Functional Groups 2: Nucleophilic Substitution & Elimination Flashcards
What is the SN1 mechanism?
Unimolecular nucleophilic substitution
Found for all alkyl halides which give stable carbocations such as tertiary halides
Rate = k[RX]
Only RX is involved in the RDS
What are the stereochemical implications of the SN1 mechanism?
Forms racemic mixture of enantiomers
Both enantiomers formed in equal amounts as the nucleophile can attack above or below the plane with equal probability
Because carbocation is planar and achiral
Why are tertiary carbocations the most stable?
Because they have the greatest inductive effects - variable alkyl groups push electron density towards the positively charged carbon
How does resonance stabilisation impact stability of a carbocation?
Increases the stability of a carbocation intermediate by delocalising the positive charge over multiple atoms
What is the SN2 mechanism?
Bimolecular nucleophilic substitution
Rate = k[RX][Nu-]
Both steps occur at the same time, so the RDS involves both the nucleophile and the alkyl halide
E.g. formation of a nitrile is SN2
What are the stereochemical implications of the SN2 mechanism?
Very precise - SN2 is stereospecific
Rear-side attack is always found for SN2
Leads to Walden inversion (inversion of the chiral centre)
Has a 100% predictable outcome on stereochemistry
What type of substrates are favoured in SN2 reactions?
Less sterically hindered, bulky substrates
Therefore primary halides are more common for SN2 reactions
What type of substrates are favoured in SN1 reactions?
Sterically hindered, bulky substrates
What type of substrates are disfavoured by SN2 reactions?
No SN2 reactions can generally occur at sp2 orbitals due to the planarity which creates steric hindrance
How does choice of solvent influence whether SN1 or SN2 is favoured?
SN1 is favoured by polar solvents due to stabilisation of the carbocation by solvation
SN2 is favoured by aprotic solvents because they don’t solvate the nucleophile so that it is still reactive
When would elimination reactions occur instead of nucleophilic substitution?
If a strongly basic nucleophile was present
What is regioselectivity?
The preference for forming one constitutional isomer over another (i.e. major and minor products) - happens in formation of alkenes
What is Zaitsev’s rule?
That more highly-substituted alkenes are more stable then less substituted alkenes because C(sp2)-C(sp3) bonds are stronger than 2 sp3s
What is the E1 mechanism?
Unimolecular elimination
The base does not appear in the slow step:
Rate = k[RX]
2 step process via intermediate carbocation therefore competes with SN1
When would SN1 be favoured over E1?
If the nucleophile was not a strong base
