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Orgo 1 > Ch 6 - Chemical Reactivity and Mechanisms > Flashcards

Ch 6 - Chemical Reactivity and Mechanisms Flashcards

1
Q

breaking a bond requires an

A

input of energy

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2
Q

enthalpy

A

used to measure the exchange of energy between the bonding MO and their surroundings
- the surrounding molecules must transfer some of their kinetic energy to the system(the bond being broken) to break a bond

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3
Q

delta H =

A

q(at constant pressure)

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4
Q

the change of energy(delta H) for any process is defined as the exchange of kinetic energy, heat(q), between

A

a system and its surroundings at constant pressure

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5
Q

delta H is primarily determined by the amount of energy required to

A

break the bond homolytically

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6
Q

hemolytic bond cleavage

A

generates two uncharged species, radicals, each which bears an unpaired electron

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7
Q

radical

A

the two uncharged species generated from a hemolytic bond cleavage
- drawn with a single barbed arrow

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8
Q

heterolytic bond cleavage

A

generates two charged species, called ions

- drawn with a double barbed arrow(one becomes + while the other becomes -)

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9
Q

bond dissociation energy

A

the energy required to break a covalent bond via hemolytic bond cleavage

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10
Q

heat of reaction

A

the total enthalpy(delta H “naught”) change of a reaction(add up all bond energy changes)

	- + delta naught = system increased in energy
	- - delta naught = system decreased in energy
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11
Q

exothermic

A

the system gives energy to the surroundings(- delta H naught)

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12
Q

endothermic

A

the system receives energy from the surroundings(+delta H naught)

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13
Q

chemists and physicist use opposite signs for delta H

A
  • chemist from the system perspective

- physicist from the surround perspective(how much work can this do?)

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14
Q

entropy

A

the measure of disorder associated with a system

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15
Q

spontaneous

A

a process that involves an increase in entropy

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16
Q

we must take into account the surroundings during a chemical reaction

A

delta Stot(total enthalpy) = delta Ssys + delta Ssurr

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17
Q

in order for a process to be spontaneous the total entropy must

A

increase

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18
Q

the entropy of a reaction can decrease IF

A

the entropy of the surroundings increases in a way that offsets the decrease

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19
Q

two dominant factors affecting the delta S sys

A
  • one mole(AB) of reactant -> two moles of product(A + B ups entropy)
    - a cyclic compound -> acyclic(more freedom of motion and conformations ups the entropy)
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20
Q

entropy is the one and only criterion that determines

A

whether or not a chemical reaction will be spontaneous

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21
Q

delta S tot =

A

delta Ssys + delta Ssurr

  • the total must always be positive(either component can be negative but entropy is always increasing
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22
Q

delta Ssurr =

A

-(deltaHsys/T)

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23
Q

delta Stot =

A

(-deltaHsys/T) + deltaSsys

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24
Q

gibbs free energy

A
  • -TdeltaStot = deltaG
    - delta G = deltaH – (T)(deltaSsys)
    - delta G = deltaH +(-T)(deltaS)
    - in some cases this nonstandard presentation will allow for a more efficient analysis of the competition between two terms
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25
Q

if deltaStot must be positive then

A

deltaG must be negative for a process to be spontaneous

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26
Q

in order for a process to be spontaneous, deltaG for that process must be

A

negative

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27
Q

Keq =

A

[products]/[reactants]

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28
Q

deltaG =

A
  • RT(lnKeq)
    - R = 8.314 J/mol*K
    - T = kelvin
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29
Q

delta G is negative then products favored

A

Keq>1

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30
Q

delta G is positive then reactants favored

A

Keq

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31
Q

in order for a reaction to be useful(products to dominate over reactants) then delta G must

A

be negative thus Keq>1

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32
Q

thermodynamics

A

the study of how energy is distributed under the influence of entropy
- the thermodynamics of a reaction specifically refers to the study of relative energy levels of reactants and products

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33
Q

spontaneous does not mean a reaction will occur suddenly

A

nothing to do with speed of a reaction but about if the reaction favors the formation of products

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34
Q

kinetics

A

the study of reaction rates

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35
Q

rate equation

A

the rate of reaction is described as this

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36
Q

Rate =

A

k[reactants]

		- k = rate constant
		- [reactants] = concentration of reactants
37
Q

first order reaction

A

the sum of exponents is 1

- rate = k[A]

38
Q

second order reaction

A

the sum of exponents is 2

- rate = [A][B]

39
Q

third order reaction

A

the sum of exponents is 3

- rate = [A]^2[B]

40
Q

Factors Affecting the Rate Constant

A
  • Energy of Activation(Ea)
    - Temperature(Celsius)
    - Steric Considerations
    - Catalyst and Enzymes
41
Q

Factors Affecting the Rate Constant

Energy of Activation

A
  • the energy barrier(the hump graphically) between the reactants and products
    - represents the minimum amount of energy required for a reaction to occur between two reactants that collide
    - at any specific temperature the reactants will have a specific average kinetic energy(some higher and some lower)
    - a low Ea(small barrier) will lead to a fast reaction
42
Q

Factors Affecting the Rate Constant

Temperature(in Celsius)

A
  • raising the temperature(increasing kinetic energy) will increase the rate of a reaction
    - rule of thumb:
    - a 10 degree Celsius increase will cause the rate to double
43
Q

Factors Affecting the Rate Constant

Steric Considerations

A

geometry and the reactants orientation during a collision can have an impact on the rate of reaction

44
Q

Factors Affecting the Rate Constant

Catalyst and Enzymes

A
  • catalyst – a compound which can speed up the rate of a reaction without itself being consumed by the reaction
    - catalyst provide an alternative pathway with a smaller Ea
    - catalyst do not change the energy of the reactants or products
    - the equilibrium is not affected by the presence of a catalyst only the rate of the reaction(lowers the Ea)
    - enzyme – naturally occurring compounds that catalyze very specific biologically important reactions
45
Q

kinetics and thermodynamics are two entirely separate concepts

A
  • don’t confuse them
    - kinetics = rate of reaction
    - thermodynamics = equilibrium concentrations of reactants and products
46
Q

often that one reaction pathway is both thermodynamically and kinetically favored

A

can be two different products though(one thermodynamically favored while the other is kinetically favored)

47
Q

intermediates are represented as

A

all local minima(valleys)

48
Q

transition states are represented as

A

all local maxima(peaks)

49
Q

transition state

A

a state through which the reaction passes

	- cannot be isolated in this state
	- bonds are literally being broken and/or formed in this high energy state simultaneously
50
Q

intermediates

A

have a certain lifetime and are not in the process of forming or breaking bonds
- very often encountered in reactions

51
Q

Hammond Postulate:

in an exothermic process the transition state is

A

closer in energy to the reactants and therefore the structure of the transition state more closely resembles the reactants

52
Q

Hammond Postulate:

in an endothermic process the transition state is

A

closer in energy to the products and therefore the structure of the transition state more closely resembles the products

53
Q

ionic reactions(polar reactions)

A

involve the participation of ions as reactants, intermediates, or products

	- typically intermediates
	- around 95% of reactions in Orgo 1
54
Q

ionic reactions occur when

A

one reactant has a site of high electron density and the other has a site of low electron density

55
Q

nucleophile

A

an electron rich center

	- “nucleus lover”
	- characterized by its ability to react with a positive charge or partial positive charge
56
Q

electrophile

A

an electron deficient center

	- “electron lover”
	- characterized by its ability to react with a negative charge or partial negative charge
57
Q

being able to identify the nucleophilic and electrophilic centers in any compound is one of the most important skills in all of organic chemistry

A

being able to predict the flow of electron density is vital

58
Q

nucleophilic center is an electron rich atom that is capable of donating a pair of electrons

A

Lewis base is synonymous with nucleophilic

59
Q

any atom that possesses a

A

localized lone pair can be nucleophilic

60
Q

pie bonds can function as

A

nucleophiles(area of high electron density)

61
Q

polarizability

A

the ability of an atom to distribute its electron density unevenly in response to external influences

	- directly related to the size of the atom(and subsequently the number of electrons that are distant from the nucleus)
	- more electrons = more polarizable
62
Q

electrophilic center is an electron deficient atom that is capable of accepting a pair of electrons

A

Lewis acid is synonymous with electrophile

63
Q

carbocation

A

has an empty p orbital which functions as a site that can accept a pair of electrons, rendering the compound electrophilic

64
Q

inductive effects of negatively charged atoms like chlorine can make

A

a carbon atom electrophilic

65
Q

the tail of every curved arrow show where the electrons

A

are coming from

66
Q

the head of every curved arrow shows where the electron(s)

A

are going

67
Q

Patters of electron pushing

A
  • Nucleophilic attack
    - loss of a leaving group
    - Proton transfers
    - Rearrangements
68
Q

Patters of electron pushing

nucleophilic attack

A

a nucleophile attacking an electrophile

69
Q

Patters of electron pushing

Loss of leaving group

A

even if a chain of arrows must be used only combination shows one arrow pushing patterns to kick off a leaving group(typically halides)

70
Q

Patters of electron pushing

Proton transfer

A
  • 2 curved arrows minimum

- can be used for either something getting protonated or deprotonated

71
Q

Patters of electron pushing

Rearrangements

A
  • many types
    - carbocations is one type
    - neighboring alkyl groups will stabilize a carbocation through hyperconjugation
72
Q

Patters of electron pushing

Rearrangements

hyperconjugation

A

the bonding MO associated with a neighboring CH bond slightly overlaps the empty p orbital of a carbocation by placing some of its electron density in the empty p orbital

73
Q

Patters of electron pushing

Rearrangements

hydride shift

A

involved the migration og H^-(hydrogen atom with an extra electron(2 electrons total)

74
Q

Patters of electron pushing

Rearrangements

primary, secondary, and tertiary refer to the number of alkyl groups attached directly to the positively charged carbon atom

A

tertiary carbocations are more stable(lower energy) than secondary which are more stable than primary

75
Q

Patters of electron pushing

Rearrangements

methyl shift

A

a methyl group shifts to convert a secondary carbocation into a tertiary carbocation
- the methyl group must be attached to a carbon adjacent to the carbocation

76
Q

all ionic mechanisms, regardless of complexity, are different combinations of the four characteristic patterns of electron pushing

A
  • proton transfer
    - loss of a leaving group
    - carbocation rearrangement
    - nucleophilic attack
77
Q

concerted process

A

when using two arrow pushing patterns simultaneously

- different than stepwise mechanisms

78
Q

Common arrow pushing sequence

A

Nucleophilic Attack -> loss of leaving group -> proton transfer

79
Q

always avoid drawing sloppy arrows

A

always avoid drawing sloppy arrows

80
Q

never place the tail of a curved arrow on

A

a positive charge

81
Q

the head of an arrow should always be placed to show either

A

the formation of a bond or the formation of a lone pair

82
Q

C,N,O can NEVER have more than

A

an octet(four orbitals)

83
Q

two common carbocation rearrangement types

A
  • hydride shifts

- methyl shifts

84
Q

allylic carbocation

A

when a carbocation has the positive charge located in an allylic position

85
Q

Reversible and Irreversible Reaction Arrows

nucleophilic attack

A

a reversible reaction arrow is generally used if the nucleophile is capable of functioning as a good leaving group

-  an irreversible reaction arrow is used if the nucleophile is a poor leaving group
86
Q

Reversible and Irreversible Reaction Arrows

Loss of leaving group

A
  • a reversible reaction arrow is generally used if the leaving group is capable of functioning as a good nucleophile
    - most leaving groups in Orgo 1 will be able to function as nucleophiles
87
Q

Reversible and Irreversible Reaction Arrows

Proton Transfer

A
  • technically all are reversible
    - generally speaking irreversible reaction arrows are used for reactions in which the acids differ in strength by more than 10 pKa units
    - if the pKa value is between 5 to 10 units either reversible or irreversible reaction arrows might be used depending on the context of discussion
88
Q

Reversible and Irreversible Reaction Arrows

Carbocation Rearrangement

A
  • generally drawn as irreversible processes

- the energy difference between secondary and tertiary is usually significant

Orgo 1 (10 decks)

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