40 Questions
What is the general formula for alkenes?
CnH2n
What is the distinguishing feature of alkene structure?
Double bond
Why are alkenes referred to as unsaturated hydrocarbons?
They contain double or triple bonds
What is the shortest member of the alkene family mentioned in the text?
Ethylene
How are most alkenes named according to IUPAC system?
By changing alkane endings to -ene
In naming alkenes, how is the position of the double bond indicated?
By using a number in the parent chain
What determines the position of the double bond in alkenes?
Distance from the end of the carbon chain closest to the double bond
Which solvent are alkenes quite soluble in?
Benzene
What effect does increasing carbon number have on the boiling point of alkenes?
Increases it
What causes cis, trans isomerism in alkenes?
Two different groups bonded to each carbon of the double bond
In cis-2-butene, where are the two methyl groups located relative to the double bond?
On one side of the double bond
Why can't cis-2-butene and trans-2-butene convert into each other at room temperature?
Restricted rotation about the carbon-carbon double bond
Which process involves converting alkyl halides into alkenes by eliminating hydrogen halide?
Dehydrohalogenation
When n-Butyl chloride undergoes dehydrohalogenation, which alkene is the major product?
1-butene
How does sec-Butyl chloride differ from n-Butyl chloride in terms of dehydrohalogenation products?
sec-Butyl chloride forms both 1-butene and 2-butene
What is the role of the hydroxide ion in dehydrohalogenation during alkyl halide conversion?
Removing a hydrogen ion
How do alkenes usually form in a dehydrohalogenation reaction compared to free radical reactions?
Unsymmetrical fashion
What type of compounds are alcohols, based on their general formula ROH?
Alcohols
Which species can accept a pair of electrons according to the text?
Lewis acid
What is the role of alumina in the dehydration process of alcohols?
Lewis acid
In the dehydration mechanism, what is formed by the union of alcohol with a hydrogen ion?
Protonated alcohol
What is lost in step 3 of alkene formation during dehydration?
Hydrogen ion
From what species is the alkene formed in the dehydration process?
Carbonium ion
Why is it stated that the alkene is not initially formed from the carbonium ion generated from alcohol?
Different carbonium ions are involved in the process.
What type of rearrangement can a carbonium ion undergo to form a more stable carbonium ion?
Alkyl shift
Which type of cation can the 2-methyl-l-butyl cation rearrange into?
Tertiary cation
What is the name for the migration of hydrogen with a pair of electrons?
Hydride shift
In the synthesis of alkenes, what method involves the dehalogenation of vicinal dihalides?
Reduction of halides
What determines whether a cis-alkene or a trans-alkene predominates in the reduction of an alkyne to the double-bond stage?
Choice of reducing agent
Why is rearrangement necessary in the preparation of alkenes from carbonium ions?
To form more stable carbonium ions
A hydride shift involves the migration of an alkyl group with a pair of electrons.
False
The dehalogenation of vicinal dihalides is a common method for the synthesis of alkanes.
False
Primary carbonium ions rearrange to form tertiary carbonium ions for increased stability.
True
The dehalogenation of vicinal dihalides involves compounds that have halogens on non-adjacent carbons.
False
The reduction of an alkyne to the double-bond stage can only yield a trans-alkene.
False
A 2,3-dimethyl-2-butyl cation rearranges to form a 3,3-dimethyl-2-butyl cation.
False
The alkyl shift involves the migration of a hydrogen atom with a pair of electrons.
False
In the preparation of alkenes, secondary carbonium ions rearrange to form primary carbonium ions for increased stability.
False
Isomeric alkenes can be formed in the dehalogenation process of vicinal dihalides.
True
The alkene formed in the reduction of an alkyne is determined solely by the position of the triple bond in the chain.
False
Study Notes
Alkenes
- General formula: CnH2n
- Unsaturated hydrocarbons obtained from alkanes by loss of hydrogen in the cracking process
- Simplest member: ethylene (C2H4)
Structure of Alkenes
- Distinguishing feature: carbon-carbon double bond
- Double bond consists of a strong σ bond and a weak π bond
- C─C distance in ethylene is shorter than in ethane
Naming Alkenes
- IUPAC system used for naming
- Rules:
- Select the longest continuous chain containing the double bond as the parent structure
- Indicate the position of the double bond in the parent chain
- Carbon-carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and direction of numbering
Physical Properties
- Insoluble in water, but soluble in nonpolar solvents like benzene, ether, and chloroform
- Boiling point rises with increasing carbon number, similar to alkanes
Cis, Trans Isomerism in Alkenes
- Restricted rotation about the carbon-carbon double bond
- Example: 2-butene has two stereoisomers, cis-2-butene and trans-2-butene
- These compounds have different physical and chemical properties
Preparation of Alkenes
- Methods:
- Dehydrohalogenation of alkyl halides
- Dehydration of alcohols
- Dehalogenation of vicinal dihalides
- Reduction of alkynes
Dehydrohalogenation of Alkyl Halides
- Involves elimination of hydrogen halide
- Hydroxide ion pulls a hydrogen ion away from carbon, and a halide ion separates to form the double bond
Dehydration of Alcohols
- Requires presence of an acid and heat
- Can be carried out in two ways:
- Heating the alcohol with sulfuric or phosphoric acid
- Passing the alcohol vapor over alumina (A12O3), which acts as a Lewis acid
- Mechanism involves formation of a carbonium ion, which loses a hydrogen ion to form the alkene
Learn about alkenes, unsaturated hydrocarbons obtained from alkanes by loss of hydrogen in the cracking process. Explore the general formula CnH2n and the distinguishing feature of carbon-carbon double bonds in the alkene structure.
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