Reaction mechanisms Flashcards
(29 cards)
What is an inductive effect?
The ability of a chemical group or individual atom to either PULL electrons towards itself or PUSH electrons away from itself.
What are the consequences of an inductive effect?
Stabilization or destabilization of the molecule.
When inductive effect stabilizes a molecule?
1) Electrons are pulled away from negative charge.
2) Electrons are pushed towards positive charge.
When inductive effect destabilizes a molecule?
1) Electrons are pushed to negative charge.
2) Electrons are pulled away from positive charge.
What is a nucleophile?
Chemical species which are:
- electron rich;
- have at least 1 lone pair of electrons;
- attracted to parts of the molecule with electron deficiency
What makes a good nucleophile?
Good nucleophiles are conjugate bases of weak acids (such as O-2 or OH-, CH3-, NH2-).
Basically, what is less stable.
What makes a good leaving group?
Good leaving groups are conjugate bases of strong acids (such as Cl-, I-, H2O, N2).
Basically, what is more stable.
Why it is useful to protonate OH when substituting it?
HCl or HBr donate protons to OH because OH- is a poor leaving group but H2O is a good leaving group.
What are the main 3 steps of SN2?
1) Nucleophile attacks the side an opposite to C–X bond.
2) Transition state (negative charge is evenly spread between nucleophile and future leaving group).
3) A chiral carbon undergoes inversion of configuration after leaving group leaves (Walden inversion).
What is a reaction profile of SN2?
“One-hill” transition state.
What is an effect of steric hindrance on SN2?
More bulky groups slow down SN2 reaction by inductive stabilizing effect (pushing electrons to partially positive carbon).
What are the stereoproducts of SN2?
Mostly a single enantiomer (enantiopure), with configuration the opposite to reagent.
What are the main 3 steps of SN1?
1) Leaving group leaves first (no extra reagent needed).
2) Intermediate trigonal planar carbocation.
3) Nucleophile can attack from either side of carbocation, so there are two products: a racemic mixture (50% retention, 50% inversion).
What is a reaction profile of SN1?
“Two-hill” transition state.
What are the stereoproducts of SN1?
Racemic mixture (50% retention, 50% inversion).
When does SN2 dominate over SN1?
- Few bulky groups (less carbon chains, mostly small chemical groups): they form least stable carbocations:
- more polarised bonds - polar, aprotic (don’t dissociate into H+) solvents:
- polar stabilise polar transition state;
- aprotic do not interact with nucleophile and leave it “naked” to
attack.
When does SN1 dominate over SN2?
- Lots of bulky groups (mostly carbon chains): they stabilize carbocations the best.
- steric acceleration additionally withdraws leaving group. - polar, protic (can dissociate into H+) solvents.
- polar additionally stabilise carbocation charge;
- protic stabilise leaving group.
What are the main 3 steps of E2 (nucleophilic elimination)?
1) Base (nucleophile) attacks and binds to Hydrogen on less “bulky” carbon (leaving group stays anti-periplanar to this Hydrogen).
2) Transition state: negative charge is concentrated and distributed from base, future double bond and leaving group.
3) Stereospecific elimination: double bond product (pi-bond is formed from a pair of electrons from the attacked H), Base–H and leaving group leaves (by taking a pair of electrons to itself).
What does determine stereoselectivity in E2?
If rotamer has bulky groups from 2 carbons close to each other during nucleophilic attack, it becomes Z configuration (bulky groups continue to stay on the same side) - MINOR PRODUCT - less energetically favourable.
If rotamer has bulky groups from 2 carbons opposite to each other during nucleophilic attack, it becomes E configuration (bulky groups continue to stay on the opposite sides) - MAJOR PRODUCT - more energetically favourable.
What are the main 3 steps of E1 (nucleophilic elimination)?
1) Leaving group leaves first (no extra reagent needed), forming carbocation.
2) Base removes proton, the electron pair from former hydrogen are used to neutralise carbocation.
3) Regioselective product with a double bond and Ba—H.
What does determine regioselectivity in E1?
When nucleophile removes hydrogen from less substituted carbon - less substituted double bond - MINOR PRODUCT (less energetically favourable because less stable carbocation is formed).
When nucleophile removes hydrogen from more substituted carbon - more substituted double bond - MAJOR PRODUCT (more energetically favourable because more stable carbocation is formed).
When does E2 dominate over E1?
Using polar aprotic solvents (aprotic do not interact with nucleophile and leave it “naked” to attack but stabilise carbocation).
When does E1 dominate over E2?
Using very polar protic solvents (greatly stabilise both carbocation and leaving group).
When SN2 dominates over E2?
When there are small strong nucleophiles (such as OH-, NH2-).
