November 29, 1939
Dear Dr. Heidelberger:
My delay in answering your letter of the 8th has not been caused from lack of interest, I assure you.  Thank you for writing me.
I agree with you that the work on cyclooctatetraene was carefully done.  What is more, it reads convincingly, but it does seem to me that the argument has loop-holes, some of which I mentioned in the paper.
The narrow boiling range (0.5 degrees) which you mentioned cannot be leaned on too heavily for isomeric dibromides should boil pretty closely together.
You mention that different effects might have been encountered at pyrolysis temperatures of 200-250 degrees than at 100-150 degrees.  Perhaps so, but I doubt it because no comparable rearrangements occur with other unsaturated hydrocarbons till much higher temperatures are reached.  1-Butene is fairly stable at 500 degrees, and rearrangement into 2-butene starts at 600-650 degrees (Hurd and Goldsby, J. Am. Chem. Soc., 56, 1813 (1934)).  Similarly, 600 degrees is the temperature required to initiate the rearrangement of 1 - into 2-pentene (Hurd, Goodyear, and Goldsby, ibid., 58, 235 (1936)).  Rearrangement of 1-alkynes into 1, 2-alkadienes does not occur at 400 degrees, but requires temperatures which are high enough to break the hydrocarbon otherwise into smaller molecules (Hurd and Christ, ibid., 59, 2161 (1937)).  These were experiments with 1-heptyne, 1-hexyne and 1-butyne.  With the butyne, it was proven that no 1, 3-butadiene came even at 56 degrees Celsius.  Hence I feel that anywhere between 100-300 degrees would be safe temperatures insofar as subsequent rearrangements are concerned when dealing with 1, 2-butadiene, 1, 3-butadiene, or 1- or 2-butyne.
You point out that our X was a 1, 2-diquaternary compound whereas Willstatter's VII (in our paper) was not.  That is a good criticism and one that I had thought of.  Nothing would be gained by taking the 1, 4-isomer that you suggested, however, namely, HOONMe3-CH2-CH2CH2NMe3OH, because 1, 3-butadiene is the only possibility here.  If the C6-analog were taken, CH3-NMe3-CHCH2CH2NMe3OH-CHCH3, I feel sure that it would pyrolyze to the mixture of 1, 5 -, 1, 4 -, and 2, 4- hexadienes the last of which is the only conjugated one.  Of course this mixture would involve greater analytical difficulties than the C4 - analogs.  Also, the original synthesis would be harder.
There is one experiment more convincing than any of these that I would like to have tried, namely, the pyrolysis of CH3NMe3OH-CH = CH2.
With one double bond already in place this would be quite analogous to Willstatter's VII.  The difficulty here is the fact that the halide, CH3CHBrCH=CH2, from which the hydroxide would be made is so unstable, yielding CH3CH=CHCH2Br.  The base corresponding to the latter should pyrolyze exclusively to methylallene, whereas the desired isomer might pyrolyze either to methylallene or 1, 3-butadiene.  The experiment, therefore, would be inconclusive.
The best escape from this dilemma, and it is an experiment I have planned to do for a long time, is to take the C5-analog.  If CH3CHBrCH=CHCH3 rearranges, the product is still the same compound.  Hence, the base, CH3CH(NMe3OH)-CH=CHCH3,  should be obtainable pure.  It should yield either 1,3- or 2,3-pentadiene or both.  A mixture of the two pentadienes could be separated by means of maleic anhydride, so the approach seems fairly definite.
I shall be pleased to keep you advised of further developments.  Thank you ever so much for your helpful suggestions.
Very sincerely yours,
Charles D. Hurd