Lecture 7 Flashcards
(19 cards)
what is the nucleophilic unimolecular substitution reaction?
it is a substitution reaction that occurs in two steps and is only dependent on the leaving group (the compound itself)
what are the characteristics of Sn1 reactions?
- the reaction is only dependent on the concentration of the alkyl — that is because the transition state for ionization (heterolytic cleavage) (3ani the bond breaking between the leaving group and the alkyl) has the highest energy barrier in the reaction making it the rate-determining step
- tertiary alkyl reacts the fastest - since the most substituted alkyl forms the most stable carbonation, also Sn2 with a tertiary alkyl would cause a lot of steric hindrance
- if the leaving group is attached to the asymmetric center the product will be a pair of enantiomers — that is because the carbonation formed allows for the nucleophile to have a 50-50 chance of attacking from either above or below forming two enantiomers and a racemic mixture
what is the mechanism of the Sn1 reaction?
slide 3
what are the factors that affect the Sn1 reaction
- electrophile
- leaving group
- solvent
Sn1 and Sn2 reactions are favored by
Sn1 - protic solvents
Sn2 - aprotic solvents
benzylic and allylic halides go through Sn1 or Sn2
it is expected for them to go through a Sn2 reaction since they are primary alkanes and they do go through Sn2, but they also go through Sn1 as the carbocation intermediate formed in both the benzylic and allylic halides would have a delocalized electron structure with resonance contributors giving the carbocation such stability that it allows for it to exist and allows for the reaction to undergo Sn1.
what are the different types of elimination reactions?
- E2-elimination — it is a concentrated one-step mechanism elimination reaction
- E1-elimination —- it is a reaction only dependent on the concentration of the leaving group and it follows a two-step mechanism with a carbonation intermediate (the more stable the carbonation is the faster the reaction)
what is Zaitsev’s elimination rule?
when there is a choice the most stable alkene is formed and in most cases that is the most substituted alkene
what is the exception to Zaitsev’s rule?
- elimination with bulky bases leads to the major formation of the less substituted alkenes since there will be a lot of steric hindrance in the transition state
- alkyl fluorine is eliminated to form a less substituted major alkene and that is because the transition state for the less substituted alkene is more stable.
what is the mechanism of both E1 and E2 reactions
E1 - 18
E2 -14
E1 vs E2
similarities
- major product is generally the most substituted alkene
- tertiary halides are the most reactive
- the better the leaving group the faster the reaction
Differences
- E2 occurs with primary, secondary, and tertiary alkyl
- E2 is favored by a high concentration of base and in polar aprotic solvent
E1 - only occurs in tertiary
E1 - is favored in weak base and polar protic solvent.
What are the two different roots of eliminations that E2-elimination can take
syn elimination - from the frontside - at eclipsed confirmation
Anti elimination - from the backside - at staggered confirmation
Anti elimination is more favoured because it enables backside attack.
one there is only one hydrogen to eliminate E2 will always go via anti-elimination
in anti-elimination when there is the possibility of the formation of E or Z isomers which will form
E will predominantly form since it is much more stable.
what is E1 Stereochemistry
it happens in two steps so it can always form the more stable E isomer as its major product
when dealing with cycloalkanes and elimination reactions it is always assumed that
Zaitsev rule is followed
also i assume that they most of the time undergo E1
when dealing with the dienes and elimination what is important to note
conjugated dienes are more easily formed so sometimes the less substituted alkene is the major product going against Zaitsev rule
what elimination would allylic and benzylic halide undergo?
both E1 and E2 due to the accessible halide and the stabilized cation that forms
Sn2 Vs E2
Primary alkyl halide - Sn2 - unless a very bulky base is present or the halide is sterically hindered and the nucleophile is a strong base.
secondary alkyl halide - Sn2 or E2 - E2 is major when the nucleophile is more sterically hindered and/or basic
Teritary alkyl halides - E2 - Sn2 is impossible but you still need a good base for E2
Sn1 vs E1
Primary - E1 - Sn1 doesnt occour
Secondary - E1 - Sn1 doesn’t occur
Tertairy - E1 and Sn1 - Sn1 is always favoured
