dehydration of ethanol? (1 Viewer)

[za]

can som1 explain how sulfuric acid acts as a dehydrating agent when added to ethanol. i just wanna know the sequence of steps in removing the water.

help would be appreciated.

 

[kheir]

its just that conct sulfuric acid is a hydrophilic substance so it absorbs the water allowing the reacton to continue in the forward reaction

 

[babydoll_]

yep

http://www.boredofstudies.org/community/showthread.php?t=11204&highlight=dehydration+ethanol

 

[za]

what im trying to ask is if, or how does it rip off a hydrogen and in wat sequence.
does it add a hydrogen to the ethanl then removes water and takes it bak or is there some other method?

 

[CM_Tutor]

Roughly speaking, it is:

CH₃CH₂OH + H₂SO₄ ---> CH₃CH₂OH₂⁺ + HSO₄^-

CH₃CH₂OH₂⁺ ---> CH₃CH₂⁺ + H₂O

CH₃CH₂⁺ + H₂O ---> CH₂CH₂ + H₃O⁺

The process requires concentrated sulfuric acid, as it requires unionised H₂SO₄ molecules

 

[za]

i was told that the H2SO4 takes bak its hydrogen at the end.


one other question doesnt a catalyst remain unchanged at hte end?

 

[CM_Tutor]

To write it as a catalytic process, you'd have to (as you suggest) reform the H₂SO₄ - in which case it is

CH₃CH₂OH + H₂SO₄ ---> CH₃CH₂OH₂⁺ + HSO₄^-

CH₃CH₂OH₂⁺ ---> CH₃CH₂⁺ + H₂O

CH₃CH₂⁺ + HSO₄^- ---> CH₂CH₂ + H₂SO₄

The problem with this is that you end up with unionised sulfuric acid molecules in a solution containing water - which will end up causing ionisation. In that sense, you could argue whether 'catalyst' is an appropriate term for H₂SO₄ in such a system.

In many ways, I'd prefer to look at this system as an equilibrium,

CH₃CH₂OH <---> CH₂CH₂ + H₂O

catalysed by H⁺ (which is a catalyst in the proper sense of that term), which lies left in dilute acid (excess water pushing eq'm left by Le C's P) and right in a concentrated dehydrating acid (as it absorbs H₂O for its ionisation, and the absorption of H₂O draws the eq'm right by Le C's P)

 

[za]

oh ok
fair enough
thanks

 

[xiao1985]

but would a catalyst change the position eqm tho cm_tutor???

isn't the definition of a catalyst one such that it speeds up the reaction but do NOT change the position of the eqlbm??? if that was the case, the dehydrating/hydrating agent would change the position of eqlbm, and is it still a catalyst???

 

[Tommy_Lamp]

i agree with CM_Tutor, it is better to write in equilibrium, with conc. H2SO4 over right arrow and dilute H2SO4 on the left arrow.

 

[CM_Tutor]

but would a catalyst change the position eqm tho cm_tutor???

isn't the definition of a catalyst one such that it speeds up the reaction but do NOT change the position of the eqlbm??? if that was the case, the dehydrating/hydrating agent would change the position of eqlbm, and is it still a catalyst???

In answer to your first question - No, it wouldn't.

Unless - didn't there just have to be an unless - unless the substance acting as catalyst has some other effect, unrelated to its catalytic function. This is often the case with concentrated sulfuric acid. It provides the catalyst (usually H₃O⁺, as in this case), but also has an effect on equilibrium position by virtue of being a dehydrating agent. The same situation occurs with esterification - the sulfuric acid acts as both catalyst (which does not effect yield) and dehydrating agent (which does alter yield by shifting the position of equilibrium).

Answers in this area need to be extremely careful not to suggest that catalysts alter yield - they don't. However, sometimes substances have more than one function.

In this particular case, you could consider that we have an equilibrium:

CH₃CH₂OH <---H⁺ catalyst---> CH₂CH₂ + H₂O

The mechanism for left to right is:

CH₃CH₂OH + H⁺ <---> CH₃CH₂OH₂⁺

CH₃CH₂OH₂⁺ <---> CH₃CH₂⁺ + H₂O

CH₃CH₂⁺ <---> CH₂CH₂ + H⁺

I have written each step in the mechanism as reversible, as the H⁺ catalyses the process in both directions, and has no effect on the position of equilibrium - which favours the ethanol side, BTW.

The source of the H⁺ catalyst is ioinisation of the sulfuric acid. However, the sulfuric acid, being concentrated, is also a dehdryating agent, absorbing the water and drawing the equilibrium position to the right, favouring the ethene product - This alters the yield. Now, you can consider that the sulfuric acid achieves this by direct reaction with the ethanol, as I posted above, or indirectly by reacting with the water product - it doesn't really matter, as both will occur, and the effect will be the same in either case.

Does this make it any clearer?

 

[xiao1985]

thanks cm_tutor... yea i understood it when u said a catalyst only accelerates the rate, whereas it can at the same time change the yield... yet there are no relationship between its catalysis function and yield alteration function...

also, what about when hydration of ethylene occurs??? i believe dilute acid or a wk acid is used as a catalyst... is it the same mechanism???

 

[Tommy_Lamp]

yes i think it is the same mechanism

 

[CM_Tutor]

I have written each step in the mechanism as reversible, as the H⁺ catalyses the process in both directions

So, yes, it is the same mechanism for ethylene hydration.