SN1 vs SN2 Flashcards
Define nucleophile
A species prepared to donate a pair of e- to form a bond
BRIEFLY describe a SN2 reaction
X leaves as Nu attaches
Is SN2 uni- or bi- molecular? Why?
Bimolecular. Rate depends on the concentration of two (both) reactants
What step is the slow step in a SN2 reaction?
The only step
What are 3 important things to note about SN2 reactions?
- Nu attacks from a specific direction (back), most δ+ region
- Causes inversion to occur
- Is stereospecific
How many enantiomers are produced in a SN2 reaction? What does this mean for polarised light?
One enantiomer .: will rotate polarised light
BRIEFLY describe a SN1 reaction
X leaves then Nu:– attaches
What step is the slow step in a SN1 reaction?
The first step (leaving group leaving)
Why is the removal of the leaving group the slow step?
Because energy needed to break bond
What role does the nucleophile play in the slow step of an SN1 reaction?
Nucleophile plays no role. SN1 reactions are independent of Nu strength - just has to be a nucleophile (prepared to donate e-)
What is the fast step of a SN1 reaction? Why?
Addition of the nucleophile.
Because attraction
Describe the hybridisation and shape of a carbocation
sp2 hybridised trigonal planar
How many enantiomers form from a SN1 reaction? Why? What does this mean for polarised light?
Flat carbocation = equal probability of Nu attacking from one side or the other
□ Equal probability of forming each enantiomer
□ Inversion and retention of shape
□ No net rotation of polarised light
Why are tertiary C more likely to react via SN1 than primary or methyl C?
Because first transition state can be stabilised.
In 3º molecules, CH3 groups attached are e- donating, e- push in, diminish charge on central C = lower Ea, rate increases.
This stabilisation does not occur (as much) in primary or methyl C molecules
What is an absolute requirement for SN1 reactions?
A good leaving group (strength of Nu:- is irrelevant)
What requirement of SN2 reactions can be made up for by a strong nucleophile?
Having a good leaving group
What determines how good a leaving group is?
How strong the C-X bond is /bond length
What does protonating an -OH group do?
O becomes +vely charged, e- closer to O, bond even more polarised, weaker
= Increases likelihood of leaving, :. becomes good leaving group
What is an absolute requirement for SN2 reactions?
A good nucleophile
What makes a good/strong nucleophile?
- Small (has to get in past any other groups attached)
- negatively charged
- Not necessarily a strong base (poor nucleophile if large/hindered)
Define protic
Protic = able to form H bonds
Is water a protic or an aprotic solvent? Why?
Protic as able to from H-bonds
What solvent types is SN1 fastest in?
Polar solvents, especially if H bonding (= polar protic)
How is the first transition state of an SN1 reaction stabilised in water? What effect does this have on the Ea of the reaction?
- polar effect around C
§ δ- O end attracted to δ+ C, pulls towards
§ Helps stabilize emerging carbocation
§ Lowers Ea - H-bonding to leaving group
§ δ+ H interacts with emerging -ve charge
§ Helps stabilize leaving group leaving as well
§ Lowers Ea
What solvents are best for SN2 reactions? Why?
Polar APROTIC solvents, so solvent cage doesn’t form around Nu, inhibit its attack
