Fischer Esterfication reaction question

[Thearchitect]

Does anyone know why acid is recommended to be used in excess in these reactions(instead of alcohol)?? Something chemically related to the reaction itself, not something like "it's cheaper" or "it's more easily removed as an impurity than an alcohol." It's just an equilibrium reaction with a 1:1:1:1 stoichiometery. R-OH + RCOOH <--H+--> H20 + RCOOR.

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[has]

acid makes alcohol a better nucleophile so that it can undergo SN2 better and form an ester!

 

[Thearchitect]

acid makes alcohol a better nucleophile so that it can undergo SN2 better and form an ester!

If assuming the reaction is catalyzed already by a strong acid?

 

[has]

Yes...but remember RCOOH are not always strong acids. Acetic acid is not as strong as HCl. That's why usually HCl is added since it is a strong acid and can make water more nucleophilic.

 

[BisMuOxo]

First of all, the Fischer esterification is not an Sn2 reaction. Second, the acid does not make the alcohol a better nucleophile, it protonates the carbonyl group so that the alcohol will attack (alcohols are weak nucleophiles). Third, who told you that the acid should be in excess, most of the time the alcohol is a solvent.

 

[Thearchitect]

First of all, the Fischer esterification is not an Sn2 reaction. Second, the acid does not make the alcohol a better nucleophile, it protonates the carbonyl group so that the alcohol will attack (alcohols are weak nucleophiles). Third, who told you that the acid should be in excess, most of the time the alcohol is a solvent.

My professor. 😕 That's why I'm asking. Makes no sense to me.

 

[captain hazel]

yah, listen to bismuoxo (esp. about how acid-catalyzed carbonyl rxns work)

and the proton donor's a catalyst, right? so:
a) in the mechanisms, the proton added to the oxygen (that makes the carbonyl carbon a better electrophile) always pops off of somewhere else later in the rxn, since catalysts are not consumed

b) you don't need to add to excess, as it is a catalyst. the amount of catalyst won't affect how much product you get, just the rate that you get it

so what would you use in excess? it's just l'chatelier principle:
excess alcohol
or excess carboxylic acid
or remove H20 as it forms


but as the OP said, generall you use excess alcohol. the steps are all equilibria rxns, so excess alcohol drives the rxn to the right towards completion (getting your ester product)

 

[Thearchitect]

So, uhh, there is no reason to use excess of one over the other even though the professor said otherwise?

 

[willthatsall]

The reason is L'Chatelier's principle. You start off with a lot of one reactant to push the reaction forward. It doesn't matter which reactant you start out with more of, but the alcohol is usually cheaper. Or you could remove one of the products as you go to drive the reaction forward, but it is easier just to add excess reactant.

 

[captain hazel]

yeah, what he said.

oh, and above I though you were saying the prof said you needed excess acid catalyst (not excess carboxylic acid). so nevermind the first half of my above response.

but yeah, either excess carboxylic acid or excess alcohol would work, since they're both reactants.

 

[theunc31]

Add excess to stop the back reaction

 

[bgreet]

acid makes alcohol a better nucleophile so that it can undergo SN2 better and form an ester!

I thought alcohol, being somewhat polar, would make an sn1 reaction run better by stabilizing a carbo cation not sn2?

 

[JohnHolmes]

acid makes alcohol a better nucleophile so that it can undergo SN2 better and form an ester!

Wow, this is dead wrong.

I am TPR orgo teacher, please don't take has' advice.

Its not Sn2 OR Sn1 ... reactions like this are called ADDITION ELIMINATION reactions (for a NET substitution, not a SUBSTITUTION, mind you ... NET) Substitution Nucleophilic biomolecular (Sn2) will only happen in a CONCERTED STEP with sp3 hybridized carbons. This is a condensation reaction.

The strong acid doesn't make the alcohol a better nucleophile, it makes the COOH (more specifically the Carbonyl carbon) a better ELECTROPHILE by protonating the carbonyl oxygen, withdrawing electron density away from the carbonyl carbon (draw resonance structures to see how this works).

 

[Thearchitect]

Sigh. I'm more confused than when I first asked the question.

Why would someone want to use an excess of either alcohol OR acetic acid as opposed to equal amounts of either one(I'm not talking about "excess reactants" but the reactants in relation to each other).Maybe that will help figure out why someone might use excess of one over the other.

 

[willthatsall]

Sigh. I'm more confused than when I first asked the question.

Why would someone want to use an excess of either alcohol OR acetic acid as opposed to equal amounts of either one(I'm not talking about "excess reactants" but the reactants in relation to each other).Maybe that will help figure out why someone might use excess of one over the other.

Here's the thing: there is no reason to use more of one reactant than the other. They react in a 1:1 ratio. So you could just use the reactants in a 1:1 ratio, say 2 moles of acid and 2 moles of alcohol. But you have to remember that you are dealing with an equilibrium reaction. Therefore, using 2 moles of each reactant will not result in the formation of 2 moles of product because a certain amount of reactants will be present at equilibrium, depending on the particular equilibrium constant of the reaction. Therefore, you add an excess of one of the reactants to "push" the reaction forward according to L'Chatelier's principle. For example, if you want to make 2 moles of product, use 2 moles of acid, but 5 moles of alcohol. The excess reactant (alcohol) causes more product to be formed. Now, like I said before, it doesn't matter which reactant is in excess, but the alcohol is usually cheaper and so it is commonly used in excess. That is why one is used in excess. You could add reactants in equal amounts and add like 4 moles of each if you wanted to end up with 2 moles of product, but it is cheaper and more practicle to use the correct amount of one reactant and add excess of the other (the cheaper one). Hope that helps.

 

[JohnHolmes]

Sigh. I'm more confused than when I first asked the question.

Why would someone want to use an excess of either alcohol OR acetic acid as opposed to equal amounts of either one(I'm not talking about "excess reactants" but the reactants in relation to each other).Maybe that will help figure out why someone might use excess of one over the other.

Ok, either excess of Acetic acid or alcohol will work to push the Rx to the right like Willthatsall said originally (Le Chat's principle, what a great guy)

Ok, so why use acid instead of alcohol in excess. Let's examine the benefits of using each:

Excess COOH:

1) Push Rx to Right
2) COOH can protonate a Carbonyl Oxygen of another Acetic acid and make it a better Electrophile, helping the Rx to proceed faster.

Excess ROH:

1) Push Rx to right.
2) That's it.

See?