Explanations 
Textbooks 
Flashcards 
91影视 AI 
Notes 
Study Plans 
Study Sets 
Find a degree 
Magazine Chapter 11: Q62P (page 591)
(a) The reaction of butan-2-ol with concentrated aqueous HBr goes with partial racemization, giving more inversion than retention of configuration. Propose a mechanism that accounts for racemization with excess inversion.
(b)Under the same conditions, an optically active sample of trans-2-bromocyclopentanol reacts with concentrated aqueous HBr to give an optically inactive product, (racemic) trans-1,2-dibromocyclopentane. Propose a mechanism to show how this reaction goes with apparently complete retention of configuration, yet with racemization. (Hint: Draw out the mechanism of the reaction of cyclopentene with in water to give the starting material, trans-2- bromocyclopentanol. Consider how parts of this mechanism might be involved in the reaction with HBr.)
Short Answer
Step by step solution
About butan-2-ol
(a) The alcohol that is made up of a carbon skeleton containing four carbons with OH group at the second carbon is butan-2-ol.This compound is a secondary alcohol.
About the reaction
There is a possibility of two mechanisms. One path will give 100% of S configuration. Another path will give 50% of S and 50% of R configurations. The two reactions are SN1 and SN2 reactions.
About the mechanism
The water blocks all the incoming bromide ions from giving the products resulting from retention of configuration. The dominant pathway is the backside attack ofSN2 type.
About trans-2-bromocyclopentanol
(b) The cyclopentanol ring consisting of bromo group in the second carbon trans to the OH group is trans-2-bromocyclopentanol.The carbon skeleton of this compound bears five carbons in a cyclic ring.
About the reaction
The reaction is reverse of the formation of bromohydrin. There will be an attack of hydrogen bromide followed by the removal of water. The rearrangement of symmetric bromonium ion leads to the formation of racemic mixture.
About the mechanism
The intermediate product in this reaction is a cyclic bromonium ion. The racemic mixture is of trans-1, 2-dibromocyclopentane. Since there is a participation of Br, it is called neighboring group assistance. The mechanism is shown below.
Over 30 million students worldwide already upgrade their learning with 91影视!
