Chapter 4 - Hydrocarbons

Question 1

Molecular Formula: aliphatic alkanes

Answer 1

C_nH_2n+2

Question 2

Molecular Formula: cycloalkanes

Answer 2

C_nH_2n

Question 3

How does branching affect an alkane’s boiling point?

Answer 3

It generally decreases.

Question 4

Order of substitution for free radical stablility

Answer 4

3º > 2º > 1º > methyl

Question 5

Describe the initiation reaction of free radical halogenation of alkanes

Answer 5

The addition of energy breaks a halogen-halogen bond homolytically to form two free radicals.

Question 6

Describe the propagation reaction of free radical halogenation of alkanes

Answer 6

Two steps:

1) the free radical halogen abstracts a hydrogen from the alkane, forming an alkyl free radical
2) the alkyl free radical abstracts an electron from a halogen-halogen bond, forming another free radical halogen and a haloalkide

Question 7

Describe the termination step of free radical halogen of alkanes

Answer 7

Two free radicals combine to form a neutral molecule

Question 8

How does reaction rate affect selectivity?

Answer 8

The slower the reaction, the more selective it is.

Question 9

What is the law regarding energy added to a system and the products that are formed?

Answer 9

As energy is added to a system, the distribution of compounds is shifted to the less stabl compounds, to absorb energy

Question 10

How does rearrangement affect free radical halogenation?

Answer 10

It does not - free radicals do not rearrange.

Question 11

Why is free radical halogenation with iodine unfavorable?

Answer 11

Iodine is so large, it is easy to break its bonds, with itself and with carbon and hydrogen. The products would be too unstable.

Question 12

How does the addition of a halogen affect an alkane’s acidity?

Answer 12

It increases acidity.

Question 13

What are the major requirements of an E₂ reaction compared to an S_N2 reaction?

Answer 13

• high temperature
• bulky base
• anti substituents (H and leaving group)

Question 14

How many steps are involved in an E₂ reaction?

Answer 14

one

Question 15

Describe an E₂ reaction’s mechanism

Answer 15

A bulky base attacks the electrophile, removing a hydrogen anti to a leaving group. The hydrogen bond electrons then shift to form a pi bond, kicking off the leaving group.

Question 16

How many steps are involved in an E₁ reaction?

Answer 16

two

Question 17

Describe the E₁ reaction mechanism

Answer 17

A leaving group leaves the electrophile, forming a carbocation intermediate. A hydrogen is then abstracted from the electrophile and its bonding electrons shift to form a pi bond.

Question 18

What are the major requirements for an E₁ reaction?

Answer 18

• acidic conditions
• high temperature

Question 19

What is the major product of elimination reactions?

Answer 19

the most stubstituted alkene

Question 20

Order of alkyl carbocation stability

Answer 20

3º > 2º > 1º > methyl

Question 21

How does resonance affect carbocation stability?

Answer 21

It increases carbocation stability. It is good to have on in a vinyllic position.

Question 22

What type of charged species undergoes rearrangement (hydride and alkyl group shifts)?

Answer 22

carbocations

Question 23

What mechanism is a Hofmann Elimination?

Answer 23

E₂

Question 24

Describe the Hofmann Elimination mechanism

Answer 24

A tertiary amine is reacted to create a quaternary amine salt and an alkene by treatment with excess methyl iodide followed by silver oxide, water, and heat. The non-Zaitsev (less substituted) product is formed.

Question 25

Hydration mechanism drawn out

Answer 25

H⁺ is catalyst

Question 26

Biochemical dehydrogenation mechanism drawn out

Answer 26

Question 27

Biochemical hydrogenation mechanism drawn out

Answer 27

Question 28

Dehydration mechanism drawn out

Answer 28

Question 29

What is the transition state of a Diels-Alder reaction?

Answer 29

aromatic (like benzene)

Question 30

What are the reactants called in a Diels-Alder reaction?

Answer 30

diene + dienophile

Question 31

How does resonance affect a Diels-Alder reaction?

Answer 31

It can change regioselectivity (which sites are more positive/negative).

Question 32

Diels-Alder mechanism drawn out

Answer 32

Question 33

Are methoxy groups electron withdrawing or donating?

Answer 33

They are electron donating through resonance.

Question 34

What types of groups increase the rate of a Diels-Alder reaction?

Answer 34

electron-donating groups on the diene and electron-withdrawing groups on the dienophile

Question 35

Cope rearrangement mechanism drawn out

Answer 35

Question 36

What is the reactant in a Cope Rearrangement?

Answer 36

1,5-diene

Question 37

Claisen Rearrangement mechanism drawn out

Answer 37

Question 38

What is the reactant in a Claisen Rearrangment?

Answer 38

vinyllic allyl ether

Question 39

What molecule are terpenes derived from?

Answer 39

Isoprene

Question 40

What is a requirement for a molecule to be a terpene?

Answer 40

It must be built from 5-carbon subunits (5, 10, 15, 20, etc.).

Question 41

What are common lab techniques to isolate terpenes?

Answer 41

steam distillation or extraction as natural oils

Question 42

Is bromination or chlorination of alkanes more selective?

Answer 42

bromination

Question 43

What is required for a sigmatropic rearragement to procede?

Answer 43

heat

Question 44

What two reactants can be involved in the biosynthesis of terpenes?

Answer 44

• isopentyl pyrophosphate
• dimethylallyl pyrophosphate