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chemistry 1A > Fundamental organic reactions > Flashcards

Fundamental organic reactions Flashcards

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1
Q

nucleophile definition

A

a negative ion or electron rich species which has an electron pair in its HOMO which it will donate to form dative covalent bonds.

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2
Q

electrophile

A

a positive ion or electron deficient species which will have a low energy antibonding or unoccupied P orbital, which will allow it to accept non bonding electron pairs to form dative covalent bonds.

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3
Q

curly arrow starting point

A

nucleophiles electron pair

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4
Q

curly arrow end point

A

electrophiles LUMO

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5
Q

strength of nucleophiles

A

n
π
σ

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6
Q

strength of electrophiles

A

P
π*
σ*

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7
Q

types of nucleophile

A

lone pair
negative charge (lone pair)
double bonded (π electron pair will act as the HOMO)

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8
Q

types of electrophile

A

positive ions
neutral atoms with a delta positive dipole from a polar covalent bond
carbonyls and imines

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9
Q

checking mechanisms

A

a reaction mechanism must have the same overall charge and ratio of species on both sides of the reaction.

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10
Q

SN1 rate order explanation

A

SN1 is 1st order because only the concentration of the molecule containing the leaving group involved will affect the rate of reaction.

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11
Q

SN2 rate order explanation

A

SN2 is 2nd order because both the nucleophile and the molecule containing the leaving groups concentrations will alter the rate of reaction.

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12
Q

SN1 rate determining step

A

the leaving group cleaving the bond and retaining the electrons.

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13
Q

SN2 rate determining step

A

the nucleophile attacking the σ* orbital of the C - X bond followed by the leaving group cleaving off and retaining an electron pair.

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14
Q

SN1 energy diagram

A

reactants
large activation energy
intermediate
small activation energy
products

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15
Q

SN2 energy diagram

A

reactants
activation energy
products

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16
Q

primary haloalkane mechanism

A

SN2

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17
Q

secondary alkyl halide mechanism

A

SN2 or SN1

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18
Q

tertiary alkyl halide mechanism

A

SN1

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19
Q

methyl halide mechanism

A

SN2

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20
Q

reason for a tertiary alkyl halide being an SN1mechanism

A

the alkyl halide has sufficient steric hinderance to allow a carbocation (tertiary) to form due to hyperconjugation, and also to prevent the nucleophile from directly attacking the carbon halogen bond.

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21
Q

reason for primary alkyl halides SN2 mechanism

A

the alkyl halide will not have enough steric hinderance to form a carbocation, due to no hyperconjugation, and the lack of bulk around the molecules will mean that the nucleophile can directly attack the carbon halogen bond.

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22
Q

SN1 mechanism

A

check book

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23
Q

SN2 mechanism

A

check book

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24
Q

which orbital does the nucleophile attack in SN2

A

σ* orbital of the C - X bond

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25
Q

which orbital does the nucleophile attack in SN1

A

unoccupied 2P orbital in the carbocation

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26
Q

what is formed halfway through an SN1 mechanism

A

an intermediate

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27
Q

what is formed halfway through an SN2 mechanism

A

a transition state

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28
Q

methods of speeding up an SN1 mechanism

A

hyperconjugation/positive inductive effect
P - π conjugation
P - π conjugation

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29
Q

methods of speeding up SN1 mechanisms explanation

A

the method of speeding up the reaction mechanism is by stabilizing the intermediate carbocation so it will form more easily.

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30
Q

hyperconjugation effect on SN1

A

parallel σ bonds to the positive unoccupied 2P orbital will donate electron density which will more evenly distribute the charge around the molecule and stabilize the carbocation.

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31
Q

p - n conjugation effect on SN1

A

the non bonding electron pair on the heteroatom will be donated to the unoccupied P orbital of the bond adjacent, which will allow for the charge from the carbocation to be transferred between the carbon and the heteroatom, which will spread the charge out and stabilize the carbocation.

32
Q

P - π resonance effect on SN1

A

electron pairs from the π orbital will be donated to the adjacent bond which will redistribute the charge to a different carbon, allowing the charge to be spread out and stabilizing the carbocation.

33
Q

types of carbon atom which can cause
p - π conjugation

A

allylic carbons and benzylic carbons
the benzylic carbon can only receive a nucleophile at the benzylic carbon, however an allylic molecule can receive a nucleophile at any +ve charge location.

34
Q

SN1 molecules

A

tertiary alkyl halides - hyperconjugation
secondary alkyl halides - hyperconjugation
allylic halides - p - π resonance
benzylic halides - p - π resonance
heteroatom one bond away from carbocation - p - n resonance.

35
Q

speeding up SN2 mechanisms methods

A

carbon double bond or carbon oxygen double bond

36
Q

carbon double bond and carbon oxygen double bond method of speeding up SN2

A

the carbon double bonds will have a π orbital - need to finish this when I understand it.

37
Q

leaving group strength order

A

RSO3 > I- > Br- > Cl- > H2O

38
Q

OH- leaving group strength solution 1

A

OH- is a very weak leaving group and wont cleave by itself, so it should be protonated to form a water leaving group which will cleave easily due to high electronegativity and its positive charge.
this is done by using a strong acid.

39
Q

OH- leaving group strength solution 2

A

alternatively you could remove the hydrogen from the OH- and bond the O to a new element such as P or S. This will allow the leaving group strength to increase.

40
Q

SN1 stereoisomerism

A

will produce a racemic mixture if the carbon of the C - X bond was a chiral centre

41
Q

SN1 racemic mixture explanation

A

the nucleophile can attack the carbocation from above or below the unoccupied 2P orbital, which will result in the formation of two enantiomers of equal proportions.

42
Q

SN2 stereoisomerism explanation

A

the nucleophile can only attack the substrate at one orientation, at the σ* orbital of the C - X bond, which will result in only one enantiomer being produced by the reaction. This is called stereospecific

43
Q

stereospecific definition

A

where one stereoisomer substrate will form a specific stereoisomer product exclusively.

44
Q

stereoselective definition

A

where one stereoisomer substrate will produce a mixture of two stereoisomer products with one being the major product and one being the minor product.

45
Q

molecules acting by and SN2 mechanism

A

methyl halides - low steric hinderance
primary alkyl halides - low steric hinderance
α carbonyl molecules - further knowledge needed

46
Q

factors promoting SN1 over SN2

A

polar solvent - stabilize carbocation
steric hindrance - stabilize carbocation

47
Q

factors promoting SN2 over SN1

A

less polar solvents
and lower steric hindrance

48
Q

steric hindrance effect on reaction rate of nucleophilic substitution

A

the large electron density of the steric hindrance will be more stable the further apart each of the branches gets, which will mean that the SN1 carbocation will be more stable and the SN2 transition state will be less stable.
slows down SN2 and speeds up SN1.

49
Q

Elimination reaction types

A

E1, E2 and E1cB

50
Q

elimination reaction definition

A

where a nucleophile will remove a proton and cause the formation of a carbon to carbo n double bond, forcing off a leaving group as well.

51
Q

E1 mechanism

A

check book

52
Q

E2 mechanism

A

check book

53
Q

E1cB mechanism

A

check book

54
Q

elimination reaction reagent

A

a nucleophile which acts as a base

55
Q

rule for nucleophile behavior and reaction mechanism

A

more basic nucleophiles will cause E reactions to occur
‘nucleophilic’ nucleophiles will cause nucleophilic substitutions.
this is because in elimination the base removes a hydrogen, whereas in nucleophilic substitution the nucleophile attacks the delta positive carbon.

56
Q

why are elimination reactions competing reactions with nucleophilic substitutions

A

because both reactions require a nucleophile as its reagent and the reaction mechanism depends on how the nucleophile acts.

57
Q

E1 rate equation

A

E1 rate = k[ leaving group ]

58
Q

E2 rate equation

A

rate = k [ leaving group] x [ base ]

59
Q

E1cB rate equation

A

rate = k [ conjugate base]

60
Q

factors promoting Elimination over nucleophilic substitution

A

more basic nucleophile
greater steric bulk of nucleophile
increasing temperature

61
Q

pKa relationship with basicity

A

the higher the pKa the stronger the base

62
Q

how to identify an E1cB reaction

A

if it contains a carbonyl 2 carbons over from the leaving group its an E1cB

63
Q

how does a more basic nucleophile promote elimination

A

the nucleophile will act to remove the hydrogen from the adjacent carbon instead of attacking the carbocation/delta positive carbon, which will promote the elimination pathway.

64
Q

how does temperature promote elimination reactions

A

need more info

65
Q

how does greater steric hinderance promote elimination reactions

A

the steric hinderance will reduce the ability of the nucleophile to attack the delta positive carbon/ carbocation, resulting in the elimination mechanism being promoted due to the nucleophilic substitution not being possible.

66
Q

factors promoting E2 over E1

A

less polar solvent - stabilize the transition state
stronger bases - will be able to remove the hydrogen 1st - which is a key part of the mechanism
more bulky base - need more info

67
Q

factors promoting E1 over E2

A

more polar solvent - stabilize the intermediate carbocation
weaker base - will mean the hydrogen will be removed after the carbocation is formed.
less bulky base - need more information

68
Q

bulky nucleophile for elimination

A

DBU
N - C = N
with two different rings attached

each nitrogen will have a non bonding electron pair which it will donate to form a mesomeric effect.

69
Q

tosylate structure and purpose

A

benzene ring attached to SO3-
used to improve the leaving group of OH-

70
Q

mesylate structure and purpose

A

methyl group attached to SO3-
used to improve the leaving group of OH-

71
Q

stereoisomerism requirement in elimination

A

for stereoisomers to form in elimination the carbon bonded to the leaving group must be a stereogenic center.

72
Q

regioisomers requirement in elimination

A

for regioisomers to form there must be two distinct hydrogens available in different positions which can be lost during the elimination mechanism.

73
Q

E1 orientation for mechanism to occur

A

The E1 mechanism requires a hydrogen to be facing in the same plane as the unoccupied 2P orbital of the carbocation, meaning a hydrogen rotates to the down position

74
Q

E2 orientation for mechanism to occur.

A

for the E2 mechanism to proceed the molecule must be anti periplanar with the C -H σ orbital and the C - X σ* orbital being parallel.

75
Q

elimination reactions type of stereoisomerism for enantiomers

A

stereoselective producing the E geometric isomer as the major product.

76
Q

elimination reactions type of stereoisomers for diasterioisomers

A

stereospecific producing one corresponding enantiomer

chemistry 1A (8 decks)

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