Chapter 7: Alkenes and Alkynes Reactions Part 1

Question 1

What is Hofmann’s Rule?

Answer 1

This rule states that the alkene that will predominate as the major product will be the one that is least substituted
- Used in E2 with bulky bases

Question 2

What is Zaitsev’s Rule?

Answer 2

This rule states that the alkene that will predominate as the major product will be the one that is the most highly substituted

Question 3

E,Z Naming System?

Answer 3

If there are 3 different substituents attached to an alkene, then cis-trans notation cannot be used
- Both the R,S and E,Z nomenclature are put in parentheses

Question 4

What is a Z isomer?

Answer 4

If two of the high priority groups at each end of the double bond are located on the same side, it is called a Z isomer

Question 5

What is an E isomer?

Answer 5

If two of the high priority groups at each end of the double bond are located on the opposite side, it is called an E isomer

Question 6

Trans isomers are always what?

Answer 6

trans isomers are always more stable than cis isomers

Question 7

Hydrogenation is what kind of reaction?

Answer 7

Hydrogenation is an exothermic reaction, in which a hydrogen molecule is added to an alkene

Question 8

The alkene that contains more substituents will be more what?

Answer 8

will be more stable (Trisubstituted alkenes are more stable than disubstituted alkenes )

Question 9

What is Dehydrohalogenation (E2)?

Answer 9

Is the process in which a beta hydrogen and a halide group is removed to form an alkene. This is an E2 type reaction
- More than one product may be generated using dehydrohalogenation

Question 10

What will happen during dehydrohalogenation if a strong small base is used?

Answer 10

If a strong small base is used, the alkene with more substituents will predominate (Zaitsev’s Rule)

Question 11

What will happen during a dehydrohalogenation is a strong bulky base is used?

Answer 11

If a strong bulky base is used, then the alkene with less substituents will predominate due to steric hindrance (Hofmann rule)

Question 12

What is Dehydration of Alcohols?

Answer 12

Dehydration is the process where a water molecule is removed, to form an alkene when it is conducted under high temperature and strong acidic conditions

Question 13

The structure of the alcohol will influence what?

Answer 13

The structure of the alcohol will influence the concentration of acid needed and heat required to dehydrate the alcohol
- rearrangements of 1° and 2° alcohols are sometimes needed to stabilize the carbocation

Question 14

1° Alcohols require what?

Answer 14

extreme conditions for dehydration to take place, such as very high concentrations of H2SO4 and 180 °C to dehydrate ethanol

Question 15

2° Alcohols require what?

Answer 15

require less mild conditions for dehydration

Ex: 85% phosphoric acid at 165-170 °C is required to dehydrate cyclohexanol

Question 16

3° Alcohols require what?

Answer 16

very mild conditions

Ex: only 20% sulfuric acid at 85% is required to dehydrate tert-butyl alcohol

Question 17

What is the E1 Mechanism in Dehydration of Alcohol?

Answer 17

1. The first step of an E1 always involves protonation of the OH group first, this is necessary because H2O is a better leaving group
2. next removal of the beta hydrogen using water, and formation of a double bond
3. The rate determining step is the step that forms the carbocation aka step 1
4. Free energy of activation is the lowest and delocalization is most efficient with tertiary carbons

Question 18

What is the E2 Mechanism in Dehydration of Alcohols?

Answer 18

1. Mainly for primary alcohols
2. This is a two step mechanism
   - First Step involves protonation of the OH group
   - Second Step is removing a beta hydrogen, formation of a double bond, and departure of a leaving group

Question 19

There are 6 different kinds of addition reactions you need to know for this exam, what are they?

Answer 19

1. Catalytic Hydrogenation
2. Hydrohalogenation (Addition Reactions for Alkenes)
3. Hydration Acid-Catalyzed (Addition of water to obtain Alcohol)
4. Hydroboration - Oxidation (anti-maokovnikov)
5. Halogenation (Br2 or Cl2) Addition of Cl2 and Br2 to alkenes
6. Halohydrination aka Halohydrin (Cl2 or Br2 with H2O)

Question 20

What is a Catalytic Hydrogenation or a Hydrogenation Reaction?

Answer 20

1. This process adds H2 to each carbon between a double or triple bond; but this process requires a catalyst (Pt, Pd, and Ni)
2. Syn Addition Reaction, hydrogens on the same side
3. Anti addition of hydrogen can occur when the alkenes are treated with Na+ or Li+ in ammonia or ethylamine at a low temperature
4. The final product will be trans or E alkane

Question 21

Alkynes have more what than alkenes and alkanes?

Answer 21

Alkynes have more acidic hydrogens than the ones in alkanes and alkenes
- Acidity will follow the trend described below:

H-OH>H-OR>HCtriplebondCR>H=NH2>HCH=CH2>H-CH2CH3

Question 22

Basicity will follow which trend?

Answer 22

OH-

Question 23

How can Alkynes be obtained from Alkenes?

Answer 23

Alkynes can be obtained from alkenes (Vicinal dihalides) using the solvent (2NaNH2)

• Vicinal dihalide is a compound that contains 2 halogens that are next to each other
  1. If a terminal alkyne is desired, then 3 molar equivalents of sodium amide is needed instead if 2
  2. The terminal Alkyne is deprotonated by sodium amide

Question 24

What are Geminal Halides?

Answer 24

Geminal Halides are compounds that have both halogen atoms attached to the same carbon. They can be converted to alkynes using dehydrohalogenation

Question 25

What is Substitution of Terminal Alkynes?

Answer 25

substitution of the terminal hydrogen of alkynes can occur by first converting it to a sodium alkynide, using sodium amide in liquid ammonia
1. Substitution of primary alkyl halide to sodium amide can easily be done, secondary and tertiary halide will proceed through an E2 reaction instead

Question 26

What Happens during the Hydrogenation of Alkynes?

Answer 26

It is the same thing as with alkenes except for it to go all the way to an alkane ONLY the catalyst Pt can be used
1. For Alkynes to get a syn product, the catalysts used must be a Lindlars Catalysts or Ni2B instead of (Pt, Pd, or Ni)