SN2 SN1 Flashcards
Steric hindrance
it is the congestion caused by the physical presence of ligands. The larger the ligand, the greater the hindrance. which will slow down the atom reaching the x.
SN1 rate law
Rate = k[electrophile]
SN2 rate law
Rate = k[nucleophile]×[electrophile]
SN1 stereochemistry
racemization on reaction center
SN2 stereochemistry
inversion on reaction center
Electrophilic Substrate on sn1
tertiary 3° > secondary 2° > primary 1° and methyl
Electrophilic Substrate on sn2
primary 1° and methyl > secondary 2° > tertiary 3°
Nucleophile SN1
weak nucleophile, solvolysis
Nucleophile SN2
strong nucleophile
for SN2 what direction does the Nucleophile react?
180
in SN2 is it the front or the back?
it is the back side
draw the transition state
pentacoordinated carbon
draw the transition state
pentacoordinated carbon
Why is SN2 second ordered?
In the mechanism, the reaction proceeds in a single step that involves both nucleophile and the substrate, so increasing the concentration of either of them makes the possibility of collision increase, that explains the second-order kinetics of SN2 reaction. With both nucleophile attacking and leaving group leaving happen at the same time, SN2 is also said to be a concerted mechanism, concerted means simultaneous.
in sn2 you must show
the arrows
the highest point at the energy profile
the transition state
what alkyl is most likely to go under sn2
methyl
steric hindrance
Steric effect is the effect that based on the steric size or volume of a group. Because of the steric hinderance of bulky groups on the electrophilic carbon, it is less accessible for nucleophile to do back-side attack, so the SN2 reaction rate of secondary (2°) and tertiary (3°) substrates decreases dramatically. Actually the 3° substrates never go with SN2 reaction mechanism because the reaction rate too slow.
Walden inversion.
the product get inverted comparing to that of the reactant, like an umbrella flipped inside out.
SN2 leaving group
the less basic the nucleophile is the better the leaving group
leaving group chart for SN2
(best leaving group) I–> Br–> Cl–> F– (weakest leaving group)
Strong bases
Strong bases such as OH–, RO–, NH2–, R– are therefore very poor leaving groups and cannot go with nucleophilic substitution reactions. For OH– or RO– however, upon protonation they can be converted to neutral H2O or ROH molecules,
bulky vs small
Smaller group is better nucleophile than bulky group.
SN2 nucleophility
Nucleophilicity decrease across a period.
what side can SN2 take place
from the back
SP2 SN2
Bonds on sp2-hybridized carbons are inherently shorter and stronger than bonds on sp3-hybridized carbons, meaning that it is harder to break the C-X bond in these substrates. SN2 reactions of this type are unlikely also because the (hypothetical) electrophilic carbon is protected from nucleophilic attack by electron density in the p bond. SN1 reactions are highly unlikely, because the resulting carbocation intermediate, which would be sp-hybridized, would be very unstable (we’ll discuss the relative stability of carbocation intermediates in a later section of this module).
the more basic
the more nucleophilic
Electronegative elements are
less basic and less basic
resonance effects on nucleophilicity
if the electron lone pair on a heteroatom is delocalized by resonance, it is inherently less reactive - meaning less nucleophilic, and also less basic.
group 7 leaving group
As Size Increases, Basicity Decreases, Leaving Group Stability Increases:
In general, if we move from the top of the periodic table to the bottom of the periodic table as shown in the diagram below, the size of an atom will increase.
As size increases, basicity will decrease, meaning a species will be less likely to act as a base; that is, the species will be less likely to share its electrons.
resonance
Resonance Decreases Basicity and Increases Leaving Group Stability
SN2 solvent
SN2 reactions are faster in polar, aprotic solvents: those that lack hydrogen-bond donating capability. Below are several polar aprotic solvents that are commonly used in the laboratory:
Frontside Orientation:
In a frontside orientation, the nucleophile approaches the electrophilic center on the same side as the leaving group. With frontside orientation, the stereochemistry of the product remains the same; that is, we have retention of configuration.
sn1
the carbocation forms when the C-X bond breaks first, before the nucleophile approaches
the carbocation is
sp2 hydridised
trigonal planar
the carbocation has a
empty P orbital
When the Nu attack in SN1
it becomes sp3 tetrahedral
a racemic solution is formed in SN1 as
Because of this trigonal planar geometry, the nucleophile can approach the carbocation from either lobe of the empty p orbital (aka either side of the carbocation).
the more stable the carbocation
the faster the reaction in SN1 as the activation energy decreases
leaving groups
A good leaving group is a weak base because weak bases can hold the charge. They’re happy to leave with both electrons and in order for the leaving group to leave, it needs to be able to accept electrons
