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CHEM 2OA3 Study Guide - Sn2 Reaction, Amide, Elimination Reaction

8 Pages
220 Views
Winter 2013

Department
Chemistry
Course Code
CHEM 2OA3
Professor
Jeff Landry

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Substitution/Elimination Reaction Properties & Conditions
Mechanism
Substrate
preference
Nu: Strength
Nu: Conc.
Solvent
Kinetics.
Sn2
1>2>3
Prefers strong
Nu:
High [Nu:]
favours Sn2
Aprotic favors
Sn2 if either of
the reactants is
charged
2nd order
r=k[Nu:][Sub]
E2
3>2>1*
Prefers strong
base
High [Nu:]
favours E2
Aprotic favors E2
if either of the
reactants is
charged
2nd order
r=k[Nu:][Sub]
Sn1
3>2>1
Not affected by
Nu: strength, but
weak Nu:
disfavors Sn2
Not affected
but low [Nu:]
disfavors a SN2
reaction
Protic favors Sn1
if the reactant is
not charged
1st order
r=k[Sub]
E1
3>2>1
Not affected by
Nu: strength, but
weak Nu:
disfavors E2
Not affected
but low [Nu:]
disfavors a E2
reaction
Protic favors E1
if the reactant is
not charged
1st order
r=k[Sub]
*For a tertiary substrate, the transition state exhibits a partial double bond that is more highly
substituted therefore the transition state will be lower in energy.
Determining Likelihood of Elimination and Substitution Reactions
E1 & Sn1
For an E1/Sn1 mechanism the stability of the substrate and of the leaving group plays the major role
in determining whether or not a reaction will occur. The Nu: does play a role to a lesser extent. The
stability of the substrate can be determined with a concept similar to ARIO, as the leaving group plays
a big role in determining whether or not E1/Sn1 reactions occur. Using ARIO, we find that the Atom
will always be a carbon, but possibility of Resonance structures, Inductive effects and Orbital
hybridization all play a big role in determining strength. The 3>2>1 rule essentially covers inductive
effects, that is extra carbon groups can donate electron density, increasing the stability of the
carbocation intermediate. The presence of a resonance structure will helps increase the stability of
the intermediate. As the number of reasonable resonance structures that are possible increases, so
does the relative stability of the corresponding carbocations. Finally the role of hybridization is
important, as sp3 carbon will more readily carry a positive charge than a sp2 carbon or sp carbon,
which makes sense considering their electronegativities. Notice how certain trends of this ARIO are
opposite when you consider ARIO with regards to hydrogen deprotonation. This is due to fact that
one forms a carbocation and the other an anion.
For determining stability of leaving groups, an easy method is to look at the pka of conjugate acid of
the leaving group. The lower the pka or stronger the acid is, the better the leaving group. Ex. HI is a
better acid than HF, likewise I- is a better leaving group than F-
. The only shortcoming of this method is
that it does not take into account kinetics, so pka trends don’t perfectly match the LG trends.
Sometimes poor LGs can be converted into better LGs (see other reactions section).
E2 & Sn2 (wip)
Size/kinetics, polarizability and electronegativity play a role in Nu: strength. The LG is important to a
lesser degree in E2/Sn2 than E1/Sn1. You’ll notice a that size and polarizability are in contest This is
beyond me.
Prediction of Elimination and Substitution Products and their
Stereochemistry
Sn2: Always undergoes 100% inversion at the chiral centre only. This may induce other chiral centres
to flip configuration, but their physical connectivity is not changed.
Sn1: Forms racemic mixtures which sometimes favor the inverted structure due to ion pairing.
Additionally, Sn1 can undergo hydride shifts, methyl shifts and resonance shifts due to formation of a
carbocation intermediate, moving the location of the + charge. This results in a mixture of products,
depending on how probable the shift is.
E2: Follows 3 rules Antiperiplanar/Anti-coplanar rule, Zaitsev's rule and E/Z Isomerism rule. The first
rule trumps the second two.
Antiperiplanar/Anti-coplanar Rule - The hydrogen to be deprotonated must be antiperiplanar (180
degrees apart) from the leaving group. In cyclic compounds only axial groups can be removed.
Zaitsev's Rule - If multiple products are possible, the the more substituted alkene is favored unless
*the base is large *the alkyl halide is an alkyl fluoride *the alkyl halide contains one or more double
bonds. In both cases the minor products occur.
E/Z Isomerism rule - If multiple products are possible, the product with Z or trans isomerism is
favoured, though minor cis products occur.
E1: This mechanism forms racemic mixtures which tend to favour Zaitsev products and trans products.
Alkene Addition Reactions
Reaction mechanisms are given under the table. Reaction mechanisms and intermediates are not
given for mechanisms you aren’t required to know, and are marked N/A. The reaction conditions
given are to favour products. Know the relationship between reversible reactions, and how reaction
conditions change to favor each side.
Reaction name
Reaction
Markov. ’
Syn/Anti &
Rxn
Conditions
s Rule
Intermediat
e
Mechanis
m and
Extra
Notes
Catalytic Hydrogenation
H2/Pt
N/A
Syn;
unnamed
intermediat
e
Mechanism
#1
Also see
Wilkinson’s
catalyst note
Hydro-
halogenation
HX
Followed
unless
R-OO-R is
present
Both;
planar
carbocation
intermediat
e
Mechanism
#2
Hydration
Dil. H2SO4/
Low Temp.
Followed
Both;
planar
carbocation
intermediat
e
Mechanism
#3
Oxymercuration-Demercurat
ion
1.Hg(OAc)2/
H2O
2. NaBH4/
H2O/Et2O
Followed
Nu: goes
on more
sub’d
carbon
Anti; cyclic
mercuriniu
m ion
intermediat
e
Mechanism
#4
See:
Demercurati
on note for
full details of
this rxn.
Hydroboration
Oxidation
1. BH3/THF
2. H2O2/NaO
H
Broken
Syn; N/A
N/A
Halogenation
Cl2 or Br2 in
CCL4
N/A
Anti; cyclic
halonium
intermediat
e
Mechanism
#5
Halohydration
Cl2 or Br2 in
H2O
Followed
OH goes
to more
sub’d C
Anti, cyclic
halonium
intermediat
e
Mechanism
#6
Epoxidation
Peroxyacids
(Ex. RCO3H)
N/A
N/A;
Unnamed
intermediat
e
Mechanism
#7
Oxidative Cleavage
(Ozonolysis)
1. O3
2. DMS
N/A
N/A; N/A
N/A

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Description
Substitution/Elimination Reaction Properties & Conditions Mechanism Substrate Nu: Strength Nu: Conc. Solvent Kinetics. preference Sn2 1>2>3 Prefers strong High [Nu:] Aprotic favors 2nd order Nu: favours Sn2 Sn2 if either of r=k[Nu:][Sub] the reactants is charged E2 3>2>1* Prefers strong High [Nu:] Aprotic favors E2 2nd order base favours E2 if either of the r=k[Nu:][Sub] reactants is charged Sn1 3>2>1 Not affected by Not affected Protic favors Sn1 1st order Nu: strength, but but low [Nu:] if the reactant is r=k[Sub] weak Nu: disfavors a SN2 not charged disfavors Sn2 reaction E1 3>2>1 Not affected by Not affected Protic favors E1 1st order Nu: strength, but but low [Nu:] if the reactant is r=k[Sub] weak Nu: disfavors a E2 not charged disfavors E2 reaction *For a tertiary substrate, the transition state exhibits a partial double bond that is more highly substituted therefore the transition state will be lower in energy. Determining Likelihood of Elimination and Substitution Reactions E1 & Sn1 For an E1/Sn1 mechanism the stability of the substrate and of the leaving group plays the major role in determining whether or not a reaction will occur. The Nu: does play a role to a lesser extent. The stability of the substrate can be determined with a concept similar to ARIO, as the leaving group plays a big role in determining whether or not E1/Sn1 reactions occur. Using ARIO, we find that the Atom will always be a carbon, but possibility of Resonance structures, Inductive effects and Orbital hybridization all play a big role in determining strength. The 3>2>1 rule essentially covers inductive effects, that is extra carbon groups can donate electron density, increasing the stability of the carbocation intermediate. The presence of a resonance structure will helps increase the stability of the intermediate. As the number of reasonable resonance structures that are possible increases, so does the relative stability of the corresponding carbocations. Finally the role of hybridization is important, as sp3 carbon will more readily carry a positive charge than a sp2 carbon or sp carbon, which makes sense considering their electronegativities. Notice how certain trends of this ARIO are opposite when you consider ARIO with regards to hydrogen deprotonation. This is due to fact that one forms a carbocation and the other an anion. For determining stability of leaving groups, an easy method is to look at the pka of conjugate acid of the leaving group. The lower the pka or stronger the acid is, the better the leaving group. Ex. HI is a better acid than HF, likewise I is a better leaving group than F.The only shortcoming of this method is that it does not take into account kinetics, so pka trends don’t perfectly match the LG trends. Sometimes poor LGs can be converted into better LGs (see other reactions section). E2 & Sn2 (wip) Size/kinetics, polarizability and electronegativity play a role in Nu: strength. The LG is important to a lesser degree in E2/Sn2 than E1/Sn1. You’ll notice a that size and polarizability are in contest This is beyond me. Prediction of Elimination and Substitution Products and their Stereochemistry Sn2: Always undergoes 100% inversion at the chiral centre only. This may induce other chiral centres to flip configuration, but their physical connectivity is not changed. Sn1: Forms racemic mixtures which sometimes favor the inverted structure due to ion pairing. Additionally, Sn1 can undergo hydride shifts, methyl shifts and resonance shifts due to formation of a carbocation intermediate, moving the location of the + charge. This results in a mixture of products, depending on how probable the shift is. E2: Follows 3 rules Antiperiplanar/Anti-coplanar rule, Zaitsev's rule and E/Z Isomerism rule. The first rule trumps the second two. Antiperiplanar/Anti-coplanar Rule - The hydrogen to be deprotonated must be antiperiplanar (180 degrees apart) from the leaving group. In cyclic compounds only axial groups can be removed. Zaitsev's Rule - If multiple products are possible, the the more substituted alkene is favored unless *the base is large *the alkyl halide is an alkyl fluoride *the alkyl halide contains one or more double bonds. In both cases the minor products occur. E/Z Isomerism rule - If multiple products are possible, the product with Z or trans isomerism is favoured, though minor cis products occur. E1: This mechanism forms racemic mixtures which tend to favour Zaitsev products and trans products. Alkene Addition Reactions Reaction mechanisms are given under the table. Reaction mechanisms and intermediates are not given for mechanisms you aren’t required to know, and are marked N/A. The reaction conditions given are to favour products. Know the relationship between reversible reactions, and how reaction conditions change to favor each side. Reaction name Reaction Product Markov. ’ Syn/Anti & Rxn Conditions s Rule Intermediat Mechanis e m and Extra Notes Catalytic Hydrogenation H 2Pt Alkane N/A Syn; Mechanism unnamed #1 intermediat Also see Wilkinson’s e catalyst note Hydro- HX Saturated Followed Both; Mechanism halogenation alkyl halide unless planar #2 R-OO-R is carbocation present intermediat e Hydration Dil. H2SO4/ Alcohol Followed Both; Mechanism *Rearrangeme Low Temp. planar #3 nt can occur carbocation intermediat e Oxymercuration-Demercurat 1.Hg(OAc) 2 Alcohol Followed Anti; cyclic Mechanism ion H O Nu: goes mercuriniu #4 2 on more 2. NaBH 4 m ion See: H O/Et O sub’d intermediat Demercurati 2 2 carbon on note for e full details of this rxn. Hydroboration 1. BH3/THF Alcohol Broken Syn; N/A N/A Oxidation 2. H 2 /2aO H Halogenation Cl or Br in Vicinal N/A Anti; cyclic Mechanism 2 2 CCL 4 dihalide halonium #5 intermediat e Halohydration Cl2or Br2in Vicinal Followed Anti, cyclic Mechanism H 2 halohydrin OH goes halonium #6 to more intermediat sub’d C e Epoxidation Peroxyacids Epoxide N/A N/A; Mechanism (Ex. RCO 3) Unnamed #7 intermediat e Oxidative Cleavage 1. O 3 Aldehydes N/A N/A; N/A N/A (Ozonolysis) 2. DMS and/or ketones *depends on how substituted
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