Effect Of R Groups Flashcards
Why do tertiary compounds not undergo SN2 reactions?
Large substituents decrease rate as steric hindrance means Nu- cannot approach and therefore tertiary do not undergo SN2
+I groups decrease positive charge therefore carbocation is less reactive
The transition state has 5 groups around it and large R groups (R instead of H) will cause steric hindrance which increases the energy of the transition state- unfavourable
What are the affect of CH3 groups in substrates of SN2 reactions?
More CH3 groups reduce the rate of reaction as they reduce the positive charge on the carbocation and so therefore make it less reactive
The stability of carbocation increases as the number of CH3 groups increase
How do the size of the substituents affect the rate of an SN2 reaction?
The larger the substituents the slower the rate of reaction
This is because the approach of the nucleophile is sterically hindered by large substituents
How is the transition state affected by the size of the R groups in an SN2 reaction?
The transition state is crowded as there are 5 groups around the central atom and so the larger the groups, the greater the steric interactions and the higher the energy of the transition state
Why do tertiary halogenoalkanes react in SN1 reactions and not primary and secondary halogenoalkanes?
Tertiary have more CH3 groups which are +I which stabilise the positive charge in the transition state so it is more stable
This means the transition state is formed faster- favourable
Primary do not have enough +I to stabilise the transition state
Under what circumstances can primary halogenoalkanes react in an SN1 reaction?
A substituents that can stabilise a carbocation by mesomeric effects will promote the SN1 reaction
- if there is a pi system to act as a +M group and stabilise the positive charge by resonance
How do the size of the R group affect the rate of an SN1 reaction?
Increasing the size of R groups increases the rate
This is due to steric acceleration
When a haligenoalkane is converted into a carbocation, there is a release of steric strain as the groups are further apart- this means the larger the R group, the greater the steric strain and the faster the rate of cleavage of CX
What is hyperconjugation?
This is the interaction of an unfilled p orbital with CH sigma bonds in adjacent carbon atoms
What is hydride shift?
This is where a hydrogen in a secondary carbocation can shift into the positive carbon to form a tertiary carbocation which is more stable- favourable
What is 1,2 methyl shift?
A CH3 on a primary carbocation can shift into a positive carbon atom to form a tertiary carbocation which is more stable- favourable
How do large R groups affect SN1 and SN2 reactions differently?
-large R groups are unfavourable for SN2 Tertiary undergoes SN1 as there would be too much steric hindrance for a Nu to enter in an SN2 reaction
- large R groups are favourable for SN1
Increase rate by steric acceleration
Apart from +I effects, how else are tertiary carbocations stabilised?
They are also stabilised by hyperconjugation
This is the donation of electron density into empty o orbitals
Describe the difference in reaction by substitution of cyclisation structures compared to linear
Cyclisation inhibit nucleophilic substitution
This is due to steric crowding which disfavours SN2 as nucleophile cannot attack and inability to form an unhindered planar sp2 hybridised carbon
For substitution, is cis or trans more favourable?
For substitution reactions, kcis»Ktrans has a faster rate and the product is more thermodynamically favourable
Although ktrans has a better reacting confirmed, kcis has a better conformer for the product and so is thermodynamically favourable