organic reaction mechanism Flashcards
electrophiles
species that want electrons
- often positive charges
nucleophiles
have excess of electrons
SN1
unimolecular nucleophilic subsitution reaction
two step mechanism
carbocation intermediate
nucleophilic substitution
nucleophile attacks electrophile
- nucleophile replaces the leaving group as a new substituent on the electrophile
how to identify the nucleophile
has one or more lone pairs and excess electrons
how to identify electrophile
partial or full positive charge that wants to accept elecrons
leaving group
typically a weak base, that will break its bond with the electrophile after nucleophilic attack with nucleophile
common leaving groups
Br- , cl- , I-, Oh-. R3N
general mechanism of nucleophilic SN1 reaction
- the nucleophilic attack
- transfer of e- to LG
- LG leaves
two major types of nucleophilic substitutions SN1 and SN2
sn1 and sn2
1 and 2 are based on the rate laws of the reaction
sn1 rate law is dependent on
the concentration of the substrate
sn2 rate law dependent on
the concentration of substrate and nucleophile
sn1 mechanism
- leaving group leaves first
- carbocation forms (unstable)
- nu attack
- nu deprotinated
rate limiting step of sn1 rxn
the carbocation formation
- very slow
how to enhance carbocation stability
- make carbocation more substituted
terciary carbocations> secondary> primary
is sn1 racemic
yes, produces 50/50 mix
sn2
backside attack and inversion of steriochemistry
tranition state includes nu bond forming and LG breaking
favor sn2
strong nu (OH-)
not sterically hindered primary> secondary >tert
polar protic solvents
hydrogen atoms attatched to polar molecule, water, etoh, meoh
polar aprotic solvent
c=o but no hydrogen in polar molecule ex: acetone and dmso
which nucleophilic reaction is favored by polar protic solvents, why
sn1 because it is able to hydrogen bond and stabilize the carbocation
which nucleophilic reactionis facored by polar aprotic solvents
sn2
what is the key to the sn2 mechanism
steric hindreance
rate law of sn2
depends on concentration of substrate and nu
strengh of nu in sn1
doesnt matter
strength of nu in sn2
strong and nonbulky
steriochemistry of sn2
inverted bc of backside attack
nucleophilic elimination
lg is kicked out and replaced with a double bond
can also be E1 and E2
E1 mechanism
similar to SN1 mechanism
- unimolecular rate law driven by carbocation formation
- nu pulls off e- from adjacent c
form double bond
1. LG leaves and C+ form
2. nu pulls hydrogen and form db
E2 mechanism
bimolecular rate law
strong base attacks electrophilic hydrogen and leaving group leave at same time
Zaitsev rule
the double bond formed is more substituted
fisher esterification
acid catalyzed conversion of COOH into an ester
COOH +ROH in presence of acid
fisher esterification mechanism
- protination of carbonyl oxygen with acid
- oxygen of alochol attack electrophilic carbonyl carbon
- protinate OH group
- then carnboyl carbon does electrophilic attack and removes h2o leaving group
- water deprotinates carbonyl oxygen
how is fischer esterification reversed
hydrolysis
OR group is leaving group and OH attacks and reforms subsitutent to form COOH and ROH
imine formation
nucleophilic substitution with NH3 group or NH2R or NHR2
c=n replaces c=o
shiff base
imine with a carbon substitioent
imine formation mechanism
- protinate the cabronyl oxygen
- amine group attacks carbonyl C
- proton transfer from n to O to make h2o grup
- N attacks cabtonul and h2o leaves
- n group is deprotinated by wter
what happens to a ketone if it undergoes nucleophilic addition
hemiketal formation
tautomerization
two structures interconvert at equilibrium
not resonance
keto - enol tautomerization
keto and enol forms c=o and c=c
acid and basic mechanisms
requitement for enol tautomer
at least one hydrogen on the adjacent alpha carbon
enolate
keto-enol intermediate with neg charge acts as a nuclephile
enolate formation can be catalyzed
by acids and bases
micheal addition
a, b unsaturated aldehydes or ketones generated
aldehyde attacks double bond and forms an a, b unsaturated aldehyde
robinson annulation
micheal addition followed by aldol condensation so it becomes ciclical and one of the aldehydes becaomes an oh
aldol condensation
nucleophilic alpha carbon attack on electrophilic carbon to form new c-c bond
retro aldol
reverse aldol condensation reaction
