Quiz 5 Flashcards
Nucleophilicty
the strenght of a nucleophile, how fast does the nucleophile attack an electrophile
Name the four factors that affect nucleophilicty
- size
- charge
- solvent
- polarizibility
How does size affect nucleophilicity?
a smaller atom is favored
excpet in polar protic solvents
How does charge affect nucleophilicty?
a nucleophile with a charge will be stronger
How does polarizibility affect nucleophilicty?
it determines how well the nucleophile can unevelnly dsitrubite a charge
How does a solvent affect nucleophilicity?
it can slow down or speed up the rxn by creating a solvent shell around the nucleophile or not
Polar protic solvent
has hydrogen bonding, bad for SN2 rxns bc it creates a solvent shell
but favored for SN1
Polar aprotic solvent
does not have hydrogen bonding, good for SN2 reactions because it doesnt create a solvent shell
Inversion of configuration
R–S or S–R
when a reactant starts with one configuration and the products end with the other configuration
happens bc back side attack
Backside attack
nucleophile attatches on opposite side of leaving grp
affects how you draw the mechanistic arrow
Beta elimination
E2 rxn
when an alykl halide is treated with a strong base and a pi bond is formed
- proton transfer + loss of a leaving group
What are the products of a beta elimination?
con A of base + solo halogen + Alkene
Is an E2 reaction bimolecular and concerted or bimolecular and stepwise?
An E2 reaction is bimolecular and concerted
- relying on the substrated and the base for its rate
There must be a — on the beta carbons or an — reaction cant happen
Hydrogen, beta elimination/E2
In an E2 rxn the alkene must form between the — and —
alpha C and Beta C
How can a base affect the E2 rxn?
It can speed it up or slow it down depending on its strenght
- the stronger the base is the faster it can deprotonate the molecule
What is the key difference between an E2 and SN2 rxn?
You want a more sterically hindered product, because they are more stable and a sterically hindered substrate so the H are accesible for the base
True or false?
A cis alkene is more stable than a trans alkene bc there is resonance
false, a trans alkene reduces interaction between substituents
put 1,2,3,and 4 substitued alkenes in order of increasing stability
monosub<disub<trisub<tetrasubstitued
Regioselectivity
when there is a major/minor product produced and one is favored over the other, the region where the reaction takes place
True or false?
The more highly substitued the base is the more likely the Zatisev product will be favored
False, the more subsituted the base is the more likely the hoffman product will be favored (the less substitued alkene)
Zatisev product
more substitued alkene
Hoffman product
less substitued alkene
Stereospecific
One beta H, produces one stereoisomer (most stable one)
SN2 and E2 under right conditions
Stereoselective
2+ beta H, will produce a major and minor product in unequal amounts
E2 rxns
Anti coplanar/anti periplanar
staggard conformation where Beta H and leaving group on opposite planes of each other
lower E makes this the favorable conformation
Periplanar/syn-planar
eclipsed conformation where Beta H and leaving group are on the same direction
very high E transistion state, not favorable for a E2 reaction
If a molecule is syn-planar/periplanar what should you do?
rotate the bonds so the molecules are anti planar
If an E2 reaction is happening with a substitued cyclohexane what has to be true of the leaving group and beta H?
- leaving group has to be axial
- the leaving group and beta H have to be anti coplanar/antiperi planar to each other (just like in non cycloalkanes)
How do unimolecular reactions differ from the biomolecular substituion and elimination reactions?
- these reactions occur stepwise: at least two transition states and one intermediate
- rate depends only on the substrate and not the solvent
- Both SN1 and E1 prefer a hindered substrate
what is the first step of a unimolecular substitution and elimination reaction and what determines what the next mechanistic step is?
first step: Loss of a leaving group
the solvent determines if a nucleophilic attack or a proton transfer will follow
True or false?
A unimolecular reaction will produce both SN1 and E1 products
True! one will just be featured more depending on the solvent
Adding — to a unimolecular reaction will increase the likelihood of a Elimation reaction?
Heat will increase the probability of an elimination reaction
Unimolecular reaction favor a — substrate
hindered
True or false?
a primary vinylic and allylic carbocation can never form
false, an allylic primary carbocation can form because of resonance. aryl and vinyllic cannot form
What kind of solvent is favored for a unimolecular reaction?
Polar protic, it can stabilize the partial charges (carbocations) that form during the reaction
If your product has a charge at the end of the reaction what should be done?
a proton transfer, a charged product is higly unlikely because it would be unstable
True or false?
Just like SN2 an SN1 reaction will always produce inversion of configuration
false, an SN1 reaction will produce both retention and inversion products
In unimolecular reactions will the hoffman or zaitsev product be favored?
Zaitsev, more substiuted and more stable
the E isomer will also likely be favored
What are the four catagories of reagant?
- Strong nucleophile, acts only as a nucleophile
- strong base, acts only as a base
- strong nucleophile, strong base
- weak nucleophile, weak base
What kind of reaction does a strong nucleophile that acts only as a nucleophile promote?
this will promote a SN2 reaction
Examples of strong nucleophiles that act only as nucleophiles
solo halogens: I-, Br-, Cl-
HS-
what kind of reaction does a strong base that only acts as a base promote?
likely will promote an E2 reaction
example of strong bases that act only as bases
DBU, DBN, tert-butoxide, strong bulky bases
What kind of reaction do strong nucleophile/strong base promote?
Bimolecular reactions
examples of strong nucleophile/ strong base
charged O- or N-, MeO-, EtO-, -OH
What kind of reactions will a weak nucleophile/ weak base promote?
unimolecular reactions
examples of weak bases/ weak nucleophiles
neutral oxygens, Water, Alcohols
MeOH, Et,OH, H2O
True or false?
A heavily subsiuted substrate will indicate any reaction that is not SN2
True, Sn2 is the only reaction that does not favor a hinderd substrate
Will the zaitsev or hoffman product be favored in an E1 reaction?
In an E1 reaction zaitsev is always favored
