In the Classroom
Teaching Structure–Property Relationships: Investigating Molecular Structure and Boiling Point Peter M. Murphy 16 Montbard Drive, Chadds Ford, PA 19317;
[email protected] Among the many challenges in teaching a chemistry course are (i) using the limited time wisely, (ii) integrating the laboratory and lecture materials, (iii) balancing the scope and depth of important principles studied, and (iv) giving students a taste of the methodology of scientific research including data analysis and literature searching. Teachers and students can benefit from resources that emphasize teaching different aspects of the scientific method. More laboratory time is often spent on experimentation and data tabulation at the expense of data analysis, hypothesis, prediction, and the iterative nature of research, which includes planning further experiments. Structure–property relationships are one theme that crosses many subdisciplines in chemistry and allows students to experience all aspects of the scientific method. In organic chemistry, students learn the effect of molecular structure and functional group on both physical properties and chemical reactivity to understand and to organize the vast number of reactions and reagents. Correia (1) provided an extensive set of boiling point data for 1-haloalkanes and showed how these data can be used to teach the structural-property effects of molecular weight, surface area or geometry, polarizability, and dipole–dipole interactions. Recent articles (2) have shown how correlations between molecular modeling and physical properties can be used to enhance students’ understanding of structure–property relationships. Research publications continue to model chemical and molecular structure as a predictor of physical and chemical properties for industrial applications (3). The determination of boiling points is an important skill for the purification and identification of organic compounds and this technique is usually taught early in a student’s career. When students combine their own experimental data with published physical properties, followed by an empirical analysis of all the data, they can further their understanding of structure–property relationships and their use of the scientific method. Textbooks (4), lab manuals (5), reference books (6), and online resources (7) have extensive physical property data but this information is scattered and not organized in the best format for teaching structure–property relationships. While spending some time compiling and organizing physical property data can benefit students, they will build a deeper understanding of scientific principles if more of their classroom, laboratory, and study time can be spent on analyzing the data and reaching appropriate conclusions and then pondering subsequent research questions. By manipulating an extensive, tabulated data set of boiling points for organic compounds organized by carbon chain and functional group, students can (i) more easily understand the process of studying structure–property relationships, (ii) learn how to develop empirical models, and (iii) begin to integrate those relationships in their thinking about the fundamental aspects of molecular structure. Through data analysis exerwww.JCE.DivCHED.org
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cises, the students can hone their analysis skills and come to “discover for themselves” various relationships between boiling point and important aspects of the chemical structure of organic compounds, such as molecular geometry, intermolecular bonding, and molecular weight. These exercises may generate unanswered questions in the student’s mind, which commonly happens in research, and may spark the curiosity necessary for more literature searching or for conducting additional experiments, thus allowing students to experience the open ended nature of research where preliminary conclusions lead to interest in further research. Tables 1 and 2 contain boiling points for a series of carbon chains (methyl, ethyl, n-propyl, phenyl, etc.) in rows and functional groups (hydroxy, chloride, amine, carboxylic acid, etc.) in columns. The tabulated boiling points were gathered from reference books (6), online resources (7, 8), and supplier catalogs (9). The tables are generally organized by increasing boiling points from top to bottom and left to right to give an overview of the trend from gases to higher boiling liquids for organic compounds. Merely having the students “color-code” the cells based on their boiling point ranges can aid the students’ understanding of the overall pattern for the impact of chemical structure on boiling point and to see the division between room temperature liquids and gases. The physical property of boiling point is commonly reported as a range of a few ⬚C, though published reports can vary widely. For example, over 30 reported values were found for the atmospheric and reduced pressure boiling point of cyclohexyl methyl ketone. The nearly two dozen published reports on the boiling point of t-butyl alcohol gave a combined range of 73 ⬚C to 96 ⬚C at atmospheric pressure (i.e., 760 mm Hg). The value in each cell in Tables 1 and 2 is a single boiling point value in ⬚C for each compound so that these data can be used as a teaching aid to facilitate quantitative exercises in empirical analysis. This single boiling point value is the mean of the reported boiling point range at atmospheric pressure whether from a single source or the average of multiple published values after excluding any values that were significantly different than the other values. More advanced students could benefit from a discussion as to why so many different boiling points are reported for the same compound and how to reconcile conflicting data in the published literature. Discussing the options and decisions for constructing Tables 1 and 2 would allow students to develop an appreciation of the complexity and potential biases of organizing even seemingly simple physical property data (such as boiling point, solubility, density, etc.) across a wide range of compounds. A number of other simplifications were made to allow students to more easily focus on correlating the effects of chemical structure on physical properties. For R⫺X compounds with the possibility of stereoisomers, (e.g., 2bromobutane), the reported boiling point of the racemic
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Boltzmann factors, the Eyring and Clausius–Claperyon equations, and the Trouton–Hildebrand–Everett rule for the entropy of vaporization as the basis for determining boiling point as a function of pressure. As a learning exercise, students could be led to speculate why no atmospheric boiling points were reported. Perhaps these compounds are unstable and may decompose at their atmospheric pressure boiling point, or perhaps no researcher has determined the atmospheric pressure boiling point of these compounds, or perhaps the boiling point has been determined but deemed uninteresting and not worth publishing, or perhaps the boiling point has been published in a source not commonly included in reference books and online databases. For some compounds, no experimental boiling point was found after checking both CAS STN online, including the databases REGISTRY, MRCK, HSDB, HODOC, DIPPR, and BEILSTEIN (9), and the CRC Handbook of Chemistry and Physics (6, 7). For such compounds, a dash was entered in Tables 1 and 2. For two compounds, the only information found pertaining to boiling point was an indication that these two compounds sublime, in the online version of the CRC Handbook of Data on Organic Compounds (HODOC) (9). These “miss-
Figure 1. Boiling point vs molecular weight for 392 organic compounds.
mixture was entered into Tables 1 and 2. For higher boiling liquids, often reduced pressure boiling points were the only values reported. Many laboratory manuals and textbooks contain nomographs to estimate boiling point as a function of pressure. For higher boiling liquids, the corresponding boiling point at 760 mm Hg was estimated using an online pressure–temperature applet and indicated in the data table by bold font. In constructing the applet, Goodman (10) used
Table 1. Boiling Points (°C) of Common Organic Compounds (R–X) at Atmospheric Pressure X
MW (g/mol)
R
Alkane
Chloride
Methyl ether
H
Cl
-OCH3
1
35.5
Amine
Bromide Aldehyde
-NH2
Br
-CHO
31
16
80
29
Alcohol
Iodide
Methyl ketone
Nitrile
-OH
I
-COCH3
-CN
17
127
43
26
1
᎑253
᎑85
65
᎑33
᎑67
᎑21
100
-35
21
26
CH 3
15
᎑161
᎑24
᎑25
᎑6
4
21
65
42
56
26
CH3CH2
29
᎑88
12
11
17
38
48
78
71
80
82 77
H
CH2=CH
27
᎑104
᎑13
6
56
16
53
---
56
87
CH2=CHCH3
41
᎑48
45
46
53
70
97
97
102
107
78
(CH3)2CH
43
᎑42
35
51
33
59
63
82
89
94
118
CH3CH2CH2
43
᎑42
46
39
48
71
75
97
102
102
97
(CH3)3C
57
᎑12
52
55
46
73
75
83
100
106
106
2-Butyl
57
0
79
60
63
91
92
98
120
117
125
(CH3)2CHCH2
57
᎑12
68
59
68
91
90
108
120
117
131
1-Butyl
57
0
78
70
78
102
103
118
130
128
140
Cyclopentyl
69
50
114
106
107
138
136
140
171
162
169
1-Pentyl
71
36
108
100
102
130
131
137
154
150
162
1-Hexyl
85
69
133
126
130
156
153
158
180
170
186
Cyclohexyl
83
81
142
133
134
166
161
160
202
180
185
1-Heptyl
99
98
160
151
157
180
171
176
204
194
205
Ph
77
80
132
155
184
156
179
182
188
202
191
91
111
179
170
184
198
195
205
242
217
234
1-Octyl
113
125
183
174
180
201
214
195
226
213
199
1-Naphthyl
128
218
260
269
301
280
318
279
302
297
299
2-Naphthyl
128
218
256
274
306
282
309
286
308
300
306
PhCH2
NOTE: For some liquids, only a reduced pressure boiling has been reported, perhaps because the compound is unstable and would decompose at its atmospheric pressure boiling point. For these compounds, an estimated atmospheric boiling point has been calculated using an applet described in ref 10 and the entry in Table 1 is bold. For compounds containing a chiral center, (e.g., 2-butanol) the entry in Table 1 is for the racemic mixture.
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Figure 2. Boiling point vs carbon chain length and functional group.
ing” boiling point data can allow the students to speculate (i) whether the boiling points of these compounds have ever been determined experimentally; (ii) whether their boiling points could ever be determined (e.g., if the compounds are sufficiently stable); (iii) whether the data might be published but not gathered in the widely used handbooks and databases for physical properties of organic compounds; or (iv) whether the lack of boiling point data might indicate that
the uninteresting nature of these compounds have left their properties so far unresearched. The overall relationship between boiling point and molecular weight for the 392 compounds contained in Tables 1 and 2 are shown in Figure 1. These data show the general trend of increasing boiling point as molecular weight increases, but the scatter in the data clearly shows that other aspects of molecular structure affect a compound’s boiling point. Figure 2 shows an example of how extracting a portion of the data in Tables 1 and 2 can be used to compare the boiling points versus carbon chain lengths or functional groups for different families of compounds. Note that Figure 2 has been constructed from the data in Tables 1 and 2 so that the total number of carbon atoms (not merely the carbon atoms in the R group) is being compared. Students can easily construct Figure 2 (or any similar comparisons) to learn that there is generally an excellent correlation of boiling point for a homologous series of compounds with the same functional group. Figure 2 also shows that most different classes of functional groups have a reasonably consistent separation in boiling point at matching carbon chain length. The three isomers of C3H6O2 shown in Table 3 indicate that at the same molecular weight, carboxylic acids have
Table 2. Boiling Points (°C) of Common Organic Compounds (R–X) at Atmospheric Pressure X
R
MW (g/mol)
Me ester
Ether
Acetate
Chloride
-CO2 CH3
-OR
-OCOCH3
-COCl
59
---
59
Et ester Phenyl -CO2 CH2 CH-Ph 3
63.5
73
77
Acid
Amide
Amine
Anhydride
-CO2 H
-CONH2
R3 N
-CO-O-COR
--
--
45
44
1
34
100
118
---
53
80
100
210
᎑33
123
CH3
15
58
᎑25
58
51
77
111
118
222
4
270
CH3 CH2
29
79
35
76
78
99
136
141
213
89
167 205
H
CH2 =CH
27
80
39
72
74
99
146
163
253
---
CH2 =CHCH3
41
106
94
104
101
119
156
163
---
150
218
(CH3 )2 CH
43
90
68
90
92
112
153
154
218
139
182
CH3 CH2 CH2
43
102
89
102
102
120
159
162
224
156
199
(CH3 )3 C
57
101
106
98
106
118
169
164
212
---
193
2-Butyl
57
116
121
112
116
133
174
177
231
---
237
(CH3 )2 CHCH2
57
117
121
112
116
132
171
176
226
192
215
1-Butyl
57
128
142
126
126
144
183
185
225
216
227
Cyclopentyl
69
158
216
180
161
174
219
212
---
---
308
1-Pentyl
71
151
188
149
152
168
205
202
255
236
247
1-Hexyl
85
174
222
171
173
188
226
223
180
264
268
Cyclohexyl
83
180
252
173
207
196
239
232
---
387
535
1-Heptyl
99
194
272
192
195
207
264
240
239
330
345
Ph
77
198
258
196
198
212
255
249
290
348
378
91
218
298
216
237
229
264
265
317
385
374
1-Octyl
113
214
287
210
236
224
262
256
sublimes
366
380
1-Naphthyl
128
395
461
351
322
378
324
300
sublimes
---
---
2-Naphthyl
128
290
426
354
331
386
---
300
---
---
---
PhCH2
NOTE: For some liquids, only a reduced pressure boiling has been reported, perhaps because the compound is unstable and would decompose at its atmospheric pressure boiling point. For these compounds, an estimated atmospheric boiling point has been calculated using an applet described in ref 10 and the entry in Table 2 is bold. For compounds containing a chiral center, (e.g. 2-butyl ether) the entry in Table 2 is for the racemic mixture.
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Table 3. Isomer Analysis for C3H6O2 Boiling Point/°C
Name Propanoic acid
141
Structure CH3CH2COOH
Methyl acetate
058
CH3COOCH3
Ethyl formate
053
HCOOCH2CH3
Table 4. Statistical Analysis of Functional Groups or Carbon Chains -OH
-OCH3
20
0
142
104
Data Analyses
n-Butyl
Number of higher boiling points within pairs
t-Butyl
19
0
128
103
Figure 3. Boiling points of C1 through C8 n-alkyl halides.
overall averages
63
78
54
54
65
᎑25
standard deviation minimum
0
᎑12
286
274
maximum
227
212
95% confidence interval for the mean difference between 29 to 47
paired comparisons (paired t-test)
22 to 29
a substantially higher boiling point than esters. Comparing the columns for -CO2H, -CO2CH3, and -CO2CH2CH3 in Table 2 show that even with higher molecular weights, methyl and ethyl esters have lower boiling points than their corresponding acids in 19 of the 21 carbon chains, the exceptions being the 1- and 2-naphthyl groups where only estimates of their boiling points at 760 mm Hg are entered into Table 2. This analysis can lead to a discussion of the importance of hydrogen bonding in intermolecular interactions. Figure 3 shows the correlation between boiling point and both carbon chain length and functional group for the C1 through C8 alkanes and the corresponding n-alkyl halides for the fluorides, chlorides, bromides, and iodides. For this graph, the boiling points for the n-alkyl fluoride were found in the CRC Handbook of Chemistry and Physics (6) to supplement the data in Tables 1 and 2 prior to constructing Figure 3. From Figure 3, we can conclude that increasing the molecular weight of the carbon chain results in a higher boiling point though the rate of increasing boiling point decreases as the carbon chain length increases. Also, for a given n-alkyl chain, increasing the atomic weight of the halide results in a compound with a higher boiling point. The data in Tables 1 and 2 allow students to hone their analysis skills and see various patterns and correlations between molecular structure and boiling point. Assigning follow up readings (such as Correia for 1-alkyl halides; ref 1 ) can explain structure–property relationships in greater detail. After the students have constructed their own Figure 2 (or similar comparisons), then they are prepared to mathematically model for themselves a limited aspect of the relationship between molecular structure and boiling point. Students can use their own model to predict the boiling points for C9, C10, and other carbon chains, and then compare their predictions to actual boiling point data reported in the literature or experimentally determined for themselves and finally discuss any discrepancies. The use of models for structure–property relationships opens the way for a discussion of the 100
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possibility of misleading predictions when estimating or forecasting beyond the range of a model. Advanced students may construct different mathematical models (e.g., linear or quadratic) and then compare the advantages and drawbacks of each model. Once the students gain these understandings, they are prepared to discuss and investigate how other aspects of molecular structure such as dipole moment, hydrogen bonding, molecular shape or size, and so forth can affect boiling point. The data in Tables 1 and 2 can lead students to explore for themselves these effects by considering sets of isomers, for example the four C4 isomeric side chains. Analyses of the six sets of paired items for the four isomeric C4 side chains across the 19 functional groups in Tables 1 and 2 showed that all pairs except the isobutyl and the 2-butyl pair have a statistically significant difference in boiling point. The data in Tables 1 and 2 form the basis for lessons and exercises in chemistry courses (high school, general college, organic, physical, etc.) and as examples of chemical data for analyses in computer, mathematics, and statistics lessons. These tabulated data can be adapted to a variety of exercises depending on the students’ scientific background, time allocation, and educational goals. These tabulated boiling point data are suitable for both individual and group assignments. Working with actual data reinforces the principles of structure–property relationships while showing that general trends are not universal predictors. Table 4 contains an example as to how the data in Table 1 can be extracted and analyzed at different levels of sophistication to compare either two functional groups or two carbon chains for their corresponding structure–property relationships. Merely calculating averages, ranges, and standard deviations may be less effective for showing the differences between various aspects of molecular structure. Comparing pairs of data (counting head-to-head differences or paired t-tests) can help the students learn to apply the appropriate statistical tool and to understand how various statistical analyses compare with their graphical representations of the same data. The exercise of completing a teacher-prepared abridged version of Tables 1 and 2 may also be an effective teaching tool by leveraging the students’ time and efforts. The complementary benefits of experimental investigation and literature searching are more understandable when students integrate their actual determination of boiling point data in the laboratory with published sources to complete their own larger data set prior to analysis. The instructors’ preparation of an abridged data table and the resources available to students to
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find published boiling points can lead the students to make temperature conversions (e.g., K to ⬚C) and to correct for reduced pressure boiling points. In addition to sharpening their literature-searching skills, students can begin to learn how scientists determine when to conduct their own experiments and when to consult published results. By wisely choosing which compounds to have the students find in the literature, teachers can also reinforce their students’ nomenclature skills, particularly with common names and indices of molecular formulas. Another way to sharpen the students’ literature searching skills would be to assign additional functional groups or other carbon chains to add columns or rows to Tables 1 and 2 as shown in Figure 3 for the n-alkyl fluorides. For example, the 1-methyl vinyl group (CH3⫺CH⫽CH⫺) would lead students to explore the structure–property aspects of cis–trans isomerization and conjugation. An advanced literature searching exercise could involve constructing an entirely new table of physical properties such as water solubility, index of refraction, density, melting point, heat of combustion, dielectric constant, viscosity, or any physical property widely reported for organic compounds. After searching for other physical properties, functional groups, or carbon chains, students will have a better understanding of the “incompleteness” of science and be more prepared to speculate why certain data remain unknown or unpublished. These literature searching exercises may be valuable preparation for laboratory experimentation. If students take note of the original publication date of the physical property data, their lesson can provide a glimpse into the modern history of chemistry. Advanced data analysis exercises may spark a student’s interest in mathematical modeling based on multiple fundamental structural features, an active research area with dedicated journals (11) containing recent articles on determining quantitative structure–property relationships, including for example boiling points (12). Conclusion By providing a concise, well-organized table of the boiling points of 392 organic compounds, this article and the corresponding online resources mentioned previously (7) facilitate inquiry-based instruction in multiple scientific principles including (i) obtaining physical property data, (ii) searching the various types of published literature for physical property data, (iii) analyzing data with varying levels of statistical sophistication, (iv) determining empirical structure– property relationships, and (v) considering how empirical modeling gives insight into fundamental structure–property relationships. Many individual or group learning activities can be derived from this tabulated data based on the instructor’s educational objectives and the students’ backgrounds.
5.
6.
7.
8.
9.
10.
11.
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