of attraction and the molecular structure of a compound. A compound with a relatively strong forces of attraction like hydrogen forces will require a relatively higher heat to help break these forces thus become loose and forms gaseous substance(Anslyn, 2012). While the compounds with relatively lower bonds and molecular structures like the weak van der Waal forces will require relatively lower heat energy to break the forces of attraction to form a gaseous compound. For the weak held bonds, they require a less heat to break the weakly held bonds to form gaseous compound (Schlaf, 2012).
These four compounds have relatively higher boiling points because all of them have hydrogen bonds. When their boiling points are reached then the hydrogen bonds are broken to form weak van der Waal forces of the respective gases. From the list given of substances 1-pentanol, 1-hexanol, 2-methyl-1-butanol, 3-methyl-2-butanone, their boiling point actually does not reduce as in the given list in the question but they reduce as below (Baev, 2013);
- 1- Hexanol – 1570C
- 1-pentanol -1380C
From the above boiling point it is clear that the boiling point reduces as follows;
1-hexanol, 1-pentanol, 2-methyl-1-butanol, 3-methyl-2-butanone. The decrease arrangement of the boiling point of these four compounds can be substantiated by the molecular formula, molecular mass and the structure of the above compound. The hydrogen bond cannot be applied here to substantiate which compound is having a higher boiling point because all the four are having the hydrogen bond. So for these compounds, the use of the molecular weight and the structure are the best parameters to substantiate for the boiling point arrangement as given above(Stoker, 2015).
1-Hexanol has a molecular weight of 102.17 g/ mol with a molecular formula of CH3(CH2)4CH2OH ( condensed molecular formula), 1- pentanol has a molecular weight of 88.168g/ mol with a molecular formula of C5H12O, 2-Methyl-1-butanol has a molecular weight of 88.148 g/mol with a molecular formula of CH3CH2CH(CH3)CH2OH (condensed molecular structure) while the 3-methyl-2-butanone has a molecular weight of 86.13g /mol having a molecular formula of C5H10O. The molecular weight of the compound fully substantiates the trend of the above boiling point. And the variation in the boiling point is proportional to the molecular weight (Brown, 2014). This can be seen where the variation between 1- Hexanol – 1570C and 1-pentanol -1380C is a relatively large range, this relatively large range is evidenced in the molecular weight where 1- Hexanol is having 102.17g/mol and 1- pentanol having 88.168g /mol.
And in the cases where the discrepancy in the boiling point is very small, the discrepancy in the molecular weight is also small. This is witnessed for the case of 1-pentanol -1380C and Methyl-1-butanol-1290C, the discrepancy in their molecular weight is very small as the compounds have the molecular weight of 88.168 g/ mol and 88.148g/mol. Relatively higher amount of heat energy will be required to break the many bonds of the relatively heavy compound as compared to the heat energy that will be required to break the bonds in the light loaded compound (Dhingra, 2011). Therefore the molecular weight is a good parameter to explain the variation of the boiling point of the four compounds. The diagram below illustrates open molecular structure 1-pentanol;
Fig 1: open structure of1-pentanol
The below shows the open molecular structure of 1- Hexanol
Fig 2: open molecular structure of 1-Hexanol
Fig 4: Showing the molecular formula of 3-methyl-2-butanone.
From the above four diagram, it can be deduced that the size of the compound will as well determine the boiling point of a compound (Hat, 2013). The small size compound will have a relatively lower boiling point because the forces of attractions to be broken are less hence they need relative less heat energy to break these bonds. While the compounds with big molecular sizes have a relatively higher boiling point since there are relatively many bonds to be broken hence relatively much heat energy is needed to break these bonds. From the above four diagrams, 1-Hexanol is having six carbon atoms( the largest), 1-pentanol is having five carbon atoms, Methyl-1-butanol is having four carbon atoms and 3-methyl-2-butanone is having three carbon atoms indicating that it is the smallest of the four compounds(Stoker, 2015).
In conclusion, the boiling point of a compound can be substantiated by the type of the molecules and bonds its forms. The molecular weight of a compound can also help to determine the trend in the variation of the boiling point in the compound as seen in the above explanations. For the organic compounds as in the above discussion the size of a compound (determined by a number of carbon atoms) enables us to understand variation in the boiling point of a different compound as seen above. For this comparison of the boiling point, the type bonds will not be employed since the entire four compounds have the same type of bond (hydrogen bond).
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Anslyn, E., 2012. Organic Chemistry. 6th ed. Hull: Cengage Learning.
Baev, A. K., 2013. The boiling point of the different organic compound. 4th ed. Hull: Springer Science & Business Media.
Brown, W. H., 2014. Boiling points of Organic Chemistry. 4th ed. Leicester: Springer.
Dhingra, S., 2011. Comprehensive Practical Organic Chemistry: Qualitative Analysis. 5th ed. Birmingham: Universities Press.
Hat, J., 2013. Liquid-Liquid InterfacesTheory and Methods. 2nd ed. Manchester: CRC Press.
Schlaf, M., 2012. Reaction Pathways and Mechanisms in Thermocatalytic Biomass Conversion. 3rd ed. Stoke: Springer.
Stoker, H. S., 2015. General, Organic, and boiling points. 2nd ed. London: Cengage Learning.