Elimination- E2 Flashcards
When does an elimination reaction occur
- When a nucleophile acts as a base and attacks hydrogen atoms beta to the leaving group to form an alkene
What is the beta position
- Not attached to atoms adjacent to leaving group
2. Attached to the atom after
Describe the structure of an alkene
- Contain a carbon-carbon double bond
- sp2 hybridised orbitals on each carbon make up 3 sigma bonds
- Remaining p-orbital on each carbon overlap to form a pi-bond
- Rotation around pi-bond is not possible
What does the restricted rotation around the pi-bond mean
- Substituted alkenes can exist in diastereomeric forms
2. Stereochemical descriptors E/Z are used to assign geometry
Describe the reaction kinetics of an E2 reaction
- Bimolecular- overall 2nd order
- The reaction rate is proportional to the concentrations of both the base and electrophile (1st order each)
- Both reactants are involved in the rate-limiting step
Describe the basic mechanism of an E2 reaction
- Simultaneous attack of the base and loss of the leaving group
- Curly arrow from base lone pair to H
- Arrow from H to C-C bond
- Arrow from C-X bond to X
- Forms alkene where C-C bond was and base.HX
Describe the transition state for an E2 reaction
- C-C solid line with dotted line above to show double bond forming
- Dotted line to delta negative X- leaving group
- Dotted line to H which has a dotted line to delta negative B
- Anti-periplanar
Write the rate equation for an E2 reaction of a halogenoalkane
- Rate = k * [halogenoalkane] * [base]
Describe the orbital requirements of an E2 reaction
- The new pi-bond is formed from overlap of sigma(C-H) with sigma*(C-X)
- The two orbitals must therefore overlap in the transition state
- Maximum orbital overlap when the hydrogen atom and the leaving group are anti-periplanar to each other
- Staggered conformation with an angle of 180 degrees between the hydrogen atom and the leaving group
What factors affect the rate of an E2 reaction
- Base
- Electrophile structure
- Leaving group
- Temperature
Why are E2 eliminations highly stereoslective
- Due to the requirement for an anti-periplanar transition state
What can be used to determine possible conformations for elimination
- Newman projections
2. Least sterically hindered conformer is favoured
What does a stereoselective reaction mean
- Multiple reaction pathways, but one stereoisomer of product is favoured
- Either the pathway with the lower activation energy is favoured and/or the most stable product is formed (thermodynamic control)
- Select most favourable pathway
What does a stereospecific reaction mean
- Stereochemistry of the starting material determines the stereochemistry of the product as a consequence of the reaction mechanism
What conformations of cyclohexanes can undergo E2 reactions
- The requirement for an anti-planar transition state means only certain conformations of substituted cyclohexanes can undergo E2 reactions
- Equatorial C-X bond means only C-C bonds are anti-periplanar so no E2 elimination
- Axial C-X bond means C-H bonds anti-periplanar to C-X so E2 elimination possible
- Leaving group must be axial for E2 elimination
How can regioselectivity of E2 eliminations be influenced
- By the steric demand of the base
- Small base- more substituted alkene favoured
- Hindered base- less substituted alkene favoured
Why do hindered bases favour less substituted alkenes
- Hindered bases attack the most accessible hydrogen atoms
- Transition state to more substituted product has greater steric demand so becomes higher in energy- thermodynamically favourable?
- Least substituted product forms faster (kinetic control)
What is the formation of the most-substituted alkene called
- Saytzeff product/elimination
- Formation of the thermodynamically favourable, most-substituted alkene
- Favoured with small bases
What is the formation of the least-substituted alkene
- Hofmann product/elimination
- Formation of the kinetically favourable, least substituted alkene
- Favoured with sterically demanding, hindered bases
Is NaOMe a big or small base
- Small
Is KOCEt3 a big or small base
- Massive
What substrates can undergo E2 elimination
- Primary alcohols can only undergo E2 as would form unstable primary carbocation
- B-hydrogen atoms are more accessible than an alpha-carbon atom
- This means tertiary alkyl substrates can undergo E2 as well as E1 whereas they can’t undergo SN2
