Effect of electronegative groups and length of alkyl chain on inductive effect among carboxylic acids

Question

According to my teacher, the inductive effect in $\ce{CH3COOH}$ exists because of the electron deficiency on $\ce{C}$ (partial positive charge $\delta +$) as a result of the electronegativity of the double bonded $\ce{O}$. Although the single-bonded $\ce{O}$ is more electronegative than $\ce{C}$, the electron density of the single bonded $O$ moves to $C$ in order to “neutralize” the $\delta+,$ leaving $\ce{H}$ less attached to the molecule:

Why do we consider that double-bonded $\ce{O}$ establishes that $\delta+$ on our $\ce{C}$ (and creates an electron withdrawing inductive effect −I) and not the single-bonded $\ce{O}$? Is this because of the double π-bond? Why?

Now supose we have the following carboxylic acids:

$$ \begin{array}{rr} \hline \text{Acid} & \mathrm{p}K_\mathrm{a} \\ \hline \ce{CH3COOH} & 4.80 \\ \ce{CH2FCOOH} & 2.66 \\ \ce{^-OOCCH2COOH} & 5.69 \\ \ce{^-OOC(CH2)4COOH} & 5.41 \\ \hline \end{array} $$

For me it makes sense that $\ce{CH2FCOOH}$ is more acidic than $\ce{CH3COOH}$ because $\ce{F}$ is an electronegative element that creates a partial negative charge attracting the electron density and leaving $\ce{H}$ in the carboxyl group less attached and easier to remove.

On the other hand, the fact $\ce{^-OOCCH2COOH}$ is less acidic than $\ce{^-OOC(CH2)4COOH}$ seems less intuitive. My teacher's presentation states that $\ce{O2^-}$ linked to an alkyl chain will create an electron donating inductive effect +I. Why? Isn't there a withdrawing resonance effect −R?

If $\ce{O}$ is an electronegative element, then why doesn't it create an −I effect instead of +I? Because if that was the case adding carbons $\ce{(CH2)4}$ in the chain would reduce the the −I effect and the $\ce{^-OOC(CH2)4COOH}$ would be less acidic than $\ce{^-OOCCH2COOH}$, and that isn't the case.

Answer 1

Why do we consider that double bonded O establishes that δ+ on our C (and creates an electron withdrawing inductive effect −I) and not the single bonded O?

Look at the resonance structures for acetic acid in the top line of the following drawing. We can see that the middle resonance structure takes the pi electrons involved in the $\ce{C=O}$ double bond and moves them to oxygen, creating a positive charge at the carbonyl carbon. In the third, or left-most resonance structure we see that the "single-bonded oxygen" is also involved in resonance delocalization, but it does not create any positive charge at the carbonyl carbon. Aside from resonance factors, both oxygens will create partial positive charge at the carbonyl carbon through inductive effects.

To answer the question about the relative $\ce{pK_{a}s}$ of these acetic acid derivatives let's proceed as follows. First, let's consider resonance effects of the substituent ($\ce{X}$) to be negligible. This is because 1) the substituent is not directly attached to the carbonyl group or the resulting carboxylate anion, 2) it turns out there is a node at $\ce{C2}$ in the carboxylate anion (zeroth order approximation), so the $\ce{X}$ group is disconnected in a resonance sense. So we need only consider the inductive effect of $\ce{X}$ upon both the reactant and the product and this should allow us to estimate how the equilibrium involving acid ionization will shift. $\ce{X=H}$ is our baseline and we will compare the other two compounds against it. When $\ce{X=F}$ the starting acid ($\ce{A}$) will be inductively destabilized due to the electron withdrawing inductive effect of fluorine. On the other hand, the inductive effect of fluorine will serve to stabilize the resultant carboxylate anion ($\ce{B}$). These two effects (destabilization of the reactant and stabilization of the product) will act in concert to push our equilibrium to the right, making fluoroacetic acid a stronger acid than acetic acid. In the case of $\ce{X=CO2^-}$, the inductive electron donating effect of the $\ce{-CH2CO2^-}$ group should be stronger than the $\ce{CH2-H}$ group in acetic acid, so our reactant should be more stabilized. On the other hand, the inductively electron donating $\ce{-CH2CO2^-}$ should destabilize the already electron rich carboxylate anion ($\ce{B}$). Here both inductive effects again work in concert, but now to stabilize the reactant and destabilize the product making this derivative less acidic than acetic acid. In addition to this inductive effect there is also an electrostatic effect. When $\ce{X=CO2^-}$, we will have two negative charges in $\ce{B}$. This electrostatic effect further destabilizes $\ce{B}$. The combination of inductive and electrostatic effects in this last case will make this acid much less acidic than acetic acid.