Alkenes

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Alkenes
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I. Preparation

• 1. Dehydration

This is intermolecular dehydration.
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I. Preparation - Dehydration

• Saytzeff Rule
• Hydrogen is preferably removed from the carbon
  with least no. of hydrogen since the alkene
  formed is more highly branched and is
  energetically more stable.

How do you which one is major product?
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I. Preparation

• 2. Dehydrohalogenation
• Example
• alcoholic KOH
• EtO- (ethoxide ion) in EtOH (ethanol)

Press
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I. Preparation

• 3. Dehalogenation

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I. Preparation - dehalogenation (application)
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I. Preparation

• 4. Hydrogenation
• This makes use of a catalyst which activity has
  been decreased by sulphur containing compound.
  E.g. Pd (palladium) in BaSO4

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Chemical Properties(It undergoes addition
reaction.)

• Electrophilic Addition Reactions
• With HX

Press
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II. Chemical Properties- (Mechanism of Addition
Reactions)

• Mechanism

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II. Chemical Properties- (Mechanism of Addition
Reactions)

• The first step is electrophilic attack of H,
  leading to the formation of carbonium ion.
• This is a rate determining step. It is then
  followed by the attack at the positive carbon
  center by X- which is a fast step.

Why first step is rate-determining (slow) while
the 2nd step fast?
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II. Chemical Properties - (Mechanism of Addition
Reactions)

• Energy Profile for electrophilic addition
  reaction

a
b
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II. Chemical Properties - (Mechanism of Addition
Reactions)

• EA1 is high, ? rate determining step.
• EA2 is low, ? once carbonium ion formed, it
  reacts quickly.

a,b are the transition states (activated
complex)
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II. Chemical Properties - (Mechanism of Addition
Reactions)
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II. Chemical Properties - (Mechanism of Addition
Reactions)

• Since alkyl group is electron donating and so
  the positive charge in carbonium ion (II) is less
  intensified.Carbonium ion (II) bears 2 alkyl
  group (i.e. secondary), which stabilize the
  carbonium ion more than that in carbonium ion (I)
  (which is primary). The rate of first step
  (r.d.s.) in forming (II) is fast and thus the
  overall reaction rate is faster in forming

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II. Chemical Properties - (Mechanism of Addition
Reactions)
The 2ry carbonium ion is more easily formed at
the r.d.s. since it is more stable.
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II. Chemical Properties

• Markownikoffs rule
• The more electronegative atom (or group of atoms)
  attached to carbon having least no. of H.In
  general, the greater the no. of alkyl grops
  present, or the larger is the alkyl group, the
  more stable is the carbonium ion.
• Stability of carbonium ion
• 3ry C gt 2ry C gt 1ry C gt CH3

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II. Chemical Properties

• Electrophilic Addition Reactions (contd)
• With conc. sulphuric acid

Hence, this is used in preparation of alcohol.
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II. Chemical Properties - With conc. sulphuric
acid

• Do you remember the Markovnikoffs Rule?What is
  / are the products of the following?

Primary alcohol, CH3CH2CH2OH, formed by which
alkene? Secondary alcohol, CH3CH(OH)CH3, formed
by which alkene? Tertiary alcohol, (CH3)3COH,
formed by which alkene?
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II. Chemical Properties

• Electrophilic Addition Reactions (contd)
• With HOX (Cl2 in aqueous)

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II. Chemical Properties

• Addition of halogen to alkene (Halogenation)

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II. Chemical Properties

• Also by electrophilic addition reaction , the
  halogen molecule is non-polar but when the
  molecule under the influence of a powerful
  electric field of a nearby CC double bond. The
  dense electron cloud of the double bond tends to
  repel the electron cloud of the halogen molecule.
  This causes the halogen atom which is near to the
  double bond to be relatively positive.
• The more positive halogen will then attach itself
  to one of the unsaturated carbon atom forming a
  carbonium ion.
• The lone pair electrons of bromine form bond with
  the carbon bearing ve charge forming the
  brominium ion. The final step is the attack
  bromide ion on the brominium ion.

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II. Chemical Properties Proof for the formation
of brominium ion
The bromide ions attack carbon of the ring from
the side opposite to that of the positive
brominium ion.
Press
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II. Chemical Properties

• Addition Reactions (NOT electrophilic)
• Hydrogenation

• It is used analytically to find the number of
• mole of double bond or triple bond by the number
  
• of mole of hydrogen absorbed per mole of
  molecule.
• b. It is used in converting vegetable oil.

Press
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II. Chemical Properties

• Oxidation (Bond breaking)
• Ozonolysis

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II. Chemical Properties

• Products are carbonyl compounds which can be
  easily identified. If RH aldehyde will be formed
  which will be further oxidised by H2O2 to organic
  acid. Since the resulting carbonyl compounds are
  used for identification, thus oxidation is an
  unfavorable process. In order to prevent this
  happens, zinc dust is added.
• e.g. if the products are identified as
• CH3CH2COCH3 and CH3CH2 CH2CHO
• The original alkene will be

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II. Chemical Properties

• Oxidation
• at room temperature (Hydroxylation) (addition)

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II. Chemical Properties

• at vigorous condition (bond breaking)

MnO4- / H
Further oxidation
In acidic condition, the products will be
oxidised to acid or ketone.
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II. Chemical Properties
MnO4- / H

• Therefore, if CO2 is present as product, the
  alkene possesses a terminal double bond.

further oxidation
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II. Chemical Properties

• Addition Polymerization
• This is a process by which simple molecules are
  joined up to form large molecule with same
  empirical formula.
• condition high temperature and pressure
• with Zieglers catalyst

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II. Chemical Properties

• Free radical addition mechanism
• Initiation
• Propagation

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II. Chemical Properties

• Termination

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II. Chemical Properties

• e.g. polythene
• moleclar mass 50000g
• melting point 126oC 135oC (having diff.
  Isomer)
• Properties
• Light, inert (strong sigma bonds) and
  water-resistance (do not form H-bonding), tough
  and capable of moulding. It is a thermo-plastic
  since chains of hydrocarbons causes the chain to
  move apart and to come closer again on cooling.

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II. Chemical Properties

• Uses
• Make water-proof sheeting
• Electrical cable insulator
• Make mould articles