--%>
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 : Molecular basis of third law. The

    The molecular, or statistical, basis of the third law can be seen by investigating S = k in W.The molecular deductions of the preceding sections have led to the same conclusions as that stated in the third law of thermodynamics, namely, that a value can be

    		•
  • Q : Problem on normality Help me to solve

    Help me to solve this problem. 0.5 M of H2AO4 is diluted from 1 lire to 10 litre, normality of resulting solution is : (a)1 N (b) 0.1 N (c)10 N (d)11 N

    		•
  • Q : Explain gels and its various categories.

    Certain sols have the property of setting to a semi-solid, jelly-like form by enclosing the entire amount of liquid within itself when they are present at high concentrations. This process is called gelation and colloidal systems with jelly-like appearance are known as gels. Some common examples

    		•
  • Q : What are electromotive force in

    The main objective of this particular aspect of Physical Chemistry is to examine the relation between free energies and the mechanical energy of electromotive force of electrochemical cells. The ionic components of aqueous solutions can be treated on the basis of the

    		•
  • Q : Molecular mass from Raoults law Provide

    Provide solution of this question. Determination of correct molecular mass from Raoult's law is applicable to: (a) An electrolyte in solution (b) A non-electrolyte in a dilute solution (c) A non-electrolyte in a concentrated solution (d) An electrolyte in a liquid so

    		•
  • Q : Number of moles present in water

    Provide solution of this question. How many moles of water are present in 180 of water: (a)1 mole (b)18 mole (c)10 mole (d)100 mole

    		•
  • Q : Describe various systems for

    Common system According to this system, the individual members are named according to alkyl groups att

    		•
  • Q : Question relatede to calculate molarity

    Select the right answer of the question. What is molarity of a solution of HCl that contains 49% by weight of solute and whose specific gravity is 1.41 : (a) 15.25 (b) 16.75 (c) 18.92 (d) 20.08

    		•
  • Q : Problem on colligative properties

    Choose the right answer from following. The magnitude of colligative properties in all colloidal dispersions is : (a) Lowerthan solution (b)Higher than solution(c) Both (d) None

    		•
  • 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

    		•