Elimination Basics
Define an elimination reaction.
Removal of atoms from adjacent carbons to form a double bond.
What is Zaitsev’s rule?
The most substituted alkene forms.
What substrates favor SN2?
Primary (1°) substrates.
Name these functional groups:
R-OH; R-O-R; three-membered cyclic ether
What is an epoxide?
A three-membered cyclic ether.
Name the two elimination mechanisms.
E1 and E2
Which alkene is more stable?
The more substituted alkene.
What conditions favor SN1?
3° substrate, weak nucleophile, polar protic solvent.
Conversion of 1° and 2° Alcohols to Alkyl Halides uses what reagents?
SOCl2 (thionyl chloride) converts alcohols into alkyl chlorides; PBr3 (phosphorus tribromide) converts alcohols into alkyl bromides.
The nucleophile opens the epoxide ring from where?
The backside.
What must a β-elimination remove?
A β-H and a leaving group.
What favors E2 over SN2?
Strong base, heat, steric hindrance.
For an alcohol to undergo nucleophilic substitution, OH must be converted into what?
OH must be converted into a better leaving group. (Unlike alkyl halides in which the halogen atom serves as a good leaving group, the OH group in alcohols is a very poor leaving group).
In an unsymmetrical epoxide, the nucleophile attacks at which carbon?
The less substituted carbon atom.
What is the difference between E1 and E2 in rate law?
E1 = unimolecular (first-order kinetics; rate = k[RX])
E2 = bimolecular (second order kinetics; rate = k[RX][base])
For E2 elimination, the C-Cl bond must be ______ to the C—H bond on a beta carbon, and this occurs only when the H and Cl
atoms are both in the _______ position.
anti periplanar; axial position.
Do strong nucleophiles favor substitution or elimination reactions?
Substitution reactions.
Strong nucleophiles favor SN2 reactions because they react quickly, directly participating in the single, rate-determining step.
Elimination requires the nucleophile to act as a strong base to pull off a proton, a path less favored unless sterically hindered or at higher temperatures where entropy drives the formation of a double bond.
The mechanism of ether cleavage is either ____ or _____ (E1/E2/SN1/SN2), depending on the identity of ________.
The mechanism of ether cleavage is SN1 or SN2, depending on the identity of R.
• When 2° or 3° alkyl groups are bonded to the ether oxygen, the C-O bond is cleaved by an SN1 mechanism involving a carbocation.
• With methyl or 1° R groups, the C–O bond is cleaved by an SN2 mechanism.
Common nucleophiles that open the epoxide ring include ____? (Name 2 out of 5)
¯OH, ¯OR, ¯CN,¯SR, and NH3.
What is the role of base strength in E1 vs E2?
Strong base → E2; weak base → E1 if carbocation forms.
Explain the difference between stereoselectivity and regioselectivity.
A reaction is stereoselective when it forms predominantly or exclusively one stereoisomer when two or more are possible (cis/trans). A reaction is regioselective when it yields predominantly or exclusively one constitutional isomer when more than one is possible.
Predict mechanism: 2° + weak nucleophile + heat.
E1
When ethers react with HBr or HI, both C–O bonds are cleaved and ______ are formed as products.
Two alkyl halides
Epoxides do not contain _____?
A good leaving group.