Hydrocarbons: Alkenes, Cycloalkenes, Dienes and Alkynes

36 Pages·2008·1.77 MB·English

ORGANIC CHEMISTRY

Hydrocarbons: Alkenes, Cycloalkenes, Dienes and Alkynes

Dr. Geetu Gambhir
E-340, Greater Kailash II
New Delhi -110048

(31.07.2006)

CONTENTS
Alkenes
Structure
Nomenclature of alkenes
Methods of preparation
Dehydration of Alcohols
Properties
Regioselectivity
Polymerization of Alkenes
Industrial Application of ethylene and Propene
Cycloalkenes
Nomenclature
Conformations
Reactions
Dienes
Types of Dienes
Nomenclature
Allenes
Methods of Preparation
Reactions
1, 3 – Butadiene
Methods of preparation
Reactions
Alkynes
Structure
Nomenclature
Methods of preparation
Properties
Acidity of terminal hydrogen
Reactions

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Alkenes
Alkenes are the unsaturated hydrocarbons having one or more carbon – carbon double bond.
These are also termed as olefins since ethene (or ethylene – common name), the simplest alkene
forms an oily liquid when treated with chlorine.

General formula: CnH n
2
Structure
The double bond geometry of alkenes is typical of that found in ethene. Each of the double
bonded carbon atom is Sp2 hybridized, molecular geometry of which requires it to have trigonal
planar geometry i.e. all the atoms surrounding each carbon atom lie in the same plane with bond
angles approximating 120°. Of the three hybrid orbitals formed by Sp2 hybridization two overlap
with s orbital of each hydrogen forming sigma bond. The third hybrid orbital overlapps with the
Sp2 hybrid orbital of adjacent carbon atom forming another sigma bond. Pure p orbital on each of
the two adjacent carbon atoms undergoes sideways overlap to form a pi bond. As a result of the
double bond the carbon bond length is shorter for alkenes in comparison to the alkanes (also
because in Sp2 hybrid orbitals there is large amount of s character (33%)and therefore density
with in Sp2 orbitals is concentrated closer to the nucleus). e.g. C H
2 2

Nomenclature of alkenes
Nomenclature of alkenes can be derived by simply modifying the alkane nomeclature.
1. An unbranched alkene is named by replacing the “ane” suffix in the corresponding
alkane with “ene”.
2. Carbon atoms are numbered from one end of the chain to other so that the double
bond receives the lowest number.
3. For the branched alkenes, the principal chain is defined as the carbon chain
containing the greatest number of double bonds even if it is not the longest (if more
than one chain with equal numbers of double bonds then the longer one is the
principal chain).
4. The principal chain is numbered from the end that results in the lowest numbers for
the carbon of the double bond.
5. Alkene containing the alkyl substituent the position of the double bond and not the
position of branch determines the numbering of the chain.
6. Position of the double bond is cited in the name after the name of the alkyl group.
7. If the compound contains more than one double bond, the “ane” ending of the
corresponding alkane is replaced by “adiene” or “atriene” and so on for two three or
more double bonds.

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Example:

Isomerism
Isomeric alkenes that differ in position of the double bond are the constitutional isomers.

Alkenes with identical connectivities that differ in the spatial arrangement of their atoms are
called stereoisomers. For any alkene with two different groups around the double bonded
carbon atom, interchanging the two groups at either carbon of the double bond gives different
molecules hence stereoisomers (The two can not be interchanged by simple rotation as any
such rotation would break the pi bond). For example:

Methods of preparation
1. Partial reduction of alkynes: to form alkenes can be brought about by number of reducing
agents like Na/liq NH , hydrogen in presence of palladium poisned with BaSO or CaCO
3 4 3
along with quinoline (Lindlar catalyst) , Hydrogen in presence of nickel boride.

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2. Dehydrohalogenation of alkyl halides: by action of strong base like ethanolic potassium
hydroxide. It proceeds by elimination of hydrogen halide leading to formation of alkene.

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Mechanism
• The reaction is referred as 1,2 elimation or β-elimation
• It is a base catalyzed reaction
• Hydrogen is abstracted by a base as a proton leaving behind it’s electron pair and the
leaving group (halogen ) leaves the substrate molecule (alkylhalide) as halide ion taking
its electron pair along with it. An extra electron pair on the carbon atom is responsible for
the pi bond between the carbon atoms.

• It is a single step bimolecular elimination reaction.
• Rate determining step involves cleavage of C - H and C - L bonds.
• The energy required to break these bonds come from the energy released due to
formation of B-H bond and pi bond
• The cleavage of C-X bond in rate determine step implies the reactivity of alkyl halides to
follow the order R-1>R-Br>RCl which matched the bond dissociation energy of C-X
bond.
• A good leaving group is a weakly basic anion or molecule.
• The reaction is not accompanied by rearrangement, which favours the given second order
kinetics.
• The reaction proceeding by E1 mechanism or first order kinetics follows the mechanism
where substrate undergoes slow heterolysis to form halide ion and a carbocation. In the
second step carbocation rapidly loses a proton to a base and forms the alkene.

The carbocation can very well combine with a nucleophile or undergo rearrangement.
Example

• Ease of dehydrohalogenation of alkyl halides is 3° > 2° >1°.
• If the alkyl halide has two or more β carbon atoms then two or more alkenes are possible
as products. It is the stability of the alkenes that decides the chief product. According to
Saytzeff Rule dehydrohalogation of alkyl halides leads to the formation of that alkene
which has maximum number of alkyl groups attached to >C = C< (more substituted
alkene is more stable)

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• If size of the base is increased it is easier to abstract proton from a less substituted β
carbon atom of alkyl halide. Therefore less stable alkene is the chief product. This is
known as Hoffmann rule

• Saytzeff product is the major product for alkyl iodides, alkyl bromides and alkyl
chlorides.
• Hoffman product is the major product for alkyl flourides.

Reason : Increases in carbon - halogen bond dissocation energy makes difficult for leaving
group to leave as Xθ. As a result reaction follows the alternate mechanism called elimination
from conjugate base. Here the base first removes proton from the β carbon atom forming
carbanion as the intermediate. The carbanion then attacks the α-carbon atom causing the
removal of F.

Dehydration of Alcohols
Alcohols when treated in presence of H SO , P O , BF or by passing the vapour of alcohol
2 4 2 5 3
over a catalyst commonly alumina (Al O ) at high temperature undergo a loss of water
2 3
molecule with the formation of alkenes.

• Mechanism:
Ist step
-OH group of alcohol is protonated in a fast reversible reaction (Acid transforms the poor
leaving group (-OH) (strongly basic) into a good leaving group O+H (weakly basic water
2
molecule).

IInd step
Water molecule is lost with the formation of carbonium ion in the rate determining step.

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IIIrd Step
Carbonium ion loses proton from its adjacent carbon atom to form stable alkene. The anion
of the acid or another alcohol molecule functions as a base and facilitates the loss of proton.

• The ease of dehydration of alcohols 3° > 2° > 1°.
Primary alcohols require strong conditions of conc. H SO and high temperature (180° –
2 4
200°) while secondary alcohols dehydrate under milder condition with 85% H PO at 160°C
3 4
or 60% H SO at 100°C. Tertiary alconds can be easily dehydrated by 20% H SO at 85°-
2 4 2 4
90°C

Reason:
• Alchol reactivity parallels carbocation stability 3° > 2° > 1°.
• More stable intermediate (carbocation) implies a lower activation energy (Ea).
This can be understood from the Hammonds postulate that for the endothermic conversions (as
dissociation of alkyloxinium ion involves bond breaking without any bond making to
compensate for energy required) as shown in the figure, the transition state resembles the high
energy intermediate or product and tracks the energy of this intermediate if it changes. The
change in transition state energy and activation energy is observed as the stability of intermediate
(carbocation changes)

Since the activation energy path to the formation of more stable carbocation (3° > 2° > 1°) is the
lowest, hence the reactivity of the alcohol also follows the similar order.
• When more than one alkene can be formed the preferred product is more stable one.

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• When carbonium in is the intermediate and structure permits rearrangement in which an atom
or a cyclic ring expands so that more stable carbonium ion is formed, the rearrangement
tends to take place example.

• Alkyl sulphonates undergo base promoted elimination closely analogues to
dehydrohalogenation.

• The leaving group (sulphonyl group) is a weak base.
4. Dehalogenation of vicinal dihalides (where two halogen atoms are attached to the adjacent
carbon atom)

• The two halogen atoms are lost in two steps and the two align themselves at 180 and in
the same plane before they are lost .

NaI and Acetone

5. Cleavage of ethers: Treatment with strong bases such as sodamide or alkyl lithium or alkyl
sodium, alkenes are generated

Mechanism: Reaction takes place through cyclic intermediate
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• Reaction is aided by e- withdrawing groups at β position
6. Pyrolysis of esters
Thermal cleavage of an ester usually acetate, involves the formation of cyclic transition state
leading to the elimination of an acid leaving behind alkene as a product.

• This is cis elimination where both the leaving groups proton and carboxylate ion are in
the cis position.

7. Hoffmann Degradation method: by heating quartenary ammonium hydroxide under
reduced pressure and at a temperature between 100ºC and 200ºC

• OHθ ion removes proton from β -carbon atom which gives more stable carbanion.
8. Wittig Reaction :Aldehydes and ketones are converted to alkenes by using special class of
compounds called phosphorus ylides or wittig regent. The alkyl halide is treated with
triphenyl phospine to produce phosphonium halide. A strong base like C H Li or n-C H Li
6 5 4 9
converts it to phosphorane, which is stabilized by resonance.

Phosphorane reacts with aldehyde or ketone to produce alkene via a cyclic intermediate.

9. By cracking of petroleum number of alkenes like ethylene propene or butene can be
prepared.

Properties
Physical properties of Alkenes
• Like alkanes, alkenes are inflammable, non-polar compounds, less dense than and
insoluble in water but soluble in nonpolar solvents like benzene petroleum ether etc.
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• Lower molecular weight alkenes (C -C ) are gases
2 4
• Sp2 hybridized carbon atom is more electronegative than Sp3 carbon atom so Sp2-Sp3
carbon - carbon bond has a small dipole directed towards Sp2 carbon atom.

Dipole moment of the compound is vector sum all the bond dipoles. This is why is cis 2-butene
has a net dipole moment, while the dipole moment of trans 2-butane is zero.

• Boiling points increases
(i) With increasing carbon content
(ii) With decrease of branching
• cis alkenes boil at some what higher temperature than trans alkene due to its higher
dipole moment.
• cis alkene have poor symmetry and do not fit in to crystalline lattice as compared to trans
isomer therefore cis alkene has low melting point.
• Thermal stability of alkenes can be compared by determination of their heats of
combustion. Lower is the – ∆H value of combustion, higher is the stability. In general
order of stability is.

• More substituted alkenes are more stable.
Chemical properties
• The most characteristic type of alkene reaction is electrophillic addition at carbon-carbon
double bond. The pi bond of the alkene and X-Y bond of the reagent are broken and new
C-X and C-Y bonds are formed

• Majority of these reactions are exothermic due to the fact that the C-C pi bond is weak
relative to the sigma bonds formed to the atoms or groups of the reagent. Consequently
bond energies of product molecules are greater than the bond energies of the reactants.

1) Hydrogenation ; Hydrogen adds to the double bond under pressure and in presence of
catalyst to form alkenes.

Heterogeneous catalyst: Catalyst such as finally divided Platinum or palladium black or nickel
are useful as hydrogenation catalyst. They are used in conjunction with solid support materials
such as alumnia (Al O ), BaSO or activated charcoal.
2 3 4

• These are heterogeneous catalyst as they are insoluble in reaction solution
• These noble metal catalysts can be filtered and reused.
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