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Questions and Answers
What is the general formula for alkenes?
What is the general formula for alkenes?
- CnH2n (correct)
- CnH2n+2
- CnH2n-2
- CnH2n-1
What is the distinguishing feature of alkene structure?
What is the distinguishing feature of alkene structure?
- Hydrogen bond
- Single bond
- Double bond (correct)
- Triple bond
Why are alkenes referred to as unsaturated hydrocarbons?
Why are alkenes referred to as unsaturated hydrocarbons?
- They contain halogen atoms
- They contain the maximum quantity of hydrogen
- They contain only single bonds
- They contain double or triple bonds (correct)
What is the shortest member of the alkene family mentioned in the text?
What is the shortest member of the alkene family mentioned in the text?
How are most alkenes named according to IUPAC system?
How are most alkenes named according to IUPAC system?
In naming alkenes, how is the position of the double bond indicated?
In naming alkenes, how is the position of the double bond indicated?
What determines the position of the double bond in alkenes?
What determines the position of the double bond in alkenes?
Which solvent are alkenes quite soluble in?
Which solvent are alkenes quite soluble in?
What effect does increasing carbon number have on the boiling point of alkenes?
What effect does increasing carbon number have on the boiling point of alkenes?
What causes cis, trans isomerism in alkenes?
What causes cis, trans isomerism in alkenes?
In cis-2-butene, where are the two methyl groups located relative to the double bond?
In cis-2-butene, where are the two methyl groups located relative to the double bond?
Why can't cis-2-butene and trans-2-butene convert into each other at room temperature?
Why can't cis-2-butene and trans-2-butene convert into each other at room temperature?
Which process involves converting alkyl halides into alkenes by eliminating hydrogen halide?
Which process involves converting alkyl halides into alkenes by eliminating hydrogen halide?
When n-Butyl chloride undergoes dehydrohalogenation, which alkene is the major product?
When n-Butyl chloride undergoes dehydrohalogenation, which alkene is the major product?
How does sec-Butyl chloride differ from n-Butyl chloride in terms of dehydrohalogenation products?
How does sec-Butyl chloride differ from n-Butyl chloride in terms of dehydrohalogenation products?
What is the role of the hydroxide ion in dehydrohalogenation during alkyl halide conversion?
What is the role of the hydroxide ion in dehydrohalogenation during alkyl halide conversion?
How do alkenes usually form in a dehydrohalogenation reaction compared to free radical reactions?
How do alkenes usually form in a dehydrohalogenation reaction compared to free radical reactions?
What type of compounds are alcohols, based on their general formula ROH?
What type of compounds are alcohols, based on their general formula ROH?
Which species can accept a pair of electrons according to the text?
Which species can accept a pair of electrons according to the text?
What is the role of alumina in the dehydration process of alcohols?
What is the role of alumina in the dehydration process of alcohols?
In the dehydration mechanism, what is formed by the union of alcohol with a hydrogen ion?
In the dehydration mechanism, what is formed by the union of alcohol with a hydrogen ion?
What is lost in step 3 of alkene formation during dehydration?
What is lost in step 3 of alkene formation during dehydration?
From what species is the alkene formed in the dehydration process?
From what species is the alkene formed in the dehydration process?
Why is it stated that the alkene is not initially formed from the carbonium ion generated from alcohol?
Why is it stated that the alkene is not initially formed from the carbonium ion generated from alcohol?
What type of rearrangement can a carbonium ion undergo to form a more stable carbonium ion?
What type of rearrangement can a carbonium ion undergo to form a more stable carbonium ion?
Which type of cation can the 2-methyl-l-butyl cation rearrange into?
Which type of cation can the 2-methyl-l-butyl cation rearrange into?
What is the name for the migration of hydrogen with a pair of electrons?
What is the name for the migration of hydrogen with a pair of electrons?
In the synthesis of alkenes, what method involves the dehalogenation of vicinal dihalides?
In the synthesis of alkenes, what method involves the dehalogenation of vicinal dihalides?
What determines whether a cis-alkene or a trans-alkene predominates in the reduction of an alkyne to the double-bond stage?
What determines whether a cis-alkene or a trans-alkene predominates in the reduction of an alkyne to the double-bond stage?
Why is rearrangement necessary in the preparation of alkenes from carbonium ions?
Why is rearrangement necessary in the preparation of alkenes from carbonium ions?
A hydride shift involves the migration of an alkyl group with a pair of electrons.
A hydride shift involves the migration of an alkyl group with a pair of electrons.
The dehalogenation of vicinal dihalides is a common method for the synthesis of alkanes.
The dehalogenation of vicinal dihalides is a common method for the synthesis of alkanes.
Primary carbonium ions rearrange to form tertiary carbonium ions for increased stability.
Primary carbonium ions rearrange to form tertiary carbonium ions for increased stability.
The dehalogenation of vicinal dihalides involves compounds that have halogens on non-adjacent carbons.
The dehalogenation of vicinal dihalides involves compounds that have halogens on non-adjacent carbons.
The reduction of an alkyne to the double-bond stage can only yield a trans-alkene.
The reduction of an alkyne to the double-bond stage can only yield a trans-alkene.
A 2,3-dimethyl-2-butyl cation rearranges to form a 3,3-dimethyl-2-butyl cation.
A 2,3-dimethyl-2-butyl cation rearranges to form a 3,3-dimethyl-2-butyl cation.
The alkyl shift involves the migration of a hydrogen atom with a pair of electrons.
The alkyl shift involves the migration of a hydrogen atom with a pair of electrons.
In the preparation of alkenes, secondary carbonium ions rearrange to form primary carbonium ions for increased stability.
In the preparation of alkenes, secondary carbonium ions rearrange to form primary carbonium ions for increased stability.
Isomeric alkenes can be formed in the dehalogenation process of vicinal dihalides.
Isomeric alkenes can be formed in the dehalogenation process of vicinal dihalides.
The alkene formed in the reduction of an alkyne is determined solely by the position of the triple bond in the chain.
The alkene formed in the reduction of an alkyne is determined solely by the position of the triple bond in the chain.
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
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Description
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.
