Ch. 6: The Reactions of Alkenes, The Stereochemistry of Addition Reactions

Question 1

If the electrophilic reagent that adds to an alkene is a hydrogen halide, what is the product?

Answer 1

An alkyl halide

Question 2

What are the hydrogen halides?

Answer 2

HF, HCl, HBr, and HI

Question 3

How does rate of reaction affect the product of a reaction of an alkene with different substituents on both sp2 carbons with a hydrogen halide?

Answer 3

If there is any difference in the rate of formation of these two carbocations, then the one that is formed faster will be the predominant product of the first step. And the carbocation formed in the first step determines the final product of the reaction.

Question 4

What is a primary carbocation?

Answer 4

has a positive charge on a primary carbon

Question 5

What is a secondary carbocation?

Answer 5

has a positive charge on a secondary carbon

Question 6

What is a tertiary carbocation?

Answer 6

has a positive charge on a tertiary carbon

Question 7

How is the stability of a carbocation effected by the number of alkyl substituents attached to the positively charged carbon?

Answer 7

Tertiary carbons are more stable than secondary carbocations, and secondary carbocations are more stable than primary carbocations. Thus, we see that the stability of a carbocation increases as the number of alkyl substituents attached to the positively charged carbon increases. These are relative stabilities, however, because carbocations are rarely stable enough to isolate.

Question 8

Why do alkyl groups stabilize carbocations?

Answer 8

Because they decrease the concentration of positive charge on the carbon.

Question 9

What is hyperconjugation and how does it decrease the positive charge on the carbon and therefore stabilize it?

Answer 9

Hyperconjugation is the delocalization of electrons by the overlap of a sigma bond orbital with an empty orbital on an adjacent carbon. The positive charge on a carbon signifies an empty p orbital. Recall that in the ethyl cation, the orbital of an adjacent C–H sigma bond can overlap the empty p orbital. This movement of electrons from a sigma bond orbital toward the vacant p orbital decreases the charge on the sp2 carbon and causes a partial positive charge to develop on the two atoms bonded by the overlapping sigma bond orbital (the H and the C). Since now three atoms share the positive charge as opposed to one, the carbocation is stabilized.

Question 10

What is a requirement for hyperconjugation to occur?

Answer 10

The orbital of the sigma bond and the empty p orbital have the proper orientation, but this is easily achieved because there is free rotation about the C–C sigma bond.

Question 11

What sigma orbitals can overlap the empty p orbital of a carbocation?

Answer 11

Only the sigma orbitals of those attached to an atom that is attached to the positively charged carbon.

Question 12

How does a halogen effect the stability of a carbocation?

Answer 12

It decreases the stability of a carbocation because it is an electronegative atom and, therefore, withdraws electrons away from the positively charged carbon. This increases the concentration of positive charge on the carbocation, which makes it less stable.

Question 13

How do the points on a reaction coordinate diagram indicate the similarity of the structures?

Answer 13

Hammond postulate: The transition state is more similar in structure to the species to which it is more similar in energy. Therefore, if two points on the uphill leg or the downhill leg of a reaction coordinate diagram have similar energies, they will also have similar structures–and the closer their energies, the more similar their structures will be.

Question 14

Is the structure of an exergonic reaction’s transition state closer in energy and structure to the products or reactants of the reaction?

Answer 14

The reactant.

Question 15

Is the structure of an endergonic reaction’s transition state closer in energy and structure to the products or reactants of the reaction?

Answer 15

The product.

Question 16

Is the formation of a carbocation an endergonic or exergonic reaction?

Answer 16

An endergonic reaction, so the structure of the transition state resembles the structure of the carbocation product.

Question 17

Why is the transition state leading to tert-butyl cation faster than the transition state leading to the isobutyl cation?

Answer 17

The same factors that stabilize the positively charged carbocation stabilize the partially positively charged transition state. Therefore, the transition state leading to the tert-butyl cation (a tertiary carbon with a partial positive charge) is more stable (lower in energy) than the transition state leading to the isobutyl cation (a primary carbon with a partial positive charge). Thus, the tert-butyl cation, with a smaller energy of activation, is formed faster than the isobutyl cation.

Basically: In an electrophilic addition reaction, the more stable carbocation is formed more rapidly.

Question 18

What is regioselectivity?

Answer 18

The preferential formation of one constitutional isomer over another.

Question 19

Are there degrees to regioselectivity?

Answer 19

Yes, a reaction can be moderately, highly, or completely regioselective.

Question 20

Which reaction is more highly regioselective: the addition of HCl to 2-methylpropene (where the two possible carbocations are tertiary and primary) or the addition of HCl to 2-methyl-2-butene (where the two possible carbocations are tertiary and secondary)?

Answer 20

The addition of HCl to 2-methylpropene because the two carbocations formed in the other reaction are closer in stability. In a completely regioselective reaction, only one of the possible products is formed, whereas there is a major and minor product in a non completely regioselective reaction.

Question 21

What is Markovnikov’s rule?

Answer 21

Only valid for addition reactions in which the electrophile is H+: The electrophile adds preferentially to the sp2 carbon bonded to the most hydrogens.

Question 22

Do alkenes react with water?

Answer 22

No, because there is no electrophile present to start the reaction by adding to the alkene.

Question 23

What is hydration?

Answer 23

An acid-catalyzed reaction, if an acid (like H2SO4) is added to the solution, then a reaction will occur because the acid provides the electrophile. The product of the reaction is an alcohol. The addition of water to a molecule is called hydration, so we can say that an alkene is hydrated in the presence of water and acid.

Question 24

Explain the mechanism for the acid-catalyzed addition of water to an alkene.

Answer 24

The product is an alcohol.

Question 25

What must be true for pH = pKa?

Answer 25

Half the compound is in its acidic form and half is in its basic form. So when pH=pKa, more than half the compound exists in the neutral nonprotonated form.

Question 26

Explain the mechanism for the acid-catalyzed addition of an alcohol to an alkene.

Answer 26

The product of the reaction is an ether.

Question 27

When do carbocations rearrange themselves?

Answer 27

only if they become more stable as a result of the rearrangement

Question 28

What is a 1,2-hydride shift?

Answer 28

A hydrogen shifts with its pair of electrons, the rearrangement is called a hydride shift. It’s a 1,2-hydride shift because the hydride ion moves from one carbon to an adjacent carbon. The major product results from adding the nucleophile to the rearranged carbocation.

Question 29

What is 1,2-methyl shift?

Answer 29

In this reaction, a secondary carbocation is formed initially. Then one of the methyl groups, with its pair of electrons, shifts to the adjacent positively charged carbon to form a more stable tertiary carbon. The major product is the one formed by adding the nucleophile to the rearranged carbocation.

Question 30

How is borane (BH3) an electrophile?

Answer 30

It has an incomplete octet. Boron uses its 3 sp2 orbitals too form three bonds with hydrogen, so it has an empty p orbital that can accept a share in a pair of electrons.

Question 31

What is the product of a hydroboration-oxidation reaction?

Answer 31

An alcohol.

Question 32

Name a key characteristic of a hydroboration-oxidation ree=action.

Answer 32

since BH3 is the electrophile, it adds to the sp2 carbon bonded to the most hydrogens. (H- is the nucleophile)

Question 33

What is a concerted reaction?

Answer 33

all the bond-making and bond-breaking processes occur in the same step

Question 34

What is a hydroboration-oxidation reaction?

Answer 34

An oxidation reaction decreases the number of C–H bonds or increases the number of C–O, C–N, or C–X bonds in a compound.

Question 35

Explain the mechanism for a hydroboration-oxidation reaction with BH3.

Answer 35

Question 36

What are the two reasons boron adds to the sp2 carbon bonded to the msot hydrogens?

Answer 36

1) there is more room at this sp2 carbon for the electrophile to attach itself because it is the less substituted carbon

2) Addition of boron to the sp2 carbon bonded to the most Hs forms a more stable transition state because the partial positive charge is on a secondary carbon.