Alkyl groups, para

If, however, a tertiary amine has two alkyl groups and also an aryl group having the para position unsubstituted, then the action of nitrous acid is to insert the nitroso group directly into this para position. Dimethylaniline, for example, when treated with nitrous acid readily gives p-nitrosodimethyl-... 

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

Shape selective catalysts, such as ZeoHtes of the H-ZSM-5 type, are capable of directing alkyl groups preferentially to the para position (18). The ratio of the catalyst to the substrate also plays a role ia controlling the regiochemistry of the alkylations. For example, selective alkylation of anilines at the para position is achieved usiag alkylatiag ageats and AlCl ia equimolar ratio (19). 

The original acid—clay developers have been largely replaced by phenohc compounds, such as para-substituted phenohc novolaks. The alkyl group on the phenohc ring is typically butyl, octyl, nonyl, or phenyl. The acidity is higher than that of a typical unsubstituted novolak because of the high concentration of 2,2 -methylene bridges. 

V-Alkylaniline and /V,/V-di alkyl aniline hydrochlorides can be rearranged to C-alkyl anilines by heating the salts to 200—300°C. In this reaction, known as the Hofmann-Martius rearrangement, the alkyl group preferentially migrates to the para position. If this position is occupied, the ortho position is alkylated. 

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. ... 

Alkyl groups which are para to strong n-donor substituents such as hydroxy or ... 

Alkyl Groups. In the class of non-conjugative positions, the observed order of the deactivating effect of the methyl group is meta > pros (2-chloroquinoline), and the fall-off factor is 1/1.3 in this case. The fall-off factor is near unity if the effects from the meta position and the conjugative cata positions are compared (4-chloroquinoline), which indicates that the deactivating effect orders are cata > epi and amphi > pros as predicted by the benzenoid order para > meta. 

Ortho- and Para-Directing Activators Alkyl Groups... 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

Aromatic hydrocarbons substituted by alkyl groups other than methyl are notorious for their tendency to disproportionate in Friedel-Crafts reactions. This tendency has previously limited the application of the isomerization of para- or ortho-) m ky -benzenes to the corresponding meta compounds. At the lower temperature of the present modification, disproportionation can be minimized. 

Relative rate coefficients for alkyl group migrations in alkyltoluenes were obtained by Allen171 for the various possible rearrangements ortho - para, para - ortho, meta - para, para - meta, ortho - meta and meta - or/Ao, and are shown in Table 8. These results were obtained by using small amounts of... 

The latter reagent also methylates certain heterocyclic compounds (e.g., quinoline) and certain fused aromatic compounds (e.g., anthracene, phenanthrene). The reactions with the sulfur carbanions are especially useful, since none of these substrates can be methylated by the Friedel-Crafts procedure (11-12). It has been reported that aromatic nitro compounds can also be alkylated, not only with methyl but with other alkyl and substituted alkyl groups as well, in ortho and para positions, by treatment with an alkyllithium compound (or, with lower yields, a Grignard reagent), followed by an oxidizing agent such as Bra or DDQ (P- 1511). 

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