Unit 2 Flashcards
1
Q
What are the different options for attach in SN2
A
- Frontside attack - attacks same side as leaving group.
- gives retention product Nu takes LG spot, permanent groups don’t move.
2
Q
Backside attack
A
- attacks from side away from leaving group.
- gives inversion product - Nu shoves groups out of the way, and group shift into open space.
3
Q
Characteristics of SN2 reactions
A
- 2 components in the RDS (Biomolecular)
- converted reaction - making and breaking bond at the same time time, one step
- will always be backside attack, inversion occurs
- stereospecific reaction
- has TS
4
Q
HOMO
LUMO
- for SN2
A
- holds the electron of the Nu that attacks.
- broken bond orbital - empty orbital - associated with the electrophile
5
Q
Leaving group trend atom effect
A
- stable CBs make great leaving groups
- bigger atoms = better stabilized electron, so leaving group ability increases ability increases down a group, and increases to the right along a row.
- higher electronegativity = better stability of electrons.
6
Q
Relationship between electron delocalisation (eg resosnance and conjugation) and leaving group ability
A
- better e- delocalisation = better leaving groups, because resonance/conjugation and inductive effect stabilize LP.
7
Q
Why are neutral leaving groups very good leaving groups?
A
- if activated, its reactants have a lower pKa (force sm to be more reactive), so that reactant will be more acidic, and thus have a more stable CB = better leaving groups
8
Q
Leaving group trends - electron delocalisation
A
- more resonance/conjugation and inductive effect = better leaving group.
9
Q
Alpha carbon
A
- carbon attached to the func group of interest - carbon with leaving group
10
Q
Beta carbon
A
- carbon attached to the alpha carbon (can have multiple)
11
Q
Why do alpha and beta carbons become less reactive as they get more substituted?
A
- more hindered - more crowded backside
- so that hindered electrophile will block the nu from attacking.
12
Q
- Leaving group in chair conformation rule
A
LG can only attack if leaving group is axial
13
Q
General nucleophility trend - charge
A
- more negative charge = more e- available for bonding, more nucleophilic
14
Q
General nucleophility trend - electronegatvity
A
- less electronegativity - more nucleophility
- because its more willing to share its electrons
- imp when considering same row atoms
15
Q
General nucleophility trend - substitution
A
- less bulky a carbon, more nucleophilic
16
Q
General nucleophility trend = neutral species
A
- more polarisable = more nucleophilic
- bigger atoms = more polarisable
- so nucleoephility increases down a column
17
Q
General nucleophility trend - charged species
A
- depends on solvent for reaction
- polar aprotic solvent: weak electrostatic attraction with Nu, so smaller atoms more basic + stronger Nu.
- polar protic solvent: strong electrostatic attraction with Nu so larger atoms are stronger Nu.
18
Q
Polar Aprotic solvents
A
- only allows for permanent dipole-dipole interactions
- has weak electrostatic interaction (doesn’t bother Nu)
- so smaller atoms more basic, more powerful Nu.
19
Q
Polar Protic solvents
A
- capable for H bonding
- has strong electrostatic interaction with ions (forms cages)
- larger atoms have better nu character
20
Q
When would you wanna use a polar protic solvents in an sn2 reactions
A
- almost never
21
Q
What makes a good nucleophilic, strong base
A
-small R group
- charge is trapped
- eg OH-, anything with a N
- C triple bond to an R group
(Overall smaller)
22
Q
What makes a bad nucleophilic but strong base
A
- Bulky - C-, N- or O-, anything thats large and is stabilized by potential hyper conjugation.
23
Q
Good Nu, weak base
A
- anything with carboxyllic acids, N=N=N, PR2-, SR-, X-
24
Q
Poor nucleophilic, weak base
A
- any neutral oxygen species
25
Solvolysis
- when solvent is acting as reagent
26
In Sn1, which substitution will have a faster reaction rate?
- the more substituted, the more reactive (opp of sn2)
27
Key characteristic of Sn1 reactions (3)
- only 1 reactant in RDS (rate equation) - rate only depends on electrophile-LG
- 2 step reaction - with carbocation intermediate.
- can form racemic mixture - as nucleophile can attack from both planes of the electrophile (or both R+S stereoisomers)
28
Structural properties of carbocations
- trigonal planar
- flat
- sp3 hybridized
29
Atomic orbital assignments carbocation
P orbital is empty
- 3 sp2 orbitals are are used for 3 sigma bonds
30
Reaction coordinate diagram analysis for sn1
- late TS, looks more like carbocation.
31
Reaction rate sn1
- Nucleophile has no impact
- better leaving group = more stable CB which leaves faster
- share e- density, to stabilize the carbocation
- eg through resonance/conjugation or more substitution (hyper conjugation)
32
Limitation with an SN1
- cannot do reaction with methyl and primary carbocation (electrophiles)
33
Inductive effect Sn1
- horrible for carbocations - destabilizes them
34
Solvent effects sn1 rate
- polar protic solvents are best
- have very strong electrostatic interactions with carbocation, so increase rate of reaction
- so late TS = looks like carbocation, which stabilizes carbocation, which stabilizes transition state which lowers activation barrier which increases rate.
35
SN2 summary
Rate eq: k[Nu][E-LG]
Concerted mechanism (only one step)
Bimolecular, 2nd order
- backside attack, always inversion
- stereospecific (only one stereoisomer will form)
- Good leaving group needed, good nucleophile
- less hindered, less crowded electrophile better
- max rate = polar aprotic solvents
36
Sn1 mechanism overview
- rate=[E-LG] - first order
- 2 step mechanism, LG leaves then Nu attacks
- first step is RDS
- carbocation intermediate
- may form 2 stereoisomers if a carbon is a sterocenter.
- still need good leaving group and Nu doesn’t matter
- electrophile = more stable carbocation is better, more substitution is better (tertiary is best, then secondary)
- max rate = polar protic
37
How do you determine with a secondary substation and which reaction its going through - Sn1 or Sn2?
- look at Nu
- if you have a good strong Nu = SN2
- if you have weak bad Nu = SN1
38
39
E1 mechanism
2 steps:
- LG leaves, bond breaks (RDS)
- Deprotonate and C=C bond forms
40
Hybridization change E1
- sp3 to sp2 carbocation
- a carbon now sp2, b carbon sp3
- ends with both C sp2
41
Reaction coordinate diagram E1
- 1st peak - LG leaves, RDS (endo, so late TS, looks more like carbocation)
- then carbocation intermediate
- 2nd peak - C=C bond forms, (product determining step, wether SN1 or E1 formed)
42
How to increase percent of E1 product formed (1)
- increased temp - allows more molecules to overcome activation barrier (more molecules have more energy equal to or more than activation energy)
- delta S more significant compared to radical halogenation (more exothermic)
43
Increase percentage of E1 products (2)
- Remove all potential nucleophiles
- eg use H2SO4 to convert an alcohol to a good LG
44
How do you determine major E1 products
- Zaitzev’s rule - small bases result in the more subbed alkene is gonna be my major product.
- (more internal a carbon = more major, more stable)
- because hyper conjugation (p orbitals of alkene overlap with sigma orbitals)
45
Relative stabilities of alkene
(In order of importance)
1. More subbed, more stable
2. Fewer steric Interactions, more stable (clashing interactions)
46
Why do carbocation rearrangements happen
- more stable carbocation could be formed
- Reduce angle strain (ring), do a ring expansion
47
Kinds of group shifts
- hydride shift (Hydride moves (again takes electrons)
- alkyl shift (C based thing with its electrons moves over)
48
E2 mechanism
- concerted
- needs a strong base!
- deprotonate, H bond to C-C bond then LG bond to LG.
- stereospecific (only stereoisomer forms)
- specific stereochemistry in product requires a specific conformation in the SM (antiperiplanar) ‘
49
Reaction coordinate diagram of E2
- early TS, looks more like SM
- exothermic RDS
- only 1 step
50
Antiperiplanar
- H and LG are anti to each other
- H-C-C-LG define a plane that cuts the molecule in half
51
