Topic 4: Alkenes and Epoxides Flashcards
rules for naming E and Z isomers
- look at the 2 substituents attached to each C and rank according to atomic no.
- if decision cannot be made > rank 2nd, 3rd or 4th atom away from double bond till clear difference found
- multiple bonded atoms equivalent to same number of single bonded atoms
- if higher ranked groups on each C at same side of double bond > Z geometry (cis), if different side then E (trans)
reagents for dibromination/dichlorination of alkenes
Br2 or Cl2
why addition of Br2 cannot follow mechanism for addition of HBr to alkene
HBr is polar > H+ easily donated as electrophile
Br2 non polar > cannot directly donate Br+ > need to form bromonium ion instead
mechanism for addition of Br2 to alkene
alkene acts as nucleophile to attack one Br
instead of forming carbocation, three membered bromonium ion is formed > stabilise +ve charge
bromonium ion highly strained > second Br- attacks from opposite side > anti addition > always result in trans product
how does dibromination follows markovnikov’s selectivity
when number of substituents are different, first Br goes to less substituted C
when number of substituents are the same, Br goes to C that has less steric hindrance
what is ring strain
small ring compounds have very high ring strain as they do not follow ideal bond angle > very reactive > easy to open the ring as it releases the strain energy
what are epoxides (ethylene oxide)
cyclic ethers with three membered ring structure
simple ethers can be opened up under acidic conditions but strong acid needed
epoxides have ring strain > more prone to nucleophilic attack > can be open by weak acidic conditions
how is epoxide prepared from alkene using two step method via halohydrin formation
- alkene reacts with halogen (X2) in presence of H2O or OH- > formation of halohydrin (compound containing OH and halogen attached to adjacent C)
- halohydrin undergoes intramolecular substitution: OH attacks C bearing X > formation of epoxide ring
how is epoxide prepared from alkene using one step method using peracids
oxygen of peracid (RCO3H) attacks pi bond of alkene > cyclic peracid intermediate > breaks down by eliminating proton from oxygen atom > form three membered epoxide ring
why does epoxidation of (R)-2-cyclohexanol only lead to epoxide on (R) side and epoxidation of (S)-2-cyclohexanol only lead to epoxide on (S) side
OH group on cyclohexanol can form H bond with peracid
H bond between OH and peracid ensures peracid attacks same face as the OH group, which is determined by configuration of starting material
if starting material is racemic mixture then get 1:1 ratio of (S) and (R)
