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Orgo 1 > Ch 7 - Substitution Reactions > Flashcards

Ch 7 - Substitution Reactions Flashcards

1
Q

substitution Reaction

A

a reaction where one group is exchanged for another

- X group is replaced with Y but the rest of the structure remains the same

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2
Q

elimination reaction

A

a reaction one group is removed via the formation of a pie bond

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3
Q

substitution and elimination reactions are often in

A

competition with each other

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4
Q

a substitution reaction can occur when

A

a suitable electrophile is treated with a nucleophile

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5
Q

substrate

A

term used to describe the electrophile in a substitution reaction

	- leaving group – A group capable of separating from the substrate
		- must be present for a substrate to function
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6
Q

a leaving group serves 2 functions with the substrate

A
  • withdraws electron density via induction creating an electrophilic carbon
    - stabilizes any negative charge which may develop as a result of the leaving group separating from the substrate
    - Halogens are very common leaving groups(fill octet easily)
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7
Q

when a chirality center is present the configuration must be indicated

A

at the start of the same in (R/S)

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8
Q

haloalkane

A

the systemic IUPAC name treats a halogen as a substituent

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9
Q

alkyl halide(organohalide)

A

the common name treats the compound as an alkyl substituent connected to a halide

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10
Q

alpha(fish sign) position

A

the carbon connected directly to the halogen

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11
Q

Beta(B) positions

A

carbon atoms connected to the alpha position

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12
Q

each carbon is described in terms of its proximity to

A

the halogen

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13
Q

an alkyl halide will have

A

1 alpha position and up to 3 beta positions

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14
Q

alkyl halides are classified as

A

primary(1 naught), secondary(2 naught), or tertiary(3 naught) based on the number of alkyl groups connected to the alpha position

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15
Q

organohalides arevery stable and often toxic

A

persist and accumulate in the environment

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16
Q

PCBs(polychlorinated biphenyls) accumulated in the environment and threated significant portions of wildlife and were banned

A

organhalide

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17
Q

organohalides serve a variety of functions in living organisms

A
  • defense mechanisms(poison = chemical warfare)

- hormones(chemical messages to specific target cells)

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18
Q

not all halogenated compounds are toxic

A

sucralose(Splenda - artificial sweetener) is a halogenated compound we eat

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19
Q

every substitution reaction involved at least 2 of the 4 arrow pushing patterns

A

nucleophilic attack and loss of a leaving group

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20
Q

in a concerted process

A

nucleophilic attack and loss of the leaving group occur simultaneously

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21
Q

in a stepwise process

A

loss of the leaving group occurs first followed by nucleophilic attack

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22
Q

remember:

NaOH is a hydroxide ion(HO-)

A

the Na+ is just a counter ion and can largely be ignored

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23
Q

always draw all groups of a tertiary carbocation as

A

far apart as possible

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24
Q

Sn2 mechanism

rate = k[substrate][nucleophile]

A
  • 2nd order reaction
    - doubling the concentration of the substrate or nucleophile will cause the reaction rate to double
    - there must be a step in which the nucleophile and substrate collide
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25
Q

bimolecular

A

a step which two chemical entities collide

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26
Q

Sn2

A
  • S = substitution
    - n = nucleophilic
    - 2 = bimolecular
    - consistent with a concerted mechanism
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27
Q

Sn2 mechanism

when the alpha position is a chirality center a change in configuration is generally observed

A

and S reactant will become an R product

28
Q

Sn2 mechanism

inversion of configuration

A

when a reactant is S configuration and results in an R configuration product(or the other way around)
- “Walden” inversion

29
Q

Sn2 mechanism

back-side attack

A

the requirement for inversion of configuration means the nucleophile can only attack from the back side(the side opposite the leaving group)

30
Q

Sn2 mechanism

2 explanations for inversion of configuration through a back side attack

A
  • the lone pairs of the leaving group create regions of high electron density that effectively block the front side of the substrate
    - nucleophile can only approach from the back side
    - according to MO theory, an attack from the back will allow the nucleophiles HOMO and the LUMO of the electrophile to overlap creating an efficient bond
    - from the front there is a node which will not bond
31
Q

the Sn2 process(inversion of configuration) is consistent with

A

a concerted mechanism because as the loss of leaving group occurs and the nucleophile attacks the backside the chirality must flip like an umbrella in the wind

32
Q

stereospecific

A

the configuration of the product is dependent on the configuration of the starting material

33
Q

the higher the Ea for the starting alkyl halide the slower the reaction

A
  • Me > primary > secondary > tertiary
    - Me is the most reactive
    - H will hold electrons less strongly than tertiary with 3 R groups
    - H will have lower Ea as its an easier bond to break
    - less sterically hindered
34
Q

stepwise reaction

A
  • loss of leaving group to form a carbocation intermediate

- nucleophilic attack on the carbocation intermediate

35
Q

rate determining step(RDS)

A

the step with the highest energy transition determines the rate for the overall process

36
Q

unimolecular

A

the step involved only one chemical entity

	- first order reaction
		- rate = k[substrate]
37
Q

Sn1

A
  • s = substitution
    - n = nucleophilic
    - 1 = unimolecular
38
Q

unimolecular means there is only one chemical entity participating in the rate determining step of the reaction

A

the rate of the reaction is not dependent on how much nucleophile is present

39
Q

an excess of nucleophile will not

A

speed the reaction up but it must be present

for Sn1

40
Q

the rate of a Sn1 reaction is highly dependent on the nature of the substrate

A
  • least stable -> most stable(methane
41
Q

tertiary substrates generally undergo substitution via Sn1 process

A

primary substrates go Sn2

42
Q

secondary substrates can proceed via

A

either Sn1 or Sn2 depending

43
Q

retention of configuration

A

Sn1 reactions form intermediate carbocations which can then be attacked from either side

	- can either keep its configuration or have an inversion of configuration
	- theoretically should produce a racemic mixture but typically slightly more form inversion product
		- slightly blocked front from the ion pair favors a backside attack by an unhindered nucleophile
44
Q

every Sn1 reaction has at least two steps

A
  • loss of a leaving group

- nucleophilic attack

45
Q

3 other possibilities in a Sn1 mechanism(loss of a leaving group followed by a nucleophilic attack)

A
  • before the two core steps – proton transfer
    - between the two core steps – a carbocation rearrangement
    - after the two core steps – proton transfer
46
Q

proton transfer before the two core steps(Sn1 reaction)

A
  • necessary whenever the substrate is an alcohol(ROH)
    - OH- is a bad leaving group but H2O is an excellent leaving group
    - if the substrate has no leaving group other than OH- then acidic conditions will be required in order to allow an Sn1 reaction
47
Q

Proton transfer after the two core steps(Sn1 reaction)

A
  • necessary whenever the nucleophile is neutral(not negatively charged)
    - the species ends positively charged and must have a proton to balance it
    - solvolysis – the solvent functions as the nucleophile in a reaction
48
Q

Carbocation Rearrangements during an Sn1 process

A
  • when carbocation is possible a mixture of products is generally obtained
    - products with and without the rearrangement
    - the ratio depends on if the rearrangement takes place more quickly than the nucleophilic attack
    - in most cases the rearranged products predominates the nucleophilic attack process
    - intramolecular process typically occur more rapidly then intermolecular processes
49
Q

there can be before and/or after Sn2(concerted process = no carbocation rearrangement possible)

A
  • proton transfer before

- proton transfer after

50
Q

a proton transfer is required before a Sn2 reaction if

A

an alcohol(to make a good leaving group)

51
Q

a proton transfer is required after a Sn2 reaction if

A

the nucleophile is neutral(charge balance the +)

52
Q

a primary substrate will be

A

an Sn2 process

53
Q

Sn1 factors to look for

A

racemization(no inversion with chirality) and rearrangement

54
Q

Sn2 factors to look for

A

inversion of configuration(chirality) and rearrangement is not possible

55
Q

4 major factors to determine if Sn1 or Sn2

A
  • the substrate
    - the leaving group
    - the nucleophile
    - the solvent
56
Q

identifying the substrate is the most important factor in determining if

A

Sn1 or Sn2

57
Q

4 major factors to determine if Sn1 or Sn2

The substrate

A
  • most important factor to determine Sn2 or Sn1
    - Sn1 is determined by carbocation stability(Tertiary best and Me worst)
    - Sn2 is determined by steric hindrance in the transition state(Me best and tertiary worst)
    - Methy and primary substrates favor Sn2
    - Tertiary substrates favor Sn1
    - Secondary substrates must be determined by the other three factors(leaving group, nucleophile, and solvent)
    - allylic halides and benzylic halides can react via Sn2 or Sn1 processes
    - Vinyl halides and Aryl halides are unreactive and do not respond to either Sn1 or Sn2
    - would generate an unstable carbocation
    - Sn2 reactions are generally not observed at sp2 hybridized centers(back side attack is sterically encumbered)
58
Q

4 major factors to determine if Sn1 or Sn2

The Nucleophile

A
  • Sn2 is affected by the concentration and strength of the nucleophile
    - a strong nucleophile will speed up the rate of an Sn2 reaction
    - a weak nucleophile will slow down the rate of an Sn2 reaction
    - Sn1 process is not affected by the concentration or strength of the nucleophile because the nucleophile does not participate in the rate determining step
    - A strong nucleophile favors Sn2
    - a weak nucleophile disfavors Sn2(thereby allowing Sn1 to compete successfully)
    - Strong nucleophiles:
    - I-, HS-, HO-, Br-, H2S, RO-, Cl-,RSH, N-=C-
    - Weak nucleophiles:
    - F-, H2O, ROH
    - F- is weak in protic solvents but strong in polar aprotic solvents
59
Q

4 major factors to determine if Sn1 or Sn2

The leaving group

A
  • both Sn1 and Sn2 are sensitive to the leaving group
    - Sn1 generally more sensitive
    - a bad leaving group will disallow either Sn1 or Sn2 reactions
    - the leaving group must be highly stabilized in order for an Sn1 reaction to be effective
    - in general, good leaving groups are the conjugate bases of strong acids
    - Iodide is one of the best leaving groups
    - Good leaving groups
    - I-, Br-, Cl-, SO3-. H2O
    - bad leaving groups
    - HO-, CH3CH2O-, (CH3)3CHO-,NH2-
    - sulfonate ions – the most commonly used leaving groups aside from halides
    - Halides
    - I-, Br-, Cl-
    - sulfonate ions
    - SO3-
60
Q

4 major factors to determine if Sn1 or Sn2

Solvent Effects

A
  • protic solvents – contain at least one hydrogen atom connected directly to an electronegative atom
    - polar aprotic solvents – contain no hydrogen atoms connected directly to an electronegative atom
    - protic solvents are used for Sn1 reactions
    - polar aprotic solvents are used to favor Sn2 reactions
    - in protic solvents:
    - I- >Br- >Cl- >F-
    - I- is the strongest nucleophile and fluoride is the weakest
    - In polar aprotic solvents:
    - F- > Cl- > Br- > I-
    - F- is the strongest nucleophile
    - Fluoride becomes the strongest because it is the least stable anion
61
Q

4 factors summary:

A
  • Substrate
    - Methyl or primary (Sn2)
    - tertiary (Sn1)
    - Nucleophile
    - Strong nucleophile(Sn2)
    - weak nucleophile(Sn1)
    - Leaving Group
    - Good leaving group(Sn2)
    - Excellent leaving group(Sn1)
    - Solvent
    - polar aprotic(Sn2)
    - Protic(Sn1)
62
Q

3 major considerations for selecting the reagent to accomplish functional group transformation

A
  • the substrate
    - nucleophile and solvent
    - leaving group
63
Q

the substrate

A
  • methyl or primary dictates an Sn2 process
    - tertiary dictates an Sn1 process
    - secondary is generally Sn2 as it avoids carbocation rearrangement
64
Q

Nucleophile and the Solvent

A
  • Sn1 requires a weak nucleophile and protic solvent

- Sn2 requires a strong nucleophile and polar aprotic solvent

65
Q

leaving group

A
  • OH is a bad leaving group and must be converted to a good leaving group
    - Sn1 process an acid is used to protonate the OH group(converting to an excellent leaving group)
    - Sn2 reactions OH is converted into tosylate(SO3-) an excellent leaving group rather than protenating the group

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