[Borek]

Simple alkene, gaseous at RTP, reacts with a highly toxic red-brown gas producing a chiral compound that contains 21.17% 20.68% of chlorine. Very similar compound, with a lower molar mass but higher percent chlorine content, is optically inactive. Identify the alkene and the gas.

[fledarmus]

I believe I've identified the gas, I have the empirical formula for the first compound, and I can't come up with a structure that matches the formula.

[Borek]

Sigh. I hate myself. I did some stupid mistake again and in a place where I would never expect it  And while I would understand many other errors, I don't understand where this one came from. I need either new glasses or new brain. Or both.

Try now, with the corrected percentage. Sorry for wasting your time

[fledarmus]

It's all good 

New empirical formula, I have two possibilities for the gas, my only question now is what counts as "very similar"?

[Borek]

Similar in terms of similar reagents and similar reaction 

[Borek]

Still too hard? Or is there still something wrong with the problem and you can't solve?

[discodermolide]

OK I'll stick my neck out.
But-1-ene and the gas is BrCl. But I am not aware of it's colour, I guess brownish.

[CrazyAssasin]

Yes, I get the same answer.

[Dan]

I guess (E)-but-2-ene. Reaction with BrCl produces racemic (2R,3S)-3-bromo-2-chlorobutane, whereas reaction with chlorine produces (2R,3S)-2,3-dichlorobutane (which is optically inactive (meso) and has a lower MW and higher Cl content).

[Borek]

I guess (E)-but-2-ene. Reaction with BrCl produces racemic (2R,3S)-3-bromo-2-chlorobutane, whereas reaction with chlorine produces (2R,3S)-2,3-dichlorobutane (which is optically inactive (meso) and has a lower MW and higher Cl content).

And that's the correct answer, or at least the one I was waiting for 

Perhaps instead of "Very similar compound" I should write "Very similar compound, produced from the same alkene".

[curiouscat]

But-1-ene and the gas is BrCl. But I am not aware of it's colour, I guess brownish.