EC2 Essay PT 2 PDF

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Essential Chemistry Essay PT 2 on Boiling Points including 1-Hexanol
C6H14O, 2-Hexanol C6H14O, Hexanal C6H12O and Pentanal C5H10O...

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The boiling point is the temperature taken when a substance turns from a liquid into a gas. It occurs when the vapour pressure of the liquid is equal to the atmospheric pressure (Dai et. al., 2013). Boiling points vary according to the strength of molecular force. The more these molecular forces coalesce, the more energy is needed to convert them into a gaseous state (Cunningham et al. 2018). The heat being added to the substance increases the temperature gaining kinetic energy which increases the molecular motion. The interactive forces between molecules hold the chemical structure together and this is called intermolecular forces. The intermolecular forces at work require more heat and energy to break the molecular chain and it is this which determines the varying boiling points for differing compounds. The larger the attraction or repulsion between molecules means a higher boiling point to separate the bonding between compounds. As the temperature increases, the molecular motion intensifies and the forces of attraction disrupt the molecules breaking them free from the liquid and form into vapour. Generally, boiling point is determined by the strength of the molecular force, the size of the carbon chain and the weight of the surface area. Name Molecular Formula 1-Hexanol C6H14O

and Molecular Structure

Boiling Point

Molecular Weight

156.9°C

102.174 g/mol

2-Hexanol C6H14O

138°C

102.174 g/mol

Hexanal C6H12O

129.6°C

100.158 g/mol

Pentanal C5H10O

103°C

86.132 g/mol

(Lide 2010) There are four types of intermolecular forces associated with chemical bonding. The attraction between opposite charges and repulsion between like charges plays a key role on the temperature required at boiling point (Abramowitz & Yalkowsky 1990). This is because the chemical bonds polarise, making the atoms experience a greater charge through greater molecular force, therefore needing a higher boiling point. The four intermolecular forces from strongest to weakest are ionic forces, hydrogen bonding, Van Der Waals dipole-dipole interactions and Van Der Waals dispersion forces. Ionic forces hold the strongest attraction between point charges and use the greatest amount of energy to dissolve ionic bonds to convert the molecular force into a gas. Hydrogen bonding is an attraction between partial point charges which are polarised with electronegative atoms. This electronegativity requires more energy to break chemical bonds to convert the liquid into a gas but doesn’t require as much energy as ionic forces. Van Der Waals Dipole-dipole interactions is weaker than hydrogen bonding because the electronegativity force is not as strong, the partial charges are reduced in size and weaker to break down, therefore requiring a lower boiling point (Buckingham, Fowler & Hutson 1988). Finally, Van Der Waals dispersion forces or London forces are the weakest in the intermolecular forces because their dipoles are generally weak and do not polarise requiring less energy to convert into gas.

Table 1 demonstrates that 1-Hexanol has the highest boiling point in comparison to the other three molecules. In order to justify this result it is important to look at the intermolecular forces that occur during boiling. Firstly, 1-Hexanol is an alcohol that has a six carbon straight chain containing a hydroxyl group at the end of the chain. It is held together by hydrogen bonding and this is the strongest type of bonding in all of the molecules (Prahlada Rao & Sunkada 2007). Although 2-Hexanol also has a six carbon chain it has a lower boiling point because the hydroxyl group branches off the second carbon, therefore, requiring less energy to convert the liquid into a gas because the hydrogen bonding is weakened due to the sphere like branching which causes dispersion. The sphere has a lower surface area and for this reason requires a lower boiling point than 1-Hexanol (Cunningham et al. 2018). Even though they share the same molecular weight of 102.174g/mol, the position of the alcohol group impacts the electronegativity because the partial charges are smaller in 2-Hexanol meaning the intermolecular force is weaker which results in less boiling time. Alcohols have hydrogen bonds present requiring higher boiling times. Both these alcohols have polar molecules and have Van Der Waals dipole dipole force present but because of the presence of oxygen it causes a much stronger bond and it has Hydrogen bonded to a small atom where the electronegativity difference will be very large and dispersion will take longer (Stone 2013). For this reason, a higher boiling point is required than Hexanal and Pentanal.

In general, aldehydes contain a lower boiling point than alcohols. Aldehydes belong to the carbonyl group and consist of a carbon and an oxygen atom linked by a double bond. Like the alcohol group, it is capable of polarizing because the C=O bond makes it a hydrogen bond acceptor but it does not contain a hydrogen acid meaning it cannot form hydrogen bonds making it a weaker chain which requires less boiling time. The electronegativity in aldehydes occurs when the forces of attraction are grouped around the oxygen atom not the carbon. Both Hexanal and Pentanal use London forces and Van Der Waals dipole dipole forces and the polarity of this molecule is in the middle rather than at the end as found in 1-Hexanol, but still acts in the same manner. The Van Der Waal dipole dipole force is stronger than the dispersion force because of the electronegativity of the oxygen requiring more kinetic energy to break down the molar as it forms a partially charged dipole. Dispersion forces usually increase with the size and weight of the molecular mass (Perkins 1995). Hexanal is a six carbon straight chain aldehyde and boils at 129 °C weighing 100.16 g/mol, whereas, Pentanal is a five carbon straight chain aldehyde boiling at 103°C and weighs 810 kg/m³, this difference in size and weight determines the difference in boiling point temperature (Lide, 2010). More energy is required to force the separation from the longer carbon chain in Hexanal and its molecular mass is larger than Pentanal, because the surface area is larger, it also requires more energy to disperse the atoms and turn the liquid into a gas. Therefore, Hexanal has a higher boiling temperature than Pentanal. In conclusion, the more kinetic energy required to disperse molecules in motion determines the boiling point needed to turn a liquid into a gas. It is also important to note that 1-Hexanol has the highest boiling point out of all the molecules because it has a six carbon straight chain with a hydroxyl group at the end and uses hydrogen bonding to force the molecular breakdown during the boiling process. The two aldehydes, Hexanal and Pentanal, require less energy to disperse because they lack a hydroxyl group.

Bibliography Abramowitz, R. & Yalkowsky, S.H. 1990, "Melting point, boiling point, and symmetry", Pharmaceutical research, vol. 7, no. 9, pp. 942. Buckingham, A, Fowler, P & Hutson, J 1988, "Theoretical studies of van der Waals molecules and intermolecular forces", Chemical Reviews, vol. 88, no. 6, pp. 963-988. Cunningham, W, Xia, I, Wickline, K, Garcia Huitron, E & Heo, J 2018, "Studying Intermolecular Forces with a Dual Gas Chromatography and Boiling Point Investigation", Journal of Chemical Education, vol. 95, no. 2, p. 301. Dai, Y., Zhu, Z., Cao, Z., Zhang, Y., Zeng, J. & Li, X. 2013, "Prediction of boiling points of organic compounds by QSPR tools", Journal of Molecular Graphics and Modelling, vol. 44, pp. 113-119. Lide, D (ed.), 2010, CRC Handbook of Chemistry and Physics, 90th edition (Internet Version). 90th edn. CRC Press/Taylor and Francis, Boca Raton USA. Physical Constants of Organic Compounds. Perkins, R 1995, "Put the Body to Them!", Journal of Chemical Education, vol. 72, no. 2, p. 151. Prahlada Rao, S & Sunkada, S 2007, "Making sense of boiling points and melting points", Resonance, vol. 12, no. 6, pp. 43-57. Stone, A 2013, The theory of intermolecular forces 2nd edn, Clarendon Press, Oxford....