Melting Point and Boiling Point of Organic Compounds

Miranda, Marilyn1, Salen, Vladimir, A.2

1Miranda, Marilyn, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology; 2Salen, Vladimir A., CHM142L/B22, School of Chemical Engineering, Chemistry and Biotechnology, Mapua Institute of Technology
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ABSTRACT
In this experiment, our attention will be focused on the effect of chemical structures of different organic compounds on two physical properties: their melting point and their boiling point. Melting point of compounds can be affected by intermolecular forces of attraction, geometric isomerism and purity. Boiling point of compounds can also be affected by intermolecular forces of attraction and by branching. The main purpose of this experiment is to (1)determine the effect of intermolecular forces of attraction and geometric isomerism on melting point of compounds, (2)determine the effect of purity on the melting point range of organic compounds and (3)determine the effect of intermolecular forces of attraction and branching on the boiling point of organic compounds. We can obtain wrong data as we encountered different errors while performing this experiment. As we go along to this laboratory report, we will able to know the effects of the different factors in melting point and boiling point of the compound and why the used compound arranged on that decreasing or increasing melting point or boiling point.
Keywords: melting point, boiling point, intermolecular forces of attraction, geometric isomerism and branching[pic]
INTRODUCTION
Determination of melting point and boiling point of organic compound can be affected by different factors like intermolecular forces, impurity and branching that can affect the temperature of the melting point and boiling point of the compound you can achieved when you performed the experiment. One error in this particular experiment will lead you into wrong true temperature of melting point and boiling point of the compounds. Structural theory states that properties of organic compound are largely influenced by the chemical structures of such compounds. Physical properties include boiling points and melting points. This experiment trying to reach or achieved the purpose which are to (1)determine the effect of intermolecular forces of attraction and geometric isomerism on melting point of compounds, (2)determine the effect of purity on the melting point range of organic compounds and (3)determine the effect of intermolecular forces of attraction and branching on the boiling point of organic compounds.
MATERIALS AND METHODS In able to know (1)the structural effect of the finely ground benzoic acid, benzoin, naphthalene, salicylic acid, and urea on its melting point in terms of intermolecular forces of attraction, (2)the structural effect of maleic acid and furmaric acid (both compound is geometric isomers or substituent) on its melting point and (3)the effect of purity on melting point range given the pure and impure urea, we must get capillary tubes and sealed the one end of each tube through the use of heat. Next is to get small amount of the solid sample needed to be analyze by putting it into the capillary tube through the use of glass tubing by dropping it inside the one meter glass tubing in which one end is placed on the floor. The height of the sample should at least 5-6mm and must be tightly packed. Use the Thomas Hoover Apparatus to determine the melting point of the compounds by placing the capillary tube containing the sample. Turn on the apparatus then adjust the heat and switch on the stirrer. Finally, start recording the temperature when the compound started to liquefy and completely liquefied. These procedure is followed until all the compounds is tested. Another thing to determine in this experiment is the boiling point. In able to determine the (1) structural effect of n-Butanol, 2-Butanane, n-Heptan e, Propanoic acid and n-Hexane on its boiling point by a micro method and (2) branching effect of n-butyl alcohol, sec-butyl alcohol and tert-butyl alcohol on its boiling point by micro method, first, fill the Thiele tube with glycerol up to the level slightly above its arm. Get a capillary tube but this time used a twisted capillary tube by heating it in the middle part. Next, place the capillary tube inside the micro test tube containing two to three drops of the test compounds. Then, Place the micro test tube containing the sample and the capillary tube side by side with the thermometer ranged from 0 to 200 degree Celsius by using a 2-mm wide rubber tubing to hold the capillary tube. Dip the thermometer in the glycerol ensuring that the rubber tubing is not immersed in the liquid. Allow the oil to cool down by removing the flame. Finally, record the temperature as the boiling point of the compound as the bubbling start to cease slowly and the liquid start to enter the capillary. Reheat the oil slowly while taking note of the temperature when the liquid start to get out of the capillary tube to obtain the boiling point range of the test compound.
RESULTS
Table 1: Melting Point Obtained of the Benzoic Acid, Benzoin, Napthalene, Salicylic Acid and Urea Needed to Know the Structural Effect on their Melting Point in Terms of Intermolecular Forces of Attraction.
Legend:
T1 = Temperature when the compound start to liquefy
T2 = Temperature when the compound completely liquefied
MP = True melting point of the compound
|COMPOUND |T1 |T2 |MP |
|Benzoic Acid |117 |126 |122 |
|Benzoin |128 |143 |134 |
|Napthalene |68 |76 |81.5 |
|Salicylic Acid |153 |161 |159.5 |
|Urea |100 |121 |133.5 |

Table 2: Melting Point Obtained of the Maleic Acid and Furmaric Acid (Geometric Isomers) Needed to Know the Structural Effect on their Melting Point
Legend:
T1 = Temperature when the compound start to liquefy
T2 = Temperature when the compound completely liquefied
MP = True melting point of the compound
|COMPOUND |T1 |T2 |MP |
|Maleic Acid |119 |135 |138.5 |
|Furmaric Acid |261 |305 |287 |

Table 3: Melting Point Obtained of Pure Urea and Impure Urea Needed to Know the Effect of Purity on Melting Point Range.
Legend:
T1 = Temperature when the compound start to liquefy
T2 = Temperature when the compound completely liquefied
MP = True melting point of the compound
|COMPOUND |T1 |T2 |MP |
|Pure Urea |100 |121 |133.5 |
|Impure Urea |93 |116 | |

Table 4: Boiling Point Obtained of n-Butanol, 2-Butanol, 2-Butanane, n-Heptane, Propanoic Acid, and n-Hexane Needed to Know the Structural Effect on their Boiling Point in Terms of Intermolecular Forces of Attraction
Legend:
T1 = Temperature when the liquid start to enter the capillary tube
T2 = Temperature when the liquid goes out of the capillary tube
BP = True boiling point of the compound
|COMPOUND |T1 |T2 |BP |
|n-Butanol |104 |114 |117 |
|2-Butanane |62 |68 |79.6 |
|n-Heptane |78 |82 |98.4 |
|Propanoic Acid |74 |82 |141 |
|n-Hexane |64 |72 |61 |

Table 5: Boiling Point Obtained of n-butyl alcohol, sec-butyl alcohol and tart-butyl alcohol Needed to Know the Structural Effect on their Boiling Point by Branching.
Legend:
T1 = Temperature when the liquid start to enter the capillary tube
T2 = Temperature when the liquid goes out of the capillary tube
BP = True boiling point of the compound
|COMPOUND |T1 |T2 |BP |
|n-butyl alcohol |104 |114 |117 |
|sec-butyl alcohol |94 |106 |99 |
|tart-butyl alcohol |74 |86 |82 |

DISCUSSION Salicylic acid to Benzoin to Urea to Benzoic Acid to Napthalene have the decreasing melting point. Intermolecular forces are forces that act between large bodies of molecules. There are basically a few types, with the most familiar probably being the London Dispersion forces (van der waals force), dipole-dipole interactions (polar attractions) and hydrogen bonding. London Dispersion exists in every molecule, Hydrogen bonding occurs when there's a hydrogen within the molecule attached to an atom with extremely high electronegativity such as Oxygen, Fluorine, and Nitrogen while Dipole-dipole exists in polar molecules. The greater the polarity (difference in electronegativity of the atoms in the molecule), the stronger the dipole-dipole attraction. Since Dispersion are dependent upon the random motion of electrons, the more electrons an atom or molecule has, the greater the Dispersion between them. The more electronegative the element the stronger the hydrogen bonding. The greater the intermolecular attraction the higher the melting point. Among the intermolecular forces of attraction, Hydrogen bonding has the strongest intermolecular forces of attraction followed by dipole-dipole forces followed by the dispersion forces. Salicylic Acid, Benzoin, Urea and Benzoic Acid have hydrogen bonding while the Napthalene had a dispersion forces as intermolecular force of attraction, that is the reason why the Salicylic acid to Benzoin to Urea to Benzoic Acid to Napthalene have decreasing melting point because Salicylic Acid is more electronegative compare to other compound and hydrogen bonding is more stronger than dispersion force. The longer and more spread out the molecule the higher the melting point. Also, for melting point, the more symmetrical the compound, the higher the melting point. Impurities decrease vapor pressure, which decreases melting point (and increases boiling point.) One way to think of it is that impurities get in the way of the bonds that would be holding the solid together. By disrupting the normal organization of atoms or molecules in that solid, the impurities weaken the bonds holding the solid together (keep in mind these are bonds between atoms and molecules, not within them). As these intermolecular bonds weaken, it takes less energy to pull the molecules apart, which means it will melt to a liquid at a lower temperature. Factors affecting the melting point of organic compounds are molecular size, molecular geometry, intermolecular attraction and impurity. Molecular size is directly proportional to surface area which is directly proportional to molecular geometry which tells us the bigger the molecular geometry the higher the melting point. The intermolecular attraction is directly proportional to melting point, the stronger the intermolecular force the higher the melting point. Impurity is inversely proportional to melting point, the higher the impurities the lower the melting point. Polarity affects boiling point of organic compounds because if an organic compound is more polar, the boiling point gets higher because the forces of attraction is now greater as the n-butyl alcohol is more polar than to sec-butyl alcohol which is more polar compare to tart-butyl alcohol. That is the reason why n-butyl alcohol has higher boiling point to sec-butyl alcohol followed by the tart-butyl alcohol. n-Heptane has a melting point and boiling point of -91 and 98.4 degree celsius respectively while n-Hexane has a melting point of -95 and 61 degree celsius respectively. We seen that the melting point of organic compound is directly proportional to boiling point of compound as the boiling point increase the melting point increase too. It is like as the melting point of organic compound increase when the molecular size increase and intermolecular forces of attraction goes stronger. The boiling point of n-butyl alcohol(117 ) is bigger than sec-butyl alcohol(99) which bigger to tart-butyl alcohol(82). The boiling point differs to each other because it is less branched. The more linear structure of n-butyl alcohol results in a larger dipole moment and stronger London dispersion forces than will be present in the other butyl molecule. Factors affecting the boiling point of organic compound are molecular size, molecular polarity and branching. Large molecules have more electrons and nuclei that create van der Waals attractive forces, so their compounds usually have higher boiling points than similar compounds made up of smaller molecules. permanent molecular dipoles generated by polar covalent bonds result in even greater attractive forces between molecules, provided they have the mobility to line up in appropriate orientations which increase boling point of the compound. Less branched the molecule increase the boiling point of the organic compound. The more linear structure of n-butyl alcohol results in a larger dipole moment and stronger London dispersion forces than will be present in the other butyl molecule.
CONCLUSIONS AND RECOMMENDATIONS In the experiment, it was failed to determined the effect of intermolecular forces of attraction and geometric isomerism on the melting point of the Salicylic acid, Napthalene, Urea, Benzoin, and Benzoic Acid for the effect of intermolecular forces of attraction on melting point and of Maleic acid and Furmaric acid for the effect of geometric isomerism on melting point as the obtained melting point temperatures is not exactly or slightly near to their respective true melting point temperature. The possible reasons why the obtained melting point temperature is not exactly or slightly near to the true melting point of the compound is not knowing the intermolecular forces of attraction of the compound, too much sample compound in the capillary tube, rapidly heating of the compound, samples is not tightly packed in the capillary tube or contaminated samples. It is also failed to determine the effect of purity on the melting point range of organic compound as obtained melting point temperature of pure urea and impure urea is not exactly or slightly near to the true melting point temperature of the compounds respectively. Possible reason for this are samples are not tightly packed, too much sample compound in the capillary tube and contaminated samples. It is also failed to determine the effect of intermolecular forces of attraction on the boiling point of n-Butanol, 2-Butanone, n-Heptane, Propanoic acid and n-Hexane and it is slightly failed to determine the effect of branching on the boiling point of n-butyl alcohol, sec-butyl alcohol and tart-butyl alcohol as the obtained boiling point temperatures is not exactly or slightly near to their respective true boiling point temperature. Possible reason for this are wrong reading of thermometer, wrong methods used, contaminated samples, not knowing the intermolecular forces of attraction and branching of the compounds, and not alert what happening. From this possible reasons may I recommend that to use many trials to determine the true melting point and boiling point temperature to have many options to be precise on the true melting point andboiling point of the compound. I also recommend that to prepare the sample compounds properly to avoid some errors like impurities/contamination. Follow the procedures as possible because this may lead you to get the true value of melting point and boiling point of the compound. Being alert also is one best method to use to be carefully observant on what happening on the compound you are doing especially when using microscale experiment.
REFERENCES
1. Baluyot, J.Y. G. and De Castro, K. Organic Chemistry Laboratory Manual for Chemical Engineering Students, Part 1, Mapua Institute of Technology, Manila, Philippines