Alkenes Final chemicals can lead to the opposite product result, but the reactive-intermediate structures and mechanisms are different.Hydrobromic acid in particular is prone to forming radicals in the presence of various impurities or even atmospheric oxygen, leading to the reversal of the Markovnikov result.
2. CONTENTS
• Preparation of Alkenes
• Structure of alkenes
• Nomenclature
• Isomerism
• Physical properties of alkenes
• Electrophilic addition reactions of alkenes
• Addition to unsymmetrical alkenes
• Other reactions
• Polymerisation
• Preparation of alkenes
• Revision check list
THE CHEMISTRY OF ALKENES
9. HYBRIDISATION OF ORBITALS - REVISION
The electronic configuration of a
carbon atom is 1s22s22p2
1 1s
2
2s
2p
If you provide a bit of energy you
can promote (lift) one of the s
electrons into a p orbital. The
configuration is now 1s22s12p3
1 1s
2
2s
2p
The process is favourable because the of arrangement of
electrons; four unpaired and with less repulsion is more stable
10. HYBRIDISATION OF ORBITALS - ALKANES
The four orbitals (an s and three p’s) combine or HYBRIDISE
to give four new orbitals. All four orbitals are equivalent.
2s22p2 2s12p3 4 x sp3
11. In ALKANES, the four sp3
orbitals of carbon repel each
other into a TETRAHEDRAL
arrangement with bond angles
of 109.5º.
Each sp3 orbital in
carbon overlaps with
the 1s orbital of a
hydrogen atom to form
a C-H bond.
THE STRUCTURE OF ALKANES
109.5º
12. HYBRIDISATION OF ORBITALS - ALKENES
Alternatively, only three orbitals (an s and two p’s) combine or
HYBRIDISE to give three new orbitals. All three orbitals are
equivalent. The remaining 2p orbital is unchanged.
2s22p2 2s12p3 3 x sp2 2p
13. In ALKANES, the four sp3
orbitals repel each other into a
tetrahedral arrangement.
HOWEVER...
In ALKENES, the
three sp2 orbitals repel
each other into a
planar arrangement
and the 2p orbital lies
at right angles to them
THE STRUCTURE OF ALKENES
14. Covalent bonds are formed
by overlap of orbitals.
An sp2 orbital from each carbon
overlaps to form a single C-C bond.
The resulting bond is called
a SIGMA (δ) bond.
THE STRUCTURE OF ALKENES
15. The two 2p orbitals also overlap to form a second bond. This is
known as a PI (π) bond.
For maximum overlap and hence the strongest bond, the 2p
orbitals are in line.
This gives rise to the planar arrangement around C=C bonds.
THE STRUCTURE OF ALKENES
16. two sp2 orbitals overlap to form a sigma
bond between the two carbon atoms
ORBITAL OVERLAP IN ETHENE - REVIEW
two 2p orbitals overlap to form a pi
bond between the two carbon atoms
s orbitals in hydrogen overlap with the sp2
orbitals in carbon to form C-H bonds
the resulting shape is planar
with bond angles of 120º
17. Alkenes are named according to standard IUPAC rules
• select the longest chain of C atoms containing the double bond;
• place the ending ENE on the basic name
• number the chain starting from the end nearer the double bond
• use a number to indicate the lower number carbon of the C=C
• as in alkanes, prefix with substituents
• side chain positions are based on the number allocated to the first C of the C=C
• if geometrical isomerism exists, prefix with cis or trans
• Number the longest carbon chain that contains the double bond in the direction
that gives the carbon atoms of the double bond the lowest possible numbers.
• Indicate the location of the double bond by the location of its first carbon.
NAMING ALKENES
18. • Name branched or substituted alkenes in a manner similar to alkanes.
• Number the carbon atoms, locate and name substituent groups, locate the
double bond, and name the main chain.
e.g. CH3 - CH = CH - CH2 - CH(CH3) - CH3 is called 5-methylhex-2-ene
19. Boiling point trends are similar to those shown in alkanes
increases as they get more carbon atoms in their formula
more atoms = greater intermolecular Van der Waals’ forces
greater intermolecular force = more energy to separate molecules
greater energy required = higher boiling point
the lower members are gases at room temperature and pressure
cyclohexene C6H10 is a liquid
for isomers, greater branching = lower boiling point
C2H4 (- 104 °C) C3H6 (- 48°C) ....... C6H10 (83°C)
Melting point general increase with molecular mass
the trend is not as regular as that for boiling point.
Solubility alkenes are non-polar so are immiscible (don’t mix with) with water
miscible with most organic solvents.
PHYSICAL PROPERTIES OF ALKENES
20. CHEMICAL PROPERTIES OF ALKENES
ELECTROPHILIC ADDITION MECHANISM
The main reaction of alkenes is addition
Because of the extra electron density in a
C=C double bond, alkenes are attacked
by species which ‘like’ electrons.
These species are called electrophiles; they
possess a positive or partial positive charge
somewhere in their structure.
Examples include... hydrogen halides
21. CHEMICAL PROPERTIES OF ALKENES
ELECTROPHILIC ADDITION MECHANISM
The electrophile, having some positive
character is attracted to the alkene.
The electrons in the pi bond come out to
form a bond to the positive end.
Because hydrogen can only have two
electrons in its orbital, its other bond
breaks heterolytically. The H attaches to
one of the carbon atoms.
22. CHEMICAL PROPERTIES OF ALKENES
Reagent Hydrogen bromide... it is electrophilic as the H is slightly positive
Condition Room temperature.
Equation C2H4(g) + HBr(g) ———> C2H5Br(l) bromoethane
Mechanism
ELECTROPHILIC ADDITION OF HYDROGEN BROMIDE
23. CHEMICAL PROPERTIES OF ALKENES
HYDROGENATION
Reagent hydrogen
Conditions nickel catalyst - finely divided
Product alkanes
Equation C2H4(g) + H2(g) ———> C2H6(g) ethane
Use margarine manufacture
Hydrogenation
Hydrogenation of alkenes produces the corresponding alkanes. The reaction is
carried out under pressure at a temperature of 200 °C in the presence of a
metallic catalyst. Common industrial catalysts are based
on platinum, nickel or palladium. For laboratory syntheses, Raney
OTHER ADDITION REACTIONS
24. nickel (an alloy of nickel and aluminium) is often employed. The simplest example
of this reaction is the catalytic hydrogenation of ethylene to yield ethane:
CH2=CH2 + H2 → CH3-CH3
Hydration
Hydration, the addition of water across the double bond of alkenes,
yields alcohols. The reaction is catalyzed by strongacids such as sulfuric acid.
This reaction is carried out on an industrial scale to make ethanol.
CH2=CH2 + H2O → CH3-CH2OH
Alkenes can also be converted into alcohols via the oxymercuration–
demercuration reaction or hydroboration–oxidation reaction.
25. Halogenation
In electrophilic halogenation the addition of
elemental bromine or chlorine to alkenes yields vicinal dibromo- and
dichloroalkanes (1,2-dihalides or ethylene dihalides), respectively. The
decoloration of a solution of bromine in water with dichloromethylene as
catalyst is an analytical test for the presence of alkenes:
CH2=CH2 + Br2 → BrCH2-CH2Br
It is also used as a quantitative test of unsaturation, expressed as
the bromine number of a single compound or mixture. The reaction
works because the high electron density at the double bond causes a
temporary shift of electrons in the Br-Br bond causing a temporary
26. induced dipole. This makes the Br closest to the double bond slightly
positive and therefore an electrophile.
Hydrohalogenation
Hydrohalogenation is the addition of hydrohalic acids such as HCl or HI to
alkenes to yield the corresponding haloalkanes.
CH3-CH=CH2 + HI → CH3-CHI-CH2-H
If the two carbon atoms at the double bond are linked to a different number
of hydrogen atoms, the halogen is found preferentially at the carbon with
fewer hydrogen substituents (Markovnikov's rule). The use of radical
initiators or other
27. chemicals can lead to the opposite product result, but the reactive-
intermediate structures and mechanisms are different.Hydrobromic
acid in particular is prone to forming radicals in the presence of
various impurities or even atmospheric oxygen, leading to the
reversal of the Markovnikov result.
CH3-CH=CH2 + HBr → CH3-CHH-CH2-Br
28. POLYMERISATION OF ALKENES
ETHENE
EXAMPLES OF ADDITION POLYMERISATION
PROPENE
TETRAFLUOROETHENE
CHLOROETHENE
POLY(ETHENE)
POLY(PROPENE)
POLY(CHLOROETHENE)
POLYVINYLCHLORIDE PVC
POLY(TETRAFLUOROETHENE)
PTFE “Teflon”
