[Organofantastic]

Ok so I'm working on a problem where we have to outline the synthesis of preparing 2-ethyl 3-methylbutanoic acid using the malonic ester route. Specifically my textbook also asks which haloalkanes should I use.

Now I know I need to use a haloethane and a 2-halopropane to alyklate the enolate ions generated from treating malonic ester with a base.

What I don't understand is that in the answers, the book recommends iodoethane for the haloethane and 2-bromopropane for the 2-halopropane.  I've been under the impression that iodine is the best leaving group- so why would it suggest to use 2-bromopropane over 2-iodopropane?  Is bromine a better leaving group when it comes to 2° haloalkanes??

 Any understanding would be greatly appreciated.

[discodermolide]

Sure iodine is a good leaving group, but is it a good leaving group in the reaction you are asking about?

[Organofantastic]

Well in this reaction we are adding two alkyl groups. An ethane group and a isopropyl group. So according to the book it seems that using iodoethane is just fine for that purpose. But for the isopropyl group 2-bromopropane is being used. The reaction conditions are the same and the only significant difference I'm seeing is that for 2-bromopropane it is a secondary haloalkane so perhaps in this case it would be a better leaving group? I'm not getting why 2-iodopropane just wouldn't be good enough if iodoethane is fine for this purpose. 

Would bromine's higher electronegativity factor in when it comes to being a better 2° haloalkane leaving group?

[discodermolide]

Perhaps it is a matter of steric bulk?

[Organofantastic]

Hmm so bulky isopropyl would have an easier time letting go of the smaller bromine atom instead of iodine?

[discodermolide]

No it is more due to the size of the anion approaching the haloalkane.

[orgopete]

I don't see any reason that isopropyl iodide would not work as well, if not better. It is not unusual to use a catalytic amount of sodium iodide with a chloride or bromide in a substitution reaction. The iodide is also a good nucleophile and can form an iodoalkane in the reaction. It will react faster regenerating iodide ion. So the iodide can be used catalyticly. Usually chlorides are cheaper than bromides which are cheaper than iodides.