FACTORS EFFECTING THE BOILING AND MELTING POINTS IN ORGANIC HOMOLOGOUS SERIES.
The idea for this investigation came about when we were learning about trends in physical properties in the homologous series. With regards to alkanes, we saw that the boiling point increases with increasing carbon number due to stronger van der Waal’s forces as the temporary dipoles increase. However, according to Brown and Fordi “the increase is not linear, but steeper near the beginning as the influence of increased chain length is proportionally greater for the small
Also, I have seen that for compounds of similar molar masses (so that the strength of van der Waal’s forces are similar) that the addition of functional groups into the hydrocarbon chain makes a very great difference to the melting and boiling temperatures. For example, the permanent dipole due to the carbonyl group in aldehydes and ketones results in stronger dipole-dipole forces and so a higher boiling or melting point. The OH group in alcohols will cause an even higher boiling and melting temperature because it causes the strongest intermolecular force, hydrogen bonding to occur. This is supported if we take three compounds of similar molar mass, propane, ethanol and ethanol and compare their boiling temperature.
|Mr (g mol-1)||Strongest intermolecular
|Boiling Temperature ii
|Propane||44||Van der Waal’s||-42|
From these values, we see that the effect on the boiling temperature of adding a carbonyl or hydroxyl group is very large. But these are small molecules and I wondered if again the effect will be reduced with increasing hydrocarbon chain length because the proportionate effect of the functional group will get less and the aldehyde, ketone or alcohol will become more “alkane” in nature as the chain length increases.
Will we see the melting and boiling points of the aldehydes, ketones and alcohols converge on those of the alkanes and if so at what chain length does the effect
of a carbonyl or hydroxyl group become insignificant?
When looking at the effects of carbonyl or hydroxyl group one other possible influencing factor is the position on the chain of the carbonyl or hydroxyl group. I can imagine that a functional group
hidden in the middle of a long hydrocarbon chain may not be able to approach and attract a close by molecule as easily as a functional group at the end of a chain. So my second aim is to see if a functional group at the end of a chain will have a greater, lesser or the same effect on the intermolecular forces as one in the middle of a chain?
(i) Will the melting and boiling points of the aldehydes, ketones and alcohols converge on those of the alkanes as we increase the carbon number and if so at what chain length does the effect of a carbonyl or hydroxyl group become insignificant?
(ii) What will be the effect on the melting and boiling points of changing the position of the functional group in the ketone of alcohol.
The dependent variables in this investigation are the melting and boiling points of the compounds in the homologous series: alkanes, aldehydes, 2-ketones, 3-ketones, 1-alcohols, 2-alcohols and 3- alcohols. The independent variables are the molar mass of the compound and the identity and position of the functional group
A control variable is that I will only look at compounds with linear hydrocarbon chains so there will be no added effect from branching.
When assessing the effect of the identity and position of the functional group I will be graphing the melting and boiling temperatures against the Molar Mass of the compound. This investigation will use data available from two databases
1. CRC Handbook of Chemistry and Physics, 85th Edition, CRC Press, 2004
2. The RSC Chemspider online chemical databaseiii
The ChemSpider database contains experimental data and predicted data from ACD/LABS (boiling point only) and EPISuite. The predicted data was only to be used where an experimentally determined value was not available in the CRC Handbook or the Chemspider website.
The CRC Handbook was the first choice source of data since it is a resource that has been available for many years and I would assume many people have cross checked the data.
Also once I was able to find a compound e.g. hexane, it was very quick and easy to read off values for hexanal, 1-hexanol, etc, since they were adjacent in the table of physical properties. In Chemspider I had to make a separate formula search for each one which was much slower.
DATA TABLE 1 Melting Pts
|Melting Point (± 1 oC)|
DATA TABLE 1 Melting Pts
|Melting Point (± 1 oC)|
Key to Data Sources in Data Tables
Blue font – CRC Handbook Green Font – Chemspider Experimental
Red Font = ACD/Labs prediction Mustard Font – EPI Suite
Uncertainty in Raw Data
The experimental data were cited with varying precision ranging from zero to three decimal places. Also the melting temperature was sometimes cited as a range. Where a range was given I have chosen the midpoint and have rounded off to the nearest integer value.
The data sources were evaluated by looking at some example compounds where experimentally determined data is available as well a prediction given
Table 3: Evaluation of Data Sources
|M.Pt (oC)||B.Pt (oC)||M.Pt (oC)||B.Pt (oC)||M.Pt (oC)||B.Pt (oC)||M.Pt (oC)||B.Pt (oC)|
Looking at Table 3 it is clear that there is usually good agreement between the CRC Handbook and Chemspider experimentally sources.
The ACD Labs predicted values for boiling temperatures appear quite close to experimental and can be used where experimental data is not available. At higher temperatures the experimental values in the CRC handbook relate to that measured at lower than atmospheric pressure. This could be because the boiling temperature at normal atmospheric pressure is very higher and the compounds may thermally decompose before the predicted boiling temperature.
The EPISuite predicted data is not so reliable. There is some variation in the boiling points and large variation in the melting temperature data. As a result I have omitted the EPI Suite data in the analysis section below.
Analysis and Discussion
Part (i) Effect of chain length on the comparative melting and boiling points of the alkanes, aldehydes, ketones and alcohols.
The first two graphs below represent all the gathered data (except the discarded EPISuite predicted data).
Graph 1 – Melting points Plotted against Molar Mass for each Homologous Series
Graph 1 (the melting temperatures) showed a trend but it is not very well defined. The trend-lines of
the different homologous series do not converge as clearly as with the boiling temperatures below.
This is not something that I was expecting at first and I cannot easily explain the fact that melting
temperatures of aldehydes, ketones and alcohols remain significantly above the melting
temperature of the alkane of similar molar mass. From the data sources the melting temperatures
were not so well defined as boiling temperatures and were often given as a range of temperatures.
This could be an area for further study.
Graph 2 – Boiling points Plotted against Molar Mass for each Homologous Series
From Graph 2 we see that the boiling temperatures are well defined and the smooth trend-lines do indicate the boiling temperature values for the alkanes, aldehydes, ketones and alcohols do appear to converge at molar mass values above 220 g mole-1. This is not surprising since the influence on the
intermolecular forces of the carbonyl and hydroxyl groups reduces as the hydrocarbon chain increases and dominates the character of the molecule.
If we look only at the CRC Handbook experimental data and not use any predicted data then the convergence is even more clear (and removes the anomalous linear aldehyde data points at 198 and 212 gmol-1) as shown in Graph 3 below.
Graph 3: CRC Handbook of Experimental Data Only
Graphs 2 and 3 also show that at low molar masses that the trend in boiling points is
alcohols > aldehydes/ketones > alkanes
which agrees with hydrogen bonding being stronger than dipole-dipole forces which are stronger than van der Waal’s forces. At higher molar masses the difference gets much less.
Part(ii). Effect on the boiling points of the functional group position in the aldehyde/ketones and alcohols.
Because the boiling points are so much clearer than the melting points the rest of my analysis will be based only on the boiling points.
Graph 3 Effect on the Boiling Points of the carbonyl position in the aldehyde and ketones
There is very little difference in the melting points between the aldehyde, 1-ketone and 2-ketone isomers at each molar mass and the graph lines are very similar. There is a strange anomaly with the data for the aldehydes C13H18O and C14H30O where the respective boiling points of 280oC and 260oC seem to be swapped around. I have checked again the experimental data on ChemSpider and those are the values given. The values are not available in the CRC Handbook to double check and this means that I am not very confident in their correctness.
If the data in the table is looked at in Data table 2 we can see that where experimental values are are available the 3-ketone has a slightly lower boiling point by between 1 and 6 oC. But the predicted values are often higher than the corresponding aldehyde and 2-ketone. I suggest that 3 ketones have a lower melting point than the aldehydes and 2-ketones but the effect is not large enough to state confidently.
Graph 4 Effect on the Boiling Points of the OH group position in the alcohols
Graph 4 shows that the boiling points of the 1-alcohols are significantly higher than the corresponding 2-alcohols and 3-alcohols. Where we have the CRC Handbook experimental data
available for all three series (up until the dodecanol C12H26O isomers) the 2- and 3- alcohols have similar boiling temps which are significantly below the 1-alcohols.
I can make the hypothesis that this is because the OH group at the end of the chain in the 1-alcohols can more easily approach from a variety of angles another OH group from another 1-alcohol molecule.
If the OH group is in the middle of a chain (like a 3-alcohol) then there are fewer ways that two molecules can align and attract each other. I have shown these possibilities in Figure 1 and 2 below with Chemsketch 3D images of 1-hexanol and 3-hexanol. It can be seen that there are more possible orientations where the 1-hexanol molecule OH groups can approach and H-bond
Figure 1: 1-Hexanol
Figure 2. 3-Hexanol
The main conclusions to this research are the answers to the two parts of the research question given earlier
(i) The boiling points of the aldehydes, ketones and alcohols do converge on those of the alkanes as we increase the carbon number and above 200gmol-1 the differences become
minimal. The melting points were less well defined and although there is some convergence it is not so clear as for the boiling points
(ii) The effect of boiling point on changing the position of the functional group in the alcohol is significant. 1-alcohols with the OH group at the end of the chain have a higher boiling
temperature than the 2- and 3-alcohols.
There were other interesting findings such as the significant differences in how well defined the melting and boiling temperatures were and the fact that some predicted values available in the web-based databases were very unreliable. These can be the basis for further study.
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