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Synthesis of 1,3,5-tri-tert. Butylbenzene
Abstract
A synthesis is described of 1,3,5-tri-tert.butylbenzene starting from 3,5-di-tert.butyltoluene.
The preparation of a number of chlorinated derivatives of meta-di-tert-butylbenzene from 3,5-di-tert-butylacetanilide and from 3,5-di-tert-butylbenzoic acid is described.
The rate of the Schmidt reaction at 20°C has been determined for a variety of substituted benzoic acids. It is shown that electronic effects of substituents affect the reaction rate, electron donation increases the rate and electron withdrawal decreases it. Ortho substituents accelerate the reaction, and the reaction is a good example of steric assistance. The buttressing effect of a tert-butyl group on an adjacent halogen atom is demonstrated in the high reaction rate of 2-halogeno-3,5-di-tert-butylbenzoic acids. An attempt has been made to separate the effect of an ortho substituent into steric, inductive and mesomeric contributions. A discussion on the mechanism of the reaction is included.
The synthesis of a number of derivatives of meta-di-tert.. butylbenzene is described. Wherever the constitution of the compounds obtained did not follow from the method of preparation, it is proved.
Following up earlier work, this paper discusses the influence of 2-methyl-6-tert.butyl substitution and/or 2,6-di-tert.butyl substitution on certain properties of compounds such as nitrobenzene, aniline, N-methylaniline, N,N-dimethylaniline, acetanilide, 4-nitro-aniline, and 4-nitro-acetanilide.
For the derivatives of nitrobenzene, N,N-dimethylaniline, and acetanilide the electronic absorption spectra indicate a complete or almost complete elimination of the mesomeric interaction. In contrast with this it has to be concluded for all the primary amines investigated that the mesomerism is fully developed; in the relevant discussion the vectorial treatment of absorption Intensities has been of decisive importance. The secondary amines take up an intermediate position; in these cases, owing to the pyramidal configuration of the nitrogen atom, the angle of twist cannot be forced to values exceeding about 60°.
The basic strength of 2,6-di-tert.butyl-substituted primary amines is lower by a factor of about 108 than might be expected on the basis of the electrical effects of the alkyl groups; the basic strength of 2-methyl-6-tert.butyl-substituted N,N-dimethylanilines is lower by a factor of about 106 than might be expected on the basis of the electrical effects of the alkyl groups and the steric inhibition of the mesomerism present in these compounds. These reductions of the basic strength have been ascribed to steric hindrance to solvation.
The basic strength of 3,5-di-tert.butyl-4-nitroaniline confirms previously obtained information on the quantitative significance of the inductive and the mesomeric effect of the nitro group in 4-nitroaniline as well as on the meta/para ratio of the inductive effect.
The isomer distributions and the partial rate factors for the nitration of eight tertiary alkylbenzenes and five tertiary cycloalkylbenzenes have been determined. The fm values for the tertiary alkylbenzenes are greater than for the primary and secondary compounds. The fp value for (1-methylcyclopropyl)-benzene shows that the pseudo conjugation between the cyclopropyl group and the benzene ring is disturbed. The steric hindrance of the nitration in the ortho-positions increases greatly as the branching of the alkyl groups increases.
By alkylation of benzene with t-butyl chloride in the presence of aluminium chloride, 1,4-di- and 1,3,5-tri-t-butylbenzene can be prepared in a simple manner. Several polyalkylbenzenes with one or more t-butyl groups have been made readily accessible.
Nine symmetrical trialkylbenzenes of good purity have been prepared by HF-BF3 catalysis. The infrared and ultraviolet bands of these compounds show an orderly displacement with changes in the alkyl groups. The boiling point and specific refraction have been related to carbon number. These correlations, along with a molecular weight determined by the mass spectrometer, help to prove the structure of a new compound, 1,3,5-tri-t-butylbenzene.
The reaction of t-butyl chloride and aluminum chloride with p-di-t-butylbenzene yields m-di-t-butylbenzene and hydrocarbons melting at 72.5-73° and 209-210°. The product of m.p. 72.5-73° is 1,3,5-tri-t-butylbenzene. It has been converted to 2,4,6-tri-t-butylaniline and 2,4,6-tri-t-butylphenol. Bromination in the presence of iron displaces one t-butyl group yielding 1-bromo-3,5-di-t-butylbenzene. The tri-t-butylbenzene has been obtained also from the condensation of pinacolone by means of metallic sodium. Potentiometric titration in 90% methanol reveals that 2,4,6-tri-t-butylaniline is not detectably basic under the conditions used, i.e., the pK A of the anilinium ion is substantially less than 2.
Further evidence for steric interference by donor alkyl substituents to the formation of halogen-aromatic complexes has been obtained through measurement of equilibrium constants for the interaction of a large number of polyalkylbenzenes with iodine monochloride. As few as three t-butyl substituents or five ethyl substituents on a benzene donor constitute a steric barrier to this type of interaction. The heats of interaction of iodine monochloride and substituted benzenes in carbon tetrachloride have been found to vary in linear fashion with the free energy and entropy changes which accompany the reaction. These variations may be described on the same graphs as have been prepared previously to summarize corresponding linear variations in thermodynamic constants for the interaction of substituted benzenes with iodine. The problem of comparing complex equilibrium constants expressed in molar concentration units with those given in mole fraction units has been considered.
The mechanism of alkyl-group transfer has been studied by the disproportionation of six alkylbenzenes in the presence of hydrogen fluoride and boron trifluoride. In experiments at room temperature with alkylbenzenes containing n-propyl, n-butyl and s-butyl groups, the alkyl group maintained its configuration while transferring from one ring to another. A bimolecular mechanism is followed wherein the migrating alkyl group, before its bond with the first ring is broken, forms a partial bond to the second arene. In support of this theory, ethylbenzene was about 90% disproportionated at room temperature, whereas neopentylbenzene was unconverted. Presumably the t-butyl segment of the neopentyl group sterically interferes with the back-side approach of the attacking arene and thereby prevents reaction. At higher temperatures, experiments with t-butylbenzene and hexaethylbenzene showed that the mechanism changes. The arene cation dissociates unimolecularly into an arene and an alkylcarbonium ion. The carbonium ion then reacts by isomerization, alkylation, polymerization and hydride-ion abstraction to yield a complex product.
It is proved that the hydrocarbon of m.p. 31-32° obtained by di-tert.-butylation of toluene is 3,5-di-tert.butyltoluene.