Transesterification

[Andiio]

Are we supposed to write a 'combined' eqn showing transesterification, or do we write 2 separate equations (one showing the triglyceride hydrolysis and the other showing the formation of methyl esters)

With the triglyceride hydrolysis, does water or methanol take part in the hydrolysis? And is the KOH catalyst used supposed to be written with (aq)?

Also, when making the methyl esters, are you supposed to write 1 fatty acid + methanol = methyl ester + water or do you use 3xfatty acids to make 3xmethyl esters?

Thanks!

[vea]

It depends what they ask but water is the thing that takes part in the hydrolysis, methanol reacts with the fatty acid to form the biodiesel. You would just write one fatty acid+ methanol = methyl ester.

[Andiio]

It depends what they ask but water is the thing that takes part in the hydrolysis, methanol reacts with the fatty acid to form the biodiesel. You would just write one fatty acid+ methanol = methyl ester.

Yeah I always thought water took part in the hydrolysis of the triglyceride, but I remember my teacher telling me that it wasn't? :S

[Sakigami]

I don't think this is made explicit how you would do these type of questions similar to a reaction pathway.

I think that important part is that:
fat + 3 water -(KOH catalyst)> 3 fatty acids + glycerol
fatty acid + methanol -H2SO4> methyl ester + water
And getting the coefficients correct.

[vea]

It depends what they ask but water is the thing that takes part in the hydrolysis, methanol reacts with the fatty acid to form the biodiesel. You would just write one fatty acid+ methanol = methyl ester.

Yeah I always thought water took part in the hydrolysis of the triglyceride, but I remember my teacher telling me that it wasn't? :S

I'm pretty sure you're teacher is wrong then, if you think about it, water is used for hydrolysis of esters, ether bonds etc. and you add the H and O atoms back to where they would normally be if the compounds were separate.

EDIT: Sometimes the question goes from triglyceride straight to the three methyl esters though and they don't add water to hydrolyse, they just use KOH as a catalyst... hmmm

[Andiio]

It depends what they ask but water is the thing that takes part in the hydrolysis, methanol reacts with the fatty acid to form the biodiesel. You would just write one fatty acid+ methanol = methyl ester.

Yeah I always thought water took part in the hydrolysis of the triglyceride, but I remember my teacher telling me that it wasn't? :S

I'm pretty sure you're teacher is wrong then, if you think about it, water is used for hydrolysis of esters, ether bonds etc. and you add the H and O atoms back to where they would normally be if the compounds were separate.

EDIT: Sometimes the question goes from triglyceride straight to the three methyl esters though and they don't add water to hydrolyse, they just use KOH as a catalyst... hmmm

Zzz.. really would like a definite answer to this.

[luken93]

Yeah, water is definitely added for hydrolysis. You have an OH from Glycerol added to the carboxyl group of the fatty acid, forming a COO ester and giving off a H2O molecule.

The only way you're gonna get it back to the original is to add water, merely adding NaOH can't give the OH back to the glycerol, and H back to the carboxyl group.

So, first part of the transesterification is to hydrolyse the triglyceride into glycerol and 3 fatty acids. Second stage involves THEN reacting each fatty acid with methanol, to once again create an ester. But rather than having all 3 esters bonded to the glycerol molecule, you now will have 3 methyl esters and in the process 3x water molecules will also be given off.

Hope this answers your question...

[davidle_10]

Transesterification is defined as swapping alkyl group of an ester with R group of an alkanol.

[vea]

Look at VCAA09 MCQ 18, it might clear things up a bit.

[VivaTequila]

WTH is transesterification?

Am I the only one who doesn't know this?

[luken93]

Transesterification is going from a triglyceride to biodiesel (methyl ester)

[VivaTequila]

Oh that's easy, but wait how can we write it in one reaction?

I thought you had to hydrolyse the bloody fat and then condense it with methanol/ethanol..

[homosapiens]

I'm a little confused about the KOH catalyst when hydrolysing the fat? I thought water was the only input needed

[luken93]

A catalyst is something that isn't used up in the reaction, hence it doesnt actually input. That is why they say "reagent" mostly in Unit 3, as reagents initiate a reaction but do actually get consumed in reaction.

[VivaTequila]

How can we write this in one equation?

NaOH's role is to help dissolve the fat because it wont dissolve in water