Williamson Ether Synthesis or Elimination?

[adizzle87]

So if i got myself a EtO- in solution with EtOH, and a alkyl halide like CH3Ch2Br, apparently this forms a ether (makes sense). My question is how do we know that this reaction is predominant over E2 elimination to form an alkene?

---

Members don't see this ad.

 

[iwannabadentist]

consider EtO- as OH- except the H is replaced with Et. So if it was OH-...it would just form an alcohol. So in this case...we have smthn similiar to an alcohol except the H is replaced with Et. Also, another trick i've sorta acquired with practicing with ochem over and over again is you gotta look at the reactions this way: Out of Sn1, Sn2, E1, and E1...Sn2 ALWAYS wins if it has the right conditions. So you look at your reactant and see if its a 1 prime halide...if it is...then its ALWAYS gonna be Sn2. If not...then you gotta look at the next factor...is it 2 prime? Then you look at the steric hinderance as well as the strength of the base/nucleophile and go from there. hope that helps

 

[adizzle87]

For Kaplan though, it says that elimination occurs....I just want to know, when, how, why.... =(

 

[iwannabadentist]

i have the kaplan bb right in front of me....can u tell what page says a 1 prime alkyl halide goes thru elimination? lol...im really suspicious of that

 

[iwannabadentist]

http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

check out this link...understand it and memorize it if you have to. but if you follow the flow chart...you won't go wrong =)

 

[krajapak]

more substituted( hinderance)= E2
Weak lewis base ( Strong Nucleophile)= SN2

 

[adizzle87]

I think that's easier said than done though. For example I was caught off guard when i came across a question that asked what KOH + CH3CH2CH2Br would yield. I thought it would be propanol but it was actually the elimination product of CH3CH=CH2. However here it says that primaries don't undergo elimination too often...and that the substrate is given priority over the reagent used. and dazed =(

http://www.chemguide.co.uk/mechanisms/elim/elimvsubst.html

 

[doc3232]

I think that's easier said than done though. For example I was caught off guard when i came across a question that asked what KOH + CH3CH2CH2Br would yield. I thought it would be propanol but it was actually the elimination product of CH3CH=CH2. However here it says that primaries don't undergo elimination too often...and that the substrate is given priority over the reagent used. http://www.chemguide.co.uk/mechanisms/elim/elimvsubst.html....confused and dazed =(

Nah, that's wrong. That would be mostly SN2. When you have a strong base/nucleophile with a primary halide, it is SN2. It is called williamson ether synthesis (if an ethoxide/methoxide instead of OH)

 

[NyCzPeter]

Just remember that when its an alcohol, it will go SN2 and form the ether. However, in presence of a strong base like KOH, the -OH is not going to SN2 which would form an alcohol. Rather, it'll pull off a proton to form H2O and the double bond.

Don't worry about the KOH because it does not apply in the williamson ether synt since the product wouldn't even be an ether.

 

[adizzle87]

Okay, so KOH and NaOH will give off elimination in an alkyl halide via E2, but ROH will give a ether via Sn2...gotcha! Thnx so much!