In the Laboratory
Identification of Primary, Secondary, and Tertiary Alcohols An Experiment in Spectrophotometry, Organic Chemistry, and Analytical Chemistry I. A. Leenson Moscow State University, Department of Chemistry, 119899 Moscow, Russia
The well-known test distinguishing between alcohols with the hydroxy group attached to primary, secondary, or tertiary carbon atoms is called the Lucas test (1). Kjonaas and Riedford checked all alcohols having 6 or fewer carbon atoms and showed that identification can be made (with a few exceptions) according to the time required for an alcohol to react with Lucas reagent (ZnCl 2 in HCl). It was shown (2) that a very simple procedure based on the conversion of an alcohol to the corresponding ester of nitrous acid (nitrite) R–OH + HNO2 → R–O–N=O + H2O enables not only distinction between different alcohols within a few minutes but also quantitative analysis of some mixtures. Furthermore, this technique can be applied more broadly than the standard Lucas test, which is limited to low molecular weight alcohols that are soluble in the HCl/ ZnCl2 solution. The only requirement is the availability of a recording spectrophotometer. The method is based on very specific absorption spectra of organic nitrites in the nearUV region: the band (n → π* transition with εmax ≈ 10 2 M{1 cm{1) has a distinct vibration structure in any organic solvent (3). All tertiary alcohols (we checked 2-methyl-2-propanol, 2-methyl-2-butanol, 2-methyl-2-octanol, 3-methyl-3pentanol, 3-methyl-3-heptanol, 3-methyl-3-octanol, and 1methyl-, 1-ethyl-, and 1-butylcyclohexanols—the ordinary
student syntheses) are found to have a maximum at ca. 400 nm (25,000 cm{1) in the spectra of corresponding nitrites in hydrocarbon solvents, whereas all primary and secondary alcohols checked (see Table 1) do not have such a maximum. The discrimination between primary and secondary alcohols is based on an empirical parameter α, the ratio of the heights of two adjacent peaks, h 1 and h 2, in the spectrum of a corresponding nitrite [h1 and h 2 are measured between a minimum at ca. 28,500 cm{1 (350 nm) and two maxima at ca. 28,000 cm {1 (360 nm) and 29,000 cm{1 (345 nm), respectively] (Fig. 1). The ratio α = h 1/h2 is dimensionless and therefore independent of the particular alcohol concentration and of absorbance (“optical density”). As we can see from Table 1, two groups of alcohols are easily distinguishable by their spectra. It should be noted, however, that the presence of a geminal or vicinal functional group in an alcohol may alter α significantly (some examples are listed in the table); nevertheless these α-values for the substituted primary alcohols are still far from those typical for the secondary alcohols.
Table 1. Parameter α for Some Primary and Secondary Alcoholsa α
Alcohol
Alcohol
α
Primary alcohols Methanol
0.98
3-Methyl-1-butanol
1.34
Ethanol
1.28
1-Hexanol
1.35
1-Propanol
1.34
1-Heptanol
1.30
1-Butanol
1.34
1-Octanol
1.29
2-Methyl-1-propanol
1.38
1-Nonanol
1.32
1-Pentanol
1.34
1-Hexadecanol
1.34
Secondary alcohols 2-Propanol
3.04
2-Octanol
3.24
2-Butanol
3.12
Cyclohexanol
3.68
2-Pentanol
3.17 Alcohols with functional groups
2-Phenylethanol
1.40
2-Propene-1-ol (allyl alcohol)
1.54
2-Methoxyethanol
1.61
aAll
424
2-Propyn-1-ol (propargyl alcohol)
1.67
2-Chloroethanol
1.84
Phenylmethanol (benzyl alcohol)
2.30
α-values are the average of 4–5 measurements of nitrite spectra.
Figure 1. Absorption spectra of (a) n-butylnitrite obtained from nbutanol; (b) sec-butylnitrite obtained from sec-butanol; (c) tertbutylnitrite obtained from tert-butanol; (d) concentrated solution of nitrogen dioxide in heptane. The concentration of NO2 here is at least two orders of magnitude higher than in the solution of nitrites to be analyzed. For convenience the absorbances for each nitrite are shifted along the y-axis.
Journal of Chemical Education • Vol. 74 No. 4 April 1997
In the Laboratory The α-values also make possible the quantitative analysis of primary and secondary alcohols in their mixtures, the accuracy of the procedure being rather sensitive to the nature of alcohols and their proportions in the mixture. Some examples are listed in Table 2 and Figure 2. We can see that in the case of mixtures of two isomeric alcohols, the α-dependence on C (the primary alcohol content in the mixture, %) is a smooth curve from which C could be derived. It is interesting that there exists a linear α vs. C dependence in logarithmic coordinates with a constant standard deviation s, provided that C ≥ 20%. This was checked on butanols and pentanols mixtures, where log C = (2.31 ± 0.054) – (2.54 ± 0.20) ? log α
Table 2. Analysis of Alcohol Mixturesa Primary alcohol (C , %)b Mixture 1-Pentanol with 2-pentanol
Introduced
Calculated
90.1
90.5
80.7
79.6
69.1
70.8
59.6
59.0
49.0
49.5
40.5
39.2
(1)
31.1
32.0
with s = 0.0336 for t0.95 = 2.02 and f = 40. It should be noted that in mixtures with C ≤ 20% the accuracy is rather low (percentage error is about 5%), and when C ≤ 10% only a rough estimation can be made (s = 0.3). The same is also true for higher alcohols, as we can see from the bottom part of Table 2. In such cases more accurate data can be obtained from a calibrating graph for the mixture. The following assignments can be proposed for students:
24.7
24.4
19.4
19.7
1. Nitrosate primary, secondary, and tertiary alcohols; extract the alkylnitrites from aqueous phase with organic solvent; record their spectra in the near-UV region; and conduct a qualitative analysis of the data. 2. Distinguish between different types of alcohols proposed by the instructor, and compare the results with those of the Lucas test. 3. Nitrosate mixtures of primary and secondary alcohols and determine α-values for these mixtures; plot graphs (α vs. C and log α vs. log C or ln α vs. ln C) and derive an analytical equation connecting α and C (eq 1); define applicability ranges of this equation with least mean squares analysis of the straight line obtained. 4. Conduct a quantitative analysis of a mixture of two isomeric alcohols provided by the instructor. 5. More advanced students can try to analyze mixtures of nonisomeric alcohols (e.g. 1-octanol and 2-butanol or 2-octanol and 1-butanol) or mixtures containing tertiary alcohol, and then plot the calibrating graphs and derive the appropriate analytical equations. They will perhaps invent some other empirical spectral parameter(s) for their purpose. The scope of activity depends only on their imagination and experience.
Experimental Procedure A few drops of an alcohol or a mixture of alcohols are placed in a test tube with a ground-glass stopper containing 5 mL of dilute (ca. 10%) hydrochloric acid and 1 mL of a solvent (hexane or heptane works very well). Then 2 mL of 10% sodium nitrite solution is added to this mixture. The stoppered test tube is shaken vigorously several times, and a few drops of the upper organic layer containing nitrite(s) 1 are transferred with a drawn-out pipet to a standard quartz cell containing the same solvent. After a short agitation the spectrum is taken between about 330 and 410 nm (30,000– 24,000 cm{1 ). The amount of nitrite solution transferred should give optical absorbances that are convenient for measurement (ca. 0.3–0.8). It is not necessary to adjust the spectrum to a zero line, as seen from Figure 1. The ordinary precautions should be taken when working with poisonous solids (sodium nitrite) and gases. (A slight odor of nitrous gases is noticed during the nitrosation of an alcohol, so this procedure, as well as making a solution for spectral analysis, is better accomplished under the hood.)
1-Octanol with 2-octanol
95.0
92.0
74.1
73.0
50.2
55.6
26.4
31.1
All α-values used in the calculations are the average of 4–5 measurements. b All mixtures were prepared by weighing, but per cent by weight and per cent by volume are essentially identical owing to the similarity in densities of isomeric alcohols. a
Figure 2. (a) Calibration curve from which the concentration of 1butanol (C, %) in the mixtures with 2-butanol can be determined (left and bottom coordinates). (b) A part of this curve in double logarithmic coordinates is shown in the region of linearity (right and top coordinates).
Note 1. The organic layer turns yellow owing to the presence of nitrogen dioxide. The latter can be easily blown off by air purging, but this is not necessary as shown by a blank run with no alcfohol added. Even the concentrated solution of NO2 has a very low absorbance in the region in question (Fig. 1a).
Literature Cited 1. Kjonaas, R. A.; Riedford, B. A. J. Chem. Educ. 1991, 68, 704–706. 2. Leenson, I. A.; Sergeev, G. B. Zh. Analit. Khim. 1977, 32, 1016– 1019; Chem. Abstr. 1977, 87, 145393m. 3. Stern, E. S.; Timmons, C. J. Gillam and Stern’s Introduction to Electronic Absorption Spectroscopy in Organic Chemistry, 3rd ed.; Edward Arnold: London, 1970; Chapter 4.
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