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CHEM214 > Reaction Mechanisms > Flashcards

Reaction Mechanisms Flashcards

1
Q

What’s the difference between thermodynamics and kinetics?

A

Thermodynamics look at how FAVOURABLE forming a product from a reactant is

Kinetics looks at how FAST a reaction occurs

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2
Q

What does labile/inert mean?

A

Labile = weak, substitution occurs quickly

Inert = resists substitution and so rate of substitution is slow

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3
Q

What makes kinetics slow?

A

High Ea, therefore complex is kinetically inert and a large CFSE

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4
Q

What makes reaction favourable?

A

A decrease in ∆G, means reactant is less stable then product

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5
Q

What is ‘dissociative’ analogous to?

A

SN1 carbon chemistry

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6
Q

What is ‘associative’ analogous to?

A

There is no analogy in carbon chemistry

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7
Q

What is interchange?

A

Where a new M-L bond is being formed whilst an old M-L bond is being broken at the same time.

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8
Q

Of the 3 types of substitution of ligands, which has an intermediate that cannot exist independently?

A

Interchange - undetectable bonds

A/D intermediates both have independent existence

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9
Q

What is ‘interchange’ analogous to?

A

SN2 carbon chemistry

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10
Q

How many transition states do A/D mechanisms have?

A

2

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11
Q

How many transition states does interchange mechanism have?

A

1 - single, undefined intermediate

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12
Q

What is dissociative interchange?

A

Id is where the M-X bond is more advanced in breaking than the new M-Y bond is in formation

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13
Q

What is associative interchange?

A

Ia is when the M-Y bond is more advance at transition state than the M-X bond breaking

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14
Q

What mechanisms do most octahedral TM complexes follow?

A

Ia or Id

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15
Q

How do most organometallic (18e-) complexes react?

A

Via D mechanisms

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16
Q

How do most square planar complexes react?

A

Via A mechanisms

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17
Q

Why is TM chemistry hard to study?

A

Because usually, M-L bonds are fairly weak, and so reactions occur very quickly

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18
Q

What is the RDS in D/Id mechanisms?

A

M-X bond breaking is key process

So rate depends on identity of X (leaving ligand)

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19
Q

What is the RDS of A/Ia mechanisms?

A

Bond formation is key process

So rate depends on identity of Y (incoming ligand)

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20
Q

What will increasing steric bulk of other ligands do to rate of mechanism?

A

Bulkier ligands speed up D/Id reactions - easier for X to leave

Will slow down A/Ia reactions - harder for Y to approach M

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21
Q

What does positive ∆S indicate about the reaction mechanism?

A

Indicates D/Id mechanism as there is more disorder at the transition state - greater entropy w bonds breaking

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22
Q

What does negative ∆S indicate about the reaction mechanism?

A

Indicates A/Ia mechanism as there is less disorder at transition state - bonds forming means less entropy

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23
Q

What’re the units of

A

cm^3 mol-1

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24
Q

What does the molar volume, ∆V, decreasing at the TS mean?

A

Decreasing ∆V at TS means applying pressure will increase rate

Indicates A/Ia mechanism

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25
Q

What does the molar volume, ∆V, increasing at the TS mean?

A

Applying pressure will decrease rate

Indicates D/Id mechanism

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26
Q

Why is measuring ∆V and rate difficult?

A

Hard to get significant variation in k with pressure, very high pressure needed

Following reactions at high pressure is hard to do

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27
Q

What does the Eyring equation give when linearised?

A

Slope = -∆H/R

Intercept = ln(kB/h) + ∆S/R

28
Q

How is the Eyring equation linearised? (What’s plotted against what?)

A

Ln(k/T) vs 1/T is plotted

Should get a straight line

29
Q

Why is varying Temp, M, L, X or Y too much a bad idea?

A

Because mechanism may change if the change is too drastic

30
Q

How should the rate of a reaction be determined?

A

Establish rate law by varying [LM-X] and [Y] and measuring effect on rate, k.

Vary M, L, X and Y

Measure effect of Temp on rate
- ∆H, ∆S

Measure effect of pressure on rate
- ∆V

31
Q

Why is Pt(II) good to study?

A

Very slow reaction - minutes or more at room temp

Means reactions are easier to study - similar to organic reactions

32
Q

Where do two-term rate laws come from for Pt(II) complexes?

A

Slow RDS is water subbing in for X

Y subbing in for water is then very fast - as water is a poor L for Pt(II)

33
Q

How does we know H2O is involved in ligand exchange for Pt(II) complexes?

A

Study reaction in MeOH instead of water.

MeOH is an even worse L for Pt(II)
- so k1 is smaller in MeOH but k2 remains roughly the same for a given Y

Value of k2 doesn’t depend on solvent (MeOH/H2O). But k1 DOES depend on solvent

34
Q

What rate mechanism is suggested in substitution of Pt(II) ligands in water/MeOH?

A

Over 10^4 difference in k2 when Y is varied
Less than 10^1 difference in k2 if X is varied

Suggests A/Ia as bond formation (of solvent) is important in transition state/intermediate.

Bonding ligand is most important

35
Q

What type of ligands does Pt(II) prefer? Give an example.

A

Soft ligands, as Pt(II) is also soft

E.g. thiourea

36
Q

What effect do bulky ligands have on the rate of Pt(II) complexes substitution?

A

They decrease the rate of substitution

Suggests A/Ia reaction mechanism

37
Q

What does a reduction in ∆S and ∆V at TS for Pt(II) complexes?

A

Means there is less disorder and less volume at TS

All values are much larger when compared to other TM reactions

Suggests A/Ia

38
Q

What is the proposed mechanism for Pt(II) complexes?

A
39
Q

Are Pt(II) 5-coordinate complexes common or rare? Compare this to other 4d/5d metals.

A

Extremely rare

For other 4d/5d square planar ions, Pd(II), Rh(I), Ir(I), are much more common

40
Q

What is the trans effect?

A

That some ligands increase the rate of substitution when positioned trans to the leaving ligand

It a KINETIC effect - says nothing about thermodynamics…

41
Q

What is the order of the trans effect series?

A
42
Q

What is trans influence?

A

Lengthening/weakening of the bond trans to it (M-X) in ground state

Thermodynamic effect

43
Q

How do ligands like C2H4 and CO exert high trans effect?

A

They have very low measured trans influence
- there is extra e- density on 5-coordinate TS (more lone pairs)

Pi-acceptor ligands can relieve this, hence stabilising TS and speeding up reaction

44
Q

Why is solvent exchange used to determine how 3d metal ions undergo substitution?

A

Because there is no net chemical change (∆G = 0 regardless of M)

Therefore there is no confusion between Thermo/kinetic effects

45
Q

How does rate relate to bond strength of ligand sub?

A

Rate is inversely proportional to bond strength

Therefore larger ions with smaller charge exchange fastest
- Cr(III) exchanges slowest

46
Q

How does CFSE affect rate of ligand sub?

A

More CFSE means slower rate

47
Q

What do most inert metal ions have in common?

A

≥3+ and all have large CFSE
- such as Cr(III) which is d3, or low-spin d5/d6 metals

48
Q

Why are activation parameters -ve on left of Periodic Table but +ve on right?

A

Because mechanism changes from Ia to Id from left to right

49
Q

Why does mechanism of ligand sub change from Ia to Id left to right of PT?

A

Early TM ion are larger - more space for associative mechanism
- also have at least one empty t2g orbital

This it’s important as H2O needs an empty orbital to donate its lone pair into

50
Q

How does large CFSE slow down reactions?

A

Consider Cr(III): -1.2∆oct CFSE

If mechanism was A, intermediate would be 7-coordinate and would lose CFSE

If mechanism is D, intermediate would be 5-coordinate and also lose CFSE

Therefore the rate is slowed regardless

(The larger the CFSE of GS, the larger the loss of CFSE on change of geometry)

51
Q

What 3d ions have the largest loss of CFSE?

A

d3 and d6

52
Q

What 4d/5d ions have the largest loss in CFSE?

A

d5 and d6 ions for 4d/5d metals - since all are low spin

53
Q

What 3d ions have the smallest loss of CFSE?

A

d0, d5 and d10
- as these all have ZERO CFSE

54
Q

Why are Cr(II) and Cu(II) so labile?

A

Because of J-T distortion
- 2 waters weakly-bonded, these exchange fast

55
Q

Give 2 examples of metals ions which are used to study rates of ligand exchange - bar Pt(II)?

A

Mainly Co(II) - cheap, inert, low-spin d6

Cr(III) - cheap, inert d3

56
Q

What does +ve ∆V at TS indicate?

A

+ve ∆V at TS indicates D/Id mechanism

57
Q

What can happen in a reaction to increase ∆V at TS?

A

Anion is generated (solvated)

Positive charge on metal increases - solvation increases

These can make ∆V at TS +ve even for reaction that are not D/Id

58
Q

If a reaction is dissociative, what should it depend on on?

A

The identity of X, not Y, as bond breaking is RDS - key step

59
Q

What is the significance of the linear free energy relationship for Co(III) sub?

A

A bigger ∆G at TS means a smaller ∆G (driving force)

This is how kinetics and thermodynamics relate to each other in the free energy relationship

F- dissociates slowest as both L and M are hard for Co(III)

60
Q

What is the mechanism for Co(III) ligand sub?

A

Essentially pure D
- based on the free energy relationship

61
Q

What is required for acid catalysis of ligand substitution?

A

That X is the salt of a weak acid, and has an additional lone pair

62
Q

How does acid catalysis of ligand substitution work?

A

M-X bond is weakened by protonation of second lone pair of X (salt of a weak acid)

63
Q

Does acid catalysis work best at higher or lower pH?

A

Lower

64
Q

At what pH does base catalysis of ligand substitution work best?

A

Higher pH

For X = Cl-, kobs is 10^6 times faster at pH 10 than at pH 7.

65
Q

What role does OH- have in base catalysis?

A

It removes a proton from an amine group, producing a common intermediate
- despite changing Y, a competing nucleophile, the rate of both reactions increases

66
Q

What is the name given to base catalysed hydrolysis?

A

Dcb - dissociative via conjugate base mechanism

67
Q

What is the mechanism for Dcb?

A

CHEM214 (7 decks)

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