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rena conquers organic chemistry > 2- SN2 > Flashcards

2- SN2 Flashcards

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1
Q

nucleophile

A
  • Chemical species rich in electrons (typically basic) and will share them with El
  • Negatively charged or neutral
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2
Q

electrophiles

A
  • Chemical species poor in electrons and will accept them from Nu
  • Positively charged or neutral
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3
Q

displacement reactions

A
  • One component is replaced by another.
  • Displacement by a nucleophile – SN reactions
  • Identify the Nu and the El
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4
Q

steps for SN2 reaction

A
  • Nucleophile reacts at the electrophilic carbon (δ+)
  • Bond breaks with the electrons moving to the more electronegative atom (the Leaving Group)
  • Nucleophile replaces the LG
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5
Q

what is the leaving group

A

the most electronegative atom on the electrophile

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6
Q

what is the carbon alpha

A

the Carbon that is sigma positive, the site where the nucleophile is going to attack

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7
Q

sn2 reactions are concerted, meaning

A

the reaction occurs in one step

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8
Q

nucleophilic attacks occur where

A

the opposite side of the leaving group

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9
Q

how many transition states are there in SN2 reactions

A

1

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10
Q

where does the Nu go? HOMO or LUMO?

A

HOMO

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11
Q

where does the El go? HOMO or LUMO?

A

LUMO

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12
Q

explain the parts of the energy diagram (reactants, ts, and products)

A
  • reactants: starting products of the reaction → higher in energy than the products, lower in energy than the transition state
  • transition state: simultaneous bond breaking and bond forming (this is the RDS)
  • products: final products of the reaction → lower in energy than the reactants, lower in energy than the transition state (this is because the reaction favours the more stable side)
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13
Q

sn2 reactions are what kind of molecular reactions

A

bimolecular (because they depend on both concentrations)

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14
Q

kinetic equation for SN2? and what order?

A

rate = k[Nu][R-x], second-order reaction

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15
Q

what happens to stereochemistry of a chiral centre during SN2 reactions?

A

the configuration inverted

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16
Q

what is double inversion? what does it do to the configuration?

A

by having 2 SN2 reactions, there is a retention of configuration

17
Q

what is nucleophilicity?

A

The ability to share electrons (for SN2 to proceed it needs to have a good nucleophile)

18
Q

what are the five effects on nucleophilicity?

A
  • Effect of negative charge
  • Effect of electronegativity
  • Effect of atom size
  • Effect of electron delocalization
  • Steric congestion near the nucleophilic atom
19
Q

what is the effect of negative charge on the nucleophile? what reaction will go faster?

A
  • More available electrons, better electron donor.

- The nucleophile that has a higher negative charge will proceed faster

20
Q

what is the effect of electronegativity on the nucleophile? what reaction will go faster?

A
  • More electronegative atom is a weaker electron donor

- The less electronegative nucleophile will proceed faster

21
Q

what is the effect of atom size on the nucleophile? what reaction will go faster?

A
  • Valence electrons in bigger atom are less tightly held. They can be attracted by nearby positive charges to form bonds.
  • Bigger atom (more polarizable) are better electron donor (better nucleophile)
22
Q

what is the effect of electron delocalization on the nucleophile? what reaction will go faster?

A

Delocalization of the lone pair of the oxygen over several atoms, reduces their ability to be donated.

23
Q

what is the effect of steric congestion near nucleophilic atom on the nucleophile? what reaction will go faster?

A

Sterically hindered nucleophile will slow down the SN2 reaction rate.

24
Q

approximate order of leaving groups?

A

Cl < Br < I, OTs < H2O < OMs < OTf

25
Q

two methods of activating bad leaving groups

A
  • It can be activated with an acid (acid catalysis) to form a better leaving group H2O.
  • It can be transformed into a sulfonate ester as a better leaving group.
26
Q

explain the activation of hydroxyl group (HO) with acid catalysis

A
  • Protonation of OH forms H2O which is a better Leaving Group.
  • H2O is a weak base, therefore a very good LG
27
Q

explain the activation of hydroxyl group HO to form sulfonate esters

A

Transforming OH via a nucleophilic addition to OMs, OTf or Ots (depending on R) which are great LG

28
Q

what are protic and aprotic solvents?

A
  • The Solvation of a Nucleophile is an important factor, SN2 depends on the nature of the solvent
    1. Protic solvent : a solvent that is a hydrogen bond donor (e.g. R-OH; R-SH)
    1. Aprotic solvent: a solvent that cannot serve as a hydrogen bond donor; no H-bond to O, S or N (e.g. DMSO, DMF, acetone)
29
Q

what makes SN2 reactions go faster and why?

A
  • SN2 reactions are much faster in polar aprotic solvents:
  • Polar aprotic solvents do not have a hydrogen bond donating group.
  • No hydrogen bonding, no solvation of the nucleophile (the anion)
30
Q

Solvants’ effect on the relative nucleophilicity of halides in polar protic solvents specifically?

A
  • the bigger the atom, there are less molecules around the atom, less chance for hydrogen bonding to occur = higher nucleophilicity
  • the smaller the atom, there are more molecules around the atom, higher chance for H bonding= higher solvation
31
Q

Solvants’ effect on the relative nucleophilicity of halides in polar aprotic solvent

A
  • No hydrogen bonding, no solvation of the nucleophile (the anion)
  • Atom size: bigger atom stabilizes better the negative charge → weaker nucleophile
  • F- is the smallest atom, it cannot stabilize negative charge as well = better nucleophile
32
Q

exception to the relative nucleophilicity of halides

A
  • Finkelstein reaction
  • subsituting Cl with an I in the presence of an aprotic solvent
  • use of acetone makes reaction possible
  • R-Cl + NaI –> R-I + NaCl
  • NaCl forms precipitate
  • NaI dissolves in acetone

rena conquers organic chemistry (6 decks)

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