DAT DESTROYER CLARIFICATION

[Studoc28]

"When comparing bond length; double bonds with resonance are not true double bonds." Does this means that double bonds with resonance will have longer or shorter bonds than an alkene with no resonance?

Also, for 2015 version number 94 (or go) can anyone explain this mechanism? I am confused why C6H5Li will remove a proton and how does an alkene form at the end!?

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[FloorMatt]

Double bonds with resonance is like having a bond length between a double bond and a single bond, or 1.5 bonds is how I think of it. So it is shorter than a single bond but longer than a double bond. Sp3>sp2 (resonance) > sp2 ---- with sp2 the shortest.

The next problem is hard to explain but you may want to look up ylide formation and wittig reaction because thats what that is. Sorry I can't be much help on that maybe someone else will give it a shot.

 

[Graffix]

I would suggest the explanation on UCD's chem wiki! it goes more into detail about the mechanism for the ylide formation and wittig reaction. I think its definitely one of the coolest mechanisms. Almost as cool as diels-alders!
http://chemwiki.ucdavis.edu/?title=...ns_of_Aldehydes_&_Ketones/The_Wittig_Reaction

 

[Endeross]

"When comparing bond length; double bonds with resonance are not true double bonds." Does this means that double bonds with resonance will have longer or shorter bonds than an alkene with no resonance?

Also, for 2015 version number 94 (or go) can anyone explain this mechanism? I am confused why C6H5Li will remove a proton and how does an alkene form at the end!?

The lithium is an ionic bond to the c6h5, it essentially stabilizes the carbocation. In the reaction the carbocation steals the hydrogen so that it can become a more stable species which forms the carbanion which is stabilized by the positive phosphorus. Definitely look at Graffix's link you can see the mechanism for it especially under the deprotonation step for mechanism of ylide formation