Final Exam Flashcards
sulfonates
- So3 groups
- S is double bonded to two oxygens and single bonded to the third oxygen
- very good leaving groups because of resonance
- made from corresponding alcohol in a reaction
alcohols in elimination and substitution reactions
they are bad leaving groups so they get converted into good leaving groups by sulfonates or strong acids
-OH as a reagent
strong nucleophile and a strong base
pyridine
non-nucleophilic base
how is the alpha carbon respresented in sulfonate reactions?
with R
stereochemistry of OH and sulfonate reactions
stereochemistry does not change because the sulfur gets attacked
how are alcohols used as starting materials in reactions?
to make alkyl halides and alkenes
What needs to occur if an alcohol is the reactant?
strongly acidic conditions are needed to protonate OH into H2O because H2O is a good leaving group
HBr as a reagent
strong acid, not a nucleophile or base
Br as a reagent
strong nucleophile only, encourages substitution
HX mechanism for alcohols
- results in a substitution reaction
- primary alcohols react Sn2
- secondary and tertiary alcohols react Sn1
what conditions are protic conditions?
strongly acidic
elimination of alcohols
-favored by H2SO4 because the byproduct of the first step
-favored by heat
-
what do protic conditions favor?
E1 for secondary and tertiary substrates
retrosynthesis
thinking and synthesizing backwards
addition reactions
- opposite of elimination reactions
- pi bonds act as lewis bases, nucleophiles or bronsted lowry bases
- at temperature dependent equilibrium with elimination reactions
- typically add 2 atoms across a pi bond
- usually exothermic
what does 🔼G represent in elimination reactions?
competition between the enthalpy and entropy term
low temperature 🔼G reactions
- entropy is small
- enthalpy dominates
- 🔼G
- K>1
- products favored
high temperature 🔼G reactions
- entropy is large
- entropy dominates
- +🔼G
- K<1
- reactants favored
why are addition reactions usually exothermic?
a pi bond is broken and 2 sigma bonds are formed
enthalpy for exothermic addition reactions
neagtive
entropy of addition reactions
entropically unfavorable(- 🔼S) because 2 molecules become one
how can an addition reaction be favorable if the entropy is unfavorable?
enthalpy magnitude outweighs unfavorable affect of enthalpy
which types of addition reactions are most closely related?
halogenation and halohydrin formation
hydrohalogenation
a reaction where H and a halide are added across a pi bond
hydrohalogenation mechanism
- Proton transfer generates a carbocation
2. nucleophilic attack
rate determining step of hydrohalogenation reactions
proton transfer/carbocation generation step(the first step)
function of an alkene in hydrohalogenation
alkenes function as a base
how are alkenes able to function as bases if they arent basic?
in hydrohalogenation, the acid is so strong that the alkene can play the basic role
characteristics of carbocation intermediate in hydrohalogenation
- flat
- can be attack fromt he top or bottom
- strong electrophile
Markovnikov’s rule
H is added to the vinylic C atom bearing more H atoms, or the less substituted carbon
regiochemistry of hydrohalogenation reactions
pro-markovnikov rule
why is markovnikov’s rule usually executed in addition reactions?
it creates the most stable possible carbocation, which stabilizes the transition state and allowing the product to form faster
What decides the activation energy of a reaction?
the transition state energy level
What stabilizes the carbocation intermediate in addition reactions?
hyperconjugation from the more substituted atom
When does hydrohalogenation result in a racemic mixture?
When a chiral center forms
What decides the regio and stereo chemistry of addition reactions?
the formation of the carbocation
methods of alkene hydration
- Acid catalyzed hydration
- Oxymercuration-demurcation
- Hydroboration-oxidation
acid-catalyzed hydration regiochemistry
markovnikov addition
stereochemistry of acid catalyzed reactions
racemic mixture of products
are rearrangements possible for hydrohalogenation?
yes
are rearrangments possible for acid catalyzed hydration?
yes
regiochemistry of oxymerucration-demercuration
markovnikov addition
stereochemistry of oxymercuration-demercuration
anti addition
are rearrangements possible in oxymercuration-demercuration reactions?
no
regiochemistry of hydroboration-oxidation reactions
anti-markovnikov
stereochemistry of hydroboration-oxidation reactions?
syn addition
are rearrangements possible for hydroboration-oxidation reactions?
no
What do acid catalyzed reactions do to a molecule?
add H2O to a bond by adding H to one side and OH to the other
how is the acid catalyst shown in acid catalyzed reactions
it is shown over the arrow and in brackets because it is regenerated rather than being a reactant
what is also present when H2SO4 is an acid catalyst?
H3O+
how do we know an acid catalyzed hydration has carbocation intermediates?
OH is added to the more substituted carbon of the alkene
Acid catalyzed hydration mechanism
- Proton transfer: alkene is protonated and carbocation intermediate is formed
- Nucleophilic attack: water functions as a nucleophile to attack the carbocation intermediate
- Proton transfer: water functions as a base to deprotonate the oxonium ion and give the product
purpose of the last proton transfer in acid catalyzed reactions
O+ is very unstable and is never the final product
what determines the rate of addition reactions with rearrangement?
carbocation stability
thermodynamics of acid catalyzed reaction
at equilibrium between alkene and addition product
what type of addition reaction do acid catalyzed reactions have the same thermodynamics as?
hydrohalogenation reactions
How can equilibrium be pushed toward addition products and away from alkenes in acid catalyzed hydration?
- Low temp: with a small entropy, enthalpy will dominate and a low temp will make △G negative
- Use dilute H2SO4 and a lot of water: this utilizes LeChatelier’s principle because water is on the elimination side of the reaction
why do acid catalyzed reactions produce racemic mixtures?
their carbocations are trigonal planar and therefore flat
what limits synthetic utility of Markovnikov reactions?
rearrangements produce a mixture of products
what is a solution to the limited synthetic ability of markovnikov reactions?
oxymercuration-demercuration reactions, no rearrangements
Oxymercuration-demercuration mechanism
Part 1-Oxymercuration
1. dissociation of mercuric acetate forming a mercuric cation: OAc leaves Hg+-OAc
- Pi bond attacks Hg+ forming two resonance structures(one with and one without a ring) as intermediates.
Part 2-Demercuration
3. NaBH4 replaces and reduces the HgoAc group
what role does a mercuric cation play oxymercuration-demercuration reactions?
it is an electrophile that gets attacked by a nucleophile(the alkene)
why don’t oxymercuration-demercuration reactions have carbocation intermediates?
in the mechanism, the resulting intermediate is not a carbocation because mercury has electrons that can interact with the nearby positive charge to form a bridge
oxymercuration-demercuration intermediate
2 resonance structures: one with and one without a ring. The lone pairs and + charge switch places in the resonance
Why don’t oxymercuration-demercuration reactions undergo carbocation rearrangments?
The more substituted carbon bears a partial positive charge instead of a full positive charge
types of stereochem in addition reactions
- Anti-addition: Nucleophile is added to the opposite side of the other added group
- Syn-addition: Added groups are added to the same side
What causes oxymercuration-demercuration reactions to undergo markovnikov addition?
the nucleophile is attracted to the more substituted side, resulting in a smaller activation energy barrier and a faster reaction due to the partial positive charge where the nucleophile is added
What happens to an oxymercuration-demercuration rings as a nucleophile gets close?
it opens up
Purpose of NaBH4 in oxymercuration-demercuration reactions
replace and reduce the HgOAc groups with a -H groups via a free radical mechanism, maintaining the stereochem of the starting molecule
which step of oxymercuration-demercuration governs the products?
oxymercuration
Which reaction is used to achieve anti-Markovnikov hydration?
hydroboration-oxidation
Hydroboration-Oxidation mechanism
- Hydroboration: Boron is added to the less substituted carbon
- Oxidation: OH replaces B H2
Similarities and differences between BH3 and a carbocation
Similar because they are both reactive, both BH3 is not asn reactive because it does not carry a formal charge
What process do BH3 molecules undergo and why?
intermolecular resonance to help fulfill their octets, since they have a broken octet. This results ins hybrid B2H6
Bonds in B2H6
- different from bonding in BH3
- bonds are called banana bonds
purpose of THF
stabilize borane(BH3) in hydroboration-oxidation reactions
What is responsible for the anti-mark property of hydroboration-oxidation reactions?
the reagent
reagent for hydroboration-oxidation reactions
BH3●THF
Reasons Boron is added to the less substituted carbon in hydroboration-oxidation reactions
- Electronics: since B is less electronegative than H, the partially negative H attacks the partially positive carbon which adds the H to the more subsituted alkene bond
- Sterics: BH2 and H are added across the double. Since BH2 is bigger, there will be less steric crowding
what do electronegativity differences result in?
dipole moments
intermediate for hydroboration-oxidation reactions
4-membered ring
What do 1 or 2 chiral centers result in with hydroboration-oxidation reactions?
racemic mixtures
What type of mechanism is hydroboration?
concerted
What type of addition do hydroboration-oxidation reactions?
syn-addition
catalytic hydrogenation
addition of H2 across a C=C bond
What happens if a chirality center is observed in catalytic hydrogenation?
syn addition
catalyst for catalytic hydrogenation
H2 with a metal catalyst