Ortho-para ratio

The catalysed nitration of phenol gives chiefly 0- and />-nitrophenol, (< 0-1% of w-nitrophenol is formed), with small quantities of dinitrated compound and condensed products. The ortho para ratio is very dependent on the conditions of reaction and the concentration of nitrous acid. Thus, in aqueous solution containing sulphuric acid (i 75 mol 1 ) and nitric acid (0-5 mol 1 ), the proportion of oriha-substitution decreases from 73 % to 9 % as the concentration of nitrous acid is varied from o-i mol l i. However, when acetic acid is the solvent the proportion of ortAo-substitution changes from 44 % to 74 % on the introduction of dinitrogen tetroxide (4-5 mol 1 ). 

The theory that the catalysed nitration proceeds through nitrosation was supported by the isolation of some />-nitrosophenol from the interrupted nitration of phenol, and from the observation that the ortho.-para ratio (9 91) of strongly catalysed nitration under aqueous conditions was very similar to the corresponding ratio of formation of nitrosophenols in the absence of nitric acid. ... 

As actually performed a 62% yield of a mixture of ortho and para nitration prod ucts has been obtained with an ortho-para ratio of about 1 3... 

The effect of low temperatures affecting the ortho para ratio is more important for light... 

Thermal conductivity is used as an analytical tool in the deterrnination of hydrogen. Because the thermal conductivities of ortho- and i7n -hydrogen are different, thermal conductivity detectors are used to determine the ortho para ratio of a hydrogen sample (240,241). In one method (242), an analy2er is described which spHts a hydrogen sample of unknown ortho para ratio into two separate streams, one of which is converted to normal hydrogen with a catalyst. The measured difference in thermal conductivity between the two streams is proportional to the ortho para ratio of the sample. 

The identification of a specific nitrating species can be approached by comparing selectivity with that of nitration under conditions known to involve the nitronium ion. Examination of part B of Table 10.7 shows that the position selectivity exhibited by acetyl nitrate toward toluene and ethylbenzene is not dramatically different from that observed with nitronium ion. The data for i-propylbenzene suggest a lower ortho para ratio for acetyl nitrate nitrations. This could indicate a larger steric factor for nitration by acetyl nitrate. 

A study of alkylations with a group of substituted benzyl halides and a range of Friedel-Crafts catalysts has provided insight into the trends in selectivity and orientation that accompany changes in both the alkyl group and the catalysts. There is a marked increase in substrate selectivity on going from / -nitrobenzyl chloride to /i-methoxybenzyl chloride. For example, with titanium tetrachloride as the catalyst, Aitoi Abenz increases from 2.5 to 97. This increase in substrate selectivity is accompanied by an increasing preference for para substitution. With /i-nitrobenzyl chloride, the ortho para ratio is 2 1 (the... 

Steric effects play a major role in determining the ortho para ratio in Friedel-Crafts alkylations. The amount of ortho substitution of toluene decreases as the size of the entering alkyl group increases along the series methyl, ethyl, /-propyl. No ortho product is found when the entering group is /-butyl. ... 

Steric fectors clearly enter into determining the ortho para ratio. The hindered 2,4,6-trimethylbenzoyl group is introduced with a 50 1 preference for the para position. Similarly, in the benzoylation of alkylbenzenes by benzoyl chloride-aluminum chloride, the amount of ortho product decreases (10.3%, 6.0%, 3.1%, 0.6%, respectively) as the branching of the alkyl group is increased along the series methyl, ethyl, i-propyl, t-butyl. ... 

One other feature of the data in Table 10.10 is worthy of further comment Notice that alkyl substituted acylium ions exhibit a smaller ortho para ratio than the various arpyl systems. If steric factors were dominating the position selectivity, one would expect the opposite result A possible explanation for this feature of the data could be that the aryl compounds are reacting via free acylium ions, whereas the alkyl systems may involve more bulky acyl chloride-catalyst complexes. 

Fluorobenzene is readily alkylated with alkenes in the presence of protic acids, however, the isomenc purity of the product is poor, and polysubstitution can result Thus, propene and sulfuric acid alkylate fluorobenzene at 20 C to yield a 45 55 ortho/para ratio of the inonoalkyl product m addition to di- and triiso propylfluorobenzene [i5] The reaction of benzene and trifluoropropene at 25 °C in HF-BF3 gives a mixture of mono-, bis-, and tns(3,3,3-trifluoropropyl)ben zene [72, 75] (equation 12)... 

A careful use of solvent effects should be of great assistance in synthetic chemistry. For example, it may be predicted from the solvent effects described above that in the reaction of 2,4-dichloroquinohne with piperidine the a y ratio should increase in the less polar solvents, although the result might be obscured by the mutual influence of the two chlorine substituents. Nitro-activated benzenes support this prediction since ortho para ratios of 4.2 in methanol and 69 in benzene were observed in the reaction of 2,4-dichloronitrobenzene with piperidine. ... 

The conversion of ortho-hydrogen to para-hydrogen is slow in the absence of a catalyst. Therefore, as one cools room-temperature hydrogen to low temperatures, the ortho. para ratio remains at 3 1, and entropy is present that results from the mixing of these two different types of hydrogen. 

The nature of the electrophile in this nitrating mixture is still not wholly agreed upon whereas kinetic evidence can be interpreted as consistent with nitration by nitronium ion, the fact that substituents with lone pairs of electrons or it-electrons give markedly different ortho para ratios from other nitrating mixtures is usually conceded to be consistent with the electrophile being something other than the nitronium ion. The balance of evidence at present is in favour of pro-tonated acetyl nitrate being the electrophile. 

Ortho para ratios for nitration of some aromatics... 

Amines and phenols can be nitrated by dilute (< 5 M) solutions of nitric acid in water or acetic acid provided nitrous acid is present1 °3. A mixture of products is frequently obtained and the ortho para ratio can vary widely according to the nitrous acid concentration104 105, thereby indicating a different electrophile under these conditions. 

The above mechanism for chloromethylation seems to be general for halo-methylation since bromomethylation gives the same ortho para ratio for toluene, ethylbenzene, and i-propylbenzene, which is entirely in accord with the halogen being substituted in a non rate-determining step of the reaction386. 

Hanstein and Traylor544 used a 4.3 M solution of trifluoroacetic acid in chloroform to measure the rates of deuteration of (PhCH2)2Hg at 35 °C, for which kx = 2,100 x 10"7. Comparison with other deuteration rates (which, however, were neither quoted nor referred to) was said to give a (CH2HgCH2Ph) = — 1.14 and the ortho para ratio was 0.84. For the compound PhCH2B(OH)j, the rate of deuteration by 3 M tartaric acid at 100 °C was found to be 5 x 10 7 (ortho para ratio = 1.1) and from this a value of ff4 (CH2B(OH)J) of —1.11 was claimed, but without the relevant data used to obtain these c+ values cautious use of them seems appropriate. 

When an ortho-para directing group is on a ring, it is usually difficult to predict how much of the product will be the ortho isomer and how much the para isomer. Indeed, these proportions can depend greatly on the reaction conditions. For example, chlorination of toluene gives an ortho/para ratio anywhere from 62/38 to 34/66. Nevertheless, certain points can be made. On a purely statistical basis there would be 67% ortho and 33% para, since there are two ortho positions and only one para. However, the phenonium ion (9), which arises from protonation of benzene, has the... 

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