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Chem 202 > Module 5 > Flashcards

Module 5 Flashcards

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1
Q

What are the 2 factors that determine the barrier height of activation energy?

A
  1. energy needed for a collision

2. orientation of that collision

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2
Q

Arrhenius plot

A

ln(k) vs. (1/T)

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3
Q

What is the slope of an Arrhenius plot?

A

m=-Ea/R

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4
Q

Kinetics

A

the rate of a reaction is determined by its path (energy heights, etc)

rates are path dependent

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5
Q

Thermodynamics

A

the free energy difference between reactants and products

path independent

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6
Q

What are the five factors that can influence the rate of a reaction?

A
  1. nature of reactants
  2. concentration
  3. temperature
  4. surface area
  5. prescence of a catalyst
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7
Q

Rate of reaction vs. rate constant

A

Rate of reaction is the change in concentration of reactant/product per time

Rate constant does not depend on the concentrations of reactants/products present and simply gives a constant of proportionality at a specific temperature

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8
Q

Units of rate of reactions

A

M/sec

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9
Q

Units of rate constant

A

change depending on the order of reaction

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10
Q

What are the two plausible mechanisms for an overall alkyl halide substitution?

A

SN1 or SN2

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11
Q

What is the barrier in an SN1 mechanism?

A

carbocation stability

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12
Q

What is the barrier in an SN2 mechanism?

A

steric hindrance

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13
Q

Alkyl halide reactivity in decreasing order (SN1)

A

3º>2º»1º

or conjugated allylic or benzylic

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14
Q

Alkyl halide reactivity in decreasing order (SN2)

A

1º>2º»3º

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15
Q

SN1 nucleophile

A

weak (typically neutral)

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16
Q

SN2 nucleophile

A

strong (typically charged)

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17
Q

SN1 solvent

A

polar protic

  • can H+ donate
  • ex: alchohols
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18
Q

SN2 solvent

A

polar aprotic

  • can’t H+ donate
  • ex: DMSO, acetone
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19
Q

Acetone chemical formula

A

C3H6O

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20
Q

SN1 stereochemistry

A

mix of retention and inversion

racemic

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21
Q

SN2 stereochemistry

A

inversion

22
Q

If you see an alkyl halide what argument are you making?

A

kinetics

looking at intermediate stability primarily

23
Q

If you see a carbonyl group what argument are you making?

A

thermodynamics

looking at most stable products

24
Q

Why in a SN1 reaction can the incoming nucleophile attack from top or bottom?

A

the carbocation intermediate is sp2 hybridized, and therefore it has an empty p-orbital

this p-orbital can be attacked from either the top or bottom

this also leads to the racemic mixture of products

25
Q

Why does a SN2 reaction have an inversion?

A

there is no open p-orbital, so the nucleophile has to attack the sigma-star orbital of the leaving group

this forces the nucleophile to attack the backside of the leaving group and causes an inversion

26
Q

Substitution reaction

A

chemical reaction where one functional group in a chemical compound is replaced for another functional group

27
Q

SN1 meaning

A

nucleophilic substitution reaction where the RDS is unimolecular

28
Q

Sn2 meaning

A

nucleophilic substitution reaction where the RDS is bimolecular

29
Q

What is the LUMO in an SN1 reaction?

A

pi

carbocation is trigonal planar, sp2 hybridized

30
Q

What is the LUMO in an SN2 reaction

A

sigma-star

tetrahedral, sp3 hybridized with no empty p-orbital

31
Q

What are relative rates reported to?

A

relative the slowest rate in a series

32
Q

Why is I- a better leaving group than F- in SN2?

A

F- has a higher pKa and is a more reactive base, therefore it is a worse leaving group

I- is bigger and can spread out the electron density better, which makes it a better leaving group

33
Q

Why would SN1 occur for (CH3)3Br, but not CH3Br or Ch3CH2Br?

A

1-butyl and 2-butyl have carbocations that are not stabilized by hyperconjugation as much and therefore are too high in energy for an SN1 reaction to occur

34
Q

Hyperconjugation

A

partial overlap between the empty p-orbital on the C+ and the sigma bond on the neighboring carbon

stabilizes the carbocation intermediate

35
Q

How can 1º alykyl halides proceed with an SN1 mechanism? (this is an execption)

A

if the carbocation can be stabilized by resonance

if not, the 1º carbocation is too high in energy to proceed with SN1

36
Q

Effect of nucleophilic strength for SN2 reaction

A

stronger nucleophile=greater rate of reaction

higher pKa = stronger nucleophile

When in the SAME row of the periodic table

37
Q

Effect of nucleophilic strength for SN2 reaction when in different rows of periodic table

A

larger molecular has more electron density to overlap during the transition state

this lowers the energy of the transition state and increases the rate of the reaction

38
Q

DMSO

A

dimethyl sulfoxide

(CH3)2SO

*polar aprotic solvent

39
Q

What molecule does hydrogen bonding from the polar protic solvent stabilize?

A

the nucleophile in a SN2 reaction

40
Q

Why does a polar protic molecule slow the speed of a SN2 reaction?

A

the nucleophile is stabilized by hydrogen bonding, which lowers it in energy

therefore, by lowering the energy of the starting product, it takes more energy to reach the peak of activation energy and takes more time

41
Q

Why do SN1 reactions prefer a polar protic solvent?

A

the H-bonding stabilizes the carbocation intermediate

42
Q

Cyclohexane rate of SN2 reaction

A

When leaving group is in the axial position, it is higher in energy than when it is in the equatorial position

therefore, the axial position has starting products higher in energy, which makes less of a energy barrier to overcome to reach the transition state

43
Q

Methyl tosylate

A

good leaving group because incredibly stabilized by resonance

it is also a very strong acid, so a very weak nucleophile leaving group

abbreviated Ts

44
Q

What is something to watch for in Sn2 reaction with CN- as nucleophile?

A

Since CN- is attached to a carbon, the priority of the assymetric carbon attachments could change

ex: CN- is less priority than CSO2

Inversion still happens, but could remain in the same configuration (i.e., S=>S)

45
Q

What do constitutional isomers in the products indicate?

A

a carbocation rearrangment

*SN1 reaction

46
Q

Allylic system

A

double bond next to the carbocation

this stabilizes the carbocation

*SN1

47
Q

Where do curved arrows have to start from for a hydride shift?

A

the sigma bond

not the hydrogen itself!

we are moving electrons!

48
Q

Why is it called a hydride shift instead of a proton shift?

A

hydrogen is bringing its electrons with it from the sigma bond

49
Q

What are the two ways that a carbocation can shift?

A

1) Hydride shift

2) through resonance

50
Q

Why do two successive hydride shifts never occur?

A

first shift must lead to a more stable carbocation

if not, it just wastes too much energy to shift to a new carbocation that is not lower in energy

51
Q

Why do rearrangements occur?

A

makes hyperconjugation of carbocation greater

carbocation is stabilized by having more carbons to conjugate with

Chem 202 (7 decks)

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