Among thioester and ester which has more electrophilic carbonyl carbon?
Reactivity of thioesters with respect to nucleophilic attack

Why is the thioester bond weaker than a regular ester bond?

Let's consider a reaction mechanism like this:



A steady-state analysis will show that the rate of formation of product is

$$r = \frac{k_1k_2[\text{ester}][\text{Nu}]}{k_{-1} + k_2}$$


$k_1$ for the thioester is larger, because the sulfur 3p lone pair overlaps poorly with the C=O π* orbital (which is formed from 2p orbitals). Recall that esters are less electrophilic than ketones because there is an ester oxygen which has a lone pair capable of overlapping with the C=O π*. In this case, since this overlap is diminished, it turns out that thioesters are typically roughly as reactive towards nucleophilic attack as ketones.
$k_2$ for the thioester is also larger because thiolates are better leaving groups than alkoxides.


Depending on the relative rates of $k_2$ and $k_{-1}$, though, the second point may or may not be relevant. For example if $k_2 \gg k_{-1}$, then the rate equation simplifies to $r \approx k_1[\text{ester}][\text{Nu}]$, in which case the comparison of $k_2$ is no longer useful.