Chapter 4 - Hydrocarbons Flashcards
(44 cards)
Molecular Formula: aliphatic alkanes
CnH2n+2
Molecular Formula: cycloalkanes
CnH2n
How does branching affect an alkane’s boiling point?
It generally decreases.
Order of substitution for free radical stablility
3º > 2º > 1º > methyl
Describe the initiation reaction of free radical halogenation of alkanes
The addition of energy breaks a halogen-halogen bond homolytically to form two free radicals.
Describe the propagation reaction of free radical halogenation of alkanes
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
Describe the termination step of free radical halogen of alkanes
Two free radicals combine to form a neutral molecule
How does reaction rate affect selectivity?
The slower the reaction, the more selective it is.
What is the law regarding energy added to a system and the products that are formed?
As energy is added to a system, the distribution of compounds is shifted to the less stabl compounds, to absorb energy
How does rearrangement affect free radical halogenation?
It does not - free radicals do not rearrange.
Why is free radical halogenation with iodine unfavorable?
Iodine is so large, it is easy to break its bonds, with itself and with carbon and hydrogen. The products would be too unstable.
How does the addition of a halogen affect an alkane’s acidity?
It increases acidity.
What are the major requirements of an E2 reaction compared to an SN2 reaction?
- high temperature
- bulky base
- anti substituents (H and leaving group)
How many steps are involved in an E2 reaction?
one
Describe an E2 reaction’s mechanism
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.
How many steps are involved in an E1 reaction?
two
Describe the E1 reaction mechanism
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.
What are the major requirements for an E1 reaction?
- acidic conditions
- high temperature
What is the major product of elimination reactions?
the most stubstituted alkene
Order of alkyl carbocation stability
3º > 2º > 1º > methyl
How does resonance affect carbocation stability?
It increases carbocation stability. It is good to have on in a vinyllic position.
What type of charged species undergoes rearrangement (hydride and alkyl group shifts)?
carbocations
What mechanism is a Hofmann Elimination?
E2
Describe the Hofmann Elimination mechanism
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.
