Looking at the diagram for the reaction mechanism, I ask why the OH- is more attractive than the original molecule end. Is it because of the negative property, and the partially positive C is that attractive? Is the highly electronegative O willing to give up electrons in a covalent bond? And moreso than the original O it’s replacing?
Also, the page says:
“Steam hydrolysis
Triglycerides are also saponified in a two-step process that begins with steam hydrolysis of the triglyceride. This process gives the carboxylic acid, not its salt, as well as glycerol. Subsequently, the fatty acid is neutralized with alkali to give the soap. The advantage of the two-step process is that the fatty acids can be purified, which leads to soaps of improved quality. Steam hydrolysis proceeds via a mechanism similar to the base-catalysed route, involving the attack of water (not hydroxide) at the carbonyl center. The process is slower, hence the requirement for steam.”
Is this a correct restatement? “Steam is used to cut the triglycerides and make glycerol and the carboxolic acid of the remaining fat molecule. A base is added to cut off the H from the acid end, yielding a slightly negative end, which is hyrdrophilic, attracted to the partially positive Hs in water molecules.” I don’t understand where a catalyst came in, or why water would want to attach to the ester end and lose an H itself.
Any better chemists than I around here who can help?
To your first question:
I don’t actually know off-hand if saponification is exergonic*. But either way, a massive excess of nucleophile (either hydroxide or water) will push the reaction in the direction of hydrolysis by way of Le Châtelier’s principle.
*It’s not really about how attractive the hydroxide is than the alkoxide. We need to look at all bonds broken and formed, plus any solvent interactions that change.
The reason the first reaction is called “base-catalyzed” is because you don’t have to have a full equivalent (with respect to all the esters you want to hydrolyze) of hydroxide for it to proceed. Hydroxide is more nucleophilic than water, so it reacts faster. There is always a little bit of hydroxide in liquid water, but adding some extra base (e.g. NaOH) will increase that concentration and speed up the reaction without being consumed.
Steam self-ionizes too, just like liquid water, but it depends a lot on temperature and pressure. I don’t know what conditions are used for saponification, but it’s likely there’s not a lot of hydroxide to go around. Water is a weaker nucleophile, so it reacts slower. Increasing the temperature and pressure helps speed up the reaction.
The product of nucleophilic attack of water on an ester may be a protonated carboxylic acid and an alkoxide. E.g.:
But that proton is going to move to the stronger base very quickly, if it doesn’t already do so as the alkoxide is being eliminated. So you end up with RC(=O)OH + HOR’
Again, I don’t know if the water “wants to” do this. The overall reaction may actually be slightly unfavorable. But that’s why you use lots of water.
Having not attempted to purify fatty acids from water and glycerol, I’ll take their word that it’s easier. But you are correct that a base is then added to the fatty acid to deprotonate it, thus forming an amphiphile with a hydrophilic end (the carboxylate) and a hydrophobic or lipophilic end (the alkyl chain.)