question_answer 1)

Explain the terms inductive and electromeric effects. Which electron displacement effect explains the following correct order of acidity of carboxylic acids? (i) \[C{{l}_{3}}CCOOH>C{{l}_{2}}HCCOOH>ClC{{H}_{2}}COOH\](ii) \[C{{H}_{3}}C{{H}_{2}}COOH>{{(C{{H}_{3}})}_{2}}CHCOOH\]\[>{{(C{{H}_{3}})}_{3}}CCOOH\]  

Answer:

Inductive effect: It is a permanent effect of the molecule in which polarisation of sigma electrons takes place. This polarity developes in a molecule as a result of electro negativity of the bonding atoms. This effect is of following two types. (i) \[\mathbf{-I}\]Effect: Atoms or groups (Electron-attracting or withdrawing) in the decreasing order is: \[-N{{R}_{3}}>-N{{O}_{2}}>-CN>-COOH>-\overset{\overset{\text{O}}{\mathop{\text{ }\!\!|\!\!\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{C}}\,-H>\] \[-\overset{\overset{\text{O}}{\mathop{\text{ }\!\!|\!\!\text{ }\!\!|\!\!\text{ }}}\,}{\mathop{C}}\,-R>-F>-Cl>-Br>-I>O{{C}_{6}}{{H}_{5}}>-OR>\]\[-OH>-{{C}_{6}}{{H}_{5}}>-H\] (ii)\[+\mathbf{I}\] Effect: Groups (Electron-releasing or repelling)in the decreasing order is: \[\underset{tert-Butyl}{\mathop{{{(C{{H}_{3}})}_{3}}}}\,,->\underset{Isopropyl}{\mathop{{{(C{{H}_{3}})}_{2}}CH}}\,->\underset{Ethyl}{\mathop{C{{H}_{3}}C{{H}_{2}}}}\,->\underset{Methyl}{\mathop{C{{H}_{3}}}}\,->D>H\]Electromeric effect: It is temporary effect which appears as per the requirement of the attacking reagent. This effect involves the complete transfer of shared pair of electrons to one of atoms joined by a multiple bond at the requirement of attacking reagent. It is indicated by E and is represented by a curved arrow showing the shifting of electron pair. This effect is of following two types: (a) When the transfer of electrons takes place towards the attacking agent, the effect is called +E effect, e.g., the addition acids to alkenes. Since, \[C{{H}_{3}}\] group is electron repelling, the electrons are transferred in the direction shown. The attacking reagent is attached to that atom on which electrons have been transferred. (b) When the transfer of electrons takes place away from the attacking reagent, the effect is called-E effect. For example the addition of cyanide ion to carbonyl compounds. The attacking reagent does not get attached to that atom on which electrons have been transferred, (i) \[-I\] effect as shown below: As the number of halogen atoms decreases, the overall\[-I\] effect decreases and the acid strength decreases accordingly. (ii) \[+I\]effect as shown below: As the number of alkyl groups increases, the \[+I\] effect increases and the acid strength decreases accordingly.