Producing Ethanol: The Role of Lewis Structures in Chemical Bonding

[bjoyful]

producing ethanol...

Homework Statement

One way to produce ethanol for use as a gasoline additive is the reaction of water vapor with ethylene: H2C=CH2 + H2O ----> CH2CH2OH Rewrite this equation on a piece of paper using Lewis structures, and use those structures to answer the following question: Was it necessary to break all of the chemical bonds in the reactants in order to form the product, ethanol? Explain your answer.

Homework Equations

The Attempt at a Solution

I rewote the equation on a piece of paper with the Lewis structures H2C= H-C-H, CH2= H-C-H, H2O= H-O-H, CH2= H-C-H, CH2= H-C-H, OH=O-H. I also wrote for ethanol: H-C=O=C-H (with single bonds of H coming off of the C on both top and bottom, due to format limitations, I can't right it..:( Is this right? Was it necessary to break all of the chemical bonds to form reactants?
Thanks!

 

[Kushal]

errmm no you don't have to break all the bonds...

do you know the mechanism for this reaction?

 

[bjoyful]

I am not sure about the mechanism for this reaction...I am totally new at this chemistry stuff! Thanks for your help:)

 

[Kushal]

this is called electrophilic addition reaction. basically electrophiles are species which are attracted by negatively charged species. so in ethene there is a pi bond in the C=C.

now, in water, there is oxygen atom which is very electronegative. it tends to pull the shared pair of electron across the O - H bond towards itself. hence, it gets a partial negative charge. the hydrogen consequently gets a partial positive charge. this is why water is polar.

when water molecules approach the pi bond, the pi electrons further repel the shared pair of electron in the O - H bond causing heterolysis. H+ and OH- are formed.

the H+ is attracted towards the pi bond(consisting of electrons). the pi electrons (in the pi bond) forms a covalent bond with the H+.

now, the ethene molecule becomes CH3C(+)H2. this is known as a carbocation. the carbon has a positive charge.

remember the OH- ion? it then forms a covalent bond with the carbocation(C+). ethanol is finally formed.

this is the simplest example of electrophilic addition reaction.

so, the only bonds broken are the pi bond of the C=C (not the sigma bond) and O - H from water.i hope this is your level of chemistry...

 

[bjoyful]

So I don't have to break all the bonds to form ethanol, just the C=C and the O-H?

 

[Kushal]

another much more simple approach was to compare the lewis structures of both ethene and ethanol, to see which bonds have changed. it would be obvious that the C = C was broken as well as H - OH bond.

and i guess this was the approach required... sorry if i complicated things...

 

[Kushal]

in short yeah...

 

[bjoyful]

No I think I am the one complicating things! I see what you are saying. I am taking this chemistry course online, and i am not sure that was the greatest idea! Oh well, thank God for people like you...

 

[Kushal]

anytime... good luck

 

[Caren vUwB]

I think Glucose + Yeast also can become Ethanol

 

[Kushal]

yeah, but in the absence of oxygen (anaerobic)
however only about 15% ethanol will be obtained...

 

[Caren vUwB]

Besides that, I noticed that Ethene + Water also can form Ethanol but 3 requirements is needed.
1. temperature 300°C
2. use H₃PO₄ as catalyst
3. pressure 65 atm