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NST Chemistry > Sn1 and Sn2 reactions > Flashcards

Sn1 and Sn2 reactions Flashcards

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1
Q

features of Sn1 reactions

A

Unimolecular - one molecule involved in RDS
sp2 planar reactive carbocation intermediate formed

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2
Q

features of Sn2 reactions

A

bimolecular - 2 molecules involved in RDS
concerted attack/leaving, transition state sp2 C
p-orbital used to share electrons between Nu and LG

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3
Q

ROR eqn Sn1

A

rate = k1[RL]

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4
Q

ROR eqn Sn2

A

rate = k2[RL][Nu]
depends on both bc two molecules involved in RDS

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5
Q

reaction energy profile Sn1 vs Sn2 reactions

A

Sn1 has a dip of lower energy at the carbocation intermediate, two higher energy transition states w partial bonds

Sn2 highest energy point is the transition state

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6
Q

four main variables to consider for sub at saturated C

A

substrate structure (think of this as equivalent to C=O reactivity)
Nu strength
LG ability
Solvent

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7
Q

two main considerations with substrate structure

A

steric hindrance (ie. ease of Nu approach)

factors influencing carbocation (Sn1) / transition state (Sn2) STABILITY

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8
Q

identify the HOMO and LUMO in Sn1 and Sn2 reactions

A

HOMO = Nu LP
LUMO = C-X sigma*

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9
Q

general rule for how substrate affects Sn1 / Sn2 pathway

A

Sn2 reactions become less likely the more substituents there are on the C centre being attacked
due to steric hindrance
Sn1 more likely the more substituents attached because of stabilisation of carbocation rather than steric hindrance

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10
Q

two factors which affect carbocation stability in Sn1 reactions

A

hyperconjugation (sigma conj)
pi-bond and lone pair donation

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11
Q

what is hyperconugation?

A

donation of adjacent sigma-bond electrons (often from alkyl groups) into the empty p-orbital of the carbocation (recall carbocation is sp2 hybridised!)

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12
Q

common misconceptions about hyperconjugation

A
  1. that stabilisation of carbocation is due to attached EDG
  2. that stabilisation occurs via the sigma bond directly attached to the C being attacked
    BOTH INCORRECT
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13
Q

relative effect of pi-bond / lone pair donation compared to hyperconjugation

A

both are stronger effects
stabilise the carbocation more than hyperconjugation

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14
Q

how does pi-bond donation work?

A

pi-bonds can donate through conjugation - +ve carbocation charge stabilised by delocalisation (draw resonance forms)

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15
Q

how does lone pair donation work?

A

assists in the departure of the leaving group (pushes it off in one step)

think about it like LG leaves, resulting in +ve charge which is moved by LP

generally, look for it when there are two atoms possessing a lone pair of e- attached to the same C

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16
Q

what is the oxonium ion?

A

+ve on O (due to its LPs donating?)
acts like an activated carbonyl group,

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17
Q

how are the partial bonds in the Sn2 transition state formed? refer to FMO

A

two partial bonds are formed from mixing the filled and vacant HOMO and LUMO orbitals together

(hence stabilising delocalisation is possible from either filled OR vacant orbitals)

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18
Q

what types of substituents stabilise the Sn2 transition state?

A

both EDG and EWG!

in particular, adjacent double bonds (pi and pi) and adjacent C=O (pi)

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19
Q

why are C=O groups better at stabilising the Sn2 transition state than C=C?

A

the pi* of C=O combines with nearby sigma* of C-LG to form a new lower energy molecular LUMO, thus making it a more reactive starting material

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20
Q

define regioselectivity

A

the preference of the reaction (bond breaking/forming) happening at one place over any other

influenced by factors like steric hindrance and stabilisation

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21
Q

name a substrate type that is bad for both Sn1 and Sn2

A

neopentyl
poorly stabilised carbocation, AND tertiary group sterically hinders

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22
Q

stereochemical outcome of Sn1 reactions

A

racemic mixture
planar carbocation - incoming Nu may attack from either top or bottom face, resulting in 1:1 enantiomers mix

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23
Q

stereochemical outcome of Sn2 reactions

A

single enantiomer produced due to back-face attack, resulting in an inversion of stereochemistry

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24
Q

are Sn1 or Sn2 reactions favoured at sp2 C centres?

A

NEITHER.
Sn1 or Sn2 Nu sub CANNOT occur at sp2 C’s (this year anyway)

25
Q

explain why Sn1 cannot happen at a sp2 C

A

LG leaves, resulting empty sp2 HAO is perpendicular to the p-orbitals in which the double bond electrons are
cannot be used to stabilise +ve charge

26
Q

three reasons why Sn2 cannot happen at a sp2 C aromatic rings

A
  1. Nu- approach repelled by e-density of double bonds
  2. back-face attack into sigma* blocked by ring
  3. ring makes inversion impossible
27
Q

importance of the nucleophile in Sn1 reactions

A

Nu NOT important in Sn1 - bc the RDS is the loss of leaving group, Nu attacks a highly reactive carbocation

28
Q

importance of the nucleophile in Sn2 reactions

A

Nu VERY important in Sn2, bc involved in the RDS

29
Q

predicting rate of reaction of a Nu at C=O vs substitution at saturated C (Sn2)

A

for C=O, easy to predict - Nucleophilicity parallels basicity hence pKa (higher pKa better Nu)

but NOT the case for Sn2

30
Q

approach to consider nucleophilicity for Sn2 reactions

A

involved in RDS hence important - but more complicated than for attack at C=O.

to compare: split into 1. atom forming the bond is the same and 2. atom forming bond is NOT the same

31
Q

nucleophilicity for Sn2 reactions - atom forming the bond is the SAME

A

eg. for comparing oxyanion Nu (O forms bond)

nucleophilicity parallels basicity – hence higher pKa = better nucleophile

(lower pKa = more acidic = more stable oxyanion)

32
Q

nucleophilicity for Sn2 reactions - atom forming the bond is NOT the same

A

Sn2 reactions are HOMO-LUMO dominated (vs C=O large dipole hence electrostatics driven) so SOFT nucleophiles will often react best

lower on table –> larger atom –> higher HOMO –> closer HOMO-LUMO –> better interaction

eg. PhS- (pKa 6.4) > PhO- (10)
I- > Br- > Cl- > F-

33
Q

two factors to consider for halide leaving groups

A

C-X bond strength
halide ion stability (pKa values)

34
Q

trend in C-X bond strength going down G17

A

decreasing bond strength
F>Cl>Br>I

35
Q

trend in acidity / pKa / anion stability going down G17

A

increasing acidity and decreasing pKa DOWN the group F–>I
hence greater anion stability and increasing leaving group ability

36
Q

leaving group ability going down G17

A

increasing leaving group ability - I- is the best LG, F- is not particularly good

37
Q

why is OH- not usually a leaving group in Nu sub @ saturated C for alcohols? (3 reasons)

A
  1. pKa of H2O is 15.7 - not a good leaving group
  2. Nu will often be basic enough to just remove the proton instead, forming NuH
  3. even if removed, hydroxide is basic enough to remove the proton from the alcohol anyway, preventing further reaction
38
Q

two ways OH can be made into a better leaving group

A
  1. protonation
  2. sulfonate ester formation

both of which lower the pK of the leaving group

39
Q

how does protonation of the -OH group impact its leaving ability?

A

protonate to OH2+, which is now a good leaving group pKa= -1.7 (for H3O+)
so the RDS in the mechanism becomes the loss of water, not the loss of hydroxide

reaction takes place in the presence of STRONG / CONC ACID

40
Q

what reagents are used to convert an OH group to a sultanate ester?

A

using para-Toluenesulfonates (tosylates, OTs)
and pyridine

write TsCl, Py above arrow
OH becomes OTs group

41
Q

why are tosylates better leaving groups than hydroxide?

A

lower pKa.
15.7 for H2O vs -1.3 for TsOH

42
Q

two steps in mechanism for making a sultanate ester from alcohol

A
  1. Nucleophilic attack of the alcohol’s O lone pair onto the sulfonyl chloride (attacks at S, -Cl leaves)
  2. proton removal by pyridine (lone pair on N removes H) giving pyridinium hydrochloride salt (Py-HCl) as byproduct
43
Q

why do the two steps in the mechanism for the conversion of an alcohol to sultanate ester not occur in reverse order? ie. protonation of alc by pyridine first, then attack of oxyanion?

A

because pyridine is only weakly basic and not strong enough to take the proton off the alcohol

pyridine pKa = 5, actually much more acidic than alcohol pKa=16

44
Q

in what cases is the tosylate the best option to use?

A

in any reaction where acidic conditions cannot be used (ie OH cannot be protonated)

for example, C-C bond formation using an organometallic reagent as nucleophile

45
Q

what is an epoxide?

A

a cyclic ether with a three-membered ring
very reactive due to substantial ring strain

46
Q

why are ethers generally unreactive in Nu substitution?

A

very little bond angle strain
strong C-O bond
pKa of alkoxide anion similar to that of hydroxide ion (pKa alcohols usually 15-18) hence not a good leaving group

(more reactive if protonated, as for alcohols)

47
Q

what makes epoxides good electrophiles in Sn2 reactions?

A

severe bond angle strain:
three membered rings - all sp3 hybridised so desired bond angle is 109.5˚ so the angle is dramatically constrained to 60˚

poorer overlap = weaker C-O bond

48
Q

how are amino alcohols formed from reaction of epoxides?

A

epoxide will react with uncharged amine nucleophiles to give amino aochols
(N lone pair attacks into a C = Sn2 and release of ring strain, then ±H+)

pg. 129

49
Q

relative stereochemistry of produce formed when epoxide ring is fused to another ring

A

the group added ends up on the OPPOSITE side of the OH group

because Sn2 attacks from one face of the ring, pushing the O- to the other face

50
Q

optimum solvent for Sn1 reactions

A

polar protic solvents

51
Q

optimum solvent for Sn2 reactions

A

polar aprotic solvents

52
Q

why are polar protic solvents best for Sn1 reactions?

A

ions formed (carbocation intermediate and its counterion) during Sn1 reaction process
polar protic solvents will solvate both the intermediate ions, ie. stop it from regenerating/reversing

∂- part stabilises carbonation, ∂+ part stabilises LG and prevents reforming SM

53
Q

examples of polar protic solvents

A

H2O
ROH
CH3COOH

54
Q

why are polar aprotic solvents best for Sn2 reactions?

A

the Nu is often an anion, which is added to the reaction with its cation counterion
polar aprotic solvents solvate CATIONS well but not anions, hence making the anions more reactive nucleophiles by “taking away” their cations

effective because no ions are formed in Sn2 (vs carbocation int in Sn1)

55
Q

examples of polar aprotic solvents

A

Acetone (propanone)
dimethylsulfoxide DMSO (acetone w S=O instead of C=O)
N,N-dimethylformamide DMF

no protons to pair w anions, hence only solvates cations with the ∂- of C=O or S=O

56
Q

what do dielectric constants show?

A

a measurement of polarity
higher dielectric constant = more polar

57
Q

order common polar protic solvents by polarity

A

most to least:
H2O, methanol, ethanol, acetic acid

58
Q

spotting retrosynthesis of epoxides

A

a structure w a Nu attached to one C and an OH attached to he next

may have arisen from Nu attack into epoxide

NST Chemistry (16 decks)

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