2- SN2 Flashcards
nucleophile
- Chemical species rich in electrons (typically basic) and will share them with El
- Negatively charged or neutral
electrophiles
- Chemical species poor in electrons and will accept them from Nu
- Positively charged or neutral
displacement reactions
- One component is replaced by another.
- Displacement by a nucleophile – SN reactions
- Identify the Nu and the El
steps for SN2 reaction
- Nucleophile reacts at the electrophilic carbon (δ+)
- Bond breaks with the electrons moving to the more electronegative atom (the Leaving Group)
- Nucleophile replaces the LG
what is the leaving group
the most electronegative atom on the electrophile
what is the carbon alpha
the Carbon that is sigma positive, the site where the nucleophile is going to attack
sn2 reactions are concerted, meaning
the reaction occurs in one step
nucleophilic attacks occur where
the opposite side of the leaving group
how many transition states are there in SN2 reactions
1
where does the Nu go? HOMO or LUMO?
HOMO
where does the El go? HOMO or LUMO?
LUMO
explain the parts of the energy diagram (reactants, ts, and products)
- reactants: starting products of the reaction → higher in energy than the products, lower in energy than the transition state
- transition state: simultaneous bond breaking and bond forming (this is the RDS)
- products: final products of the reaction → lower in energy than the reactants, lower in energy than the transition state (this is because the reaction favours the more stable side)
sn2 reactions are what kind of molecular reactions
bimolecular (because they depend on both concentrations)
kinetic equation for SN2? and what order?
rate = k[Nu][R-x], second-order reaction
what happens to stereochemistry of a chiral centre during SN2 reactions?
the configuration inverted
what is double inversion? what does it do to the configuration?
by having 2 SN2 reactions, there is a retention of configuration
what is nucleophilicity?
The ability to share electrons (for SN2 to proceed it needs to have a good nucleophile)
what are the five effects on nucleophilicity?
- Effect of negative charge
- Effect of electronegativity
- Effect of atom size
- Effect of electron delocalization
- Steric congestion near the nucleophilic atom
what is the effect of negative charge on the nucleophile? what reaction will go faster?
- More available electrons, better electron donor.
- The nucleophile that has a higher negative charge will proceed faster
what is the effect of electronegativity on the nucleophile? what reaction will go faster?
- More electronegative atom is a weaker electron donor
- The less electronegative nucleophile will proceed faster
what is the effect of atom size on the nucleophile? what reaction will go faster?
- Valence electrons in bigger atom are less tightly held. They can be attracted by nearby positive charges to form bonds.
- Bigger atom (more polarizable) are better electron donor (better nucleophile)
what is the effect of electron delocalization on the nucleophile? what reaction will go faster?
Delocalization of the lone pair of the oxygen over several atoms, reduces their ability to be donated.
what is the effect of steric congestion near nucleophilic atom on the nucleophile? what reaction will go faster?
Sterically hindered nucleophile will slow down the SN2 reaction rate.
approximate order of leaving groups?
Cl < Br < I, OTs < H2O < OMs < OTf
two methods of activating bad leaving groups
- It can be activated with an acid (acid catalysis) to form a better leaving group H2O.
- It can be transformed into a sulfonate ester as a better leaving group.
explain the activation of hydroxyl group (HO) with acid catalysis
- Protonation of OH forms H2O which is a better Leaving Group.
- H2O is a weak base, therefore a very good LG
explain the activation of hydroxyl group HO to form sulfonate esters
Transforming OH via a nucleophilic addition to OMs, OTf or Ots (depending on R) which are great LG
what are protic and aprotic solvents?
- The Solvation of a Nucleophile is an important factor, SN2 depends on the nature of the solvent
- Protic solvent : a solvent that is a hydrogen bond donor (e.g. R-OH; R-SH)
- Aprotic solvent: a solvent that cannot serve as a hydrogen bond donor; no H-bond to O, S or N (e.g. DMSO, DMF, acetone)
what makes SN2 reactions go faster and why?
- SN2 reactions are much faster in polar aprotic solvents:
- Polar aprotic solvents do not have a hydrogen bond donating group.
- No hydrogen bonding, no solvation of the nucleophile (the anion)
Solvants’ effect on the relative nucleophilicity of halides in polar protic solvents specifically?
- the bigger the atom, there are less molecules around the atom, less chance for hydrogen bonding to occur = higher nucleophilicity
- the smaller the atom, there are more molecules around the atom, higher chance for H bonding= higher solvation
Solvants’ effect on the relative nucleophilicity of halides in polar aprotic solvent
- No hydrogen bonding, no solvation of the nucleophile (the anion)
- Atom size: bigger atom stabilizes better the negative charge → weaker nucleophile
- F- is the smallest atom, it cannot stabilize negative charge as well = better nucleophile
exception to the relative nucleophilicity of halides
- Finkelstein reaction
- subsituting Cl with an I in the presence of an aprotic solvent
- use of acetone makes reaction possible
- R-Cl + NaI –> R-I + NaCl
- NaCl forms precipitate
- NaI dissolves in acetone
