Alkenes are a class of hydrocarbons (e.g, containing only carbon and hydrogen) unsaturated compounds with at least one carbon-to-carbon double bond. Another term used to describe alkenes is olefins. Alkenes are more reactive than alkanes due to the presence of the double bond.
4. Preparation Of Alkenes:
There are different methods pf
preparation of alkenes given below;
• By Dehydration Of Alcohols
• By Dehydrohalogenation Of Alkyl Halides
• By Dehalogenation Of Vicinal Di Halides
• From Alkynes
5. By Dehydration Of Alcohols:
• Dehydration means the removal of molecule water from a reactant molecule.
• When an alcohol is heated in the presence of sulphuric acid, a molecule of water is
eliminated, and an alkene is formed.
R—CH—CH2 H2SO4 R—CH CH2 + H2O
H OH heat
6. By Dehydrohalogenation Of Alkyl
Halides:
• Dehydrohalogenation means removal of the elements of hydrogen halides (H-X)
from the adjacent carbon atom.
• When an alkyl halides is heated with an alcoholic solution of sodium or potassium
hydroxide, a molecule of hydrogen halide is eliminated, and an alkene is formed.
H H
H—C C H + NaOH alcohol CH2 CH2 + NaBr + H2O
H Br heat
7. By Dehalogenation Of Vicinal Dihalides:
• Dehalogenation involves the removal of a halogen molecule (X-X) from a reactant
molecule.
• A compound having two halogen atoms on adjacent carbon atoms is called a
vicinal dihalide.
• The treatment of vicinal dihalide with zinc dust using ethyl alcohol as a solvent,
results in dehalogenation, and an alkene is formed.
H H
CH3 C C H + Zn ethanol CH3 CH CH2 + ZnBr2
Br Br
8. From alkynes:
• Alkynes can be hydrogenated by using Raney nickel or finely divided platinum or
palladium to give alkenes.
R C C H + H2 Ni R CH CH2 +H2 R CH2 RCH3
9. Saytzeff rule:
• A reaction that produces an alkene would
favor the formation of an alkene that has
greatest no substituents attached to the
C=C group
11. PHYSICAL PROPERTIES OF ALKENES:
(BOILING POINT)
- Most physical properties of alkenes are similar to those alkanes.
- Example: the boiling points of 1-butene, cis-2-butene,
trans-2-butene and n-butane are close to 0oC.
- Densities of alkenes: around 0.6 or 0.7 g/cm3.
- Boiling points of alkenes increase smoothlywith
molecular weight.
- Increased branching leads to greater volatility and
lower boiling points.
12. POLARITY:
• - relatively nonpolar.
• - insoluble in water but soluble in non-polar
• solvents such as hexane, gasoline, halogenated
• solvents and ethers.
• - slightly more polar than alkanes because:
• i) electrons in the pi bond is more polarizable
• (contributing to instantaneous dipole moments).
• ii) the vinylic bonds tend to be slightly polar
• (contributing to a permanent dipole moment).
13. REACTIVITY OF ALKENES:
More reactive than alkanes because:
• A carbon-carbon double bond consists of a σ and a π
bond. It is easy to break the π bond while the σ bond
remains intact.
•The π electrons in the double bond act as a source of
electrons (Lewis's base). Alkenes are reactive towards
electrophiles which are attracted to the negative charge
of the π electrons.
• π bond will broken, each carbon atom becomes an
active site which can form a new covalent bond with
another atom. One π bond is converted into 2 σ bonds
15. REACTION OF ALKENES:
(Catalytic Hydrogenation)
Addition of hydrogen
to a double bond and
triple bond to yield
saturated product.
Alkenes will combine
with hydrogen in the
present to catalyst to
form alkanes.
Platinum (Pt) and
palladium (Pd) –
Catalysts
- Pt and Pd:
temperature 25-90oC
- Nickel can also used
as a catalyst, but a
higher temperature
• of 140oC – 200oC is needed.
16. Addition of halogens in alkenes:
A halogen addition
reaction is a simple
organic reaction
A halogen molecule is
added to the carbon–
carbon double bond of
an alkene functional
group
C=C + X2 → X−C−C−X
(X represents the
halogens bromine or
chlorine, and in this
case, a solvent could be
CH2Cl2 or CCl4).
The product is a vicinal
dihalide.
17. Addition of
hydrogen
halides:
• Addition reaction with electrophilic reagents.
• - Alkenes react with hydrogen halides (in gaseous state or
• in aqueous solution) to form addition products.
• - The hydrogen and halogen atoms add across the double
• bond to form haloalkanes (alkyl halides).
• - Reactivity of hydrogen halides : HF < HCl < HBr < HI
18. MARKOVANIK
RULE:
• There are 2 possible products when
hydrogen halides react with an
unsymmetrical alkene.
• It is because hydrogen halide molecule can
add to the C=C bond in two different ways
• The addition of HX to an unsymmetrical
alkene, the hydrogen atom attaches itself
to the carbon atom (of the double bond)
with the larger number of hydrogen atoms
19. Addition reaction with concentrated sulfuric
acid: hydration of alkenes
• the alkene is absorbed slowly when it passed through concentrated sulfuric acid in the cold 0-15oC).
• involves the addition of H atom and HSO4
• group across the carbon-carbon double bond.
• - follows Markovnikov’s rule
20. ANTI-MARKOVNIC RULE:
Peroxide-catalysed addition of HBr occurs through a free
radical addition rather than a polar electrophilic addition
- also observed for the reaction between HBr and many
different alkenes.
• - not observed with HF, HCl or HI.
• combustion and oxidation follows anti- markonic rule
