W20 Nucleophilic substitution SN1 and SN2 Flashcards
What is the Transition state?
Theoretical representation of the high energy point between starting materials and products. It does not physically exist
Bimolecular reactions
two species are involved in the rate determining step (defined as the slowest step of a reaction)
Factors Affecting SN2 reaction: nucleophile
Strength
Bulkiness
Basicity definition:
Defined by the equilibrium constant for abstracting an acidic proton;
forms a new bond to a proton, it has reacted as a base
Nucleophilicity definition
Defined by the rate of attack on an electrophilic carbon atom;Forms a new bond to carbon, it has reacted as a nucleophile
Periodic table:
Does nucleophilicity inc or dec?
Across the period
Down the period
Across= Decreases following the increase in electronegativity (tightly held non-bonding electrons that are less reactive toward forming new bonds)
Down= increases down the periodic table, following the increase in size and polarisability (more electrons far from the nucleolus)
How does bulkiness affect an SN2 reaction?
Nucleophile: an ion or a molecule must get close to a carbon atom to attack it
Bulky groups on the nucleophile hinder this close approach, SLOWS the reaction rate
What are Protic solvents?
-OH/NH groups (i.e. alcohols)
-H-bonds to negatively charged nucleophiles
-Very good in solubilising reagents
-Reduce Nucleophilicity of small anions
What are Aprotic solvents?
-Without OH/NH groups (i.e. hexane).
-Enhance nucleophilicity of anions
-Anions are more reactive
-Relatively weak solvating ability
What are Polar aprotic solvents?
- No OH or NH
- Enhance solubility
- Acetonitrile, dimethylformamide (DMF), acetone, and dimethyl sulfoxide (DMSO)
- From alcohol to DMSO often enhances the rate of an SN2 reaction
- Reverse the nucleophilicity order (F->Cl-..)
Resonance stabilization
Because resonance allows for delocalization, in which the overall energy of a molecule is lowered since its electrons occupy a greater volume, molecules that experience resonance are more stable than those that do not. These molecules are termed resonance stabilized.
What is the Cis/trans system?
Only applies when both ends of the double bond have two different groups attached, but groups at one end of double bond are identical to those at the other
What happens in an SN2 reaction? (nutshell)
Nucleophiles convert alkyl halides to a wide variety of functional groups (FGI)
How does Strength of the Nucleophiles affect an SN2 reaction?
Strong Nucleophile: react faster in a SN2 reaction
Basicity Vs Nucleophilicity
Basicity: defined by the equilibrium constant for abstracting an acidic proton; forms a new bond to a proton, it has reacted as a base
Nucleophilicity: defined by the rate of attack on an electrophilic carbon atom;
