--%>
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 : 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 : Carnot cycle show how a mathematical

    show how a mathematical definition of entropy can be obtauined from a consideration of carnot cycle?

    		•
  • Q : Problem on endothermic or exothermic At

    At low temperatures, mixtures of water and methane can form a hydrate (i.e. a solid containing trapped methane). Hydrates are potentially a very large source of underground trapped methane in the pole regions but are a nuisance when they form in pipelines and block th

    		•
  • Q : Liquid Vapour Free Energies The free

    The free energy of a component of a liquid solution is equal to its free energy in the equilibrium vapour.Partial molal free energies let us deal with the free energy of the components of a solution. We use these free energies, or simpler concentration ter

    		•
  • Q : Importance of organic chemistry

    Describe the importance of organic chemistry?

    		•
  • Q : Concentration of urea Help me to go

    Help me to go through this problem. 6.02x 1020 molecules of urea are present in 100 ml of its solution. The concentration of urea solution is: (a) 0.02 M (b) 0.01 M (c) 0.001 M (d) 0.1 M (Avogadro constant, N4= 6.02x 1023mol -1)<

    		•
  • Q : Basic concept Give me answer of this

    Give me answer of this question. The volume of water to be added to 100cm3 of 0.5 N N H2SO4 to get decinormal concentration is : (a) 400 cm3 (b) 500cm3 (c) 450cm3 (d)100cm3

    		•
  • Q : What is Flash Photolysis Reactions.

    An example illustrates the type of mechanism that can be written to explain the development of flash photolysis reactions. Often, as the reactions in the ozone layer of the earth's atmosphere, we are interested in the kinetic behavior of species that are not a

    		•
  • Q : Question based on maximum vapour

    Provide solution of this question. Which has maximum vapour pressure: (a) HI (b) HBr (c) HCl (d) HF

    		•
  • Q : Molarity of HCl solution 20 ml of HCL

    20 ml of HCL solution needs 19.85 ml of 0.01M NaOH solution for complete neutralization. Morality of the HCL solution is:  (i) 0.0099 (ii) 0.099 (iii) 0.99 (iv) 9.9 Choose the right answer from above.

    		•