The lowest strength of acid is shown by 1)CH3COOH 2)CH2ClCOOH 3)CHCl2COOH 4)CCl3COOH
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Answer: I think of 3) option dear..
Answer:
The ranking of these acids in order of increasing acidity is: CH3CH2COOH<CH3COOH<CH2ClCOOH<CHCl2COOH
First of all, you should start by comparing the protons held by these compounds. As you should know, acidity stems from the ability of a compound to release protons in the medium , so acids are proton donors.
By looking at the compounds you’re dealing with, all of the protons are bonded to oxygen atoms, which are more electronegative. So by trying to compare conjugate bases, all carboxylate anions have similiar resonance stabilization, so we will need to compare another aspect to determine which acid is stronger or weaker.
First of all note that acetic acid, chloro-acetic acid, as well as dichloro-acetic acid only differ by the number of hydrogen and chlorine atoms bonded to the last carbon atom (the first is the one holding the functional group of the molecule). On the other hand, propionic acid has an additional methyl group. Now methyl groups are considered to be electron donating groups because of the +I inductive effects. So that methyl group in propionic acid is going to push electrons towards the central carbon and therefore give the molecule more of a nucleophilic character, thus less of an electrophilic one, and finally, make it to be a weaker acid. This is related to the definition of an acid which is a proton donor. So a proton donor would need to be more positively charged. By its electron donating effect, the methyl group will create some kind of partial negative charge on the central carbon atom, so more negatively charged= less positively charged= less of a proton donor.
Having established that propionic acid is the weakest acid in the list, we are left to deal with the other three acids. Now we have already said that resonance will not help us in our comparison, so let us talk more about the inductive effect.
Cl is more electronegative than carbon (3 compared to 2.5 generally). By stating so, a more electronegative atom has a greater tendency to pull electrons towards it.
So when chlorine pulls electrons towards it, it creates a momentary positive charge, which is an indicator of a greater acidic effect through the -I inductive effect. Seeing as dichloro-acetic acid has two chlorine atoms, it is going to have more of that -I I.E. which will therefore make it more acidic. By contrast, acetic acid has a methyl group which does quite the opposite than Cl as explained before, so acetic acid, even though stronger than propionic acid, is weaker than its chlorinated peers.