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Organic Chemistry 2423 > Chapter 8 > Flashcards

Chapter 8 Flashcards

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1
Q

A tertiary alpha carbon rules out…

A

SN2

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2
Q

A primary alpha carbon will result in what type of reaction?

A

E2

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3
Q

If the alpha carbon is primary it cannot be what type of reaction?

A

E1

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4
Q

What type of reactions will most likely result from a tertiary alpha carbon

A

SN1 or E1

Rule out SN2

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5
Q

What does SN1 stand for? And what is it in competition with?

A

Unimolecular nucleophilic substitution

E1

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6
Q

What is the mechanism for SN1 reactions?

A
  • step wise*
  • Leaving group breaks away to create Carbocation intermediate
  • The solvent (acts as a nuclophile) replaces the leaving group
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7
Q

What does SN2 stand for?

A

Bimolecular nucleophile substitution

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8
Q

What is the mechanism for SN2?

A
  • two Molecules reacting at the same time in ONE concerted step (fast)
  • no intermediates
  • in acheiral molecule you will get an inversion
  • nucleophile replaces the leaving group
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9
Q

What does E1 stand for? And what is it in competition with?

A

Unimolecular beta elimination

SN1

This reaction is favored at higher temps

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10
Q

What is the mechanism for E1?

A
  • leaving group breaks away to form carbocation intermediate (slow, rate determining step)
  • nucleophile looks for a beta hydrogen and uses its electrons to pluck off the hydrogen, the electrons that bind hydrogen to carbon will stay on the molecule and collapse towards the positive charge to give the pi bond between the alpha and beta carbon
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11
Q

What are two clues for an E1 reaction?

A

Carbocation intermediate and a pi bond between alpha and beta carbons

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12
Q

What is the slow step and rate determining step for SN1?

A

Leaving group breaks away to create Carbonation intermediate

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13
Q

What does E2 stand for?

A

Bimolecular beta elimination (two molecules reacting at the same time)

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14
Q

What is the mechanism for E2?

A
  • no intermediate, fast one step reaction
  •  The base grabs the hydrogen on the most substituted beta carbon
  • The electrons between hydrogen and carbon collapse in the direction of the leaving group, eliminates LG
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15
Q

Two clues for E2 reactions?

A
  • no carbocation Intermediate

- pi bond between the alpha and more substituted beta carbon (Z)

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16
Q

Elimination reactions always form a…

A

Pi bond or Alkene

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17
Q

What is the rate for E2?

A

Rate = K [Substrate] [base]

2nd order
but first order in respect to both?

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18
Q

What is the mechanism for E2?

A
  • proton transfer
  • pi bond formation
  • loss of leaving group
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19
Q

In an E2 reaction, what would happen if the substrate increases? What would happen if the base increases? What about if they both increase?

A

Doubles
Doubles
Rate x4

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20
Q

The more __________ the alkene, the more stable the alkene is.

A

Substituted

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21
Q

What is Zaitsevs rule?

A
  • the more sub. Alkene = major (Zaitsev)

- less sub, = Minor (Hoffman)

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22
Q

Three exceptions to Zaitsev’s rule?

A

1) bulky base with bulky substrate
2) Poor leaving group (F-)
3) no anti-periplanar hydrogen

Results in Hoffman product

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23
Q

What is the stereospecificity for E2 reactions?

A

leaving group and hydrogen must be anti-periplanar

Close to 180 degrees apart

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24
Q

Are E2 reactions stereospecific or stereoselective?

A

stereospecific

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25
E2-when there is only one beta hydrogen, only E OR Z will be formed, not _____.
Both
26
How do you double the rate of an E1 reaction?
Double the alklohalide (first order)*
27
is E1 step-wise or concerted?
Step-wise
28
What is the mechanism for E1 reactions?
- loss of leaving group, carbocation formation | - proton transfer and pi bond formation
29
E1 reactions typically involve a ______ base and ______ nucleophile
Weak, weak Water and alcohols are the most common Generally the weaker the base, the better the leaving group
30
Single biggest difference between E2 and E1 is the _______
Base
31
Zaytsev‘s rule, the more substituted alkene is the major product. ____ is generally favored over _____ due to stability and steric effects
E, Z
32
Difference between a base and a nucleophile in reactions?
Base-removes a proton in a reaction | Nucleophile- A species that has a pair of electrons that can be used to form a bond to a carbon
33
What is the Z isomer?
When the two highest priorities are on the same side
34
In cyclic Alkenes with less than 8 atoms in the ring, only Cis Alkenes are stable. Why? When applied to bicycloakenes, the rule is called…
Bond angles Bredt’s rule
35
When assigning E or Z to a stereoisomers, how are the groups prioritize that are attached to the carbon carbon double bond?
Atomic number***
36
Because of __________, cis isomers are generally less stable than trans
Steric strain
37
 The more substituted the Alkene, the more _______
Stable Tetrasubstituted > trisubstituted > disubstituted > Monosubstituted
38
In general, elimination reactions eliminate…
- a hydrogen atom - a leaving group (Always see loss of LG, and proton transfer) Rearrangement is still a possibility
39
Which elimination reaction is stepwise and which elimination reaction is concerted?
Step-wise —-> E1 | Concerted —> E2
40
What order is E2?
Second order
41
Tertiary substrates are unreactive towards _____ while they react readily with ____
SN2, E2
42
in E2 reactions, believing group and hydrogen must be __________
Anti-periplanar, close to 180 degrees | sterospecific
43
The kinetics of E2 and SN2 are quite similar. However, tertiary substrates are under reactive towards SN2 while they will react readily by E2. Why?
Nucleophiles cannot reach the alpha carbon due to stearic effects, The base can reach the hydrogen on the outside and can react easily from the outside
44
Tertiary substrates are more reactive towards E2 than primary substrates even though primary substrates are less hindered. Why?
Tertiary substrates should proceed through a more stable transition state and are therefore kinetically favored and a more stable product, thermodynamically favored
45
Would a primary, secondary, or tertiary substrate react fastest in an E2 reaction? Which one has the highest activation energy?
Tertiary | Primary
46
If there are multiple reactive sites or regions on a molecule, _________ products are possible
Products
47
This occurs when one constitutional product is formed predominantly over the other?
Regioselectivity
48
The identity of the ______ can affect the radioselectivity Ex?
Base Pokey bases will favor the Hoffman product over Zaitsev
49
Why is the zaitsev product favored?
More substituted is more thermodynamically stable
50
What is the difference between stereoselective and stereospecific?
Stereospecific there is only one product outcome (E product). Stereoselective there is multiple product outcomes
51
Why do E2 reactions proceed exclusively through the anti-coplanar structure?
Steric effect of the base and leaving group Syn-coplanar does not work for elimination
52
Is a strict 180° angle necessary for E2 mechanisms?
No 175 to 179 degrees is sufficient
53
The term, __________ is generally used instead of anti-coplanar to account for slight deviations from coplanarity 
Anti-periplanar
54
Although the E isomer is usually more stable because it is less sterically hindered, The requirement for an anti-Periplanar transition state can often lead to…
The less stable Z isomer
55
The reaction rate for both E1 and SN1  is not affected by the concentration of the ______
Base Rate = k [substrate]
56
Given the rate law for E1, which step in the mechanism is the rate determining slow step?
Loss of leaving group (carbocation formation)
57
Is tertiary or primary more reactive for E1?
Tertiary because it forms a more stable carbocation Smaller activation energy needed
58
Because E1 and SN1 proceed by the same first step, their competition will generally result in…
a mixture of products
59
How could you promote E1 over SN1?
Increase the temperature
60
The final step of the E1 mechanism determines the … | E1 reactions generally produce the ________ product.
Regioselectivity Zaitsev
61
The maximum Number of steps in an E1 mechanism is generally
Four
62
Which step of E1has the biggest activation energy?
Loss of LG
63
Higher pKa means…
Stronger acid/ weaker base
64
If the base is neutral, access the strength of its conjugate acid qualitatively using…..
ARIO (Atom, resonance, induction, orbital) If the base carries a negative formal charge, qualitatively assess the strength of the base using ARIO
65
Reagents that act as ___________ only are either highly polarizable and/or they have very strong conjugate acids
Nucleophiles
66
Reactants that act as ________ only have either very low polarizability and/or they are sterically hindered
Bases
67
The more sterically hindered reagents are more likely to promote
Elimination rather than substitution
68
Three things to look for when predicting products
1. Analyze the function of the reagent (nucleophile and/or base) 2. Analyze the substrate (primary, secondary, or tertiary) 3. Consider regiochemistry and stereochemistry
69
________ solvent it is important for SN1 or E1 because we need ionization for the cation
Protic (OH, NH, FH)
70
Only carbons ________ to the carbon with the leaving group are involved in reactions
Adjacent

Organic Chemistry 2423 (6 decks)

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