Nucleophilic substitution: SN2 Flashcards
What does the 2 stand for?
Bimolecular
What does bimolecular mean?
Two molecules are involved in the rate determining step
What does the rate of reaction depend on?
Both the concentration of the nucleophile and the electrophile
What happens to the reaction graphs when you change either concentration?
The rates increase with both
What can donate electron density?
anything with a lone pair
What does the donation of electron density do?
Stabilises the molecule and lowers the energy
How many transition states are there in an SN2 reaction?
1
How many steps is an SN2 reaction?
1
What type of electrophiles are likely to be bad at SN2 reactions and why?
Bulky electrophiles
There is a steric hinderance as the necessary orbital overlap is poor
What is the influence of conjugating groups on an SN2 reaction?
They speed up the reaction by stabilising the transition state and therefore lowering the activation energy
What do the rates of SN2 reactions depend on?
Sterics
What is the relative rate of reaction for a molecule with no blocking groups?
200
Unhindered
What is the relative rate for a molecule with one blocking group?
cli
What is the relative rate for a molecule with one blocking group?
1
A little bit hindered but not massively so still a fast rate of reaction
What is the relative rate for a molecule with two blocking groups?
0.02
Secondary molecules, more hindered
What is the relative rate of reaction for a molecule with 3 blocking groups?
0
Too sterically hindered doesn’t take part in SN2 reactions
What is the relative rate of reaction for a molecule with a phenyl ring?
200
Benzene ring makes for a better conjugated system than an isolated c=c
What is the relative rate of reaction for a molecule with a carbonyl group?
100,000
C=O conjugation is even more effective
Why does the leaving group affect both SN1 and SN2 reactions?
The C-X bond is broken in the rate determining step for both
What are the two factors which affect the leaving group’s ability?
Bond strength of C-X
Anion stability of X-
What is the stability of X- related to?
The pKa of X
Which pKa value is the most stable leaving group? Use halogens as example
The highest one
F = 3
Which halogen is the best leaving group and why?
Iodine
pKa = -10 so strongest acid
It is the largest atom = weakest bond due to poor overlap of orbitals.
How does the nucleophile affect the Sn2 reaction?
Determines how fast the reaction will be as it is part of the rate determining step
The more basic the nucleophile…
The more nucleophilic
Why is -OH the fastest nucleophile? What is the relative rate?
12000
The pKaH is the relatively basic (15), it wants to delocalise the negative charge
Why is PhO- a slower nucleophile than -OH? What is the relative rate?
2000
It wants to delocalise within itself so it is less likely to donate as a Nu- than -OH
Why does AcO- have a slow relative rate? What is the relative rate?
900
Because of conjugation, a lot of the molecule is already delocalised which means it is not a strong nucleophile
Why does a reaction with ClO4- as the Nu- not happen?
The whole molecule is already delocalised
How do the rates of PhO- and PhS- compare as nucleophiles?
PhO- = 2000 (pKa = 10) PhS- = 50,000,000 (pKa = 6)
Why is it surprising that sulfur is a better nucleophile than carbon for attack at a saturated carbon?
It has a lower pKa, less basic than -OH
Why is sulfur a better nucleophile than oxygen?
The LUMO of the C-X bond is on the same level as the filled sulfur orbital whereas the filled oxygen level is one below
It takes less energy to transfer electrons from HOMO of sulfur into the LUMO of C-X
What are the best nucleophiles to use for SN2 reactions?
Soft nucleophiles
Why is it better to use nucleophiles which are further down the periodic table?
They are a better match in orbital energy
Describe soft nucleophiles?
Large Not basic Have a diffuse Have an easily polarised electron cloud Have a high energy HOMO
In SN2 reactions with nitrogen nucleophiles, what is the reactivity of the starting material like compared to the reactants?
They are the same
When you react a nitrogen nucleophile, what can you do?
React the initial product of a primary amine again as it is still nucleophilic
You can form primary, secondary, tertiary amines and quaternary ammonium salts
What must you do if the primary amine is your desired product in SN2 reaction with a nitrogen nucleophile?
Use a large excess of NH3
What must you do if the quaternary ammonium salt is your desired product in SN2 reaction with nitrogen nucleophile?
Use at least one equivalent of the alkyl halide relative to the amount of ammonium used
What happens to the stereochemistry during an SN2 reaction?
It becomes inverted
Why does stereochemistry become inverted?
Large nucleophile
Pushes the R groups causing them to be inverted
Attacks at 180 degrees to the leaving group
What is the steric reason the nucleophile attack 180 degrees to the leaving group?
Approach from the 180 degrees is the least hindered direction for Nu- attack the carbon
What is the electronic reason for the nucleophilic attack at 180 degrees to the leaving group?
The HOMO of the nucleophile must overlap with LUMO of electrophile.
For an alkyl halide, the sigma star orbital of the C-X bond and this is orientated at 180 degrees to the C-X bond
What is the HOMO-LUMO gap like for soft nucleophiles?
The gap is smaller so there is better overlap
