Enols and Enolates Flashcards
instead of the chemistry happening on the carbonyl, where is the chemistry with enols?
in alpha carbons
what is an alpha carbon
a carbon adjacent to the carbon of the carbonyl
what are alpha protons
hydrogens attached to alpha carbons
are alpha protons mildly acidic or basic? what is the effect of this
mildly acidic, so they may be deprotonated with a strong base
after an alpha proton is deprotonated, what is left behind
a conjugate base that has resonance
the conjugate base of a deprotonated molecule has resonance. Where is the negative charge?
on the O
when a ketone has an alpha proton and is in the presence of an acid or a base, what happens
the ketone exists in equilibrium with an enol
what is an enol
alkane alcohol
is the ketone/enol equilibrium centered?
no, it lies heavily towards the ketone
what word is used to describe the relationship between ketones and enols
tautomers
define tautomer
the two molecules differ by an alpha proton
the ketone has it on a carbon, the enol has it on the oxygen
how might the ketone/enol equilibrium be shifted to the right
acid or base catalysis
what is it called when the enol is missing a proton
enolate
describe acid catalyzed tautomerism
ketone attacks H+ to make the enol. Both ketone and enol have resonance. Water can attack the enol resonance contributor
describe base catalyzed tautomerism
OH- attacks the ketone to make the enol. Both form resonance contributors, and the one from the enol can attack water
how might we add an electrophile onto the carbon of an enol
must do acid/base catalysis first to make the enol, and then the enol can attack the electrophile from the pi bond, electrons move around, giving us a ketone with the E attached to an alpha carbon
what is it called when we add an electrophile onto a ketone
alpha addition
what happens in the Hell-Volhard-Zelinsky mechanism
it’s alpha addition onto a carboxylic acid (not a ketone this time)
describe the mechanism of the HVZ
convert carb acid into an acyl halide, then tautomerize, then use Br2 (which is the alpha addition step), then convert back into an acid via hydrolysis
how do we make enolates
deprotonate an enol (gives the alpha position a negative charge)
T or F: you can draw the enolate with one structure
false; you need to show both resonance contributors
in making enolates, what side of the equilibrium is favored when OH- is used
the ketone
in making enolates, what side of the equilibrium is favored when LDA is used
the enolate, because LDA is stronger than OH-
describe the strength and size of LDA
strong and bulky
which will be more selective and why: OH- or LDA
LDA is more selective because it’s bigger and needs to consider sterics
what type of enolate is OH- best used for making
double enolates or the haloform reaction
what is the haloform reaction
you start with a methyl ketone, use OH- and Br2 to make a carb acid. It’s used as a test to see if we started with a methyl ketone
what two bases are used to make enolates (not OH-)
LDA and NaH
describe the strength and size of NaH
super small and strong
compare the size and strength of NaH and LDA
NaH: small and strong
LDA: big and strong
describe what a kinetic product is
a less stable product, to make it there isn’t interference
describe what a thermodynamic product is
a more stable product, to make it there is more sterics
which base is used to make a kinetic product
LDA (bulky)
which base is used to make the thermodynamic product
NaH (small)
how do we grow the carbon chain of an enolate
first we make the enolate, then react it with RX and R will attach to the alpha carbon
what are enolates analogous to
enamines
describe why enamines are analogous to enolates
enamines can also do alpha addition. If we push the lone pair from the N into the ring, we can move electrons from the double bond to form an enolate (neg charge on the ring, pos charge on N)
describe how to do alpha addition on an enamine
enamine attacks RX, adding R onto the ring, then we do hydrolysis to turn the =N part into a carbonyl
