ORGANIC CHEMISTRY 171

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ORGANIC CHEMISTRY 171

• Section 201

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Alkenes,Chapter 3
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Alkenes and Cycloalkenes

• Unsaturated hydrocarbons can be
• 1-open-chain (linear and branched alkenes)
• 2- cyclic (cycloalkenes)

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Unsaturated Hydrocarbons
Hydrocarbons that contain at least one CC (
alkenes) are called unsaturated hydrocarbons
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• Alkenes are acyclic unsaturated hydrocarbons that
  contain at least one CC

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• Generic formula Start with CnH2n2 and minus two
  for each CC
• one CC ? e.g., C2H4, C3H6, etc.

CnH2n
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Nomenclature of Alkenes
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Common Names

• Usually used for small molecules.
• Examples

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IUPAC Nomenclature of Alkenes

• 1. Find the longest continuous chain containing
  the double bond.
• 2. Name the corresponding alkane and change the
  ane ending to ene for alkenes.
• 3. Number the chain so as to give the double
  bond the lowest number. Place a numerical prefix
  in front of the parent name to indicate the
  position of the first carbon in the double bond.
• Number and name alkyl groups as with alkanes.

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Nomenclature

• alkenes parent chain contains CC
• CC gets lowest numbers
• position of CC indicated by lower of the two
  numbers

1-butene 4-methyl-1-butene 3-bromocyclohexen
e
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Nomenclature

• CC and OH alkenol

higher priority group (OH) gets last suffix and
lowest number
2-propen-1-ol 5-methyl-4-hexen-2-ol 2-cyclo
hexenol
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Nomenclature

• as side groups

Examples
vinyl chloride allyl alcohol isopropenyl
bromide
ethenyl (vinyl) 2-propenyl (allyl) 1-methylethe
nyl (isopropenyl) methylene
1-vinylcyclohexene methylenecyclopenta
ne
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Name the following compound
The longest continuous chain containing the
double bond is 5 carbons long and is indicated in
blue, below
The parent compound is derived from pentane. The
parent alkene is pentene.
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Nomenclature
E-Z notation

• Determine the higher priority group on each end
  of the alkene.
• If the higher priority groups are
• on opposite sides E (entgegen opposite)
• on the same side Z (zusammen together)

CH3 gt H Cl gt CH2CH3
(E)-3-chloro-2-pentene
(Z)-3-chloro-2-pentene
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Name These Alkenes
1-butene
2-sec-butyl-1,3-cyclohexadiene
2-methyl-2-butene
3-n-propyl-1-heptene
gt
3-methylcyclopentene
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ALKENE STRUCTURE AND BONDING
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ALKENE STRUCTURE AND BONDING
sp2
R
R
sp2
C
C
R
R
SHAPE IS TRIGONAL PLANAR
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Orbital Description

• Sigma bonds around C are sp2 hybridized.
• Angles are approximately 120 degrees.
• No nonbonding electrons.
• Molecule is planar around the double bond.
• Pi bond is formed by the sideways overlap of
  parallel p orbitals perpendicular to the plane of
  the molecule.
  gt

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THE BOND ANGLE OF AN ALKENE
120o
120o
120o
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Bond Lengths and Angles

• Hybrid orbitals have more s character.
• Pi overlap brings carbon atoms closer.
• Bond angle with pi orbitals increases.
• Angle CC-H is 121.7?
• Angle H-C-H is 116. 6? gt

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(No Transcript)
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Isomerism in alkenes
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Cis and Trans Isomers

• Some alkenes can have the same connection of
  atoms, but have a different arrangement in three
  dimensional space.
• This is due to the lack of free rotation about
  the double bond.
• The different arrangements are geometric isomers.
• One of the isomers is cis- the other is trans.

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CIS / TRANS ISOMERS
substituents on the same side of main chain
substituents on opposite sides of main chain
cis
trans
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COMPARE cis / trans ISOMERS IN RING COMPOUNDS
cis
trans
In alkenes and rings cis / trans isomers are
called stereoisomers or geometric isomers.
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2-butene
CH3 groups opposite sides
CH3 groups same side
mp -139oC
mp -106oC
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Geometric Isomers of 2-butene
Insert figure 19.11
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Physical Properties

• Low boiling points, increasing with mass.
• Branched alkenes have lower boiling points.
• Less dense than water.
• Slightly polar
• Pi bond is polarizable, so instantaneous
  dipole-dipole interactions occur.
• Alkyl groups are electron-donating toward the pi
  bond, so may have a small dipole moment.
  
  gt

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Polarity Examples
? 0.33 D
? 0
gt
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Preparation
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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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Dehydration Mechanism
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I. Preparation

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

Press
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Hofmann Product

• Bulky bases abstract the least hindered H
• Least substituted alkene is major product.

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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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Reactions of Alkenes
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Reactivity of CC

• Electrons in pi bond are loosely held.
• Electrophiles are attracted to the pi electrons.
• Carbocation intermediate forms.
• Nucleophile adds to the carbocation.
• Net result is addition to the double bond.
  
  
  gt

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• 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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(It undergoes addition reaction.)

• Electrophilic Addition Reactions
• 1-With HX

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(Mechanism of Addition Reactions)
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• Electrophilic Addition Reactions (contd)
• 2-With conc. sulphuric acid

Hence, this is used in preparation of alcohol.
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• 3-Addition of halogen to alkene (Halogenation)

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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.
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• Addition Reactions (NOT electrophilic)
• 4-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.

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• 5-Ozonolysis

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

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• 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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• 7-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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• Free radical addition mechanism
• Initiation
• Propagation

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• Termination

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