Acetyl perchlorate

Burton and Praill were the first to prqjare this ecies both by the interaction of perchloric acid with acetic anhydride and the metathetic reaction of sflver perchlorate with acetyl chloride , in a comprehensive study of acylation reactions. They did not attempt a characterisation of the compound and assumed that in acid solutions it existed in a fully ionised form. A few years later, Jander and Surawski followed the formation of acetyl perchlorate in acetic anhydride by measuring the electrical conductivity changes which took place when acetyl bromide was added to silver perchlorate or vice versa. In both titrations an inflection point was observed for a mixture of equimolar quantities of the two reactants indicating the formation of acetyl perchlorate. Moreover, it was clearly drown that this compound was at least partly ionised since its conductivity was higher than that of acetyl bromide. Avedikian and Commeyras characterised acetyl perchlorate by infrared and Raman spectroscopy both in acetic anhydride and carbon tetrachloride. They concluded that some ionisation was present but could not assess its extent. Molecular acetyl perchlorate was also detected. As far as we are aware, no other study of the structure and extent of dissociation of acetyl perchlorate in solution has been conducted. 

The detrimental effect of tetrabutylammonium iodide and tetrafluoroborate on the rate of polymerisation of styrene in the presence of acetyl perchlorate can easily be ex-plaiited by an anion-exchange reaction with the propagating esta to give a much less active chain carrier. 

A subsequent more important paper by Higashimura and Kishiro dealt with the system styrene-acetyl perdilorate from the point of view of the kinetics of polymerisation and DP distribution. At 0°C a simple kinetic pattern was observed resembling closely that characteristic of the classic tystem styrene-perchloric acid (see Sect. III-E-13-a). The polymers displayed bimodal distributions and the relative inqjortance of the two peaks was studied as a function of temperature, added water, dielectric constant of the medium, catalyst concentration, and common anion concentration. Two alternative possibilities must be considered in the overall interpretation of these results  

Most probably, the real situation in these systems is one of compromise between the two possibilities discussed above, Le., the initiator is a mixture of perdhloric acid and acetyl perchlorate. It seems obvious to us that the mechanism of these polymerisations is psoidocationic and the minor effect of added perchlorate salts is simply due to homoconjugation of perchloric acid present at the beginning of the reaction or liberated in ontaneous transfer processes, and not to common-ion effects suppressing the concentration of free ionic chain carriers. 

Further work by Higashimura s group on the polymerisation of styrene and styrene derivatives by acetyl perchlorate does not provide any substantial new 

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Acetyl perchlorate should be a strong acid in acetic anhydride, but it decomposes too rapidly for accurate measurement.203... 

Acetylium tetrailuoroborate, 4 144t N-Acetyl-p-aminophenol (acetaminophen), acetic anhydride used in production of, 1 158. See also Acetaminophen Acetyl perchlorate, 1 157 Acetylsalicylic acid (aspirin), 22 17-21 acetic anhydride used in production of, 1 158... 

Cationic polymerizations can be initiated with protic acids (e.g., sulfuric, perchloric, trifluoroacetic acid), with Lewis acids (see Sect. 3.2.1.1), and with compounds that form suitable cations (e.g., iodine, acetyl perchlorate). Some monomers are also polymerized by high-energy radiation according to a cationic mechanism. 

A very important category of mistaken identity concerns the polymerisations allegedly initiated by acetyl perchlorate. Unless such experiments are conducted in high vacuum systems with really good technique, the residual water to be found under almost all other conditions produces a mixture of acetic and perchloric acids. The HCIO4 will participate in the reaction together with any unhydrolysed acetyl perchlorate. The real trouble... 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

Ibid, 1953,827 (Action of acetyl perchlorate on benzene and related compds)... 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The first reported controlled/living polymerization of p-methylstyrene with acetyl perchlorate in the presence of HBU4NCIO4 [221] was based on the added salt method. Later, it was reported that cumyl acetate/ BC13 and related initiating systems induce controlled/living polymerizations of p-methylstyrene [117,222] and 1,3,5-trimethylstyrene [223]. The HI/ZnC(2 system, suited for vinyl ethers and p-alkoxystyrenes, can also be used for p-methylstyrene [224], but the lower reactivity of the monomer requires a much higher concentration of the zinc activator (ca. 100 mJW for 10 mM HI) to obtain a sufficient polymerization rate. These systems function without added nucleophilic additives and can be classified under the counteranion method. 

Refs l)Beil—not found 2)H-Mackenzie E. Winter, TrFaradSoc 44,169(1947) CA 42, 6623(1948) 3)H.Burton P.Praill, JCS 1950, 1203 2034 4)Ibid, 1953,827 (Action of acetyl perchlorate on benzene and related compds)... 

If one supposes that the real initiator is acetyl perchlorate, then it must be accepted that this compound attacks styrene in a fast initiation reaction and thereafter the situation is in any case totally equivalent to that of a polymerisation induced by perchloric acid. Should this be so, the explanation given by Hamman et concern-... 

Higashimura and Nishi have recently studied the dimerisation of styrene and a-methylstyrene by acetyl perchlorate. An appropriate choice of temperature and solvent allowed the c timisation of linear dimers formation. These authors postulate a transition state for the formation of the linear dimers which resembled closely the spontaneous-transfer mechanism proposed thirteen years earlier by Gandini and Hesch in the pseudocationic polymerisation of styrene by perchloric acid. 

Kohjiya and Yamadiita investigated the polymerisation of cyclopentadiene by acetyl perchlorate in toluene-methylene chloride mixtures. They found a kinetic behaviour entirely simUar to that of the system cyclopentadiene-perchloric acid, which they had studied previously. In fact, in the same solvent mixture and at the same temperature the time-conversion curves were practically identical. For a criticism of the method... 

Acetyl perchlorate has also been used as an initiator in the cationic polymerisation of substituted butadienes in methylene chloride and toluene (possible acylation of this solvent by the catalyst was ignored). These studies do not advance the limited knowledge on the properties of this catalyst in solution and on its mode of initiation. 

It is disappointing to see that all the work carried out with acetyl perchlorate to the present, has failed to provide any real fundamental information about the way this catalyst operates. It seems in fact that its use was simply decided in order to avoid the more laborious preparation of solutions of anhydrous perchloric acid. In this respect, it might have served a purpose. We think that acetyl perchlorate would be a very interesting catalyst to investigate under strictly controlled experimental conditions. 

Hasegawa and Higashimura reported an interesting application of their recent studies on the selective dimerisation of styrenes. By choosing the conditions which give an optimum yield of linear dimer, they polymerised divinylbenzene with acetyl perchlorate to a linear polymer with the structure... 

Diphenylcyclopropenecarboxylic acid gives the corresponding diphenylcyclopro-penyl cation using acetyl perchlorate however, the major product is durene when the tetrafluoroborate salt is used. With triphenylmethyl perchlorate in methylene chloride and lithium carbonate this acid is converted into l,2-diphenyl-3-triphenylmethylcyclopropenyl cation (15) (equation 7). 

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