Orgo Quention E2 reaction

[i4everpaki]

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What is a possible product of the reaction shown?

CHDCH3-CHBrCH3

__________D___Br__________
__________|___|________________________OH-
________H-C----C-CH3__________________---------->
__________|___|_____
-________CH3_H________
It is an Elimination reaction E2, Br is taken off first, then which is is taken off from C1, D or H? i thought it was going to be H, but the ans showed it was D. can any one explain why D is taken off instead of H?

 

[UndergradGuy7]

What is a possible product of the reaction shown?

CHDCH3-CHBrCH3

__________D___Br__________
__________|___|________________________OH-
________H-C----C-CH3__________________---------->
__________|___|_____
-________CH3_H________
It is an Elimination reaction E2, Br is taken off first, then which is is taken off from C1, D or H? i thought it was going to be H, but the ans showed it was D. can any one explain why D is taken off instead of H?

For E2, the Br and the H or D on the next carbon have to be antiplanar. So if Br is a solid wedge, H or D must be a dashed line. Check to see which is like that?

 

[i4everpaki]

For C1 D is dotted line, H is solid wedge, and CH3 is in the Plane
For C2 Br is in the plane, CH3 is dotted line, and H is solid wedge.

 

[UndergradGuy7]

For C1 D is dotted line, H is solid wedge, and CH3 is in the Plane
For C2 Br is in the plane, CH3 is dotted line, and H is solid wedge.

Here is my guess...

So at first you are like on the left. The molecule is more stable when the two biggest groups are anti to each other like in the second picture (the two CH3). So if you rotate the carbon in the back clockwise they are anti. Then Br and D are anti.

 

[RCT PC CRN]

Here is my guess...

So at first you are like on the left. The molecule is more stable when the two biggest groups are anti to each other like in the second picture (the two CH3). So if you rotate the carbon in the back clockwise they are anti. Then Br and D are anti.

👍

You will have to rotate the bond in order to do E2 - so which way would you rotate? My answer is more stable way. I'm with UndergradGuy7.

 

[orgohacks]

Fun question. For the E2 to take place, the Br has to be anti to the hydrogen atom you are removing with strong base.

Draw the Newman diagram where H and Br are anti.
Now draw the Newman diagram where D and Br are anti.

Do you notice a difference in energy between the two? You should.

What is a possible product of the reaction shown?

CHDCH3-CHBrCH3

__________D___Br__________
__________|___|________________________OH-
________H-C----C-CH3__________________---------->
__________|___|_____
-________CH3_H________
It is an Elimination reaction E2, Br is taken off first, then which is is taken off from C1, D or H? i thought it was going to be H, but the ans showed it was D. can any one explain why D is taken off instead of H?

 

[i4everpaki]

hmm it makes sense, i never thought of Newman projection. but it makes perfect sense. Thank you