4 Analyzing Organic Reactions Flashcards
Lewis Acid
electron acceptor in formation of covalent bond
often electrophiles
vacant p-orbitals
positively polarized atoms
Lewis Base
electron donor in formation of a covalent bond
often nucleophiles
coordinate covalent bond
Lewis acid + Lewis base
both electrons come from the same starting atom (Lewis base)
Bronsted-Lowry Acid
species that can donate a proton [H+]
Bronsted-Lowry Base
species that can accept a proton
amphoteric molecule
example?
ability to act as an acid OR base
ex. water
acid dissociation constant (Ka)
equation
strength of acid in solution
Ka = ([H+][A-]) / [HA],
for HA <–> H+ + A-
pKa = ?
-logKa
more acidic solution’s = ?(higher/lower pKa)
lower pKa
pKa < -2
strongly acidic
pKa = -2 to 2
weak organic acids
alpha-hydrogens
VERY acidic
connected to alpha-carbon of carbonyls
stabilized by enol form resonance
nucleophiles
nucleophilicity and charge
nucleophilicity increases with increasing electron density (more negative charge)
nucleophilicity and electronegativity
nucleophilicity decreases as electronegativity increases (atoms less likely to share electron density)
nucleophilicity and steric hindrance
bulkier molecules are less nucleophilic
nucelophilicity and solvent
polar protic solvents?
polar aprotic solvents?
protic solvents can hinder nucleophilicity by protonating the nucleophile or through hydrogen bonding
polar protic solvents: nucleophilicity increases DOWN periodic table (protons are attracted to nucleophile and get in the way)
polar aprotic solvents: nucleophilicity increases UP periodic table (directly relates to basicity)
“electron-loving”
species with positive charge/positively polarized atom
tend to be good acids
electrophiles
increasing electrophilicity
more positive charge
better leaving groups
leaving groups
molecular fragmetns that retain electrons after heterolysis
best LGs are able to stabilize extra electrons
best leaving groups
weak bases (conjugate bases of strong acids lie I-, Br-, Cl-)
increased resonance
inductive effects from electron-withdrawing groups
delocalize/stabilize negative charge
SN1 reactions
- rate-limiting step: LG leaves, positively-charged carbocation remain
- nucleophilic attack on carbocation yields subsitution product
SN1 reaction passes through a planar intermediate and so will result in a ___________.
racemic mixture
SN1 reaction is more likely to proceed with a _______(more/less) subsituted carbon.
more
(more stable carbocation)
rate of SN1 reaction
rate = k [R-L]
[R-L] is alkyl group containing LG
1st order reaction: anything that accelerates formation of carbocation (Step 1) increases rate of SN1 reaction
SN2 reactions
1 step (concerted reaction)
“backside attack”
requires STRONG nucleophile and non-sterically hindered substrate
SN2 reaction proceeds best with _____ (more/less) substituted carbon.
less
SN2 reactions are stereospecific. What does this mean?
the configuration of reactants determines the configuration of the products
inversion of relative configuration
AND if nucleophile and LG have same priority: inversion of absolute configuration
rate of SN2 reaction
rate = k[Nu][R-L]
oxidation-reduction reaction (redox)
oxidation states of reactants change
oxidation state
indicator of hypothetical charge that an atom would have if all bonds were completely ionic
can be calculated from molecular formula
oxidation
increases oxidation state
loss of electrons
reduction
decrease in oxidation state
gain of electrons
oxidizing agent
accepts electron from other species
high affinity for electrons (ex. O2, O3, Cl2)
unusually high oxidation states (ex. MnO4-, CrO4 2-)
oxidation reagent?
alcohol –> aldehyde
PCC
CrO3/pyridine
oxidation reagent?
alcohol –> ketone
PCC
CrO3/pyridine
oxidation reagent?
aldehyde –> carboxylic acid
H2CrO3
KMnO4
H2O2
oxidation reagent?
alcohol –> carboxylic acid
KMnO4
H2CrO4
oxidation reagent?
alkane –> carboxylic acid
KMnO4
oxidation reagent?
alkene –> aldehyde/ketone
O3, then Zn
O3, then CH3SH3
oxidation reagent?
alkene –> carboxylic acid/ketone
O3, then H2O2
KMnO4, heat, H3O+
oxidation reagent?
alkyne –> carboxylic acid
O3, then H2O2
KMnO4, heat, H3O+
oxidation reagent?
alkene –> diol (vicinal diol)
OsO4
KMnO4, HO-
oxidation reagent?
alkene –> epoxide
mCPBA
oxidation reagent?
diol –> aldehyde
NaIO4
Pb(OAc)4
HIO4
oxidation reagent?
ketone –> ester
mCPBA
reducing agents
sodium, magnesium, aluminum, zinc (low electronegativities and ionization energies)
metal hydrides (H- ions)
reduction reaction
aldehyde –> primary alcohol
LiAlH4/NaBH4
reduction reaction
ketone –> secondary alcohol
LiAlH4/NaBH4
reduction reaction
amide –> primary amine
- LiAlH4/ether
- H+/H2O
reduction reaction
carboxylic acid –> primary alcohol
- LiAlH4/ether
- H2O
reduction reaction
ester –> primary alcohols
- LiAlH4/ether
- H2O
chemoselectivity
preferential reaction of one functional group in presence of other functional groups
increased oxidation of functional group increases reactivity (nucleophile-electrophile rxns and oxidation-reduction rxns)
common reactive sites for chemoselective reactions
SN1?
SN2?
carbonyl carbon
substrate carbon in substitution reaction
SN1: tertiary/secondary C (stable carbocation)
SN2: methyl/primary C (decreased steric hindrance)
NO tertiary carbons
steric hindrance
prevention of reactions at particular location within a molecule due to the size of the substituent groups
steric protection
useful tool for synthesis of desired product
prevents alternate products
