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chem 24 > SN > Flashcards

SN Flashcards

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1
Q

what is a SN reaction

A

it is a nucleophillic substitution where the nucleophile reacts with the substrate and the substrate is undergoing the change

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2
Q

What is a nucelophile

A

it is an electron rich substance that wasnts to be positive but is negative, neutral with lone pairs, or partially negative

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3
Q

what is an electrophile

A

it is an electron poor substance that wants to be negative but is positive or has a partial positive dipole

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4
Q

how do the electrons flow in a mechanism

A

from electron rich to electron poor regions

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5
Q

what type of hybridizations are involved in substitution and elimination reactions?

A

sp3 hybridized carbons

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6
Q

what is an SN2 reaction

A

it is a bimolecular concerted mechanism that occurs in a single step

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7
Q

what does it mean to be a bimolecular concerted mechanism

A

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

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8
Q

What occurs in a SN2 reaction?

A

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

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9
Q

when a nucelophile attacks from the back, what happens to the groups and the configuration?

A

the attached groups invert (like an umbrella) and there is an inversion of configuration

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10
Q

what are examples of intermediates

A

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

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11
Q

what does inversion of configuration mean

A

if the configuration was initially R, it is now S

if the configuration was initially S, it is now R

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12
Q

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

A

the rate increases by a factor of 4

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13
Q

in an SN2 reaction, what structure of the substrate is the best and what is the worst?

A

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

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14
Q

if you have a 3° substrate, will an SN2 reaction proceed? Why or why not?

A

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

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15
Q

how does the nucleophile affect the rate of the SN2 reaction

A

some nucleophiles are better than others.
a negatively charged nucleophile will react more quickly than a neutral nucleophile.

So negatively charged nucleophiles are better

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16
Q

Which nucleophile is better: H20 or (-)(OH)

A

hydroxide, it has a negative charge that allows it to react faster which increases the rate of the reaction

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17
Q

which nucleophile is better: NH3 or (-)NH2

A

NH2 is a better nucleophile because it is negative

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18
Q

What type of solvent is best for SN2

A

polar aprotic solvents are the best

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19
Q

what type of solvent is the worst for SN2

A

polar protic solvents are the worst

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20
Q

what is a polar protic solvent

A

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

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21
Q

why are solvent effects important for SN2

A

they can minimize the solvent’s interaction with the nucleophile in an SN2

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22
Q

what is a polar aprotic solvent?

A

they do not have hydrogen directly bonded to an electronegative atom

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23
Q

Why is a aprotic solution for a nucleophile like NaOH important?

A

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”

24
Q

what does it mean to be a “naked anion” in a aprotic solution

A

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

25
Q

Why are polar protic solvents bad for SN2 reactions?

A

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

26
Q

what does it mean for a proton to “cage” a nucleophile in a protic solvent?

A

it means that the protons surround the negatively charged nucleophile and prevent it from reacting

27
Q

what are some examples of polar protic solvents

A

MeOH, EtOH, H20

28
Q

What are some polar aprotic solvents

A

DMSO (dimethyl sulfoxide) , DMF (dimethyl formamide), Acetone

29
Q

what makes a good leaving group

A

the substituents must be able to be a stable, weakly basic molecule or ion

30
Q

why are halogens good leaving groups?

A

they are the weak conjugate base to a strong acid

31
Q

what is the best leaving group out of the halogens and why?

A

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)

32
Q

what is the best leaving group

A

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

33
Q

What is SN1 reaction

A

unimolecular nucleophillic substitution

34
Q

what is the rate law of an SN1

A

K[substrate]

35
Q

in an Sn1 reaction, does the rate of the reaction depend on the nucleophile at all?

A

no, because the rate-determining step occurs with the loss of a leaving group

36
Q

What are the steps of the SN1 mechanism

A

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

37
Q

what is the slow step in a SN1 reaction?

A

the loss of a leaving group and formation of a carbocation, it is an endergonic process

38
Q

what structure of a substrate is the fastest with an SN1 reaction

A

3° > 2°>1°> Methyl

39
Q

why are tertiary structures the fastest with an SN1 reaction?

A

the presence of the three methyl groups stabilizes the positive charge on the carbocation since they act as electron donating groups

40
Q

why is a methyl structure the slowest with an Sn1 reaction?

A

there are no electron rich groups to stabilize the carbocation

41
Q

What solvent is the SN1 fastest in?

A

polar protic solvents are better because they stabilize the carbocation, meaning it has a lower energy and a higher rate of reaction

42
Q

what is true about the first transition state of a SN1 Reaction?

A

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

43
Q

what happens to the stereochemistry in an SN1 reaction?

A

there is a racemic mixture of each enantiomer

44
Q

what happens to the hybridization of a carbon when its leaving group leaves

A

it becomes sp2 and is trigonal planar with an empty p-orbital

45
Q

why do we get a racemic mixture in an SN1 reaction

A

the nucleophile can attack from 2 trajectories

46
Q

what happens when the nucleophile attacks from the top

A

the resulting substitution will be on a wedge

47
Q

what happens when the nucleophile attacks from the bottom

A

the resulting substitution will be on a dash

48
Q

What is an exergonic reaction?

A

When ∆G is (-) and energy is released, this is a favorable reaction and usually occurs in a substitution reaction after the transition states

49
Q

what is an endergonic reaction

A

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

50
Q

what is a transition state and where is it on a free energy diagram

A

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

51
Q

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

A

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)

52
Q

Why is the activation energy lower for a tertiary alkyl halide in comparison to a methyl halide in a SN1 reaction?

A

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

53
Q

what is true about carbocation stability and SN1 reactions?

A

carbocation stability defines how fast an SN1 reaction will occur, a tertiary carbocation forms more favorably and faster than a methyl

54
Q

What is the resonance effect on carbocations?

A

a carbocation stabilized by resonance is more stable and favorable

55
Q

what are examples of primary alkyl halides that will proceed in an Sn1 reaction and why?

A

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

chem 24 (22 decks)

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