Ortho, para-directing substituents

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... 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Crafts alkylations are, of course, well known from the benzene series, where large proportions of the meta isomers are obtained upon alkylation of benzenes with ortho-para directing substituents. H. C. 

This is a relatively straightforward synthetic problem. Bromine is an ortho, para-directing substituent nitro is meta-directing. Nitrate first, and then brominate to give 1-bromo-3-nitrobenzene. 

Methoxy is an ortho, para-directing substituent. All that is required to prepare p-methoxy-benzenesulfonic acid is to sulfonate anisole. 

Direct nitration of the prescribed starting material cumene (isopropylbenzene) is not suitable, because isopropyl is an ortho, para-directing substituent and will give the target molecule... 

Qualitatively, the effect of the electropositive heteroatom of arsabenzene appears to be comparable to that of an activating ortho-para directing substituent on benzene. Presumably, the electrophile attacks only at the 4- (and 2-)positions because only these positions allow efficient electronic stabilization of the intermediate cr-com-plex 64. 

Cyclizations meta to an ortho, para directing substituent are known. For example, the 7-substituted tetralones XXIV were prepared16 from the corresponding arylbutyric acids XXIII. 

A number of authors have studied substitution with the nitro group in benzene derivatives containing ortho-para directing substituents, when nitric acid with acetic anhydride was used. The experiments led to the conclusion that replacement of water in the nitrating mixtures by acetic anhydride produces an increase of the ratio of ortho- to para- isomers. 

The first compound has one electron-withdrawing substituent (CO2H), which is metn-directing md deactivating. The second has two identical ortho, para-directing substituents (alkyl groups), hich activate all positions. Steric hindrance decides where the nitro group will go. 

The remaining two compounds have two competing ortho, para-directing substituents, but in each case the one with lone pair electrons (NHAc or OR) beats the simple alkyl group. In the first... 

Titanium tetrachloride and tin tetrachloride are milder catalysts and have the important advantage of solubility in organic media such as carbon disulfide, so that reaction in a homogeneous phase becomes possible. Cullinane and Leyshon514 recommend titanium tetrachloride particularly for alkylations since polyalkylation is then not observed. When the aromatic reactant carries an ortho-para-directing substituent, alkylation under the influence of titanium tetrachloride usually occurs only at the para-position. 

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