Question

Which acid of each pair shown here would you expect to be stronger? (i) CH3CO2H or CH2FCO2H (ii)CH2FCO2H or CH2ClCO2H (iii) CH2FCH2CH2CO2H or CH3CHFCH2CO2H (iv)

Answer 1

Answer:

1) Ch2FCOoH because it has a Fluoride group attached which acts as an electron withdrawing group or -I effect showing group which helps in localising the negative charge formed in the conjugate base making it stronger than CH3COOH.

2) Ch2FCoOH will be a stronger acid because -F is a stronger -I effect showing group than -Cl.

3) Ch3Ch2F COOH will be a stronger acid since here, -F being the -I showing group is nearer to the carboxylic group, also since we know that this inductive effect is distance dependent, hence nearer the -I group more easy delocalisation takes place and hence Ch3ChFcooh than ch2Fch2cooh.

Hope this helps you !

Answer 2

Answer:

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(I) The +I effect of -CH3group increases the electron density on the O-H bond. Therefore, release of proton becomes difficult. On the other hand, the -I effect of F decreases the electron density on the O-H bond. Therefore, proton can be released easily. Hence, CH2FCO2H is a stronger acid than CH3CO2H.

(ii)

F has stronger -I effect than Cl. Therefore, CH2FCO2H can release proton more easily than CH2ClCO2H. Hence, CH2FCO2H is stronger acid than CH2ClCO2H.

(iii)

Inductive effect decreases with increase in distance. Hence, the +I effect of F in CH3CHFCH2CO2H is more than it is in CH2FCH2CH2CO2H. Hence, CH3CHFCH2CO2H is stronger acid than CH2FCH2CH2CO2H.