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If one of the hydrogens in the alkyl side chain of ethanoic acid is replaced with an atom of Chlorine, the acid has a larger Ka value. My book says this is since the Cl "borrows" electron density from the Carbon it is connected to. And, in this example, this carbon borrows some electron density from the Carbon in the acid group, and thus the oxygens of the acid have less charge density. The bold part is what I don't get.

The C to which this 1 chlorine is attached gets a bit electron deficient but not as much deficient as the C in the acid group (since this has two oxygen atoms attached). So how can it borrow density away from the C in the acid group? In my mind, to borrow some charge density you have to be more electronegative than what you're attached to, and the more deficient carbon should have a greater pull.

What am I missing in this?

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    $\begingroup$ OK, think of it this way: to say that it borrows density away from the acid group is an abuse of language, actually supposed to mean that it gives density to the acid group, but less willingly than -CH3 did. Imagine a guy who says he was robbed because his welfare check is a tad less than it was the other month. Same thing here. $\endgroup$ – Ivan Neretin Apr 2 at 7:39
  • $\begingroup$ And, though the principle is the same, it is easy to see this effect as stabilization of a real charge. So it is easy to discuss the conjugated bases situation, and in general that with a real charge. $\endgroup$ – Alchimista Apr 2 at 9:25
  • $\begingroup$ @Ivan Neretin, I thought about what you said. One question though : the data shows that the acid with the the chlorine is also stronger than methanoic acid. So, all in all, this CH2Cl is a worst donor than both the CH3 and the H? $\endgroup$ – Sal_99 Apr 2 at 22:07
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    $\begingroup$ So it is, then. $\endgroup$ – Ivan Neretin Apr 2 at 22:23
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    $\begingroup$ Yes.$\mathstrut$ $\endgroup$ – Ivan Neretin Apr 7 at 20:01
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The result is correct but the explanation is bad.

The main reason is that when you compare CH3COOH and CH3ClCOOH , the main difference is that the alpha carbon in the former has more electron density than the latter one because of which when the conjugate base gets formed, it is more stabilised than ethanoic acid because of low electron density.

You are correct about this "In my mind, to borrow some charge density you have to be more electronegative than what you're attached to, and the more deficient carbon should have a greater pull".

Edit 1 - The orbitals of CH3COOH can provide more electron density than CH2ClCOOH due to the former having more electron rich orbitals which interferes constructively with the orbitals of other C to satisfy its demands. Think in terms of how Hyper conjugation or resonance works in terms of interference of wave functions.

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  • $\begingroup$ @Sal_99 The reason is overlapping of electron containing orbitals which decreases the deficiency of electrons. Think this like Hyper conjugation or Resonance. Interference of various wave functions take place constructively or destructively. $\endgroup$ – Ankit Kumar Apr 6 at 8:38
  • $\begingroup$ I will improve the answer over time. First try to think yourself. $\endgroup$ – Ankit Kumar Apr 6 at 8:42

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