STRUCTURE AND REACTIVITY: HYDROCARBONS

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• STRUCTURE AND REACTIVITY HYDROCARBONS

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Aim

• Study the reactivity of few structurally
  identical hydrocarbons.
• Characterize the unknown using its reactivity and
  spectroscopy.

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Terms

• Hydrocarbons
• Bromination
• Free Radicals
• Substitution Reaction
• Addition Reaction

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Introduction

• More than 18 millions organic compounds, that are
  known today, are classified into just a few
  general families on the basis of chemistry that
  follows a simple pattern.

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Introduction

• A group of atoms within a large molecule that has
  a characteristic structure and chemical behavior
  is called FUNCTIONAL GROUP.
• Functional groups allow us to group vast number
  of organic molecules in to few classes.

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Hydrocarbons

• Organic compounds that contain only carbon and
  hydrogen.
• No heteroatom.

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Hydrocarbons

• Alkanes Have only single bonds and saturated
  hydrocarbons.
• Alkenes Contain a carbon-carbon double bond
  functional group and unsaturated hydrocarbons. It
  is also called OLEFINS

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Introduction

• Alkynes Contain a carbon-carbon triple bond
  functional group.
• Arenes Compounds contain a six-member ring of
  carbon atoms with three alternating double bonds.
  Its also called AROMATIC COMPOUND

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Part 1

• Reaction with Bromine (Br2).
• Reaction with aqueous Potassium Permanganate (aq.
  KMnO4).
• Reaction with strong acids (con. H2SO4).

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Reaction with Bromine

• Different classes of hydrocarbon show widely
  different reactivity with bromine.
• Some yield HBr as side product.
• Bromination can be easily detected by naked eye.

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Alkanes with Bromine

• Alkanes dont react with acids, bases, or most
  other common laboratory reagents.
• Their only major reactions are with oxygen and
  with halogens.
• Combustion Reaction of alkanes with oxygen.
  Carbon dioxide and water are produced in this
  combustion reaction.

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Alkanes with Bromine

• Initiation of halogenation reaction requires
  heat or light.
• Replacement of a hydrogen of alkanes by a
  chlorine or bromine.
• Alkanes are incapable of undergoing addition
  reaction since they are saturated.

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Alkanes with Bromine

• Active free radicals are formed in presence of
  light.
• Its an example for a SUBSTITUTION REACTION
• e.g., CH4 with Br2 in presence of light.

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Alkenes with Bromine

• Alkenes readily react with bromine.
• No light or heat is required.
• No replacement reaction takes place.
• Alkenes form a Bromonium Ion intermediate with
  bromine.
• Its in an example of ADDITION REACTION.
• e.g., CH2 CH2 with Br2

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Arenes with Bromine

• Arenes have double bonds like alkenes.
• Reactivity is completely different from alkenes.
• Benzene does not undergo the typical addition
  reaction like alkene with Br2.

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Arenes with Bromine

• It needs a catalyst.
• They follow substitution reaction.
• e.g., C6H6 with Br2 in presence of FeBr3

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Arenes with Bromine

• Alkyl derivative of benzene reacts with bromine
  like alkanes under light.
• Active free radicals are responsible for
  bromination.
• Its also an example of substitution reaction.
• e.g., Toluene with Br2 in presence of light.

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Bromination Procedure

• Add 20 drops (1 mL) of the hydrocarbon to be
  tested.
• Add 15 drops of 5 Br2.
• Use Red and Blue litmus to test for HBr.
• Keep the test tube in the dark place if it is not
  reacting immediately.
• Prepare identical sample and place it under
  light.
• Note the results.

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Reaction with aq. KMnO4

• KMnO4 is a strong oxidizing agent.
• It has the capability of oxidizing number organic
  functional group.
• Easily detected by the color change.

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Alkanes with aq. KMnO4

• Normally alkanes do not react with aq. KMnO4.
• Usually oxidizes at very high temperature with
  oxygen.

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Alkenes with aq. KMnO4

• More readily oxidized than alkanes.
• It attacks the ? electrons of the double bond.
• Very useful test to detect the presence of double
  bond.

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Arenes with KMnO4

• Unsubstituted arenes do not oxidize easily by aq.
  KMnO4 like alkenes.
• Alkylarenes react with aq. KMnO4 under hot
  condition.
• e.g., Toluene with aq. KMnO4 to for benzoic acid.

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Oxidation Procedure

• Take 1 mL (20 drops) of aq. KMnO4 in a test tube.
• Add five drops of the hydrocarbon to be tested.
• Mix it well and note the results.
• Note the results.

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Reaction with Con. H2SO4

• Number of organic compounds react with
  concentrated sulfuric acid.
• Evidence of reaction
• 1. Dissolution
• 2. Evolution of Heat
• 3. Color change (from yellow to black)
• If a hydrocarbon does not react with the acid,
  they will be insoluble.

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Reaction with Con. H2SO4

• Place 1 mL of concentrated sulfuric acid in a
  clean test tube.
• Add 5 drops the compound being tested.
• Mix the contents carefully.
• Note the results.

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Part B

• Get your unknown from the instructor.
• Write the unknown number in your lab note book.
• Do the reaction of unknown with Br2, aq. KMnO4
  and concentrated sulfuric acid.
• Get the IR and 13C NMR spectrum
• From the reactivity, IR and 13C NMR, Characterize
  your unknown.

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Caution!

• Bromine (Br2) and concentrated sulfuric acid
  (H2SO4) very, very dangerous and that can cause
  serious chemical burns.
• Use Red and Blue litmus instead of Congo Red.
• Work in pairs.