40 Questions
What is the general formula for the alkene family?
CnH2n
What is the distinguishing feature of the alkene structure?
Carbon-carbon double bond
What is the strongest bond in the carbon-carbon double bond?
σ bond
What is the name of the simplest member of the alkene family?
Ethylene
According to the IUPAC system, what is the parent structure of an alkene named?
The longest continuous chain that contains the carbon-carbon double bond
What is the ending of the alkane name changed to when naming an alkene?
-ane to -ene
How is the position of a double bond designated in an alkene?
By the number of the first doubly-bonded carbon
What takes precedence over alkyl groups and halogens in determining the main carbon chain and the direction in which it is numbered?
The presence of a carbon–carbon double bond
What is a characteristic of the physical properties of alkenes?
They are insoluble in water, but soluble in nonpolar solvents
What is the reason for cis, trans isomerism in alkenes?
Restricted rotation about a carbon–carbon double bond
What is the difference between cis-2-butene and trans-2-butene?
The arrangement of methyl groups and hydrogen atoms around the double bond
Why are cis-2-butene and trans-2-butene different compounds?
Because they cannot be converted into one another at room temperature
What is the process of converting alkyl halides into alkenes?
Dehydrohalogenation
What is the primary product when sec-Butyl chloride undergoes dehydrohalogenation?
A mixture of 1-butene and 2-butene
What is the role of the hydroxide ion in the dehydrohalogenation mechanism?
To pull a hydrogen ion away from carbon
What is the difference between the breaking of C-H and C-X bonds in dehydrohalogenation?
The bonds break in an unsymmetrical fashion
What is the general formula for alcohols?
ROH
What is the second method of preparing alkenes mentioned in the text?
Dehydration of alcohols
What is required for dehydration to occur?
The presence of an acid and the application of heat
What is the role of alumina in dehydration?
It acts as a Lewis acid
What is a Lewis acid?
A species that accepts a pair of electrons
What is the first step in the mechanism of dehydration of alcohols?
The union of the alcohol with a hydrogen ion
What is lost in the second step of the dehydration mechanism?
H2O
What is the final product of the dehydration mechanism?
The alkene
What is the driving force behind the rearrangement of a carbonium ion?
The formation of a more stable carbonium ion
What is the result of the dehalogenation of vicinal dihalides?
The formation of alkenes
What is the type of migration where a hydrogen atom moves with a pair of electrons?
Hydride shift
What is the outcome of the reduction of an alkyne to the double-bond stage?
The formation of either a cis-alkene or a trans-alkene
Why does the 3,3-dimethyl-2-butyl cation rearrange to the 2,3-dimethyl-2-butyl cation?
To form a more stable carbonium ion
Which type of carbonium ion is the most stable?
Tertiary carbonium ion
A carbonium ion can rearrange to form a less stable carbonium ion.
False
The dehalogenation of vicinal dihalides is a method for the synthesis of alkanes.
False
A hydride shift is a migration of an alkyl group with a pair of electrons.
False
Reduction of an alkyne can yield only a cis-alkene.
False
The 2-methyl-1-butyl cation rearranges to the 2-methyl-2-butyl cation.
True
Alkyl halides can be converted into alkanes through dehydrohalogenation.
False
The 3,3-dimethyl-2-butyl cation rearranges to the 2,3-dimethyl-1-butyl cation.
False
Isomeric alkenes can be formed in the dehalogenation of vicinal dihalides.
True
Tertiary carbonium ions are less stable than primary carbonium ions.
False
An alkyl shift is a migration of hydrogen with a pair of electrons.
False
Study Notes
Alkenes
- Alkenes are unsaturated hydrocarbons with the general formula CnH2n.
- The simplest alkene is ethylene, C2H4.
- Alkenes have a carbon-carbon double bond, which is made up of a strong σ bond and a weak π bond.
- The C─C distance in ethylene is less than in ethane, and the double bond is shorter than the carbon-carbon single bond.
Names of Alkenes
- Common names are rarely used, except for three simple alkenes: ethylene, propylene, and isobutylene.
- Most alkenes are named by the IUPAC system.
- The IUPAC system rules:
- Select the longest continuous chain that contains the carbon-carbon double bond as the parent structure.
- Indicate the position of the double bond in the parent chain by a number.
- Carbon-carbon double bonds take precedence over alkyl groups and halogens in determining the main carbon chain and direction.
Physical Properties
- Alkenes possess physical properties similar to those of alkanes.
- They are insoluble in water, but soluble in nonpolar solvents like benzene, ether, and chloroform.
- Boiling point rises with increasing carbon number.
Cis, Trans Isomerism
- Alkenes show cis, trans isomerism due to restricted rotation about the carbon-carbon double bond.
- Example: 2-butene has two stereoisomers, cis-2-butene and trans-2-butene.
- These compounds cannot be converted into each other at room temperature and have different physical and chemical properties.
Preparation of Alkenes
-
- Dehydrohalogenation of alkyl halides:
- Alkyl halides are converted into alkenes by elimination of hydrogen halide.
- Dehydrohalogenation involves removal of the halogen atom and a hydrogen atom from a carbon adjacent to the one bearing the halogen.
-
- Dehydration of alcohols:
- Alcohols are compounds of the general formula ROH.
- Dehydration requires the presence of an acid and heat.
- Mechanism involves the formation of a protonated alcohol, dissociation into water and a carbonium ion, and the loss of a hydrogen ion to form the alkene.
-
- Dehalogenation of vicinal dihalides:
- Vicinal dihalides are compounds that have halogens on adjacent carbons.
- They are prepared by the reaction between a halogen and an alkene.
-
- Reduction of alkynes:
- Reduction of an alkyne to the double-bond stage can yield either a cis-alkene or a trans-alkene.
- The choice of reducing agent determines which isomer predominates, except when the triple bond is at the end of a chain.
Test your knowledge of alkenes, a type of unsaturated hydrocarbon with a carbon-carbon double bond. Learn about their general formula, simplest member, and more. This quiz covers the basics of alkenes in organic chemistry.
Make Your Own Quizzes and Flashcards
Convert your notes into interactive study material.
Get started for free
