ALKENES

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ALKENES
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• Unsaturated chemical compound containing at least
  one carbon-carbon double bond, where rotation
  about the CC is very difficult.
• To show the presence of the double bond, the ane
  suffix from the alkane name is changed to ene.
• Also called olefins( fat dissolving)
• sp2 atomic orbitals
• Trigonal planar, 120o degree

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Geometric Isomerism

• Cis-trans isomerism
• -isomers that have same order of atom attachment
  but a different arrangement of their atoms in
  space.

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General Formula

• CnH2n
• where n is the number of carbon atoms in the
  molecule

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Physical Properties

• Physical state -The first lower member like
  ethene, propene and butene are colorless gases.
•  Density - lighter than water.  
• Solubility - insoluble in water and soluble in
  nonpolar organic solvents.
• more reactive than alkanes due to their double
  carbon-carbon bond.

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• Boiling point -The boiling points of alkenes
  gradually increase with an increase in the
  molecular mass.
•  The cis isomer ( example cis-2-butene, b.p.
  3.7C) is higher in bpt than its trans isomers
  (example, trans-2-butene, b.p. 1C) 

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• Melting point 
• The melting points of alkenes increase with an
  increase in the molecular mass.

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Natural Sources

• Isolated from petroleum.
• Plant material like plant hormone, like Ethylene
  a natural ripening agent and Terpenes found
  in essential oil.

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• Some Common Alkene Polymers and their Uses
• Ethylene H2CCH2 Polyethene, Polythene Packaging,
  cable insulation, films and sheets
• Tetrafluoroethene F2CCF2 Polytetrafluoroethene,
  PTFE, Teflon Coatings, gaskets.
• Chloroethene (vinyl chloride) H2CCHCl Polyvinyl
  chloride, PVC, Tedlar Insulation, films, pipes
• Styrene H2CCHC6H5 Polystyrene, Styron Foam for
  packaging etc.
• Vinyl acetate H2CCHOCOCH3 poly(vinyl acetate),
  PVA
• Paints, adhesives.

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Preparations of Alkenes

• 1. Dehydrohalogenation of Alkyl halides
• 2. Dehydration of Alcohol
• 3. Dehalogenation of Vicinal Halides
• 4. Reduction of Alkynes

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Preparations of alkenes

• 1. Dehydrohalogenation of alkyl halides

3 gt 2 gt 1
Example
n-butyl chloride 1-butene
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Dehydrohalogenation of Alkyl Halides

• Is an Elimination reaction. The term
  "elimination" describes the fact that a small
  molecule is lost during the process.
• Different mechanisms are possible
• Loss of the LG to form a carbocation,  removal of
  H and formation of CC bond
• Simultaneous H removal, CC bond formation and
  loss of the LG
• Removal of H to form a carbanion, loss of the LG
  and formation of CC bond.

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2. Dehydration of alcohol
3 gt 2 gt 1
ex.

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n-butyl alcohol 1-butene 2-butene
(chief product)
sec-butyl alcohol 2-butene
1-butene (chief product)
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Dehydration of Alcohols

• It is the elimination of water molecule from
  alcohol to convert into alkene.
• Lost of H and OH from adjacent carbons
• An acid catalyst alkene.

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Mechanism of Alcohol Dehydration

• Step 1 Alcohol unites with a hydrogen ion to
  form the protonated alcohol
• Step 2 Alcohol associates into water and
  carbonium ion.
• Step 3 The carbonium ion then loses a hydrogen
  ion to form alkene.

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• Dehalogenation of vicinal dihalides
• (same side)

Example

2,3- Dibromobutane 2- butene
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4. Reduction of alkynes
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Reduction of Alkynes

• Reducing Alkynes to form trans or cis Alkenes.
• Using Na/ NH3
• Step 1
• Sodium transfer an electron to the alkyne giving
  a radical anion.
• Step 2 The radical anion removes a proton from
  the ammonia in an acid/base reaction

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• Step 3
• A second atom of sodium transfers another
  electron to the alkyne giving an anion.
• Step 4
• the anion removes proton from the ammonia in an
  acid/base reaction.

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Reactions of Alkene

• Halogenation
• Hydration
• Hydrogenation
• Addition of hydrogen Halides
• Addition of sulfuric acid
• Addition of Carbenes
• Addition of Free Radical

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• Allylic Hydrogenation
• Dimerazation
• Alkylation
• Polymerization
• Hydroxylation
• Halohydrins formation
• Ozonolysis

• Hydroboration-oxidation
• Epoxidation

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Reactions of Alkenes 1. Addition of Halogens
(X2)
Example.
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Halogenations Addition of Halogens

• When an alkene is treated at room temperature
  with a solution of bromine or chlorine in carbon
  tetrachloride or some other inert solvent, the
  halogens adds rapidly to the double bond of the
  alkene to give the corresponding vicinal dihalide
  ( two halogens attached adjacent carbons

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2. Addition of Hydrogen (catalytic
hydrogenation)
Ex.
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Hydrogenation of Alkenes

• The relationship between reactants and products
  in addition reactions can be illustrated by the
  hydrogenation of alkenes yield alkanes.Hydrogenati
  on is the addition of H2 to a multiple bond.

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3. Addition of hydrogen halides
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Ex.
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Ex.

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Addition of hydrogen halides

• In the addition of an acid to the CC of an
  alkene, the hydrogen of the acid attaches itself
  to the carbon that already holds the greater
  number of hydrogens
• The reactivity of alkene, with halogen acids is
  in the order.
• HI gt HBr gt HCl

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4. Addition of sulfuric acid
Ex.
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5. Addition of water. HYDRATION
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Ex.
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Addition of Water - Hydration

• When heated with water in the presence of an acid
  catalyst, alkenes yield alcohol ROH.
• The process is called hydration of alkenes
  because it involves the addition of water across
  the double bond.
• The addition of the HOH across the double bonded
  carbon that bears the greater number of hydrogen
  atoms and the hydroxyl groups goes to the other
  double-bonded carbon

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6. Halohydrin formation
Ex.
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Sterospecific
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7. Dimerization (di two, mer part, product
contains exactly twice the of C H atom
as the original).
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Mechanism
Addition of the tert-butyl cation to iso
butylene the orientation of addition is duch to
yield the more stable tertiary cation. Step(2)
brings about the union of two isobutylene
units, which is of course necessary for the
product.
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8. Alkylation
ex.
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mechanism
Addition of a hydrogen ion to form carbocation
Addition of a tert-butyl carbocation to
isobutylene
Carbocation abstracts a hydrogen atom with its
pair of electrons from a molecule of alkane. This
abstraction of hydride ion yields an alkane of 8
carbons and a new carbocation to continue the
chain.
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A carbocation may a.) combine with a negative
ion or other basic molecule b.) rearrange to a
more stable cabocation c.) eliminate a hydrogen
ion to form an alkene d.) add to an alkene to
form a larger carbocation e.) abstract a hydride
ion from an alkane
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9. Oxymercuration - demercuration

• Oxymercuration involves addition to the CC of
  OH and HgOAc (mercuric ion)
• Demercuration the HgOAc is replaced by H

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Ex.
Undergo the process of oxymercuration, involves
addition to the carbon carbon double bond of
OH -HgOAc
Follows Markovnikov addition
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10. Hydroboration oxidation

With the reagent Diborane, alkenes undergo
hydroboration to yield alkylboranes, which on
oxidation give alcohols.
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Mechanism
Hydroboration involves the addition of the double
bond of BH3 w H becoming attach to one doubly
bonded carbon and boron to the other. The
alkylborane can then undergo oxidation in which
the boron is replaced by OH. Thus, the 2 stage
reaction process of hydroboration oxidation
permits the effect. The addition to the carbon
carbon double bond of elements of H-OH.
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11. Addition of free radicals
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Mechanism
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Electrophilic addition Markonikov orientation
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Free radical Addition Anti Markovnikov
orientation
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12. Polymerization of Alkenes Polymerization
the joining together of many small molecules to
form very large molecules. Monomers the simple
compounds form which polymers are made.
Ex.

• 5 processes of polymerization
• 1. Free - radical polymerization
• 2. Cationic polymerization
• 3. Anionic polymerization
• 4. Condensation polymerization
• 5. Coordination polymerization

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Free radical polymerization

• Polyvinyl chloride - use to make phonograph,
  records, plastic pipes, when plasticized with
  high boiling esters raincoats, shower curtains
  and coatings for metal and upholstery fabrics.
• Peroxide initiator, required in small amount in
  polymerization
• Free radical initiator

Mechanism
Free radical adds to molecule of alkenes which
for another free radical
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This radical adds to another molecule of alkene
to generate another free radical. This radical
adds to another molecule of alkene to generate a
still larger radical
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13. Addition of Carbenes. Cycloaddition carbenes
derivative of methylene
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Methylene exist into 2 different forms
singlet methylene unshared electrons are
paired, less stable generated first in
photolysis stereospecific addition
triplet methylene unshared electrons are not
paired, free radical (diradical)
nonstereospecific addition
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Cycloaddition addition of the carbon carbon
double bond
Sterospecific (addition of methylene can occur
with 2 different kinds of stereochemistry.)
Photolysis of diazomethane into in liquid
And in liquid
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Singlet methylene Stereospecific Electrophilic
addition Electron deficient and can find
electrons at the C-C double bondingle


Triplet methylene Non - Stereospecific Free
radical addition ff. by addition
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Methylene undergoes intersection
Addition of substituted carbenes 1,1 -
elimination
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Mechanism
Reaction involves a divalent carbon compound, a
derivative of methylene dichlorocarbene
CU2. Generated in 2 steps, initiated by attack on
chloroform by the strong base tert-butoxide ion
and then adds to the alkene.


Because of the presence of halogen atom, the
singlet form is the more stable form of
dichlorocarbene and is the one adding to the
double bond.
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Addition of Carbenes

• Carbenes are intermediates of the general formula
  CH2. The derivatives of methylene (CH2) are the
  carbenes.
• Methylene is formed by the photolysis of either
  diazomethane, CH2N2 or ketene, CH2CO.

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14. Hydroxylation. Glycol formation
Example.

• Oxidizing agents that bring about hydroxylation
• cold alkaline potassium permanganate, KMnO4
• b. peroxy acids, such as peroxyformic acid HCO2OH

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15. Halogenation. Allylic substitution ( same
mechanism with substitution in alkenes)
Ex.
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• Can we direct the attack to just one of these
  sites? Yes, by our choice of rxn. Conditions.
• Conditions
• alkenes undergo substitution by halogen at high
  temp. or under the influence of UV light,
  generally in gas phase.
• it can also undergo addition of halogen at low
  temp. in the absence of light and generally in
  liquid state(phase).

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N bromosuccinimide a reagent used for the
specific purpose of brominating alkenes at the
allylic position provides a constant low conc. of
bromine.
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Vinylic hydrogen- hydrogens attached to CC
Alylic hydrogen hydrogens attached to a carbon
atom next to a double bond
Ease of abstraction of hydrogen atoms
Allylic gt3º gt 2º gt 1º gt CH4 gt Vinylic
Ease of formation of free radicals allyl gt 3º gt
2º gt 1º gt CH3. gt vinyl
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• Example

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16. Ozonolysis (Cleavege rxn)
Cleavage a rxn in which the double bond is
completely broken and the alkene molecules
converted into 2 smaller molecule.
Reducing agent (Zn) prevent formation of
hydrogen peroxide will not react with aldehyde
and ketone (aldehyde are often converted to
acid, RCOOH for ease of isolation.)
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Ozonolysis is a typical means of degradation
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17. Cleavage with periodate ( cleavage with a
diol)
RCOOH are generally obtained instead of
aldehydes, RCHO a terminal CH2 group is
oxidized to CO2.
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Example.
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Cleavage of cycloalkenes
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18. Epoxidation of Alkenes
O
O
CC CH3COOH C C CH3COH
O
Carboxylic acid
Alkene
Peroxyacetic Acid
Epoxide
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Example
O
O
H2CCH(CH2)9CH3 CH3COOH
CH2-CH(CH2)9CH3 CH3COH
1-Dodecane
1,2-epoxydodecane
Acetic acid
Peroxyacetic acid