ANALYTICAL CHEMISTRY
1250
Figure 1. %Methylnaphthalene (50 X) hfc.
Right.
Cmstals obtained by careful maoraublimation Thin crystalline film from fusion
OPTICALPROPERTIES (deterzined by W. McCrone). Refractive Indices (5893 A,; 25" C.), CL = 1.494 * 0.001, 0 = 1.640 0.004. y = 1.77 0.01. Optic Axid Angles (5893 b.; 25" C.). 2V, = 81"; 2Er = 108". f
f
Dispersion. v > II (on centered obtuse hisedrix figure). Optic A.xidalPlrtne. L!OO. Sign of Double Refraction. ( Acute Bisectrix. In the 100 p
form hrientatia& orientation. These cryst& crystals, ne& never show angles ,eit,her either growing or in mixed fusion. 2. All crystals from the melt show a centered obtuse hisectrix interference figure with isogyres just included by N . A . = 1.25; 2 H y = 108'; v > r on this view and the sign is negative. A mixed fusion with Citrgille index liquid, 1.414,oauses the crystals cryst,als t o distlppear in the position showing alpha at 2C"
-
I CITED
uramiscne nnsrauographie." Vol. Engelmann, 1910. (2) Neuhaus, Z. R ~ i s t .101, , 177-92 (1939).
(1) CTrocn,
0,
p.
*LA,
~ieiyuig.
meter Analysis SIR: The reoent article on "Mass Spectrometer Analysis" by Brown et al. [ANAL.CHEM.,20, 5 (19481 contains mme excellent work on the application of this technique to liquid samples. The d s h presented in Tablea XI a i d XI1 introduce into the literature for the first time the conclusion that 3-ethylpentane and other ethylparaffins are present in oonsiderable quantities in cracked naphthas. The data. submitted on this point should, therefore, be examined oritioally. The following data are presented in Table XI: NaBtha. B, Vol. % Naphtha A, Vol. % Fraotion No. 2 3 4 2 3 4 3-Methylhersne 6.06 1.78 2.94 4.74 3.57 1.10 3-Ethyl~enthne %-Heptane
.. .,
0.26 1.62
ii.92
.. ..
0.822.26 li.78
In distillation outs of the type being analyzed here. the abundanoe ratio 01 the compounds found on analyzing successive fractions has been ioun'd to follow a regular pattern determined by boiling points. I n the case of naphtha A, for example, if 3-ethylpentane were present to the reported extent between 3-methylhenme and n-heptane in fraction 3, it would normally be found in much IcLrger amount in fraction 4, where both the adjacently boiling oompounds are increased. In the oorresponding cuts from naphtha. B a similar situation appears, and the amount of 3-ethylpentme would be expected to be roughly the same in fraotions 3 and 4. Accordingly, the total amounts of this compound in the original samples are probably much larger ,than those reported here if the No. 3 cut analyses &re reliable. or much less if the No. 4 cut analyses are correct. In previous naphtha analyses by fractional distillation, infrared, or Raman spectroscopy, it has usually been impossble to demonstrate that the ethylparaffins'me present in more than trace amounts. Specific statements hcwe been made with regard to the apparent absence of 3-ethylpentane.in the seven virgin naphthas examined by A.P.I. Project 6 (I), and the absence of the methylethylpentams in alkylates and hydropolymers (2). I n view of the data previously reported and the internalinconsistency of the present data, it Seems that 8 further study of these samples may be in order. The above situation suggests t h t a major defioiency of the method
a8 described may be the uncertainty of the qualitative analysis of thr samples. In Table XII, for example, it is not clear why 2,2.3-trimethylbutane was included in the analysis and 2,Z-dimethylpentane left out. The authors report that these compounds have very similar maas spectra, and previous data indicate that the 2.2-dimethylpentane is .a likely and 2,2,3-trimethylbutane an unlikely constituent. The vduelues reported for the trimethylpentanes in.Ta,ble XI1 and 3,3-dimethylpentane in Table XI &re also interesting in this oonneotion. It would appear that further experiments are warranted to determine whether or not these compounds are as rare in naphtha samples 88 has commonly been supposed. Hohma J. HAIL Esso Laboratories, Standard Oil Development Co., Elisabeth, N. J.
LITERATURE CITED
(1) Foruiati, A. F., Willingham. C. B., Mair. B. J.. and Rossini, F. D., J . Resewch Nail. Bur. Standards, 32,31 (1944). (2) Glasgow, A. R., Streiff. A. J., Willingham, C. B., and Rossini, F. D., Proc. Am. Petrolwm Inst., 26.111. 169 (1946). SIR: It is well recognized that the ability of the mass speotrometer aoourately to resolve an isomeric mixture is largely dependent on both the nature of the mixture and the relative concentrations of individual components therein. To verify independently the presenoe of 3-ethylpenhne in naphtha BI cut 3 was analyeed by an infrared spectrometer subsequent to the receipt of Hall's communication. At the wave length 01 11.13 microns, 3-ethylpentane exhibits a unique absorption relative to other hydrooabons present in the 92' to 96O C. distillate cut. The data show positive infrared confirmation of the maas spectrometer analysis, thereby establishing beyond reasonable doubt the presence of 3-ethylpentane in the cut in question. In out 4,*however,the total volume of the sample was oonsiderably larger, with the oonoentration of heptanes present in excess of 75%. thereby rendering the
