funky question (2 Viewers)

[kwabon]

the molecular formula of butanoic acid and ethyl ethanoate are same C4H8O2. which of the following is the best chemical test to distinguish between these compounds?

A) bromine water
B) dilute sodium carbonate
C) concentrated sulphuric acid
D) solubility in water

answer is B, and i dont understand why.

thanks

 

[Aerath]

Bromine water is just wrong (write out the structural foruma for ethyl ethanoate, and try to add Br2 across the double bond, just doesn't work)

Concentrated sulfuric acid is a dehydrating agent, and since both have H2O in their structural formula, it's not gonna help distinguishing.

Solubility is a physical test (methinks).

 

[kwabon]

Bromine water is just wrong (write out the structural foruma for ethyl ethanoate, and try to add Br2 across the double bond, just doesn't work)

Concentrated sulfuric acid is a dehydrating agent, and since both have H2O in their structural formula, it's not gonna help distinguishing.

Solubility is a physical test (methinks).

i too got it by the process of elimination, but do u know the actual reason why does that happen?

 

[Aerath]

Butanoic acid is an acid, and hence it reacts with sodium carbonate, a weak base.
Ethyl ethanoate is an ester, and I guess, it's neutral (I have no idea)

 

[lychnobity]

I think it has something to do with the sodium carbonate reacting with all the products left over from reflux.

 

[kwabon]

:-s

 

[lychnobity]

Ok, esterification is messing with me, so... disregard that other post.

 

[Aerath]

Hahaha, yeah, esterification isn't part of this question - it's just the ester.

 

[emmcyclopedia]

So the test with the sodium carbonate - it's just doing the general reaction:

acid + metal carbonate --> metal salt + CO2 + H2O

so from there, you'd have to collect any gas that came out of the reaction vessel and bubble it through some limewater.

If it goes milky, you've confirmed the presence of CO2, which means that the original solution had to be an acid - thus showing that the substance is butanoic acid and not the ester.

Does that make sense? =]

 

[Aerath]

Just wondering, what would happen if you added metal carbonate to the ester?

 

[Ruptured]

The sodium carbonate solution ( basic salt solution) will supply carbonate ions which will neutralise the acid, releasing heat. I'm about 90% sure thats why, I dont think it woudl have anything to do with other products of their creation.

 

[emmcyclopedia]

Just wondering, what would happen if you added metal carbonate to the ester?

I don't really know aye... ?
Esters aren't highly reactive, so erm I don't think anything would happen. Anyone feel free to correct me here!

The sodium carbonate solution ( basic salt solution) will supply carbonate ions which will neutralise the acid, releasing heat. I'm about 90% sure thats why, I dont think it woudl have anything to do with other products of their creation.

Good point - all neutralisation reactions are exothermic, it's all coming back!
That sounds a little easier than messing with the CO2, but if asked that in the exam, I believe both answers would valid =]

 

[kwabon]

I don't really know aye... ?
Esters aren't highly reactive, so erm I don't think anything would happen. Anyone feel free to correct me here!

yeh i dont think anything would happen.
kk thanks guys.