Try it with (S,S).
The cis isomer has one axial and one equatorial bromine. Only the axial bromine has the necessary anti arrangement for elimination.
The trans does have the diaxial conformation, albeit unstable, available to it. Now both bromines are ani to a hydrogen and can eliminate. Try drawing the structure.
The cis isomer has the bromine in an axial position as the t-butyl group will be equatorial due to steric considerations. In order to get the bromine axal in the trans isomer, the t-butyl group must also occupy an axial position which is sterically hindered – hence the difference in rate
Two units of unsaturation
Two units of unsaturationThe management feels that the compound might be useful as a pesticide, but they need to know its structure. You have been called in as a consultant at a handsome fee. Compound "A", when treated with KOH in ethanol, yields two compounds "B" and "C", each with molecular formula C10H16.
Three units of unsaturation now present. A dehydrobromination has occurred to give a new double bond.
Compound "A" rapidly reacts in aqueous ethanol to give an acidic solution which, in turn, gives a precipitate of AgBr when tested with AgNO3 solution.
Suggests a tertiary bromide which forms a tertiary carbocation and bromide ion
Ozonolysis of "A" followed by treatment with (CH3)2S gives (CH3)2C=O as one of the products plus an unidentified halogen-containing product.Original hydrocarbon contains (CH3)2C= group
Hydrogenation of either "B" or "C" gives a mixture of both trans- and cis-1-isopropyl-4-methylcyclohexane.
This is the carbon skeleton of "A"
Compound "A" reacts with one equivalent of Br2 to give a mixture of two separable compounds "D" and "E", both of which can be shown to be achiral. Finally, ozonolysis of "B" gives (CH3)2C=O and diketone:
Propose structures for compounds A through E.
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This page is maintained by Dr. Ed Blackburn (Ed.Blackburn@UAlberta.CA), course instructor.Updated August 23, 2002