another E1 vs E2...

[Tina324]

CH3CH2O-Na+ + R-CH2Br -->

When I first did this prob I did not recognize it as a Williamson ether sysnthesis (which is SN2). I thought it was an E2 reaction yielding R=CH2.
According to the ans key, the major product is indeed the ether.

I don't understand why though...i mean isn't CH3CH2O- a bulky base?...if not then what REALLY is a bulky base?

---

Members don't see this ad.

 

[doc3232]

CH3CH2O-Na+ + R-CH2Br -->

When I first did this prob I did not recognize it as a Williamson ether sysnthesis (which is SN2). I thought it was an E2 reaction yielding R=CH2.
According to the ans key, the major product is indeed the ether.

I don't understand why though...i mean isn't CH3CH2O- a bulky base?...if not then what REALLY is a bulky base?

Not at all a bulky base...the question will make a bulky base be obvious...
Like t-butyl oxide which is shaped like a X...or LDA which is a Nitrogen with 2 alkyl groups coming off it.

 

[GCT]

CH3CH2O-Na+ + R-CH2Br -->

When I first did this prob I did not recognize it as a Williamson ether sysnthesis (which is SN2). I thought it was an E2 reaction yielding R=CH2.
According to the ans key, the major product is indeed the ether.

I don't understand why though...i mean isn't CH3CH2O- a bulky base?...if not then what REALLY is a bulky base?

It's not bulky - it's linear. In addition it's not much of a base.

 

[Tina324]

umm i thought CH3CH2O- is a strong base. it is an alkoxide.
also here is a similar reaction that results in elimination like i thought the one i posted would too.

RCH3CHBr + NaOC2H5 --> R-CH=CH2 (from destroyer)

so obviously CH3CH2O- is considered "bulky" if this elimination could occur.
am i right?

 

[Tina324]

help...pleaseee lol

 

[Mamona]

umm i thought CH3CH2O- is a strong base. it is an alkoxide.
also here is a similar reaction that results in elimination like i thought the one i posted would too.

RCH3CHBr + NaOC2H5 --> R-CH=CH2 (from destroyer)

so obviously CH3CH2O- is considered "bulky" if this elimination could occur.
am i right?

Where is that rxn in destroyer?? If the 1)NaOC2H5 is follow by something like 2)CH3OH for instance, yes it will be a elimination rxn.

 

[Tina324]

yes..it is followed by alcohol...why would it then be E2? and it is in road map 2 at the bottom left of the page

 

[keliao]

a bulky base is connected to the # of Carbons. let say CH3CH2O (tha's O connected to one carbon) (CH3)3O (thats bulky, O connected to three carbons)

CH3CH2O is a weak base

F- , H2O, CH3OH are STRONG BASE-weak nucleophile
-SH I- -CN -OH CH3O (CH3CH2)2NH are WEAK BASE-strong nucleophile

do you see the difference?

 

[Tina324]

a bulky base is connected to the # of Carbons. let say CH3CH2O (tha's O connected to one carbon) (CH3)3O (thats bulky, O connected to three carbons)

CH3CH2O is a weak base

F- , H2O, CH3OH are STRONG BASE-weak nucleophile
-SH I- -CN -OH CH3O (CH3CH2)2NH are WEAK BASE-strong nucleophile

do you see the difference?

ohh ok i understand the bulky base part now...basically anything not linear? so let's say the base is NOT bulky, but the alkyl halide is, will elimination still occur?
for example:

Ph-CHCH3Br + -OC2H5/C2H5OH --> Ph-CH=CH2
this is E1 because the benzene is bulky and the solvent is protic?

Also...I don't really understand your explanation about weak and strong nuc's =/ ...ahhh im gettin so confuseddd

 

[GCT]

ohh ok i understand the bulky base part now...basically anything not linear? so let's say the base is NOT bulky, but the alkyl halide is, will elimination still occur?
for example:

Ph-CHCH3Br + -OC2H5/C2H5OH --> Ph-CH=CH2
this is E1 because the benzene is bulky and the solvent is protic?

Also...I don't really understand your explanation about weak and strong nuc's =/ ...ahhh im gettin so confuseddd

First of all ethanol has a pKa which means that the conjugate base isn't very much of a base.

The product has an increased level of conjugation from the original phenyl reagent and is thermodynamically favored and substitution is also not favored because of the large phenyl group.

In protic solvents an agent that has good deprotonating capabilities can also be better surrounded by water e.g. F- compared to I- .