Dehydration of Ethanol (1 Viewer)

[Jaserius]

Hello fellow chemistry sufferers,

Why on earth does the dehydration of ethanol to ethylene require a catalyst, and what catalyst is this?

Any help would be much appreciated.

 

[Slidey]

Well, one catalyst is concentrate sulfuric acid.

Catalysts are objects which cause a reaction to take place, but are not actually integrated into the product; they are preserved.

Many reactions require catalysts - catalytic cracking, as the name implies, is one.

The reason? Probably something to do with energy states, bonds and stability and such.

 

[nit]

ok, ethanol can't spontaneously convert to ethene without a high enough temperature and a catalyst. To answer your question of why it requires a catalyst, you can think of it this way: in order for ethanol to be converted into ethene, the -OH group and a hydrogen atom from different carbons needs to be eliminated. -OH isn't a good leaving group nor is a hydrogen atom - they simply can't drop off the ethanol molecule regardless of temperature. Thus a catalyst is required in order to strip the two functional groups off the ethanol molecule. The catalysts used include the mineral strong acids - eg sulfuric acid/phosphoric acid. They basically release H+ ions in aqueous solutions (you'll do this later, if you haven't akready). The OH group is protonated as a result, forming an unstable (OH2)+ group attached to the ethanol molecule. This is a good leaving group and will readily be eliminated from ethanol. Once it has left, the water molecules formed will be protonated by a hydrogen atom from the adjacent carbon. Depending on conditions, this will all occur in one step or two (ie E1 or E2 reactions occur). So the electrons in the C-H bond now broken will be donated to the pre-existing C-C single bond, to form the double bond. What I've described is a simplification of the mechanism by which ethanol is dehydrated to form ethene.

Another thing to note is that water is a by-product in the reaction, and this reaction is an equilibrium between ethanol on the one side, and ethene and water on the other. Hence, because sulfuric acid is a good dehydrating agent, and will suck up water, it is used in its concentrated (18.4M??) form, pushing the equilibrium to the right and increasing ethene yield.

Edit: Damn, slide you beat me!!

 

[Xayma]

Catalysts are required, in most cases to lower the activation energy of a reaction, or in some cases allow the reaction to proceed such as in the hydration of ethene.

Concentrated sulfuric acid, severs multiple roles in the dehydration of ethanol. It acts as a catalyst allowing the reaction to proceed by severing the OH group from the ethene molecule but it also acts as a dehydrating agent, pushing the equilibrium to the right as the water is drawn to deconcentrate it.

 

[wanton-wonton]

I think if you write in the exam 'The catalyst acts as a dehydration agent'...that should be enough. I think.

 

[Xayma]

I think if you write in the exam 'The catalyst acts as a dehydration agent'...that should be enough. I think.

Actually it won't. The dehydration is not part of its catalytic properties.

 

[wanton-wonton]

Actually it won't. The dehydration is not part of its catalytic properties.

So how do you answer it then?

EDIT* The dehydration agent answer I got from Chemistry Pathways.

 

[Xayma]

The dehydration of ethanol to ethene requires a catalyst as the ethanol molecule will not decompose by itself.

Concentrated sulfuric acid or phosphoric acid is used which enables the hydroxy group to be removed from the molecule then stripping a H atom off the CH₃-CH₂ molecule.

Concentrated sulfric or phosphoric acid also acts as a dehydrating agent, which increases the yield of ethene.

 

[wanton-wonton]

describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used

The reaction requires a catalyst as the water molecule itself will not attack the electrons in the ethene double bond.

Is that enough?

 

[Xayma]

describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used

The reaction requires a catalyst as the water molecule itself will not attack the electrons in the ethene double bond.

Is that enough?

Except that is hydration not dehydration.

 

[wanton-wonton]

Except that is hydration not dehydration.

Yes I know. I was referring to another dot-point.

 

[xiao1985]

lolz i'd rep u again if i can xayma =p
well said, u are truly a bos treasure =)

nit: doubt u will need to go into detail at hsc level the mechanism of how the catalyst works... =p but yeh ur answer is awesome too =)

agreed on xayma =)

 

[Casmira]

jesus christ you people know youre shit!

 

[mitochondria]

Depending on conditions, this will all occur in one step or two (ie E1 or E2 reactions occur)...

Isn't this reaction an E2 reaction? Bimolecular... I thought whether a reaction is E1 or E2 has nothing to do with how many steps it undergoes.. What steps anyway?

 

[nit]

*goes to get his mcmurry*

 

[shannonm]

wouldnt it be unimolecular (E1) ?

Isn't this reaction an E2 reaction? Bimolecular... I thought whether a reaction is E1 or E2 has nothing to do with how many steps it undergoes.. What steps anyway?

 

[nit]

ok, I'd say E2 because we're dealing with a primary alcohol, and carbocations are stabilised mostly by the polarizable alkyl groups of a tertiary substrate.

In terms of the steps, well E2 is the elimination analogue of Sn2, and E1 is the elimination analogue of Sn1. For E1, the leaving group leaves, leaving behind a carbocation substrate. Elimination thus occurs in two steps. In E2, the leaving group leaves at the same time that the other functional group is attacked by a base (the stronger the better) - ie the backside attack with anti-periplanar geometry. This mechanism occurs in 1 step. This much can be deduced from the rate laws of the observed reactions. E2 rates depend both on the conc of the substrate and nucleophilic base, while the E1 reaction doesnt depend on the amount of base present, as the elimination of the leaving group is the rate-limiting step.

 

[shannonm]

ok, I'd say E2 because we're dealing with a primary alcohol, and carbocations are stabilised mostly by the polarizable alkyl groups of a tertiary substrate.

In terms of the steps, well E2 is the elimination analogue of Sn2, and E1 is the elimination analogue of Sn1. For E1, the leaving group leaves, leaving behind a carbocation substrate. Elimination thus occurs in two steps.

is that not what is occuring ? :S

i dont really get you why its E2


but hey im not majoring in chem so i wouldnt kno for sure. i dont think e1,e2,sn1,sn2 reactions are even in the hsc syllabus?

 

[nit]

if its E1, then there needs to be a tertiary (possibly benzyl or allyl) carbon in order for the carbocation to be inductively stabilised. Theres no such carbon in ethanol, which only contains primary carbons. Thus E2 must be the mechanism here. It's not a terribly great example of E1 (think EthOTos or Iodoethane with ethoxide ions floating around an aqueous solution), but theres no other mechanism that could be used, unless you can think up a new one

 

[mitochondria]

^____________^ gracias mucho...

ummm... is it page 3... 3... three hundred and something in McMurray? I don't seem to find that much detail in McMurray... headache doesn't help..