what is electronic effect exerted by ocH3,cl on acidic strength of phenol
Answers
Answer:
NO2 > F > Cl > Br > I > OMe > C2H5
The inductive effect is the effect on electron density in one portion of a molecule due to electron-withdrawing or electron-donating groups elsewhere in the molecule
Illustrated Glossary of Organic Chemistry
A more electronegative atom polarizes the bond with its neighbors by pulling the shared electron pair closer to itself. This causes unequal distribution of the electron density and gives rise to partial charges on atoms. Thus, fluorine (electronegativity 4.0) when bonded to a carbon atom (electronegativity of 2.5) pulls an electron pair of C-F bond closer to itself, polarizing the bond: C→F.
As the fluorine atom has higher electronegativity and withdraws the electron density, it has a negative inductive effect -I. The inductive effect of hydrogen is assumed to be zero. The inductive effect of silicon and alkyl groups is positive.
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When the substance loses or transfers a proton we call it an acid, whereas the formed residue after the loss of the proton - a conjugate base (HCOO- in example below):
HCOOH = HCOO- + H+ pKa 3.7 (pKa = - lg Ka, thus the lower pKa corresponds to a stronger acid).
The more stable conjugate base corresponds to a stronger acid since the equilibrium shifts more right, increasing Ka and decreasing pKa.
The electron donors like alkyl substituents destabilize the conjugate base by pumping the electron density onto already electron rich fragment.
CH3COOH = CH3→COO- + H+ pKa 4.8
By replacing an H with a methyl group, CH3-, we decreased the acidity as pKa went up from 3.7 to 4.8.
In contrast, the electron withdrawing groups containing the electronegative atoms like halogenes, oxygen and nitrogen will stabilize the anion and enhance the acidity.
ClCH2COOH = Cl←CH2←COO- + H+ pKa 2.8
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The I-effect of substituents was quantified by Robert Taft with a sigma I constant (JACS 1958, p. 2436) . The electron releasing group (+I effect) were assigned the negative values whereas the electron acceptors (-I effect) received the positive sigma I constants:
Et -0.05
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H 0.00
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OMe 0.27
I 0.39
Br 0.44
Cl 0.46
F 0.50
NO2 0.65
So that you can expect that nitro group NO2- with the highest value of the Taft’s constant of 0.65 will enhance acidity (of X-CH2COOH, for example) more than any other substituent in this series whereas ethyl group is expected to decrease acidity by pushing electron density onto neighboring atoms of COOH group.
The inductive effect can also be evaluated from Hammett meta sigma constants (for benzoic acids) on assumption that the resonance effect is transmitted negligibly from the meta position to the carboxyl group.
Please see review on this matter by Hansch, Leo and Taft at Chem. Review 1991, 165 for comprehensive summary and various constants.