# E1 and E2 elimination reactions of isotopically labeled bromocyclohexanes

Question
a) What would the product of the reaction be for an E2 mechanism

b) What would the products be for an E1 mechanism

The Br coming out of the page is the 79 isotope while the Br going into the page is the 81 isotope.

There is no mention of reaction conditions.

I am not totally sure what the effect of the isotope will have on bromine's ability to leave. I am guessing that the bromine-81 will leave since it is bigger and can stabilise the negative charge better.

I am also not sure how the products for the E1 and E2 mechanism will be different. Won't they be the same?

• By any reasonable chemical definition of "size" the isotopes should be the same, so that can't be it. The first thing to think of when looking at isotopes is that the heavier isotope makes slightly stronger covalent bonds due to reduced zero-point energy of bond vibrations. For such high atomic masses, though, the difference must be extremely slight, so I don't know if that's the right path to go down. – Nicolau Saker Neto Feb 23 '16 at 9:45
• @NicolauSakerNeto Thanks for that. I wasn't aware of the fact that heavier isotopes made stronger covalent bonds. This might be the way to go because I can't think of anything else that will effect its leaving ability. – Nanoputian Feb 23 '16 at 11:11
• This is commonly called a "kinetic isotope effect". The wikipedia article is decent.. – Lighthart Feb 23 '16 at 14:45
• I have a feeling that the isotopes are only there to distinguish between the two bromine atoms. Although there is an isotope effect, with the high mass bromine the reactivity difference will be negligible. Think about the difference in mechanism between E1 and E2. I would also guess that the stereochemistry is given to you, and that will play a critical role in an E2 reaction. If you provide the full structure (and reaction conditions, if possible) I can give a less speculative answer. – jerepierre Feb 23 '16 at 17:34
• By way of comparison, the kinetic isotope effect for the E2 elimination of $\ce{HCl}$ from (1-chloropropan-2-yl)benzene is only $k(\ce{^35Cl}):k(\ce{^37Cl}) = 1.0059$. The relative mass change for $\ce{^79Br}/\ce{^81Br}$ is even smaller. – Loong Feb 23 '16 at 21:40