Organic Chemistry for Chemical and Physical Data - myassignmenthelp

Question: Discuss about theOrganic Chemistry for Chemical and Physical Data. Answer: Introduction The boiling point is defined as the temperature at which a pure substance under normal pressure of 1 atmosphere changes from liquid phase to gaseous phase (Ophardt, 2003). The molecules of a liquid are packed closely and held together by inter-molecular forces of attraction. When a liquid is heated, the molecules acquire kinetic energy resulting to increased vibrations. As more energy is supplied, the vibrations become intense enough to overcome the intermolecular forces and the molecules break free becoming a gas (Ophardt, 2003). Gas molecules are not in contact with each other. The boiling points of organic compounds depend on the strength of the inter-molecular forces between the molecules. Stronger intermolecular forces require a lot of energy to overcome resulting to higher boiling points. Among the common inter-molecular forces include hydrogen bonds, dipole-dipole interactions and Van der Waals (London dispersion forces) (Reusch, 1999). The strongest intermolecular forces are Hydrogen bonds while the weakest are London dispersion forces (Clayden, 2012) The nature of inter-molecular forces of attraction between molecules is dependent on the polarity of the molecules (Ophardt, 2003). Highly polar molecules are held together by very strong intermolecular forces of attraction (Hydrogen bonds and dipole-dipole interactions) leading to high boiling points while non-polar molecules have very weak intermolecular associations (London dispersion forces) leading to low boiling point. The degree of polarity of a molecule is determined by the nature of the functional group present (Ophardt, 2003). Comparison of the boiling points of given compounds .The boiling points of 1-pentanol, 2-pentanol, 2-pentanone and 1-hexanol together with their formulae and molar masses are as tabulated in table 1. Table1: formula, mass and boiling point of given organic compounds (Lide, 2005; ONeil, 2001; Haynes, 2014) Compound Molecular formula Structural formula Molar mass (g) Boiling point (0C) 1-pentanol C5H12O CH3(CH2)3CH2OH 88.15 138 2-pentanol C5H12O CH3(CH2)2CHOHCH3 88.15 119 2-pentanone C5H10O CH3(CH2)2COCH3 86.134 102 1-hexanol C6H14O CH3(CH2)4CH2OH 102.177 157 The boiling points of the four compounds increase in the order: 2-pentanone2-pentanol1-pentanol1-hexanol. 2-pentanone has the lowest boiling point while 1-hexanol has the highest boiling point. The differences in the boiling points arise due to differences in the molecular structures of the compounds (Brown, 2000) and the type of intermolecular forces existing between the molecules of each compound (Carrey, 2001). Boiling points of organic compounds increase with increase in molecular mass (Brown, 2000). For compounds with comparable molecular masses, the boiling points depend on the nature of the inter-molecular forces between the molecules. The molecular structures of the four compounds are as shown below. 2-pentanol 1-hexanol 1-pentanol, 2-pentanol and 1-hexanol are all alcohols. In alcohols, the oxygen atom is directly bonded to a hydrogen atom. Because O is highly electronegative, it attracts the electrons of the O-H bond, acquiring a partial negative charge and the H acquires a partial positive charged. The positive H attracts lone-electrons from O atom of neighboring molecules resulting to the formation of hydrogen bonds. Hydrogen bonds are the strongest intermolecular forces and this explains why alcohols have unusually high boiling points when compared to other organic compounds of comparable molecular size (Solomons, 2008). Among alcohols, the boiling point increase with increase in molecular weight. As the carbon chain increases, the van der Waals dispersion forces between the molecules become stronger due to additional electrons and these results to increased boiling point for longer chain alcohols (wade, 2014). 1-pentanol and 2-pentanol have same molecular mass (88.15 g) because they both have 5 carbon atoms. 1-hexanol on the other hand, has 6 carbon atoms and a higher molecular mass (102.177 g). The boiling point of 1-hexanol (157 0C) is higher than that of both 1-pentanol (138 0C) and 2-pentanol (119 0C) due to stronger van der Waals arising from the increased length of the carbon chain. For alcohols with the same molecular weight, the boiling points vary with the strength of hydrogen bonds which is directly impacted by the extent of exposure of the O-H bond. Generally, primary alcohols have higher boiling points, followed by secondary alcohols and tertiary alcohols have the lowest boiling points (Wade, 2014). In primary alcohols, the O-H is more exposed and can readily interact with other molecules leading to stronger hydrogen bonds and higher boiling point. 1-pentanol and 2-pentanol have the same molecular mass. However, 1-pentanol is a primary alcohol and 2-pentanol is a secondary alcohol. In 1-pentanol, the O-H group is located at the terminal end of the chain and linked with only 1 alkyl group. The O-H group is more exposed and can interact with the O-H of many neighboring molecules leading to strong hydrogen bonds and this explains why the boiling point of 1-pentanol is higher than that of 2-pentanol. In 2-pentanol, the O-H is linked to two alkyl groups. The presence of many alky groups hinders the interaction of the O-H group with many adjacent molecules, leading to relatively weaker hydrogen bonds as compared to those formed by 1-pentanol. Consequently, the boiling point of 2-pentanol is relatively lower than that of 1-pentanol. 2-pentanone has the lowest boiling point when compared to the rest of the compounds which are alcohols with comparable masses. 2-pentanone is a ketone with 5 carbon atoms. In 2-pentanone, carbonyl bond (O=C) is highly polar. Since O is more electronegative than C, it pulls the electrons of the bond towards itself, making the O end of the bond partially negatively charged and the C end partially positive. This results to the formation of a dipole. Positive end of one molecule attracts the negative end of neighboring molecules and thus, the molecules are held together by dipole-dipole forces of attraction between the molecules (Solomon, 2008). Dipole-dipole attractions are relatively strong and this gives 2-pentanone the significantly high boiling point (102 0C). The boiling point of 2-pentanone is lower than that of the corresponding alcohols because the dipole-dipole attractions are not as strong as the hydrogen bonding found in alcohols. Conclusion The boiling points of the given compounds are in the order: 2-pentanone2-pentanol1-pentanol1-hexanol. 2-pentanone has the lowest boiling point while 1-hexanol has the highest boiling point. Alcohols have high boiling point because of the hydrogen bonding between O and H of neighboring molecules. Primary alcohols have higher boiling point than secondary alcohols because the O-H is more exposed in 1o alcohol forming stronger hydrogen bonds. Ketones have lower boiling point than alcohols because they lack hydrogen bonds. References Brown,W.H., Iverson,B.L., Anslyn,E.V., Foote,C.S. (2000). Organic chemistry. Carey,F.A., Giuliano,R.M. (2001). Organic chemistry. New York: McGraw-Hill Education. Clayden,J., Greeves,N., Warren,S.G. (2012). Organic chemistry. Oxford: Oxford University Press. Haynes, W. M. (2015). CRC handbook of chemistry and physics. Boca Raton, FL, CRC Press. Lide, D. R. (2005). CRC handbook of chemistry and physics: a ready-reference book of chemical and physical data. Boca Raton, Fla, CRC Press. Bottom of Form Oneil, M. J. (2001). The Merck index. Whitehouse Station, N.J., Merck Ophardt,C. (2003). Polarity of Organic Compounds. Retrieved from https://chemistry.elmhurst.edu/vchembook/213organicfcgp.html Reusch,W. (1999). Virtual textbook of organic chemistry. Retrieved from https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/intro1.htm Solomons, T. W. G., Fryhle, C. B. (2008). Organic chemistry. Chichester, John Wiley Sons. Wade, L. G. (2014). Organic chemistry. Harlow, Pearson Education.