destroyer ochem #105

[tizom]

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hey everyone,

this might be dumb, but help me out

cyclohexane + br2/hv gives bromocyclohexane

now i react that with C2H5O-Na+/EtOH and that goes through E2..

can anyone help me with that? its acting on secondary halide, in a protic solvent with a good leaving group.. is it just because the Nuc is so basic?

 

[americanpierg]

hey everyone,

this might be dumb, but help me out

cyclohexane + br2/hv gives bromocyclohexane

now i react that with C2H5O-Na+/EtOH and that goes through E2..

can anyone help me with that? its acting on secondary halide, in a protic solvent with a good leaving group.. is it just because the Nuc is so basic?

this is a potential williamson ether synthesis. williamson ether synthesis only occurs when there is a primary alkyl halide, if the alkyl halide is secondary or higher, E2 will occur in competetion with the SN2 reaction.

 

[wantVCUdental]

i thought of it this way:
cyclohexane is a ring, which is more sterically hindered than normal secondary carbons... so think of it as a secondary and tertiary hybrid, there ain't no way that base is getting behind to do an Sn2, but it still wants to participate in rxn, so the only way to do it is through the hydrogens and that can only give you an E2
hope that helps

 

[americanpierg]

hey everyone,

this might be dumb, but help me out

cyclohexane + br2/hv gives bromocyclohexane

now i react that with C2H5O-Na+/EtOH and that goes through E2..

can anyone help me with that? its acting on secondary halide, in a protic solvent with a good leaving group.. is it just because the Nuc is so basic?

So if you REALLy wanna decide between which of the 4, SN1, SN2, E1, or E2 it is, and don't want any gimmicks or easy way to remember it, its quite simple just to think of it all the way from the beginning.

bromocyclohexane has a secondary alkyl halide thats non allylic or benzylic, so SN1 and E1 can be ruled out. The solvent is protic, so SN2 can be ruled out. That leaves E2. Its a base acting on a secondary non allylic non benzylic alkyl halide in a protic solvent= E2.

 

[dentrilla]

So if you REALLy wanna decide between which of the 4, SN1, SN2, E1, or E2 it is, and don't want any gimmicks or easy way to remember it, its quite simple just to think of it all the way from the beginning.

bromocyclohexane has a secondary alkyl halide thats non allylic or benzylic, so SN1 and E1 can be ruled out. The solvent is protic, so SN2 can be ruled out. That leaves E2. Its a base acting on a secondary non allylic non benzylic alkyl halide in a protic solvent= E2.

i never knew that SN1 and E1 REQUIRED allylic or benzylic alkyl halides? I know they stabilize the carbocation but I dont think they're absolutely necessary, is that the only reason you ruled out SN1 and E1 so quickly?

 

[loveoforganic]

No. Protic solvent, strong base, non-primary substrate = E2.

SN1 and E1 don't require allylic or benzylic alkyl halides.