Reaction Reagents 10 Flashcards
what is the hybridization of ketones and aldehydes? what does this mean?
trig planar = enantiomers bc nuc attack either side
why are aldehydes more reactive than ketones (towards nucs)?
- less sterically hindered
- aldehyde has larger delta + on carbonyl C
What is the general mechanism for nucleophilic addition under BASIC CONDITIONS?
what should you use at nuc?
- nuc attacks delta + C and e- from double bond transfers to O making O- negative intermediate
- O- base attack a proton source
made an alcohol
use H- (LAH or NaBH4) or C- (Grignard - CMgBr)
ketone + Grignard reaction + H2O
basic condition nucleophilic attack
C- of Grignard attacks C+ of carbonyl
water protonates O- intermediate in second step
C- or H- (grignard or hydride)
when do you use acidic bs basic conditions for nuc attack?
strong nuc = basic conditions
weak nuc = acidic conditions
what is the general mechanism for nuc addition at acidic conditions?
- O of carbonyl attacks H of acid (this protonates the carbonyl = better electrophile) - positive intermediate
- weak nuc attacks C of carbonyl and e- in double bond transfers to O
makes alcohol
can nucleophilic addition be reversible?
yes, if nuc is a good LG
examples of H- donors?
LiAlH4 or NaBH4
can you make a racemic mixture with ketone/aldehyde + H-/C-?
yes! aldehydes + unsymmetrical ketones are sp2 = H-/C- attacks either side
what is the strength of an Oxygen nucleophile?
strong under basic (HO-, RO-)
weak under acidic (water, ROH)
ketone/aldehyde + water
equilibrium with hydrate (2OH) but generally doesn’t favour hydrate
acidic cond. = water becomes O+ = is better electrophile
basic cond. = water becomes OH- = better nuc
under acidic conditions, a mech is reasonable if…
avoids use/formation of strong base
and vice versa
what is an acetal?
C with 2 OR groups
rxn to make acetals?
carbonyl + 2ROH under acidic conditions
acidic by TsOH or H2SO4 rep by H+
ROH is a weak nuc = acidic conditions needed to protonate first to make C a better electrophile
mech to make acetal?
NEED TO PROTONATE
1. TsOH or H2O4 protonates O of carbonyl
2. C is a good electrophile = O of ROH attacks C
3. ROH deprotonates O of ROH that is attached to C of carbonyl
this is a HEMI ACETAL
1. protonate carbonyl O with H2OR generates water LG and double bond to OR and C becomes electrohphile
2. ROH attacks electrophilic C
3. ROH gets deprotonates by another ROH
this is ACETAL
all steps are reversible, need 2 ROH
acidic condtions
how can you ensure that acetal formation occurs?
enough ROH alcohol present
(if too much water/proton, drives back to carbonyl) remove water as it is being generated
carbonyl + diol under acidic conditions?
both equivalents of alcohol compes from same compound and a cyclic acetal is formed
hemi acetyl under basic conditions?
- base attacks C of carbonyl
- protonate O of carbonyl
no LG = can’t make acetal
carbonyl + 2RSH
thioacetal or cyclic thioacetal if acidic conditions and diol used
acetal
acetals can be used as
protecting groups; protect ketone from LAH and H3O will deprotect
how can you isolate hemiacetal?
make cyclic when compound contains both carbonyl and hydroxy group
electrophile and nuc in same molecule
intramolecular
imine formation?
carbonyl + CH3NH2
acidic condition protonates CH3NH2
primary amine
enamine formation?
carbonyl + 2º amine (R2NH)
acidic conditions
no proton to deprotonate after water leaves, so beta H is depronotated and makes double bond with C
acetal + base
no rxn
