Chapter 8: Addition Rxns of Alkenes Flashcards

(34 cards)

1
Q

How is the determination whether an alkene will undergo addition or elimination affected by temperature?

A
  • Addition favored by low T.
  • Elimination favored by high T.
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2
Q

What are the two possible functions of a double (π) bond in reactions?

A

It can act as a base or as a nucleophile.

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3
Q

Do addition reactions typically have a positive or negative ∆H?

A

Negative

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4
Q

Acid catalyzed hydration of alkenes

A

The result of adding a water molecule across a double bond with Markovnikov regiochemistry and no specific stereochemistry.

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5
Q

How does the stability of the carbocation on an alkene affect the rate of hydration rxn?

A

Alkenes that yield more stable C+ react faster. With each additional alkyl group substituted on the alkene, the rxn rate increases by many orders of magnitude.

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6
Q

What are the two ways of indicating a hydration reaction?

A
  1. H3O+ on top of the arrow.
  2. An acid catalyst on top of the arrow (such as [H2SO4]) and H2O as a reactant.
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7
Q

Microscopic reversibility

A

If we know the individual steps, transition states, and intermediates for a forward rxn, then we precisely know them for the other direction.

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8
Q

Hydrohalogenation

A

Results in the addition of HX across a double bond with Markovnikov regiochemistry and no specific stereochemistry.

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9
Q

Markovnikov addition

A

When HX is added, the H goes to the carbon of the alkene with the most hydrogens already present.

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10
Q

Anti-markovnikov addition

A

When HX is added, the H goes to the carbon of the alkene with the least hydrogens already present.

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11
Q

In hydrohalogenation, the addition of what reagent will result in an anti-Markovnikov product?

A

Anti-Markovnikov products are observed when reactions are performed in the presensce of peroxides (O2 2-) such as H2O2 (ROOR).

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12
Q

What is the rate-determining step of hydrohalogenation?

A

The first step–proton transfer.

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13
Q

Oxymercuration-demurcuration

A

Adds an H and an OH across a double bond to form a Markovnikov product.

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14
Q

Hydroboration-oxidation

A

Adds an H and an OH across a double bond to form an anti-Markovnikov product.

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15
Q

How is oxymercuration-demurcuration an alternative to acid-catalyzed hydration of alkenes

A

OD yields Markovnikov products without the possibility of rearrangements of the carbocation intermediate.

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16
Q

Hydroboration-oxidation is an alternative method to do what to alkenes?

A

To “hydrate” alkenes. Even though H and OH are added across the double bond, this rxn does not use H2O to do so.

17
Q

Is a carbocation intermediate formed with hydroboration-oxidation?

18
Q

What is the stereochemical result of hydroboration-oxidation?

A

Syn addition.

19
Q

Catalytic hydrogenation

A

Adds two Hs across a double bond with syn stereochemistry.

20
Q

What are the necessary reagents for catalytic hydrogenation?

A
  • A catalyst that remains in the solid phase (heterogenous catalysis)
  • A solvent to dissolve the alkene (typically ethanol, hexane, or acetic acid)
21
Q

Wilkinson’s catalyst

A

A common homogenous catalyst for the hydrogenation of alkenes.

22
Q

Halogenation

A

Adds Br2 or Cl2 across a double bond with anti stereochemistry.

23
Q

Halohydrin reactions

A

Adds a halogen and an OH across a double bond with anti stereochemistry.

24
Q

Does carbocation rearrangement occur in bromination reactions?

A

No, but some rearrangement occurs in chlorination reactions.

25
What is the regioselectivity for the addition of halogens in H2O?
When water is used as the solvent, the halogen is added to the less substituted C and OH to the more substituted C.
26
What is the stereoselectivity for the addition of halogens in H2O?
Anti addition
27
Syn dihydroxylation
Two OH groups are added to the same side of a double bond.
28
Anti dihydroxylation
Two OH groups are added on opposite sides of a double bond.
29
What is the first step of anti dihydroxylation?
The creation of an epoxide.
30
Epoxides
3-membered ether rings: 2 C atoms and 1 O atom in the ring
31
Peroxy acids
Peroxy acids resemble carboxylic acids with one additional O atom. They are strong oxidizing agents. Examples: peroxyacetic acid or MCPBA
32
How does the number of steps in the mechanism differ between syn and anti dihydroxylation?
Anti dihydroxylation occurs in multiple steps. In contrast, like other syn additions, syn dihydroxylation adds across the C=C double bond in one step.
33
What type of dihydroxylation can be achieved with KMnO4?
Syn dihydroxylation, but only under mild conditions (cold temperatures).
34
Ozonolysis
Breaks the pi and sigma bonds of a double bond and forms carbonyls in their place.