--%>
Assignment Help - homework help

What is ortho effect?

Orthosubstituted anilines are generally weaker bases than aniline irrespective of the electron releasing or electron withdrawing nature of the substituent. This is known as ortho effect and may probably be due to combined electronic and steric factors.

The overall basic strength of ortho, meta and para substituted anilines, however, depends upon the electron,-donatingelectron-withdrawing resonance effect as well as inductive effect as discussed below:

(a) If the substituent has electron withdrawing inductive (-I) as well as resonance (-R) effect. Then all the substituted anilines are weaker bases with ortho isomer being the weaker base. Then m-isomer in this case is relatively stronger base because R-effect does not operate at m-positive. For example, basic strength of o, p, m-nitro anilines are given as follows:

(b) If the substituent has electron donating inductive (+I) as well as electron donating resonance (+R) effect, then among the substituted anilines, the ortho substituted anilines are weaker bases than aniline whereas p- and m- isomers are relatively stronger bases. However, the p-isomer is still stronger than m-isomers. This is clear from the basic strength of toluidines as given below:

(c) If a substituent has electron donating resonance effect (+R) but electron withdrawing inductive effect (-I), the overall basic strength depends upon the relative predominance of R-effect or I-effect.

(i) When a substituent has strong (+R) effect and weak (-I) effect (For example, -OCH3 group). At meta-position it exerts only (-I) effect causing base weakening effect. Among o- and p-isomers, ortho isomer is weaker base than aniline due to ortho effect while para-isomer is stronger base than aniline due to dominance of + R effect. The basic strength of o-, p- and m-anisidines are as under:

Similar trends are observed in amino-phenols with any ortho aminophenol is stronger base than aniline due to stabilization of o-hydroxy anilinium ion because of intramolecular H-bonding. The basic strengths of aminophenols are as under:

-NH2 group has much stronger (+R) effect and much weaker (-I) effect than -OH group and -OCH3 groups. The decreasing order of basic strengths of phenylenediamines is as given below:

(ii) when the substituent has a weak +R effect but a strong -I effect Chloro group (-Cl) is common example. Since -I- effect outweighs the +R effect, therefore, all the three o-, m- and p- chloroanilines are weaker bases than aniline. However, due to ortho-effect, o-chloroaniline is the weakest base. Further in p-chloroaniline is the weakest base. Further in p-chloroaniline both +R only the -I effect operates; therefore, p-chloroaniline is relatively stronger base than m-chloroaniline. Thus, the basicity of o-, m- and p- chloroanilines relative to aniline follows the sequence as given below:

   Related Questions in Chemistry

  • Q : Cations Explain how dissolving the

    Explain how dissolving the Group IV carbonate precipitate with 6M CH3COOH, followed by the addition of extra acetic acid.

    		•
  • Q : What are heterogenous catalysis? Give

    When the catalyst exists in a different phase than that of reactants, it is said to be heterogeneous catalyst, and the catalysis is called heterogeneous catalysis. For example, SO2 can be oxidized to SO3

    		•
  • Q : Explain methods for industrial

    The important methods for the preparation of alcohol on large-scale are given below:    

    		•
  • Q : Problem based on molarity Select the

    Select the right answer of the question. If 18 gm of glucose (C6H12O6) is present in 1000 gm of an aqueous solution of glucose, it is said to be: (a)1 molal (b)1.1 molal (c)0.5 molal (d)0.1 molal

    		•
  • Q : Describe Enzyme Catalyzed reactions

    Many enzyme catalyzed reactions obeys a complex rate equation that can be written as the total quantity of enzyme and the whole amount of substrate in the reaction system. Many rate equations that are more complex than first and se

    		•
  • Q : Amines arrange in order of

    arrange in order of basicity,pyridine,pipyridineand pyorine

    		•
  • Q : Procedure for separating the components

    Briefly describe the procedure for separating the components of the gun-powder?

    		•
  • Q : Question based on normality Provide

    Provide solution of this question. A 5 molar solution of H2SO4 is diluted from 1 litre to 10 litres. What is the normality of the solution : (a) 0.25 N (b) 1 N (c) 2 N (d) 7 N

    		•
  • Q : Question based on strength of solution

    Help me to go through this problem. On dissolving 1 mole of each of the following acids in 1 litre water, the acid which does not give a solution of strength 1N is: (a) HCl (b) Perchloric acid (c) HNO3 (d) Phosphoric acid

    		•
  • Q : Solutions The relative lowering of

    The relative lowering of vapour pressure of 0.2 molal solution in which solvent is benzene

    		•