jetbot33

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From what I've read online, when alcohols are reacted with acid (H+, usually H2SO4) and heat, 2 reactions can occur:

Acid-catalyzed Alcohol Condensation: 2 Alcohol + acid + heat -> ether

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch15/ch15-4-4-1.html

Alcohol Dehydration (Elimination): Alcohol + acid + heat -> alkene

http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Synthesis_of_Alkenes/Alkenes_by_Dehydration_of_Alcohols

So if the same reaction conditions are used to form both the alkene and ether, shouldn't these reaction conditions form a mixture of these two products? I do not find this online anywhere. For example, if I am given a question that gives me methanol + H2SO4 + heat, would the answer be an alkene or ether?
 
A

akog

The introduction of the UCDavis link seems to suggest that the ether product prevails when less heat is used.

When two explanations seem to conflict, but only one of the explanations accounts for the other explanation, I tend to put more stock in that one. In any case, alcohol + heat + H2SO4 was always mentioned in the context of making alkenes in all of my study materials, and from a test prep standpoint I would care more that one.

Interesting though... I'd never heard of the ether product
 

orgoman22

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From what I've read online, when alcohols are reacted with acid (H+, usually H2SO4) and heat, 2 reactions can occur:

Acid-catalyzed Alcohol Condensation: 2 Alcohol + acid + heat -> ether

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch15/ch15-4-4-1.html

Alcohol Dehydration (Elimination): Alcohol + acid + heat -> alkene

http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Synthesis_of_Alkenes/Alkenes_by_Dehydration_of_Alcohols

So if the same reaction conditions are used to form both the alkene and ether, shouldn't these reaction conditions form a mixture of these two products? I do not find this online anywhere. For example, if I am given a question that gives me methanol + H2SO4 + heat, would the answer be an alkene or ether?
I hope I can settle this.....I have done these very reactions for the past 25 years, lol. If the alcohol is primary......we often have BOTH ether and alkene formed.....and yes.....this is a real pain !!!! Rarely do we get exclusivity. In the real world....by- products and competing reactions are troubling indeed. Heating, solvents, and many other factors such as alcohol structure can vary the outcome.

For the DAT exam......If you have a primary alcohol, for example ethanol and use 130 degrees Celcius, we would get ...diethyl ether as the major product. If the primary alcohols are different you get a mixture of different ethers and it results in a poor experiment. If we went to 180 Celcius, we would favor the alkene. For secondary and tertiary alcohols, we normally form the alkene if acid and a bit of heat is added. I hope this helps. Free to ask any further questions on this very interesting and exciting topic !!!!

Dr. Jim Romano
 
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