Nucleophilic substitution: SN1 Flashcards
What are the basic steps of an SN1 reaction?
Leaving groups departs, leaving the carbenium ion which is attacked by Nu-
Which step is the rate determining step?
The slowest step
Is the rate determining step reversible or irreversible?
Reversible
Why is the fastest step irreversible?
There is electrostatic attraction between the negative nucleophile and the positive carbenium ion
What is hyperconjugation?
Electrons from the sp3 hydrogen are donated into the empty p orbital in the same plane
What is more effective hyperconjugation or pi conjugation
Pi conjugation
What does S stand for?
Substitution
What does N stand for?
Nucleophilic
What does the 1 stand for?
Unimolecular
What does unimolecular mean?
Only one molecule involved in the rate determining step
What do the graphs for rate kinetics show?
If you change one molecule and keep the other constant, the rate increases
If you change the other while you keep the other constant, the rate stays the same
Shows only one is involved in RDS
How many transition states are there for SN1 kinetics diagram?
two
What are the transition states?
R—X
Nuc —- R
Which transition state is higher in energy?
R—-X
Which activation energy is the highest?
The one which causes the loss of the leaving group meaning that one is determining
What are the two axes for a reaction diagram?
Energy on Y (With a scale)
Reaction co-ordinate on X
What are the two SN1 substrate effects?
Stability of the carbenium ion intermediate
Leaving group ability of X-
How does the stability of the carbenium ion intermediate effect?
The stability of the first transition state determines the rate of reaction, partially positive charge on the carbon atom is stabilised
What is able to stabilise a carbenium intermediate?
Any electron donating group attached to the electron deficient carbon
What are the most to least stable carbenium ions?
Tertiary
Secondary
Primary
Methyl
How do alkyl groups help to stabilise the carbenium ion?
They are electron donating
How does hyperconjugation stabilise the carbenium ion?
The alkyl groups stabilise the carbenium ion by donating electrons from pi bonds when overlapped with the p orbital
Which type of conjugation is more effective at stabilising carbenium ions; pi or hyper?
Pi conjugation
What does pi conjugation result in?
A positive carbenium ion where the positive charge is spread across two or more atoms
How does pi conjugation stabilise a carbenium ion?
electrons in the filled pi bond are donated to the empty p orbital
Where can a nucleophile attack in pi conjugation?
Occur at either end of the delocalised carbenium ion and mostly attacks at the less hindered end of the allyl cation
What are the rates of SN1 reactions controlled by?
Electronics
What is the relative rate of a primary alkyl and why?
0.07
all sn2 reactions are likely - unstable carbenium ion intermediate
What is the relative rate of a secondary alkyl and why?
0.12
Poor SN1 but can still take place, not very stable carbenium ion
What is the relative rate of a tertiary alkyl and why?
2,100
Very good SN1 due to stability of the tertiary carbenium ion
What type of conjugation do primary, secondary and tertiary alkyls have?
Hyperconjugation
What is the relative rate of an allylic compound?
1
Good SN1, pi conjugation
What is the relative rate of an allylic compound which is also tertiary?
130,000
Extremely good SN1, has both a tertiary centre and pi conjugation
What is the relative rate of an allylic compound with a phenyl ring?
7,700
Good SN1, pi conjugation
Why are pi conjugated systems good?
Conjugation means a primary carbenium ion could become a tertiary carbenium ion (resonance forms) which is more stable and causes a really fast reaction rate
How can heteroatoms stabilise carbenium ions?
By delocalisation
They possess a lone pair pf electrons
How much of the lone pair of the heteroatoms can be donated to stabilise the carbenium ion?
100%
What is the order of effectiveness for pi, sigma or lone pair?
Lone pair»_space; Pi»_space;» sigma
Why is chloro methyl ether sometimes used?
Alkyl halide which reacts with a range of nucleophiles by SN1 reactions
What conditions does acetal hydrolysis take place in?
Acid hydrolysis
In acetal hydrolysis, how is the carbenium ion stabilised/
By the adjacent oxygen atoms
What is the influence of the nucleophile in the SN1 reaction?
The nucleophile doesn’t affect the rate of reaction as they aren’t involved in the rate determining step
Why do nucleophiles affect the nature of the SN1 product? eg alcohols
Nucleophile may attack C+ even though there is more water present but if the water attacks it will produce the starting product again
What is the stereochemistry like for an SN1 reaction?
They cause racemisation (50:50 mixture of products)
How is racemisation caused in an SN1?
There is no stereochemistry in the planar carbenium ion which means there is equal attack on either face by the nucleophile
When will the product be racemic?
Always even if the starting material is enantiomerically pure
What does reaction from starting material to product go via? (racemisation)
A planar intermediate
