Chapter 8: Addition Rxns of Alkenes

Question 1

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

Answer 1

• Addition favored by low T.
• Elimination favored by high T.

Question 2

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

Answer 2

It can act as a base or as a nucleophile.

Question 3

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

Answer 3

Negative

Question 4

Acid catalyzed hydration of alkenes

Answer 4

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

Question 5

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

Answer 5

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.

Question 6

What are the two ways of indicating a hydration reaction?

Answer 6

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

Question 7

Microscopic reversibility

Answer 7

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

Question 8

Hydrohalogenation

Answer 8

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

Question 9

Markovnikov addition

Answer 9

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

Question 10

Anti-markovnikov addition

Answer 10

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

Question 11

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

Answer 11

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

Question 12

What is the rate-determining step of hydrohalogenation?

Answer 12

The first step–proton transfer.

Question 13

Oxymercuration-demurcuration

Answer 13

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

Question 14

Hydroboration-oxidation

Answer 14

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

Question 15

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

Answer 15

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

Question 16

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

Answer 16

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

Question 17

Is a carbocation intermediate formed with hydroboration-oxidation?

Answer 17

No.

Question 18

What is the stereochemical result of hydroboration-oxidation?

Answer 18

Syn addition.

Question 19

Catalytic hydrogenation

Answer 19

Adds two Hs across a double bond with syn stereochemistry.

Question 20

What are the necessary reagents for catalytic hydrogenation?

Answer 20

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

Question 21

Wilkinson’s catalyst

Answer 21

A common homogenous catalyst for the hydrogenation of alkenes.

Question 22

Halogenation

Answer 22

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

Question 23

Halohydrin reactions

Answer 23

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

Question 24

Does carbocation rearrangement occur in bromination reactions?

Answer 24

No, but some rearrangement occurs in chlorination reactions.

Question 25

What is the regioselectivity for the addition of halogens in H2O?

Answer 25

When water is used as the solvent, the halogen is added to the less substituted C and OH to the more substituted C.

Question 26

What is the stereoselectivity for the addition of halogens in H2O?

Answer 26

Anti addition

Question 27

Syn dihydroxylation

Answer 27

Two OH groups are added to the same side of a double bond.

Question 28

Anti dihydroxylation

Answer 28

Two OH groups are added on opposite sides of a double bond.

Question 29

What is the first step of anti dihydroxylation?

Answer 29

The creation of an epoxide.

Question 30

Epoxides

Answer 30

3-membered ether rings: 2 C atoms and 1 O atom in the ring

Question 31

Peroxy acids

Answer 31

Peroxy acids resemble carboxylic acids with one additional O atom. They are strong oxidizing agents. Examples: peroxyacetic acid or MCPBA

Question 32

How does the number of steps in the mechanism differ between syn and anti dihydroxylation?

Answer 32

Anti dihydroxylation occurs in multiple steps. In contrast, like other syn additions, syn dihydroxylation adds across the C=C double bond in one step.

Question 33

What type of dihydroxylation can be achieved with KMnO4?

Answer 33

Syn dihydroxylation, but only under mild conditions (cold temperatures).

Question 34

Ozonolysis

Answer 34

Breaks the pi and sigma bonds of a double bond and forms carbonyls in their place.