Microscale Organic Laboratory, Ill A Simple Procedure for Carrying Out Ultra-Micro Boiling Point Determinations Dana W. Mayo, Ronald M. Pike,' Samuel S. Butcher, and Marcia L. Meredith Bowdoin College. Brunswick, ME 0401 1 Historically, the boiling point of a liquid substance has provided an important ~hvsicalconstant used as a criterion both of product identity and purity. In the development of techniques to be emploved in an introductorv microscale organic laboratory piogiam (1,2), i t became bbvious that conventional procedures described in the literature (3-8) for acquiring bofiing point data would not be practical. The microscale preparations carried out a t Bowdoin generally yield quantities of liquid products in the 30-70 fiL range. The allocation of 5 fiL or less to boiling point measurements, therefore. became hiehlv desirable. We ha;e now estaLlished that reproducible and reasonablv accurate (f1'C) hoiline points can be observed on 3-4 p ~ b many f liquids employl'ng a modified Wiegand procedure (9). Sophomore organic students readily master the necessary manipulations to make these determinations successfully. Procedure Ultra-micro boiling points can be conveniently determined in standard (90-mm length) Pyrex glass capillary meltinn.. . noint tubes. The melting.. noint . tube re~lacesthe convenriunnl 3-4-mm (u.d.) tubing used in the Siwdoboff un~cedure( 3 ) .The samule (3-1 uLi is loaded into the melting-point ci&lary viaai0-;L syringe and centrifuged to the bottom. A small glass bell (which replaces the conventional melting point tube as the bubble generator) is formed by first drawing out [microburner] 3-mm (0.d.) Pyrex tubing to a diameter small enough to be accepted by the melting-point capillary. Second, a section of the drawn capillary is fused [microburner] and then cut to yield two small glass cavities annroximatelv 5 mm lone (see Fin. la). One of the elass ., ca\.ities is inserted into t f e loaded'melt'ing-point capiiiary, open end iirst (down),~ n alluwed d to fall to the buttom. The aiseml~ledsystem wee Fig. 111)is then inserted into thestage of a Thomas-Hoover Uni-Melt Caoillaw Meltina Point ADparatus (10) or similar system. p he temperature is rapidly raised to 15-2O9 below the expected boiling point and then adjusted to a 2"Imiu rise rate until a fine stream of bubbles is emitted from the glass bell. (The temperature should be monitored carefully in the case of unknown substances.) The heater is then adjusted to drop the temperature. The boiling point is recorded when the last escaping hubhle collapses. This sequence may then he repeated by quickly adiustine the heater to eive a 2'1min increase which induces a seeon; stream of bubhes. he above cycle generally may be carried out several more times if desired. Observed boiline points for a series of compounds having boiling points that cover a wid9 range of temperatures are summarized in the table. Discussion
Utilizing a melting point capillary as the boiler tube has the particular advantage that the system is ideally suited for
' Visiting Charles Weston Pickard Professor of Chemistry, Spring
1984.
1114
Journal of Chemical Education
observatiun in a conventional melting-point apparatus. The illumination and maenifiratiun available make the identification of rate changes in the bubble stream readily apparent. Utility gas chromatographic syringes (10 fiL) appear to he the most successful instrument in student hands for dealing with the small quantities of liquids involved in these transfers. The 3-in. needles normally supplied with the 10-fiL barrels will not reach the bottom of the capillary. Liquid samnles denosited on the wall of the tube. however. are easiiy and efficiently moved to the bottom by &rifuga;ion. Aftrr n~ckinnthe samole in the bottom of the ranillarv tuhe. the giass hey1 is introduced. The use of the glass bell i$ necessitated by the fact that if a Wiegand capillary insert, which normally exceeds the length of the outer tube, is emploved capillarv action between the "boiler" tube wall and the capillary insert invariably draws the majority of the sample from the bottom up onto the inner wall of the outer tube. Thus, the formationof a hubhle stream often is prevented. We have observed that little loss of even low-boiling liquids (see the table) occurs during trial runs. If the boiling point is overrun and the sample flashed from the bottom section of the "boiler" capillary, i t rapidly condenses on the u m e r cooler sections of the tuhe which extend above the hkit transfer liquid. The sample easily can be recentrifuged to the bottom of the tube and a new determination of the boiling point attempted. Indeed, if the bell cavity fills completely during the cooling point of a cycle, it is often difficult to reinitiate the bubble stream without first emptying the entire cavity by overrunning the boiling point. In our experience, materials that are thermally stable a t their boiling point will give identical values on repeat determinations. Substances that begin to decompose will give values that slowly drift after the first few measurements.
-
Observed Bolllng Polnts (OC)' Compound
Observed
Literature Value
Methyl iodide lsopropyl sl~ohol 2.2-Dimethaxyprapane 2-Heptanone (I-lrL sample) Cumene Mesilylsne pCymsne Benzyl alcohol Diphenylmethane observed values uncorreued for changes in atmosuhsrlc pressure (oareotlonr all esamslsd to be le3s than M.S°C). "cRc Hardbwk of ChemisQ and Physics;' 62nd ed.. CRC h s , lnc.. 6-Ratan. FL. 1981. Y9082.0. G373.
a..~iuimaryof ~ r g i n i c ~ o m ~ o u 41h nd~ s ~1 . oxford . un~vasityRW. ~ Val. I, p. 11.
m nIS(U; ,
"CRC Handbeak o l Chemlnry and Physics." 62nd ed., CRC Ress, Inc. Bocs Ratm, FL, 1981,17827. p. C-321. '"CRC Handbark," 65394, p. C-244. 9 "CRC Handbook," 68987. p. G370. "CRC Handbwk," 62192. p. C-138. '"CRC Handbook." 63180, p. G169. I"CRC Handbook." 66282. G274.
The observation of color and/or viscosity changes plus a variable boiling point all signal caution in interpreting repeat measurements. Students in our microscale organic laboratory program have successfully utilized this procedure in determining the boiling points of a number of liquid products. Acknowledgment
We wish to acknowledge the work of Janet R. Hotham, Paulette M. Fickett, and David J. Butcher. They are primarily responsible for the successful adaptation of this technique to the microscale teaching laboratory. Because of their enthusiasm and innovation a technique that normally would have remained in the research laboratory has made a smooth
transition into the undergraduate laboratory experience. The authors are grateful to the Surdua Foundation and the ARC0 Foundation for generous support of the development of this laboratory program. Literature Cited (11 Butcher.S. S.,Mayo,D. W.,Pike.R. M.,Fmte,C. M.,Hotham.J.R..Pspe.D.S..J. CHEM.EDUC.,62.147 (19851. (21 MWO. D.w., B U V ~S.~s.. ~ pike, , R. M., ~ ~ tC. eM.,, ~ ~ tJ. %Page, h ~ D. ~ s..J. , CHEM.EDUC., 62.119 (1985). (3) s i ~ ~ ~ ~ b ~19,795 f r ,(1886). ~.,~~~., (41 schumann.~h.,z.anoi. ch..~.249(1870!.
'"
(6) (7) (8) (,, (10)
O'Dowd, L., perkin. F M., sSE"mS""~Th''Amer J~Phnrm"4"527i1869). ram ~ a m d o soe., y 4.95 (1909). 81itc H..&r.. 30.1208 (189'11. Smith, A., Menzies, A. W. C., il Amer, Chem. Soc, 32.897 (1910). ,iepand,c.,n Chem,, 61,77 (19551,
ThomasSeienti(ic,VineStroetatThird.P.O.Rox779.Philadelphia,PA19105-W79.
Volume 62
Number 12
December 1965
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