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CHEM333 (RBL) > 2. Kinetics > Flashcards

2. Kinetics Flashcards

1
Q

What are the two types of elementary reactions?

A

Unimolecular and bimolecular

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2
Q

In unimolecular reactions, the reaction order is _____ in reactant.

A

first

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3
Q

What is the rate equation for a pseudo-first order reaction involving fluoride and methyl iodide?

A

rate = k[fluoride][MeI]

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4
Q

How is the observed rate constant for a pseudo-first order reaction calculated?

A

k_obs = k[fluoride]

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5
Q

For multistep reactions, the rate equation may not resemble the _____ equation.

A

stoichiometric

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6
Q

What is the rate expression for the cyanide-catalyzed reaction involving benzaldehyde?

A

rate = k[benzaldehyde][CN]

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7
Q

What does the rate limiting step indicate in a reaction mechanism?

A

It is the slowest step that limits the overall reaction rate

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8
Q

What is the significance of the highest energy transition state in a reaction pathway?

A

It determines the overall reaction rate

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9
Q

True or False: The rate of a multistep reaction can be influenced by the steps following the rate limiting step.

A

False

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10
Q

What does an effectively irreversible step in a reaction do?

A

Stops products from reverting to reactants

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11
Q

Fill in the blank: The reaction with the highest energy transition state is usually the _____ step.

A

slowest

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12
Q

What is a common characteristic of synthetic reactions in relation to reversibility?

A

They often have at least one effectively irreversible step

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13
Q

What does AG* represent in a free energy diagram?

A

The difference in energy between the initial state and the highest point

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14
Q

What is the relationship between the energy barriers and the rate of reaction in multistep processes?

A

The rate is determined by the highest energy barrier

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15
Q

True or False: Steps leading up to the rate limiting step do not contribute to the overall rate of the reaction.

A

False

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16
Q

What role do intermediates play in multistep reactions?

A

They are transient species formed during the reaction pathway

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17
Q

What happens to the rate of reaction if one of the reactants is in large excess?

A

It simplifies the rate equation, making it pseudo-first order

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18
Q

What is the significance of the term ‘termolecular’ in reaction kinetics?

A

It refers to collisions involving three molecules, which are rare at normal concentrations

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19
Q

What assumption is made about the rate limiting step in the bromination of ketones?

A

The last step is likely irreversible since bromine doesn’t spontaneously fall off bromoketones.

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20
Q

Write the expression for the rate of formation of product in the bromination of acetone.

A

Rate = k[acetone][Br2]

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21
Q

What is the assumption made by the steady state approximation?

A

It assumes that the concentration of high energy intermediates will be constant and low.

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22
Q

Fill in the blank: The sum of the rates of reactions that produce the intermediate equals the sum of the rates of reactions that _______.

A

destroy it

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23
Q

What happens if the theoretical equation cannot reproduce the experimental results?

A

The proposed mechanism is wrong.

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24
Q

What does it indicate if the theoretical equation can reproduce the experimental rate expression?

A

The mechanism may be right.

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25
Q

What is the significance of enolization in the rate of bromination of acetone?

A

Enolization is the rate limiting step because it is the slowest.

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26
Q

True or False: The kinetic isotope effect confirms that enolization is not the rate limiting step.

A

False

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27
Q

What does a higher value of k2 relative to k1 imply in the context of bromination?

A

Bromination reacts with the enol much faster than with the ketone.

28
Q

What does the energy diagram for the reaction illustrate?

A

Enol formation has the biggest barrier since it is the slow step.

29
Q

What does the rate constant determine in a reaction?

A

The rate constant determines the flux of molecules over the energy barrier

30
Q

In a multistep reaction, what does the rate depend on?

A

The rate depends on the concentration of reactants

31
Q

What is an equilibrium constant K defined as?

A

K = k_forward / k_reverse

32
Q

What does the rate determining step represent in a reaction?

A

The rate determining step corresponds to the highest energy barrier in the reaction

33
Q

True or False: The observed rate constant can be made up of several elementary rate constants.

A

True

34
Q

Fill in the blank: The kinetic isotope effect confirms that _______ is not rate limiting.

A

enolization

35
Q

What is the significance of the steady state approximation in reaction mechanisms?

A

It allows for the analysis of reaction intermediates

36
Q

What is the experimental rate law for the nitration of benzene?

A

Rate = k[benzene][NO2+]

37
Q

What are the components of the rate law for the last step in the nitration mechanism?

A

Rate = k[I]

38
Q

What does the term ‘kinetic analysis’ refer to in the context of chemical reactions?

A

Kinetic analysis refers to the study of reaction rates and mechanisms

39
Q

What is a limitation of kinetic analysis?

A

Different mechanisms can produce identical rate equations

40
Q

What does the term ‘rate constant’ refer to?

A

The rate constant is a proportionality factor in the rate equation

41
Q

True or False: The concentration of a reagent can change during a reaction without affecting the rate constant.

A

False

42
Q

What does the term ‘postulated mechanism’ refer to?

A

The postulated mechanism is a proposed sequence of elementary steps in a reaction

43
Q

What does the height of the energy barrier correspond to in a reaction?

A

The height of the energy barrier corresponds to the free energy difference between reactants and products

44
Q

What determines the rate-limiting step (RLS) in a reaction?

A

The smallest rate constant corresponding to the step

The RLS can be influenced by the stability of intermediates and the mechanism of the reaction.

45
Q

In the context of reaction mechanisms, what is the significance of the partitioning factor?

A

It determines the pathway taken in a reaction when the C-H bond does not break in the rate-limiting step

This factor can indicate how the reaction proceeds under different conditions.

46
Q

What is an example of a stable cationic intermediate in a reaction?

A

The nitration of anthracene

The stability is due to resonance stabilization across the three rings.

47
Q

What is the main product of the base-catalyzed aldol reaction of acetaldehyde?

A

A p-hydroxy ketone

This reaction proceeds through multiple reversible steps.

48
Q

Fill in the blank: The rate-limiting step of the aldol reaction may involve the formation of the _______.

A

enolate of acetaldehyde

49
Q

What is assumed about the second step in the aldol reaction mechanism?

A

It is irreversible

The product does not revert to reactants under the reaction conditions.

50
Q

What is the rate expression for the aldol reaction involving acetaldehyde and water?

A

Rate = k[aldehyde][H2O]

This expression is consistent with the assumption that the reformation of the enolate is faster than its reaction with acetaldehyde.

51
Q

True or False: The equilibrium constant for enolization in the aldol reaction is influenced by the concentration of water.

A

True

The concentration of water in water is approximately 55.6 M.

52
Q

What happens to the energy levels during a reaction?

A

Each energy level is the sum of the energies of all molecules present

The initial state reflects the total energy before the reaction proceeds.

53
Q

In the base-catalyzed aldol reaction, what is the significance of the hydroxide ion (OH-)?

A

It acts as a catalyst in the formation of the enolate

The hydroxide ion is essential for the initial step of the reaction mechanism.

54
Q

What is the role of water in the aldol reaction mechanism?

A

It participates in the reformation of enolate and affects the reaction rate

Water’s presence impacts the kinetics of the reaction significantly.

55
Q

What does KIE stand for?

A

Primary kinetic isotope effect

KIE is measured as the ratio of the rate constants for reactions involving isotopes, typically hydrogen (H) and deuterium (D).

56
Q

What is the energy difference between C-D and C-H bonds?

A

Approximately 5 kJ/mol

This energy difference contributes to the kinetic isotope effect observed in reactions involving these bonds.

57
Q

In the bromination of nitromethane, what does a KIE of kH/kD = 1.5 suggest?

A

Enolization is not the rate-limiting step

This small ratio indicates that the breaking of the C-H bond is not the primary step in the reaction mechanism.

58
Q

Fill in the blank: The strength of the base used in a reaction affects the ______ of the isotope effect.

A

Magnitude

The weaker the base, the longer the isotope effect observed in the reaction.

59
Q

What occurs during the initial step of bromination of nitromethane?

A

Removal of a proton by a base to give an anion

This step is crucial in many synthetic transformations involving nitromethane.

60
Q

True or False: The KIE is always the same regardless of the strength of the base used.

A

False

The KIE varies with the strength of the base, illustrating the influence of reaction conditions on isotope effects.

61
Q

What does it imply if the KIE is largest when the isotope transfer is symmetrical?

A

The isotope effect is more pronounced when the breaking and forming of bonds are balanced in the transition state

This reflects the nature of bond breaking and making in the reaction pathway.

62
Q

What is the typical value of KlEs for aromatic nitration?

A

Usually close to 1

This indicates that the rate-limiting step (RLS) does not involve breaking a C-H bond.

63
Q

What can make the second step of aromatic nitration become rate limiting?

A

When the cationic intermediate is particularly stable

For example, in the reaction of anthracene with NOz’BF4 in MeCN, k/ko = 6.1.

64
Q

What reagents are commonly used to oxidize alcohols?

A

CrOs-based reagents (PCC, PDC, Jones)

These reagents are effective for alcohol oxidation.

65
Q

What does the deuterated compound (CHs)CDOH oxidizing seven times more slowly indicate?

A

Elimination across the C-O bond is the RLS

This observation (k/kD = 7) suggests that the bond breaking step is crucial for the reaction rate.

CHEM333 (RBL) (7 decks)

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