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Organic Chemistry > Physical Organic Chemistry > Flashcards

Physical Organic Chemistry Flashcards

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1
Q

equilibrium constant (K)

A

concentration of products / concentration of reactants

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2
Q

Gibbs free energy formula

A

ΔG = -RTlnK

R = gas constant (8.314 J K-1 mol-1)
T = temp. (in kelvin)
K = equilibrium constant

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3
Q

positive ΔG

A

reactants are favoured at equilibrium

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4
Q

negative ΔG

A

products favoured at equilibrium

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5
Q

what does ΔG tell us?

A

equilibrium position (not how long it takes to get there)

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6
Q

how to manipulate the equilibrium position?

A

constant at a particular temperature but we can alter the equilibrium position by changing concentrations of components

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7
Q

positive ΔH

A

endothermic

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8
Q

negative ΔH

A

exothermic

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9
Q

ΔS

A

change in disorder between reactants and products

positive = more disorder (1 molecule -> 2 molecules)
negative = less disorder (2 molecules -> 1 molecule)

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10
Q

how can we favour products in a reaction?

A

[i.e. shift equilibrium to RHS]

ΔG = negative (= larger K)

TO ACHIEVE THIS:

ΔH = negative (exothermic reaction)
ΔS = positive (reaction becomes more disordered)

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11
Q

when K varies with temperature

A

combine ΔG = ΔH - TΔS and ΔG = -RTlnK then dividing by -RT

FORMULA:
lnK = ΔH/RT + ΔS/R

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12
Q

what does an intermediate represent?

A

local energy minimum of a reaction

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13
Q

rate equation

A

k [A]^x[B]^y

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14
Q

reactions under thermodynamic control

A

outcome depends on position of equilibrium ∴ depends on relative stability of possible products

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15
Q

reactions under kinetic control

A

outcome depends on rate at which reaction happens ∴ depends on relative energies of transition states

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16
Q

what must we make sure we do when predicting reactions?

A

make sure only ONE factor is being changed

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17
Q

isotopic labelling

A

earlier work used radioactive labels + then used degradation reactions to based on well-known reactions to locate labelled atoms

nowadays, non-reactive labels used with spectroscopic methods to locate labelled atoms

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18
Q

ESR/EPR

A

[electron spin/paramagnetic resonance]

detects radicals at low conc. -> provides info about shape/structure of unpaired e-

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19
Q

crossover experiments

A

intramolecular product = no mixture (re-arrange in 1 motion)

intermolecular product = mixtures (due to fragments)

^ distinguishable by NMR

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20
Q

stereochemical evidence for Sn2 mechanism

A

each enantiomer of the product can be formed stereospecifically from same configuration of enantiopure alcohol (proves inversion)

Sn2 follows 2nd order kinetics
-> measure rate of incorporation of radioactive iodine
-> measure rate of racemisation by observing loss of optical activity (double rate of inversion)
-> rate of sub. = rate of inversion

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21
Q

Sn1 - solvent effects

A

involves polar, carbocation intermediate - stabilised by polar solvents

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22
Q

Sn2 - solvent effects

A

no overall charge type (transition state is just charge distribution) - not very sensitive to solvent polarity (usually better in non-polar solvents)

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23
Q

dielectric constant

A

one measure of solvent polarity

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24
Q

neighbouring group participation

A

interaction of a reaction centre with a l.p. of e- in an atom

or

e- present in pi bond contained within the parent molecule but not conjugated with reaction centre

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25
Q

what does a transition state represent?

A

energy maximum (most unstable structure)

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26
Q

zero order

A

changing [R] has no effect on rate

27
Q

first order

A

if [R] doubles, rate doubles

28
Q

second order

A

if [R] doubles, rate quadruples

29
Q

what does the transition state resemble in SN2 reactions?

A

reactant (“early” transition state)

30
Q

what does the transition state resemble in SN1 reactions?

A

product (“late” transition state)

31
Q

what does the transition state resemble (in general)?

A

adjacent reactant, intermediate or product that is closest in energy

32
Q

what is the product ratio for a reaction that has 2 or more rapidly interconverting intermediates or reactants?

A

product ratio is determined by relative heights of highest energy barriers leading to the products

33
Q

when is the Curtin-Hammett applicable?

A

when energy barriers interconverting the different intermediates are much lower than the barriers to form products

34
Q

how to perform kinetic experiments

A

vary conc. of each reactant + measure rate

determine if order is 0/1/2

rate depends on [species] involved, which changes with time -> so need to measure rate at equivalent points

35
Q

systematic changes

A

[e.g. sterics, electronics]

made to starting material in the expectation of a predictable change in RoR

36
Q

example of systematic change - investigating Sn1 by changing EG to EW

A

if C+ intermediate is involved (e.g. Sn1) it will be much less stable

reaction will be slower

37
Q

Hammett relationship

A

relationship between reaction rates and equilibrium constants for reactions involving benzoic acid derivatives with meta/para subs.

38
Q

σ values for different sub

A

EWG = positive σ

EDG = negative σ

39
Q

Hammett sub. constant (σ) - meta vs para

A

conjugation = more effective in para

σp > σm

40
Q

how to determine Hammett sub. constant (σ)

A

equilibrium constant for ionisation of benzoic acid and benzoic acid with sub. determined

σx = pKa(C6H5CO2H) - pKa(XC6H4CO2H)

41
Q

what happens to the p value if reaction centre moves away from aromatic ring bearing EDG/EWG?

A

becomes smaller because anions can’t be delocalised around the ring

42
Q

moderate - ve p value

A

e- flow out of TS

positive charge near ring

loss of conjugation

43
Q

large -ve p value

A

positive charge on ring or delocalised around ring

44
Q

scenarios for small p value

A
  1. aromatic ring is too far away from reaction site
  2. reaction is not dependent on e- flowing in/out of aromatic ring
  3. two steps in mechanism with contrasting character
45
Q

moderate +ve p value

A

electrons flow into TS

negative charge near ring

loss of conjugation

46
Q

large +ve p value

A

negative charge on ring or delocalised around ring

47
Q

non-linear Hammett plots

A

indicates change in reaction mechanism or single mechanism with change in RDS

48
Q

kinetic isotope effect (KIE)

A

when RoR can change if isotope is changed for another isotope

most significant when relative difference of masses is greater
-> e.g. changing H for D = 100% increase in mass

49
Q

KIE formula

A

KIE = kH/kD

50
Q

origin of KIE

A

covalent bonds = vibrating

zero point energy = minimum vibrational energy
-> depend on mass of atoms attached to bond
-> heavier masses = lower zero-point energy

to break covalent bond, energy is required to separate nuclei (i.e. need to raise vibrational energy of bond to the point where it breaks)

zero point energy for C-D is smaller (required more energy input to break bond)

51
Q

which will be faster - reactions that break C-H or C-D bonds?

A

C-H (bond is weaker)

52
Q

primary kinetic isotope effect (PKIE)

A

KIE when isotopically sub. bond is directly involved in bond-making/bond-breaking process

53
Q

drug discovery - replacing H with D

A

if metabolic pathway involves C-H cleavage, it may be possible to slow down the breakdown of the drug (increasing effectiveness)

54
Q

secondary kinetic isotope effect (SKIE)

A

KIE when isotopically sub. bond is NOT directly involved in bond-making/bond-breaking process

55
Q

how does a catalyst lower the Ea of a reaction?

A

lowers energy of TS or raising energy of reactants

56
Q

specific acid catalysis (SAC)

A

rate unaffected by nature of any acidic species present + only depends on conc. of conjugate acid of solvent

protonates electrophiles to make them more electrophilic

57
Q

specific base catalysis (SBC)

A

rate unaffected by nature of any basic species present + only depends on conc. of conjugate base of solvent

deprotonates nucleophiles to make them more nucleophilic (rapid deprotonation followed by RDS, resulting in anion)

58
Q

what happens to catalysis at neutral pH?

A

no possible reaction

59
Q

general base catalysis

A

depends on pH + conc. of other bases

=> weak bases (too weak to deprotonate nucleophile) can still act as catalyst

60
Q

general acid catalysis

A

depends on pH + conc. of other acids

transfer of a proton from a weak acid during RDS

61
Q

when is GAC/GBC most effective?

A

when one component is present in large excess

62
Q

nucleophilic catalysis

A

reacts with an electrophilic centre as a nucleophile (not a base)

involves formation of intermediate

63
Q

requirements for nucleophilic catalysis

A

catalyst must be:

  1. more nucleophilic than nucleophile in non-catalysed reactions
  2. must become better leaving group in intermediate than original LG in substrate
  3. less thermodynamically stable than product
64
Q

what is implied if graph goes from negative slope to positive (V shape)?

A

change in reaction mechanism

Organic Chemistry (13 decks)

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