Organic Chemistry: Alkenes, Benzene & Alkanes
Chemistry
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Alkenes
General Formula: C n H 2 n
Ending: ene
Functional group: C = C
Unsaturated hydrocarbons: they can still have hydrogen atoms added to their formula.
Contain a carbon – carbon double bond somewhere in their structure.
Example 1: Name of Alkenes
Example 2: Skeletal formula of two isomers of C4H8 which are not alkene.
Physical Properties of Alkenes
1. Boiling Point
The boiling points of the alkenes increase as the number of carbon atoms increases.
Increases as molecular mass and size increases – number of electrons increases,
increases VDW forces.
For isomeric alkenes the greater the degree of branching, the lower the boiling
point.
2. Solubility
Being non-polar alkenes are insoluble in water but soluble in organic solvents.
Alkenes are soluble in non-polar solvents because their intermolecular forces are weak
and are of a similar nature and magnitude to the intermolecular forces between the
solvent molecules.
[1]
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Alkenes do not dissolve in water because water is a polar solvent with strong
intermolecular forces.
The forces between the water and alkene molecules will be weak dipole induced dipole
forces.
Reactivity
Why alkenes are more reactive than alkanes given that double bonds are stronger than single
bonds?
Only the pi-bond is broken during a reaction and more energy is released when bonds are
formed.
How is a pi-bond formed?
By sideways overlapping of two p-orbitals. It has two regions, one above and one below the
plane of the molecule.
Preparation of Alkenes:
1. From Alcohols by elimination
Reaction Reagent: Conc. Sulfuric acid or conc. Phosphoric acid
Conditions: Reflux.
[2]
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Mechanism Dehydration (elimination)
Protonation of the alcohol Using lone pair on oxygen (alcohol acts as a Lewis Base)
Loss of a water molecule to generate a Carbonium ion
Loss of a proton (H+) to give the alkene.
2. From Haloalkanes – Elimination
Mechanism Elimination
[3]
Chemistry
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Alkenes
General Formula: C n H 2 n
Ending: ene
Functional group: C = C
Unsaturated hydrocarbons: they can still have hydrogen atoms added to their formula.
Contain a carbon – carbon double bond somewhere in their structure.
Example 1: Name of Alkenes
Example 2: Skeletal formula of two isomers of C4H8 which are not alkene.
Physical Properties of Alkenes
1. Boiling Point
The boiling points of the alkenes increase as the number of carbon atoms increases.
Increases as molecular mass and size increases – number of electrons increases,
increases VDW forces.
For isomeric alkenes the greater the degree of branching, the lower the boiling
point.
2. Solubility
Being non-polar alkenes are insoluble in water but soluble in organic solvents.
Alkenes are soluble in non-polar solvents because their intermolecular forces are weak
and are of a similar nature and magnitude to the intermolecular forces between the
solvent molecules.
[1]
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Alkenes do not dissolve in water because water is a polar solvent with strong
intermolecular forces.
The forces between the water and alkene molecules will be weak dipole induced dipole
forces.
Reactivity
Why alkenes are more reactive than alkanes given that double bonds are stronger than single
bonds?
Only the pi-bond is broken during a reaction and more energy is released when bonds are
formed.
How is a pi-bond formed?
By sideways overlapping of two p-orbitals. It has two regions, one above and one below the
plane of the molecule.
Preparation of Alkenes:
1. From Alcohols by elimination
Reaction Reagent: Conc. Sulfuric acid or conc. Phosphoric acid
Conditions: Reflux.
[2]
, Organic Chemistry |Alkenes, Benzene, Alkanes |Chemistry
Mechanism Dehydration (elimination)
Protonation of the alcohol Using lone pair on oxygen (alcohol acts as a Lewis Base)
Loss of a water molecule to generate a Carbonium ion
Loss of a proton (H+) to give the alkene.
2. From Haloalkanes – Elimination
Mechanism Elimination
[3]
