Substituents locations

Nucleophilic substitution of benzene itself is not possible but the halogeno derivatives undergo nucleophilic displacement or elimination reactions (see arynes). Substituents located in the 1,2 positions are called ortho- 1,3 meta- and 1,4 para-. 

The greater stability of an equatorial methyl group, compared with an axial one, is another example of a steric effect (Section 3.2). An axial substituent is said to be crowded because of 1,3-diaxial repulsions between itself and the other two axial substituents located on the same side of the ring. 

For the Birch reduction of mono-substituted aromatic substrates the substituents generally influence the course of the reduction process. Electron-donating substituents (e.g. alkyl or alkoxyl groups) lead to products with the substituent located at a double bond carbon center. The reduction of methoxybenzene (anisole) 7 yields 1-methoxycyclohexa-1,4-diene 8 ... 

Rehahn et al. [35] recently presented the synthesis of constitutionally homogeneous oligophenylenes, 22a/b, with 2,5-alkyl substituents located on the central aromatic ring, generated via the cross-coupling reaction of Suzuki. They are exclusively linked in the para-positions and composed of 3 -15 benzene rings. 

Alkenes to Alkanes. The ionic hydrogenation of many compounds containing carbon-carbon double bonds is effected with the aid of strong acids and organosilicon hydrides following the n-route outlined in Eq. 2. A number of factors are important to the successful application of this method. These include the degree and type of substituents located around the double bond as well as the nature and concentrations of the acid and the organosilicon hydride and the reaction conditions that are employed. 

This result indicates that the substituent located on one bond affects electronically the process of electron removal from the other double bond which is not bearing the substituent. 

The only difference between these two models is the first descriptor. Given the location and disparity between the descriptors, both can be considered false bringing attention to a region that is not important to the model. In Model 2 the descriptor adds to the model, yet in Model 4 the descriptor subtracts from the model. Most striking is that the descriptors are separated by 2.24 A as illustrated in Fig. 10 and more than 6 A from either of the other descriptors at the substitution locations of interest. The two descriptors at positions (0, 2, 4, any) and (0, 4, 1, any) at the me la and para substituent locations of the benzene ring are the most important of those displayed. The descriptors represent steric properties because they will accept any type of heavy atom at those two locations the addition of a heavy atom at either of these two locations will improve the bioactivity of a compound from the series. 

The role of steric hindrance and of conjugation of the substituent with the ring in connection with the ease of ipso attack is still an open question. Conjugation may be quite effective with electron-releasing substituents located a or y to the aza group. 

Generally, substituents located on saturated groups attached to nitrogen influence base strengths through their inductive effects in the same way that these substituents influence the strengths of carboxylic acids (see Section 18-2). 

The methyl esters of the long-chain fatty acids can be subjected to exactly the same examination as outlined earlier in this chapter. In the usual case, these fatty acid esters will be composed largely of unsaturated (olefinic) linkages with little or no saturated components. This follows the pattern noted before for the substituents located at the C-2 position in other phosphoglycerides. A quantitative analysis of these fatty acids will show that there is 1 mol per mol of lipid phosphorus. The lysoalkenyletherphosphatidylcholine can then be studied further. 

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