week 1 Flashcards
(32 cards)
give the 4 greek letters and corresponding to C atom position relative to the C=O
alpha = 1 beta = 2 gamma = 3 delta = 4
give an important property of alpha hydrogens
very acidic
where does the C=O have the most e- withdrawing influence?
alpha carbon
why do alpha H have increased acidity?
the CB formed is very stable
what does the pKa of an acid (ie it’s strength) depend on and why?
the stability of the CB formed.
this makes it easier to ionize the acid.
which side of the equilibrium is favoured for the reaction of a strong acid?
RHS
explain the relationship between the electronegativity of an element and the stability of the CB it is part of
the more electronegative an element the more stable the CB
the -ve charge stabilises the CB
explain the relationship between the value of pKa and acid strength
low pKa = strong acid
high pKa = weak
pKa = -log (Ka)
negative log
how does resonance stabilisation of a CB affect acid strength?
delocalistaion of -ve charge increases acid strength
why are aldehydes stronger acids than ketones?
inductive effect of a CH3 group slightly destabilises CB
why are ketones stronger acids than esters?
e- donating resonance effect destabilises (more than inductive effect)
why does a 2nd C=O make an acid stronger?
2 C=O are more stabilising than just 1
what is a tautomer?
structural isomer of a compound which differs only in the position of a single H+
how is an enol formed from a ketone?
a H+ of the central CH2/3 is removed by a OH
give the 2 most important factors in determining the ratio of products of ketone + water
eqm constant and pH of solution
give the 2 products formed in ketone + water
ketone hydrate and enol
which catalysis mechanism, acidic or basic, has an enolate intermediate?
basic
describe the enol content in solutions of simple C=O compounds in neat liquid
low
how can enols be stabilised?
resonance effect or H bonding
why does the eqm favour simple aldehydes/ketones over enols? (neat liquid)
combination of C=O + C-H bonds is stronger than combination of C=C + O-H bonds.
give 2 examples, with reasoning, of systems with 100% enol content
dimedone- 100% in solid state because of resonance and H bonding
phenol- aromaticity stabilises
how do you generate an enolate?
complete deprotonation of a keto
how do you quantitatively generate enolates?
- slowly add ketone to excess base to stop equilibriation
- LDA (non-nucleophilic strong base)
- 78 degrees celsius
- aprotic solution (eg THF)
give the 2 types of product formed from enolization of unsymmetrical ketones
Kinetic enolate
Thermodynamic enolate
