Para attack

Like hydroxyl groups and ammo groups however halogen substituents possess unshared electron pairs that can be donated to a positively charged carbon This electron donation into the TT system stabilizes the intermediates derived from ortho and from para attack... 

Sections How substituents control rate and regioselectivity m electrophilic aro 12 10-12 14 matic substitution results from their effect on carbocation stability An electron releasing substituent stabilizes the cyclohexadienyl cation inter mediates corresponding to ortho and para attack more than meta... 

Regioselectivity in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables.51 In general, para attack predominates for... 

Through resonance, halogen tends to stabilise the carbocation and the effect is more pronounced at ortho- and para- positions. The inductive effect is stronger than resonance and causes net electron withdrawal and thus causes net deactivation. The resonance effect tends to oppose the Inductive effect for the attack at ortho- and para-positions and hence makes the deactivation less for ortho- and para-attack. Reactivity Is thus controlled by the stronger Inductive effect and orientation Is controlled by resonance effect. 

The sigma complexes arising from ortho, meta, and para attack. 

Regioselectivity in Friedel-Crafts acylations can be quite sensitive to the reaction solvent and other procedural variables.45 In general, para attack predominates for alkylbenzenes.46 The percentage of ortho attack increases with the electrophilicity of the acylium ion, and as much as 50% ortho product is observed with the formylium and... 

The exceptions to the general rule of preference for para-attack are cases in which, according to calculations, the para-product is destabilized relative to the observed ort/ro-product (85f) by at least 35 kcal/mol, or the observed conjugated product of attack on the S-carbon of a 4-stilbenyl ion (44k,1) is stabilized by ca. 5 kcal/mol relative to the product of para-attack. In the latter case calculations at the RHF/6-31G //3-21G level suggest that attack of H2O on the /S-carbon may be facilitated by the large magnitude of the LUMO coefficient at that carbon. ... 

The obvious way to obtain this information is to carry out reactions with various Z groups and to analyze the products for percent ortho, meta, and para isomers, as has so often been done for electrophilic substitution. However, this procedure is much less accurate in the case of free-radical substitutions because of the many side reactions. It may be, for example, that in a given case the ortho position is more reactive than the para, but the intermediate from the para attack may go on to product while that from ortho attack gives a side reaction. In such a case, analysis of the three products does not give a true picture of which position... 

The observed para-selectivity might be due either to the bulkiness of the non dissociated N02 N03 ion pair in solution or to the adsorption of the same nitrating agent onto the polar surface of the dessicant. The later would indeed produce a strong increase in steric hindrance and could explain the para-attack of the incoming substrate. 

Consider the nitration of toluene (Following fig.). The amount of meta substitution is very small as expected and there is a preference for the ortho and para products. The formation of more ortho substitution compared to para substitution is due to the fact that there are two ortho sites on the molecule to one para site and so there is double the chance of ortho attack to para attack. Based on pure statistics it would be expected that the ratio of ortho to para attack to be 2 1. In fact, the ratio is closer to 1.5 1. In other words, there is less ortho substitution than expected. This is because the ortho sites are immediately next door to the methyl substituents and the size of the substituent tends to inference with ortho attack— a steric effect. The significance of the steric effect will vary according... 

The electron-withdrawing effects also destabilises the reaction intermediate and makes the reaction more difficult. This destabilisation is more pronounced in the intermediates arising from ortho/para attack and so meta attack is favoured. 

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