SN Flashcards
what is a SN reaction
it is a nucleophillic substitution where the nucleophile reacts with the substrate and the substrate is undergoing the change
What is a nucelophile
it is an electron rich substance that wasnts to be positive but is negative, neutral with lone pairs, or partially negative
what is an electrophile
it is an electron poor substance that wants to be negative but is positive or has a partial positive dipole
how do the electrons flow in a mechanism
from electron rich to electron poor regions
what type of hybridizations are involved in substitution and elimination reactions?
sp3 hybridized carbons
what is an SN2 reaction
it is a bimolecular concerted mechanism that occurs in a single step
what does it mean to be a bimolecular concerted mechanism
if it is bimolecular (SN2) the rate of the reaction is dependent on the concentration of the substrate and the concentration of the nucelophile
It is concerted because it happens in a single step with no intermediate
What occurs in a SN2 reaction?
the nucleophile attacks the electrophillic carbon center from the backside (the side opposite of the leaving group)
the molecule temporarily has an overabundance of electrons, causing the leaving group to leave, taking a lone pair of electrons with it and the nucleophile forms a bond with the carbon and inverts the attached groups
when a nucelophile attacks from the back, what happens to the groups and the configuration?
the attached groups invert (like an umbrella) and there is an inversion of configuration
what are examples of intermediates
an intermediate is something that appears between the starting material and the formation of the products
typically we see the formation of a carbocation or a carbanion
what does inversion of configuration mean
if the configuration was initially R, it is now S
if the configuration was initially S, it is now R
what happens to the rate of the reaction in an SN2 reaction if we increase the concentration of both of the substrate and nucleophile by 2x
the rate increases by a factor of 4
in an SN2 reaction, what structure of the substrate is the best and what is the worst?
a methyl substrate (CH3Br) is the best/ fastest >
1° is the second best > 2° works for both Sn1 and Sn2 > 3° is the worst/ slowest
if you have a 3° substrate, will an SN2 reaction proceed? Why or why not?
3° are unreactive for SN2 reactions. so it will not occur. This is because there is a steric effect that hinders the nucleophile from attacking the carbon quickly
in a methyl halide, the only thing blocking the nucleophile is hydrogen, which is too small to have any significant difference on the nucleophile being able to attack so it occurs very quickly
how does the nucleophile affect the rate of the SN2 reaction
some nucleophiles are better than others.
a negatively charged nucleophile will react more quickly than a neutral nucleophile.
So negatively charged nucleophiles are better
Which nucleophile is better: H20 or (-)(OH)
hydroxide, it has a negative charge that allows it to react faster which increases the rate of the reaction
which nucleophile is better: NH3 or (-)NH2
NH2 is a better nucleophile because it is negative
What type of solvent is best for SN2
polar aprotic solvents are the best
what type of solvent is the worst for SN2
polar protic solvents are the worst
what is a polar protic solvent
they have a hydrogen directly bonded to an electronegative atom
a polar protic solvent has at least one hydrogen atom capable of participating in a hydrogen bond
why are solvent effects important for SN2
they can minimize the solvent’s interaction with the nucleophile in an SN2
what is a polar aprotic solvent?
they do not have hydrogen directly bonded to an electronegative atom
Why is a aprotic solution for a nucleophile like NaOH important?
the Na+ acts as a positive counterion that affects the reactivity of the hydroxide nucleophile
but when in an aprotic solution, the solvent solvates the counterion and surrounds it, leaving the hydroxide as a “naked anion”
what does it mean to be a “naked anion” in a aprotic solution
it means that the counterion has been solvated and is no longer interacting with the anion nucleophile, leaving it naked and free to react with the alkyl substrate
Why are polar protic solvents bad for SN2 reactions?
they slow down the reaction since the partially positive protons in the protic solvent surround the negatively charged nucleophile and slow it from reacting with the substrate
what does it mean for a proton to “cage” a nucleophile in a protic solvent?
it means that the protons surround the negatively charged nucleophile and prevent it from reacting
what are some examples of polar protic solvents
MeOH, EtOH, H20
What are some polar aprotic solvents
DMSO (dimethyl sulfoxide) , DMF (dimethyl formamide), Acetone
what makes a good leaving group
the substituents must be able to be a stable, weakly basic molecule or ion
why are halogens good leaving groups?
they are the weak conjugate base to a strong acid
what is the best leaving group out of the halogens and why?
Iodide is the best because it comes from the strongest acid HI
Fluoride is the worst because HF is the weakest acid out of the halogens (atomic size)
what is the best leaving group
OTS (tosylate), this is due to the negative charge on the oxygen (-ate) and the resonance effects
OMS (mesylate) this is due to the negative charge on the oxygen and the resonance effects. But there are less resonance effects than OTS so its slightly worse
What is SN1 reaction
unimolecular nucleophillic substitution
what is the rate law of an SN1
K[substrate]
in an Sn1 reaction, does the rate of the reaction depend on the nucleophile at all?
no, because the rate-determining step occurs with the loss of a leaving group
What are the steps of the SN1 mechanism
the leaving group leaves and causes the formation of a carbocation intermediate
then the nucleophile attacks
if water is the nucleophile, there will be a third step that involves the deprotonation of the alcohol, creating hydronium as a byproduct
what is the slow step in a SN1 reaction?
the loss of a leaving group and formation of a carbocation, it is an endergonic process
what structure of a substrate is the fastest with an SN1 reaction
3° > 2°>1°> Methyl
why are tertiary structures the fastest with an SN1 reaction?
the presence of the three methyl groups stabilizes the positive charge on the carbocation since they act as electron donating groups
why is a methyl structure the slowest with an Sn1 reaction?
there are no electron rich groups to stabilize the carbocation
What solvent is the SN1 fastest in?
polar protic solvents are better because they stabilize the carbocation, meaning it has a lower energy and a higher rate of reaction
what is true about the first transition state of a SN1 Reaction?
the formation of the carbocation begins to form as the leaving group begins to leave,
this creates partial positive and negative charges that are stabilized by the polar protic solvent
what happens to the stereochemistry in an SN1 reaction?
there is a racemic mixture of each enantiomer
what happens to the hybridization of a carbon when its leaving group leaves
it becomes sp2 and is trigonal planar with an empty p-orbital
why do we get a racemic mixture in an SN1 reaction
the nucleophile can attack from 2 trajectories
what happens when the nucleophile attacks from the top
the resulting substitution will be on a wedge
what happens when the nucleophile attacks from the bottom
the resulting substitution will be on a dash
What is an exergonic reaction?
When ∆G is (-) and energy is released, this is a favorable reaction and usually occurs in a substitution reaction after the transition states
what is an endergonic reaction
when ∆G is (+) and energy is required to start the reaction,
in a free energy diagram, the activation energy required to remove the leaving group is endergonic and uphill as it requires energy
what is a transition state and where is it on a free energy diagram
In an Sn1 reaction, the transition state is the period when the leaving group is starting to leave and the formation of the carbocation is ongoing. This is typically represented by the first peak in a reaction coordinate diagram.
This is an endergonic process and is the rate determining step as it takes the longest
when looking at the free energy diagrams of a tertiary alkyl halide and a methyl halide, what are the primary differences and what does it say about an SN1 reactio
the tertiary alkyl halide will have a lower activation energy than a methyl alkyl halide. This means that it is more favorable for a reaction to overcome the activation energy of a tertiary alkyl halide rather than a methyl halide (it will occur faster)
Why is the activation energy lower for a tertiary alkyl halide in comparison to a methyl halide in a SN1 reaction?
because the formation of a tertiary carbocation is more stable than a methyl carbocation, this stability is caused by the inductive effect of the methyl groups attached in a tertiary carbocation
what is true about carbocation stability and SN1 reactions?
carbocation stability defines how fast an SN1 reaction will occur, a tertiary carbocation forms more favorably and faster than a methyl
What is the resonance effect on carbocations?
a carbocation stabilized by resonance is more stable and favorable
what are examples of primary alkyl halides that will proceed in an Sn1 reaction and why?
allylic carbocations stabilize the postive charge by the presence of a carbon next to a double bond (not vinyl)
benzyllic carbocations are even more stable than allylic carbocations due to the resonance delocalization of a charge across 5 resonance structures
