Sciencemadness Discussion Board

What reaction mechanisms do this (Alkane to alkene; alkyl halide to alkane)?

Edward Elric - 15-11-2009 at 12:29

So, I'm looking through various reaction mechanisms, and I'm trying to create a cyclic chart. This isn't homework; I'm trying to create a conceptual map.

What I don't have on the map is a way to turn an alkane to an alkene. I also don't have a way to turn an alkyl halide to an alkane.

I can turn an alkane into an alkyl halide. I believe that's free-radical halogenation. And the ability to turn an alkene to an alkane is ... hydrogenation of an alkene with a catalyst.

So, what kind of reaction mechanisms am I looking for?
I probably don't know them yet.
But I'd like to complete my chart. :)

Alkyrisk - 15-11-2009 at 15:53

You can turn an alkane to an alkene by cracking. Actually this is the only way to turn directly an alkane to an alkene but this process doesen`t observe the cliché "the only and the best" (or "*" i don`t know the alternative in english) because in cracking is involved dehydrogenation and that means it will be more reaction products.
Alkanes are heat at 650. It`s pretty hard for me to explain in english this process at this hour. I will write in our common language;) after you will conclude.

Temperature >650 degree C:

CH4 is stable before 900 degree C

CH3-CH3 --> CH2=CH2 + H2

CH3-CH2-CH3 -->CH2=CH-CH3 + H2

2CH3-CH2-CH2-CH3 --> CH2=CH-CH2-CH3 + CH3-CH=CH-CH3 + 2H2
(two diastereomers

2CH3-CH2-CH2-CH2-CH3 --> CH2=CH-CH2-CH2-CH3 + CH3-CH=CH-CH2-CH3 + 2H2

Noow...CRACKING TIME(temperature <650 degree C). Cracking it would be first but is not wrong is a little bit complicated.

CH3CH3 → 2 CH3•
CH3• + CH3CH3 → CH4 + CH3CH2•
CH3CH2• → CH2=CH2 + H•
CH3CH2• + CH2=CH2 → CH3CH2CH2CH2•
CH3• + CH3CH2• → CH3CH2CH3
CH3CH2• + CH3CH2• → CH2=CH2 + CH3CH3

So after cracking you will obtain a mix of alkanes and alkenes.

I hope this explanation help you to finish your economical map:)

* Please give the right expression for my unstuck attempt.

Edward Elric - 15-11-2009 at 18:18

I see. Thank you.

How would a person control that?
Is that not controllable?
It seems at such higher temperatures that the ability to control such a formation would be unlikely.
To reduce the problem of little control, should a chemist take this path: Alkene to alkyl halide to alkane?

Do most chemists go from alkene to alkyl halide to alkane when desiring an alkane?

[Edited on 16-11-2009 by Edward Elric]

sonogashira - 15-11-2009 at 18:22

I remember reading this for a finals exam several years ago, and it is still very interesting now also with clever terminal alkane activation chemistry too.

Edit - the review makes more sense than my sentence:P and I would recommend also to look for Larock's Comprehensive organic transformations if you're interested in inter-conversions as it is very good! :D

Attachment: Organometallic alkane CH activation.pdf (323kB)
This file has been downloaded 7056 times,

[Edited on 16-11-2009 by sonogashira]

[Edited on 16-11-2009 by sonogashira]

[Edited on 16-11-2009 by sonogashira]

Arrhenius - 15-11-2009 at 20:39

alkyl halide -> alkane = sodium amalgam (Na/Hg) amongst others.

Edward Elric - 15-11-2009 at 21:49

Think you could be more specific?
Is the Na a catalyst?

Your search - "sodium amalgam" "alkyl halide to alkane" mechanism - did not match any documents.

[Edited on 16-11-2009 by Edward Elric]