11/21 Flashcards
if you start with a chiral molecule, and have it undergo acidic or basic conditions, what happens to the chiral carbon upon enolization
it will form an enol, which is sp2 hydridized and it loses its chiralty
what happens to the chiral molecule after it becomes enolized and then is transformed back into the keto form?
it becomes sp3 again and is racemized to form a 50/50 mix of enantiomers
what types of ketones can be racemized
originally chiral sp3 alpha carbons on the ketone will be racemized and lose their stereochemistry
will you see racemization with non-chiral alpha carbons?
no, it only occurs when the molecule is chiral
what are the conditions for a haloform
a methyl ketone treated with a X2 halogen and excess base
what does X2 and excess base form with a methyl ketone
it forms a carboxylate salt and a haloform
what is an indicator that a haloform reaction has occured
yellow ppt forms in the reaction
what does the OH do after we add three additions of halide to an alpha carbon on a keto?
the three halides form a good LG so the hydroxide can perform nucleophilic acyl substitution
When reacting a carboxylic acid with 1.) X2 and phosphorous and 2.) H20
you form a alpha halo carboxylic acid
when reacting a alpha halocarboxylic acid with NH2, what is the product
an alpha amino acid
when reacting an alpha halocarboxylic acid with 1.) OH and 2.) H30, what is the product
an alpha-hydroxy carboxylic acid
which way does equilibrium lie when trying to enolize a keto with NaOEt
it favors the formation of the weaker acid so it will favor the reactants
why is ethoxide not a good base for enolization?
the pka of a keto acid is 19-20 while the pka of ethanol is 16 pka, since ethanol is the stronger acid the equilibrium is not favorable and does not favor the formation of ethanol and the enolate
why is LDA better to use for enolization than EtO
because of pkas, when using LDA, the weaker acid is formed on the product side which makes the equilibrium favorable in the forward direction
what is the pka for a protonated LDA? what about EtO? which is the weaker conjugate acid?
the pka for a protonated LDA is 38, for EtOH it is 16. The weaker acid is LDA and favored by equillibrium
how do you use LDA
it needs to be prepared in-situ and acts as a strong base for enolization
how do you prepare LDA in situ
you use n-butyl lithium and diisopropyl amine to create butane which bubbles off in solution and LDA
what is the thermodynamic and kinetic product of an enolate
the thermodynamic product is the predominate product at equillibrium and it is the more substituted enolate
the kinetic product is the faster forming product from the least hindered alpha proton
when is the thermodynamic product favored
when using a non sterically hindered base like OH
when is the kinetic product favored
when using a sterically hindered base like NDA
what type of reaction is alkylation with an enolate
SN2 reaction
why can an enolate perform nucleophillic substitution?
it is nucleophillic
what type of substrates cannot undergo alkylation with an enolate
3° alkyl halides
what type of substrates undergo alkylation the fastest with an enolate
methyl> 1° > 2° alkyl halides
why are beta dicarbonyls more acidic than regular ketos
they have more opportunities for resonance so they form a more stable conjugate base
