quayle

Question 1

what is tautomerism

Answer 1

the reversible inter conversion of constitutional isomers via a transfer of a stable mobile group

Question 2

what are constitutional isomers

Answer 2

compounds with the same molecular formula but have different connectivity e.g. keto-enol isomerism

Question 3

what is the favourable angle for attack for nucleophiles on carbonyls ad why

Answer 3

109 as this allows the best overlap between the HOMO of the nucleophille and the LUMO (pi*) of C=O

Question 4

Racemization

Answer 4

a reaction which will make the reactant lose its optical activity

Question 5

what affects keto-enol isomerism?

Answer 5

-structural features (aldehyde vs Ketone)
aldehydes generally have a higher enol content than ketones
- solvent

Question 6

what are isolable structures

Answer 6

2 isometric structures that are interchangeable meerly by re positioning a single H atom

Question 7

when generating an Enol tautiomer of a carbonyl compound, is the keto tautiomer catalysed by acid or base

Answer 7

it can be catalysed by either

Question 8

how can you form extended enolate anions?

Answer 8

removal of H(gamma) with a base forming either a beta gamma unsaturated compound or an alpha beta unsaturated compound

Question 9

Factors affecting the Pka of organic compounds

Answer 9

1. Hybridisation - the more s character a compound has the higher its PKa
2. Resonance - the more resonance structures that are formed means a lower PKa as the positivecharge can be spread out
3. Inductive effect - if an electronegative atom attatched it can lower the Pka through a withdraving inductive effect

Question 10

chemistry of enols

Answer 10

1) Low concentration but reactive
2) can exist in equilibrium with ketones
3) reactions often proceed by the generation of very low concentrations of the Enol tautomer
4) highly nucleophillic enabling functionalisation at c(alpha) by reaction with electrophiles
5) the C=C bond in enols is more nucleophillic than that of an isolated C=C bond
6) Reactivity is similar to phenols

Question 11

An acid’s pKa depends upon:

Answer 11

i. the stability of the conjugate base (i.e. anion A-
) – substituents?
ii. The bond strength A-H;
iii. The solvent

Question 12

Kinetic Enolate

Answer 12

(K.E.): Less stable –
but formed more
rapidly
formed on least hindered carbon

Question 13

Thermodynamic enolate:

Answer 13

thermodynamically more
stable. Generated from
K.E. upon equilibration
formed on more hindered carbon

Question 14

PKa

Answer 14

The term “pKa” is a measure of the strength of an acid in solution. It is defined as the negative base 10 log of the acid dissociation constant. A lower pKa value indicates a stronger acid.

Question 15

Chemistry of enolates

Answer 15

1. very reactive nucleophiles
2. both O and C have partial negative charges so electrophiles attack
3. less reactive electrophilles attack at C, more reactive electrophiles attack at O
4. carbon centred electrophiles can attack at either

Question 16

what is torsional strain

Answer 16

Torsional strain or eclipsing strain is the increase in potential energy of a molecule due to repulsion between electrons in bonds that do not share an atom