Nucleophilic substituions = SN1 and SN2 Flashcards
nucleophile always attacks the ______
electrophile
Explain what the Transition state is:
theoretical representation of the high energy point between starting materials and products. It does not physically exist
(a moment when a bond is forming and breaking)
How many steps in the Sn2 reaction?
1 with NO intermediate formation
What is SN2?
Substitution Nucleophilic Bimolecular reaction
Bimolecular reactions
two species are involved in the rate determining step (slowest step of a reaction)
What is involved in the rate-limiting step?
BOTH - Nucleophile and electrophile
Transition state: bond to the nucleophile (hydroxide) is partially ______ and the bond to the leaving group (bromide) is partially _______
formed
broken
Omeprazole is a….
proton pump inhibitor that decreases the amount of acid produced in the stomach
Factors Affecting SN2 reaction: nucleophile
1) strength of nucleophile
2) Nucleophilicity increases
3) Nucleophilicity decreases
4) Bulkiness of the nucleophiles
define Basicity
the equilibrium constant for abstracting an acidic proton;
forms a new bond to a proton, it has reacted as a base
define Nucleophilicity
the rate of attack on an electrophilic carbon atom;
forms a new bond to carbon, it has reacted as a nucleophile
- strength of nucleophiles
Species with negative charge: stronger nucleophile than a similar neutral species. Base is a stronger nucleophile than its conjugate acid
- Nucleophilicity decreases
from left to right in the periodic table, following the increase in electronegativity (tightly held non-bonding electrons that are less reactive toward forming new bonds)
- Nucleophilicity increases
down the periodic table, following the increase in size and polarizability (more electrons far from the nucleolus)
e- are more ______ held > move more freely toward a +ive charge > stronger bonding, enhancing atom’s ability to begin to form a bond at a relatively long distance
loosely
- Bulkiness of Nucleophiles
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, slow the reaction rate
What is a steric hindrance?
he slowing of chemical reactions due to steric bulk
manifested in intermolecular reactions, whereas discussion of steric effects often focus on intramolecular interactions
exploited to control selectivity, such as slowing unwanted side-reactions
Protic solvents meaning and characteristics?
OH/NH groups (i.e. alcohols)
H-bonds to negatively charged nucleophiles
Very good in solubilising reagents
Aprotic solvents meaning and characteristics?
without OH/NH groups (i.e. hexane).
Enhance nucleophilicity of anions
Anions are more reactive
Relatively weak solvating ability
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-…)
Factors Affecting SN2 reaction: substrate
Leaving-Group Effects on the Substrate
Polarise C-X bond (making the carbon atom electrophilic)
Leaves with the pairs of electrons
Factors Affecting SN2 reaction: substrate
name 2
- Electron withdrawing: polarise the carbon atom and stabilises the negatively charged transition state. Halogen, oxygen, nitrogen, sulphur
- Stable once has left:
weak base (i.e. Cl-, Br-, I-), so conjugate bases of strong acids (i.e. HCl, HBr, HI)
______ bases are poor leaving groups
strong
(-OH/-NH2)
neutral molecules can be good leaving groups example =
H2O, alcohols, amines
+NMe3 > TsO- > I— > Br — > +OH2 > Cl— > F— > OH—
