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Title: Concept of organic chemistry
Description: There is a brief description of organic chemistry
Description: There is a brief description of organic chemistry
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Unsaturated Hydrocarbons
(Alkenes and Alkynes)
• Hydrocarbons in which all the four valencies of carbon are not
fully satisfied, contain less than maximum number of hydrogen
atoms, and have carbon-carbon multiple bonds, are called
unsaturated hydrocarbons
...
• They have the general formula
CnH2n (n = number of C atoms)
• As a class alkenes are commonly known as olifines (Latin; oleum
= oil ; ficare = to make) because the lower members form oily
products on treatment with chlorine or bromine
...
• They are called:
alkadienes
– if the number of double bonds is two
alkatriens
– for three double bonds etc
• If two double bonds are, adjacent, they are called
comulated double bonds and the alkenes with
comulated double bonds are called allenes
...
• If the two double bonds are separated by a single bond,
they are called conjugated double bonds and the diene is
called conjugated diene
...
• The term alkene is specifically used for those
unsaturated hydrocarbons which have isolated carboncarbon double bonds
...
• The unsaturated hydrocarbons that contain a triple
bond (-C≡C-) in their molecules are called alkynes
and are isomeric with alkadienes
...
• The general formula for alkyne is CnH2n-2
...
Structure of ethylene
• Let us consider (H2C=CH2) for illustrating the orbital make up of
alkenes
...
• They are attached to each other by a σ bond and a π bond
...
e
...
• The π bond is formed from overlap of the unhybridized p orbitals
...
• Ethylene is a planar molecule
...
(a) Formation of σ bonds; (b)
Formation of the π bond ; (c) Geometry of the molecule
...
Such reactions are called elimination
reactions
...
• There are three common types of elimination reactions
1
...
Dehydrohalogenation of alkyl halides
3
...
Dehydration of Alcohols
• Alcohols on heating in the presence of a Bronsted
acid such as sulphuric acid, phosphoric acid or a
Lewis acid such as alumina (Al2O3) lose a molecule
of water to form an alkene
...
Orientation in Elimination
• Alcohols containing more than one β-carbon bearing
hydrogen atoms, on dehydration gives mixture of
alkenes in accordance with Saytzeff rule
...
• For example
Carbonium Ion Rearrangement
• Some primary and secondary alcohols also undergo
rearrangement of their carbon skeleton, involving
carbonium ion as an intermediate
...
• The tertiary carbonium ion then undergoes
deprotonation to form a mixture of isomeric alkenes
in accordance with the Saytzeff rule
...
Dehydrohalogenation of alkyl halides
• Elimination of hydrogen halide molecule from alkyl
halide is called dehydrohalogenation
...
3
...
• Vicinal dihalide on heating with Zn dust in ethanol or
NaI in acetone gives alkenes
...
4
...
• n-Propyl acetate on heating to 500°C gives propene
...
Sodium xanthate
• The xanthate ester is then heated to 180 - 200 oC to
produce alkene
...
• The amine oxide when heated to 120 – 150 °C gives
alkene via a cyclic CH3 transition state
...
• Phosphorus ylides, which are hybrids of two canonical forms, are usually
prepared from triphenylphosphine and alkyl halides which react together
to form phosphonium salts which on treatment with a strong base as
NaOH or n-butyllithium or phenyllithium in dry ether, gives a phosphorus
ylide
...
By Controlled Hydrogenation of Alkynes
• Alkynes react with hydrogen in the presence of Lindlar's catalyst to
give alkenes
...
6
...
Chemical Properties (Reactions) of Alkenes
1
...
• The most commonly used catalysts are platinum, palladium
and nickel
...
• Alkenes react with hydrogen under pressure and in the presence of
Ni, Pt or Pd catalyst to form alkanes
...
The reaction depends on high affinity of hydrogen gas for certain
metals - Ni, Pt, Pd
...
• When a hydrogen molecule and an alkene molecule lie next to
each other, their closeness and the weakening of their bonding
permit a rearrangement of electrons to form new bonds
...
• This is to be expected because the H2 and alkene molecules lie
side by side on the catalyst surface at the time of reaction
...
Addition of Halogens
• Alkenes react with chlorine or bromine in an inert
solvent like CC14 at room temperature to form vicinal
dichloride or vic-dibromide
...
3
...
Halohydrin
vic-dihalide
Halohydrin
• In case of unsymmetrical alkenes the positive halogen
of the hypohalous acid, is attached to the carbon atom
having the greater number of hydrogens in
accordance with the Markovnikov's rule
...
4
...
5
...
• The hydration of alkenes can be brought about as follows:
(a) Addition of sulphuric acid
• Alkenes react with cold concentrated sulphuric acid to form alkyl
hydrogen sulphates
...
• Alkyl hydrogen sulphate solution on dilution with water followed
by heating gives an alcohol
...
• The overall result of the reaction is the addition of H2O
(hydration) to the double bond
...
• The addition of water to the double bond follows
Markovnikov's rule
...
• Diborane (B2H6) reacts with alkenes to form
trialkylboranes
...
• Diborane is commercially available in THF solution
in the form of THF: BH3
...
Oxidation of alkenes
• Alkenes react with various oxidizing agents to give different
oxidation products depending on the nature of the oxidizing
agent, the reaction conditions and extent of oxidation
...
• Epoxides are cyclic ethers with three membered rings
...
(ii) Hydroxylation
• The hypothetical addition of hydrogen peroxide (HOOH) to the alkene double bond is called hydroxylation
...
• The pink colour of KMnO4 is discharged and a brown
solid, MnO2 is formed
...
• Usually each doubly bonded carbon is oxidized to
C=O, while any hydrogen attached to these carbons is
oxidized to -OH group
...
• The process is called ozonolysis and is carried out by
passing ozone containing air through a solution of an
alkene in an inert solvent like chloroform, the ozone
molecule adds to the double bond forming, initially
an addition product, a molozonide, which rearranges
rapidly to form an ozonide
...
• They are, threrefore, are not isolated, but are reduced
directly by treatment with zinc dust in a dilute acid
...
Molozonide
Ozonide
7
...
• A hydrogen on a carbon atom which is directly
attached to a double bonded carbon atom is called
allylic hydrogen and the reaction in which allylic
hydrogen is replaced by halogen is known as
substitution reaction
...
Alkylation (Addition of alkanes)
• Some alkanes add to alkenes in the presence of
H2SO4 or HF to produce higher alkanes
...
• For example, 2-methyl propane adds to 2methylpropene in the pesence of H2SO4 to form 2,2,
4-trimethy]pentane - the well known antiknock fuel
used in petroleum industry
...
Polymerization
• The process by which small molecules (monomers)
join together to form large molecules (polymers) is
known as polymerization
...
• Polymerization involves a large number of monomers
and gives rise to a macromolecule, commonly known
as polymer
...
Simmons-Smith reaction (Carbene addition)
• A carbene is a highly reactive species with the general
formula
• Carbenes possess only six electrons and are
electrophilic because of their electron deficiency
...
• They are prepared, by various methods, within the
reaction mixture in which they will be consumed (in
situ)
...
• A useful approach of adding methylene to an alkene is
by treating alkene with diiodomethane in the presence
of Zn-Cu alloy, Zn-Cu couple alloy generates a carbene
complex which reacts with alkenes to form
cyclopropanes
...
11
...
• An alkene with 2π electrons, called dienophile
...
Alkenes react rapidly with a dilute solution of bromine
in CC14 and as a result the reddish brown colour of
bromine is discharged
...
Alkenes are oxidized by cold dilute aqueous solution of
potassium permanganate and as a result the pink colour
of KMnO4 solution disappears
...
Alkenes dissolve in cold concentrated sulphuric acid
and forms a single phase
...
4
...
Tetranitromethane
Spectroscopic identification of alkenes
• The UV absorption spectra of alkenes show λmax at 171 nm
...
• Carbon-carbon double bonds give stretching absorption peaks at
1620-1680 cm-1
...
Title: Concept of organic chemistry
Description: There is a brief description of organic chemistry
Description: There is a brief description of organic chemistry