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organic chem > lecture 6: substitution > Flashcards

lecture 6: substitution Flashcards

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1
Q

what is homolyssi

A

bond dissociateion and each atom takes one electron (forms radicals)

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2
Q

what is heterolyssi

A

bond breaks and 1 atom takes both electrons

forms ions

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3
Q

heterolysis can lead to what two types of carbon ions

A

carbocations

carboanions

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4
Q

if c is more electroneg what tpy of ion do you have

A

carbanon

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5
Q

if c is less electroneg what tpy of ion do you have

A

carbocation

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6
Q

carbocations are acids or basis

A

they are acids since they are electron deficient

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7
Q

carboanions are lewis acids or bases

A

bases (wants to lose e pair)

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8
Q

electrophiles are acids or bases

A

acids (seeking negative charge (

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9
Q

true or false: does the C need to have full postiive charge

A

no it can have partial charge due to bond polarity

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10
Q

nucleophiles are seeking positive or negative charges

A

position chair

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11
Q

are nucleophiles bases or acids

A

bases (seeking positive chair)

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12
Q

what are the 4 main raction types

A

sub
elim
addition
rearrange

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13
Q

in addition, do you form or lose bonds

A

go from double to single

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14
Q

in eliminattion are you adding or losing bonds

A

you are forming bonds

single to double

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15
Q

how can an organic compound act as a base

A

as long as it has an unshaired electron pair it can be a base

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16
Q

true or false: lone/unshared electron pairs are the only things available for basicity

A

false, pi bonds or alkenes can also be available for protenattion

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17
Q

what is an alkyl halide

A

it has a halogen bonded to an sp3 hybridized (tetrahedral) carbon aotm

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18
Q

alkyl halides make bonds polairized or neutral

A

polarizes since the halongen is more electrogenegative

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19
Q

what is the classification of carbons based on

A

how many c groups attached to the c directly bonded to the halogen

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20
Q

wwhatt happens to halogen size as we go down periodic table

A

atomic size increases

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21
Q

what happens to the carbon halogen bond length and strength as we go down the periodic table and why

A

the carbon halogen bond length increases and the strength decreases because larger attoms are more polarizable and have less overlap (weaker bond)

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22
Q

CI bond is weaker or stroenger than CF

A

weak

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23
Q

SN2 is concerted or multistep

A

concerted

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24
Q

sn1 is concerted or mult step

A

mult step

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25
Q

what is the rate equation for SN2

A

rate= K (NU)(SUB)

depemnds on conc of nu and sub

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26
Q

what is the preference for SN2 reactions in terms of substrates

A

methyl 1 2 3

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27
Q

why is methyl preffered for sn2 recations

A

beacuse less steric hinderance

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28
Q

wgat is the rate equation for sn1 reactions

A

rate= k (sub)

only depends on the concerntration of substrate

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29
Q

what is the rate limiting step in sn1 reactions

A

the first step (formation of the carbocations)

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30
Q

true or fals: sn2 reactions are solvolysis reactions

A

false , sn1 are

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31
Q

true ro false: sn2 can attack from any side

A

false, only backside attack

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32
Q

rtue or false: sn1 reactions can approach from either side and why

A

treu because the carbocation is trtigonal planar

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33
Q

what must you do with a neutral NU

A

3 steps, must deprotonate

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34
Q

what is the substrate preference for sn1

A

3 2 1 methyl

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35
Q

why is 3 degree more preffered in sn1 reactions

A

because it forms the best stable carbocations

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36
Q

rearragnements can occur in what type of reactions

A

sn1

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37
Q

in a nucelophilic sub rxn, what replaces what

A

the nu replaces the LG

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38
Q

the nucleophile is always a what

A

a lewis base (electron pair donor)

either negative or neutral

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39
Q

what is the main mechanism of substitution reactions

A

the nuclophile is a lewis base and donates the electron pair to the substrate

the bond b/w the C and LG breaks giving both electrons from thebbond to the LG (heterolotically)

the NU uses its electron pair to form a new covalent bodn wit hthe substraite

the LG gains the pair of electrons

40
Q

in substaition reactions does the LG dissoacted heterolytically or homolytically

A

hetero

41
Q

in sn1 reactions is there inversion of stereochem

A

no only sn2

42
Q

how does the NU approach in an SN2 reaction

A

nu approaches the sub from the back side (180) into the anti bonding portion

43
Q

what reaction results in inversion of sterochem

A

sn2

44
Q

what is the reactivty of the alkyl halifes in sn2

A

methyl 1 2 3 because of steric hinderance

45
Q

what are aryl halides

A

halogen bonded to sp2 carbons or to aromatic rings

46
Q

is sn2 bimol or unimol

A

bimol (requires 2 steps in the rate limiting step)

47
Q

the rate of rnx in sn2 is dependient on what two facors

A

the concetrnation of nucleo and electron (substates)

48
Q

what is basicity

A

measure of how well a compound shres its lone pair (tendency of conj acid to lose a proton)

49
Q

what is nucleophilicity

A

measure of how readily a copound is able to attach to e deficient atom

50
Q

what are the 3 factors affecting NU reactiviety

A

periodic trends
basicicity for same element
steric hinderance decreases nucleo

51
Q

what is the NU reactivity trend across the table

A

basicity of conj base increases from left to right NU gets stronger going to the left

52
Q

what is the NU reactivity trend down the table

A

nuc increases down a group in POLAR PROTIC SOLVENTS

more polarizable = better nucleophile

53
Q

more polarixable is a better or worse LG

A

better

54
Q

wha tis NU trend in polar protic solvent

A

NU increases going down table (incrase side)

55
Q

what is NU trend going down table in polar APROTIC

A

NU decreases as you go down

56
Q

what is the trend of basiciity for the same element in terms of NU reactivity

A

nucleo directly correlates with incrase lewis base strength when all compounds have the same attacking atom

57
Q

list the relative nuclophilicity from streong to weak

A 
RS-
I-
CN-
CH3O-
Br-
NH3
Cl-
F-
ch3oh
58
Q

what is the trend bwetween NU and steric hinderance

A

steric hinderance decrease nucleohpilicity

59
Q

rate of reaction is not affected by leaveing group (true or F)

A

false, it is dependant on relative stability of LG

60
Q

why does I have faster reaction rate than F

A

beacuse it is more stable (larger therefore can distrubute negative charge better)

61
Q

rank these as least to most reactive (RI, RCL, RF, RBR)

A

RF
RCL
RBr
RI

62
Q

why is HI tthe most stable conj base

A

most reactive due to long and weak bond

63
Q

what determines whether or not something will be a good LG

A

the stability of the conjugate base determines

64
Q

a LG that is a stronger base than the approaching group will do what

A

a LG that is a stronger base than the approaching NU will not be displaced

65
Q

in sn2 reactions, which substrate have no measurable rate of reaction

A

3 beacuse of steric hinderance

66
Q

in sn1 reactions, which substrate have no measurable rate of reaction

A

1

3 is the highest because 3 carbocations are the most stable because of hyperconj and inductive effect

67
Q

true or false: sn1 is bimol

A

uni

68
Q

alkyl halide reactivity is the same or oppsoite for sn1 and sn2

A

same beacuse weaker base is held less tightly to the carbon atoms

69
Q

a carbocation is what hybridiization

A

sp2 (planar

70
Q

what is the relative stability of carbocations related to

A

number of alkyl groups attached to the positive carbon (tertiaty most stable)

71
Q

explain why more alkyl groups makes a more stable carbocations

A

1) inductive effect: alkyl groups donate electrong density to stabilize the positive charge)

2) hyperconjugation (dumping e density from sigma bonds into empty p orbitals of carbocations)
=negative charge paritally dispersed to the postive C bnecause more overlap with empty p orbitals

72
Q

what is the facial preference for sn1 reactions

A

due to trigonal planar sym of carbocations in sn1 there is no facial preference

73
Q

sn1 will lead to racemic mis=xuteres?

A

true

74
Q

what is racemization

A

takes place when the rnx causes a chral molecule to be converted to achiral

75
Q

what is solvolysis

A

sibstituion where the nucleophile is a molecule of the solvent

76
Q

rate of sn1 rxn is unaffected by what and why

A

unaffected by NU

sicne bu does not particapte in rate limiting step (formation of carbocation)

77
Q

the strenght of the nu depends on what structural features

A

1) a neg charge nu is always streonger than conj acid (ho stronger than h20)
2) in a group of nucleo where the atom is the same, nucleo corrleates with basicity

78
Q

sn2 reactions are favoured by what solvents and gvie examples

A

favoured by polar aprotic (acetone, DMF, DMSO)

79
Q

sn1 reactions are favoured by what solvents and give examples

A

polar protic solvenets

ETOH, MEOH, H20)

80
Q

explain why polar protic solvents are bad for sn2

A

the sovlent will sovlate the NU (stabilizing and making it less reactive)

strong hydrogen bonding will encumber the NU and the NU must shed it before raecting )

81
Q

in the same column, what is the relationship between atom size and solvent nu interaction

A

smaller atoms have more solvent nu interanctions

82
Q

what is halide nucleophilicity in protic solvents

A

I bettwe than br, cl, F

83
Q

why is I a better nucleophile than F

A

it is more polarizble and less electronegative therefore there is lesss solvent nu interactions

84
Q

what is the relationship between polar protic solvents and sn1

A

sn1 prefers protic solvents because it forms H bodns with the carbos =stabilizing it (lowering energy of transition state)

85
Q

rate of sn2 reactions genereally increase or decreases with polar aprotic

A

increase

86
Q

why does sn2 favour polar aprotic

A

because they do not have any H bonded to highly electromneg atoms

nucleos are only weakely attracted to the solvent

nu do not need to break any strtong solvent intcatoions

87
Q

what is the order of halide reactivity in APROTIC SOLVENTS

A

F
CL
BR
I

88
Q

compare substrate factor for sn1 vs sn2

A 
sn1= prefer 3
sn2= prefer methyl
89
Q

compare nu factor for sn1 vs sn2

A 
sn= weak nucleophile or neutral (solvolysis9
sn2= strong lowis base
90
Q

compare solvent factor for sn1 vs sn2

A 
sn1= polar protic (eg: water, alcohol)
sn2= polar aprotic (DMSO, acetomo, DMF)
91
Q

compare LG factor for sn1 vs sn2

A

same for both

I better than Br, Cl , F

92
Q

true or false: vinylic and aryl halidies are unreactive in sn1 and sn2

A

true

93
Q

what is an alkenyl halide / vinylic halide

A

a halogen bonded to 1 carbon of a couble bond

94
Q

what is a aryl/phenyl halide

A

halogen attached to a benzyne ring

95
Q

what are aryl/vinyl halides unreactive in sn1

A

due to instability of their carbocations

96
Q

what are aryl/vinyl halides unreactive in sn2

A

beacuse the electrons of the ring or double bond repele the NU

organic chem (4 decks)

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