Essential Chemistry 2 PDF

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Boiling point is closely associated with intermolecular force of attraction (Peckham and McNaught, 2012), when temperature increase causes an increase in molecular motion (Bettelheim, Brown and March 2001; Spencer, Bodner and Rickard, 2012), until the molecular motion reach a point where molecules acquired enough kinetic energy through heating (Bettelheim, Brown and March 2001), the intermolecular attraction between molecules are overcome by thermal energy where molecules are turned form liquid state into gaseous form (Cooper, Williams and Underwood, 2015). Intermolecular force is force of attraction between atoms and ions with in the molecules (Cooper, Williams and Underwood, 2015). Three types of intermolecular forces include; Hydrogen bonding which is the strongest types of intermolecular force, where the Interaction of a hydrogen atom creating a bond with another electronegative atom e.g. oxygen in a different molecule leading to high molecular force. (Peckham and McNaught, 2012). The second largest intermolecular force, dipole-dipole is where one negative charged part of molecule forms attraction with a positive charged part of another molecule, forms polar bond between the molecule that creates dipole movement leading to dipole-dipole (Cooper, Williams and Underwood, 2015). However, the electronegativity between carbon and oxygen atoms are less than the direct hydrogen bonding (Spencer, Bodner and Rickard, 2012; Bettelheim, Brown and March 2001). Dispersion force forms temporary dipole in most polar and non- polar molecules, where boiling point and force acting between the molecule is determined by the number of electrons present in the molecule, thus makes dispersion force having the weakest intermolecular force (Peckham and McNaught, 2012; Stone, 2013). The stronger force of attraction between molecules the more energy is required to break the bond, e.g. thermal energy released in boiling (Bettelheim, Brown and March 2001). However, when molecules

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are present with the same intermolecular force, the molecular structure can play an important role in determining the boiling point of the molecule, by observing the structural composition and the functional group present in the molecule (Murphy, 2007).

Structure and boiling point of assigned molecules Table 1: Compound name/ boiling point

Molecular structure

Butanoic acid 163.7 C (CRC Handbook of Chemistry and Physics, 2017)

O || CH3 – CH2 – CH2 – C – OH

Haxanal 129.6C (CRC Handbook of Chemistry and Physics, 2017)

O || CH3 – CH2 – CH2 – CH2 – CH2 – C – H

Pentanal 103 C (CRC Handbook of

O

Chemistry and Physics, 2017)

|| CH3 – CH2 – CH2 – CH2 – C – H

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3-methylbutanal 92 C – 93 C (The

CH3

Merck Index, 2017)

|

O ||

CH3 – CH – CH2 – C – H

Hexanal with the second highest boiling point of 129.6 C (CRC Handbook of Chemistry and Physics, 2017), a polar carbonyl group which forms carbon oxygen double bond and single hydrogen bond on the last carbon, leading to the aldehyde functional group that exhibits strong dipole- dipole movement and dispersion force which connects the aldehyde molecules together (Peckham and McNaught, 2012). Aldehydes with a longer the carbon chain, have more electrons present to create positive force of attraction between molecules, to raise the boiling point (Farrell et al., 2010). Therefore, hexanal have higher boiling point compare to pentanal and 3-metylbutanal which also contains the aldehyde functional group. (Peckham and McNaught, 2012). Despite that hexanal have six carbons, the most amount of carbon atoms and electrons, which creates the strongest dispersion force (Farrell et al., 2010), it cannot form hydrogen bond due to that it only has hydrogen acceptor not hydrogen donator and the aldehyde group (Stone, 2013). Butanoic acid contain carboxylic acid with polar OH bond, and carbonyl and hydroxyl group which forms relatively weak dispersion

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force due to that it only contains four carbon atoms (Bettelheim, Brown and March 2001). However, it exhibits strong dipole- dipole and hydrogen bond (Bettelheim, Brown and March 2001). When the hydrogen bond between an oxygen and a hydrogen atom in carboxylic acid increase the force of attraction and gains more energy when boiling this increases the boiling point (Stone, 2013). Therefore, under the condition where both compound are polar and have a double bonded oxygen, have dispersion, dipole- dipole association (Prigogine, 1958), only butanoic acid can form hydrogen bonding due to its carboxylic acid group and OH bond (Stone, 2013). Despite the temporary dipole in dispersion force and opposite attraction present in dipole-dipole, hydrogen bond has stronger intermolecular force due to its intermolecular force between positive and negative ends of molecule (Cooper, Williams and Underwood, 2015; Spencer, Bodner and Rickard 2012). Consequently, the force of attraction between molecules is stronger for butanoic acid compared to hexanal (Bettelheim, Brown and March 2001). Therefore, higher thermal and kinetic energy is required to break greater attraction force between molecules (Peckham and McNaught, 2012). Hence, hexanal 129.6C have lower boiling point compare to butanoic acid 163.7C (CRC Handbook of Chemistry and Physics 2017; Shemesh, Lan and Gerber, 2013; Farrell et al, 2010).

Pentanal contains polar bond with partially charged positive and negative ions, form polar bonding (Spencer, Bodner and Rickard 2012), it is an aldehyde with a hydrogen atom attached to the carbonyl group forming dipole-dipole force as well as dispersion force (Shemesh, Lan and Gerber, 2013), its intermolecular force is respectively lower than butanoic acid and hexanal due to difference in functional group, molecular size and shape

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(Stone, 2013). Therefore, requires less thermal energy input to break the bonds between the molecule (Prigogine, 1958), thus, pentanal has relatively lower boiling point 103 C (CRC Handbook of Chemistry and Physics, 2017). Compare to pentanal 3-methylbutanal have slightly lower boiling point of 92 C – 93 C (The Merck Index, 2017) it is polar with one double bonded oxygen, dipole-dipole bonding and dispersion forces throughout the fivecarbon chain (Cooper, Williams and Underwood, 2015). When both molecules are chain isomers both contain five carbon atoms, have the same intermolecular force, same aldehyde functional group with dipole-dipole and dispersion force and similar boiling points (Bettelheim, Brown and March 2001), such as Pentanal 103 C and 3-methylbutanal 92 C – 93 C (CRC Handbook of Chemistry and Physics, 2017; The Merck Index, 2017; Farrell et al., 2010; Stone, 2013; Shemesh, Lan and Gerber, 2013). Despite the similarities, there is different molecular structures between the two compounds, where straight carbon chained pentanal have larger surface area and molecules more intact compare to a branched chain for 3-methylbutanal (Bettelheim, Brown and March 2001). Hence, branching of carbon changes decrease the intermolecular force required to break the bond between the molecule of the carbon chain, weakens the dispersion force due to decreased surface area which cause the atoms to be more tightly stacked in a branch (Farrell et al., 2010). Conclusively, resulting in lower boiling point for 3-methylbutanal (Peckham and McNaught, 2012). The longer the carbon chain the greater the dispersion force, therefore, the intermolecular force for pentanal and 3-mthylbutanal is weaker than hexanal due to length of carbon chain Cooper, Williams and Underwood, 2015). Thus, lowers the boiling point of the molecule (Farrell et al., 2010).

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Compound that has the greater intermolecular force have higher boiling point, the different types of intermolecular force are also important in determining the boiling point between different molecule. Likewise, molecules with specific molecular structure have better force of attraction between molecules, leading to a higher boiling point (Murphy, 2007; Spencer, Bodner and Rickard, 2012).

Bibliography:

Bettelheim, F., Brown, W. and March, J. (2001). Introduction to organic & biochemistry. 9th ed. Fort Worth: Harcourt College Publishers, pp.2-6.

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Cooper, M., Williams, L. and Underwood, S. (2015). Student Understanding of Intermolecular Forces: A Multimodal Study. Journal of Chemical Education, [online] 92(8), pp.1288-1298. Available at: http://pubs.acs.org/doi/pdf/10.1021/acs.jchemed.5b00169 [Accessed 12 Aug. 2017].

CRC Press, Taylor & Francis Group, an Informa Group company 2017, CRC Handbook of Chemistry and Physics 2017 ,98th Edition, viewed 15 august 17. http://west-sydneyprimo.hosted.exlibrisgroup.com/primo_library/libweb/action/dlDisplay.do?vid=UWSALMA&docId=UWS-ALMA51133136700001571

Farrell, S., Brown, W., Bettelheim, F., Torres, O. and Campbell, M. (2010). Introduction to general, organic and biochemistry. 7th ed. Belmont, CA, USA: Mary Finch, pp.192-195.

Murphy, P. (2007). Teaching Structure–Property Relationships: Investigating Molecular Structure and Boiling Point. Journal of Chemical Education, [online] 84(1), p.97. Available at: http://pubs.acs.org/doi/pdfplus/10.1021/ed084p97 [Accessed 20 Sep. 2017].

Peckham, G. and McNaught, I. (2012). Teaching Intermolecular Forces to First-Year Undergraduate Students. Journal of Chemical Education, [online] 89(7), pp.955-957. Available at: http://pubs.acs.org/doi/pdfplus/10.1021/ed200802p [Accessed 14 Aug. 2017].

Prigogine, I. (1958). Advances in chemical physics, volume 1, 1958-volume 6, 1964. 12th ed. New York: Interscience Publishers, pp.1-5.

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Shemesh, D., Lan, Z. and Gerber, R. (2013). Dynamics of Triplet-State Photochemistry of Pentanal: Mechanisms of Norrish I, Norrish II, and H Abstraction Reactions. The Journal of Physical Chemistry A, [online] 117(46), pp.11711-11724. Available at: http://pubs.acs.org.ezproxy.uws.edu.au/doi/abs/10.1021/jp401309b [Accessed 13 Aug. 2017].

Spencer, J., Bodner, G. and Rickard, L. (2012). Chemistry: structure and dynamics. 5th ed. Hoboken, N.J.: Wiley, p.94.

Stone, A. (2013). The theory of intermolecular forces. 2nd ed. Oxford: Oxford University Press, p.52-55.

Royal Society of Chemistry 2013. Published by Merck and Company, Rahway, New Jersey, 1940. The Merck Index 2017. Edition 5. Available at: https://www-rscorg.ezproxy.uws.edu.au/merck-index [Accessed 13 Aug. 2017].

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