[Vidhya]

Hello,

I've been agonizing over an assignment question that I think I've answered however, I would love some feedback to see if I am correct. Here it is:

When a sample of 2-butene was reacted with cold aqueous potassium permanganate, the product formed was meso-2,3-butanediol.  Treatment of a second sample of 2-butene from the same source with m-chloroperbenzoic acid produced an epoxide which, when subjected to acid hydrolysis, gave a racemic mixture of 2,3-butanediol. Was the 2-butene used the cis or trans isomer?

My Answer:
Now from reading up on alkene hydroxylation it seems that cis addition always occurs, so formation of the meso diol product occured as expected. When an acid-catalyzed epoxide cleavage occurs, it takes place by backside attack and a trans diol always occurs. Again we obtained the expected product.

However, I have been reading all over and I can't seem to find any evidence that the isomer of the original reactant (2-butene) in any way affects this process. Therefore, I can only assume that the question is essentially a trick question in that the 2-butene could have been either cis or trans and we would have still obtained the same complementary products. Is this correct?

[azmanam]

no.  the geometry of the double bond is crucially important.

draw both cis and trans double bonds.  Then draw the mechanism for, say, the epoxidation and epoxide opening - note that the double bond geometry influences the stereochemistry of the product.  Note that one double bond geometry leads to meso diol and one leads to racemic diol.

but which gives which?

[Vidhya]

Thanks for the reply. After reading your post, I thought I had it figured out, but then I ran into a problem and I think it must be with my drawings.

When I drew the reactions for the alkene hydroxylation (reaction 1) it looks like the cis-2-butene results in the meso-diol.

However, when I draw the reactions for the epoxide ring reactions (reaction 2), I am not obtaining a meso-diol for either cis or trans 2-butene, however one must be a meso.

I've attached three pics for the two reactions however I only included the products of each step. Any advice would be a huge help. Thanks!

[blind]

Perhaps it would be easier for you to visualize whether or not it's meso or racemic if you drew the methyl groups on the plane of the paper, then evaluate the two products using that as a reference.

Hope that helps.

[Vidhya]

When I draw the methyl groups on the plane of the paper for the cis Reaction 2, the final product looks meso to me. That would mean the cis-2-butene results in meso products for both reaction 1 and 2. 

However, according to the question the same reactant should result in a meso product for reaction 1 and a racemic product for reaction 2. 

I'm fairly certain I'm correct in how I visualized the product from reaction 1, so therefore the cis-2-butene results in a meso-diol for alkene hydroxylation. If this is correct, then I expected to obtain the racemic product for the cis reactant in reaction 2 and I seem to be getting the opposite.   

It seems like such a simplistic problem, yet for some reason I can't figure out why my products for reaction 2 are turning out opposite then I'm expecting. Did I actually get reaction 1 wrong somehow?

[blind]

Keep in mind about where the hydroxyl group is with respect to the methyl group when you're rotating the bonds to the "trans" position after the reaction, then compare with the trans product.

You're right about the first one though.

[Vidhya]

Thanks for all the help, I've finally figured out the problem which seems so simplistic now...

I seem to run into problems when attempting to visualize a 3-D molecule in 2-D space. It's fine when I have a couple days to work over it, but I certainly worry about this come test time.