13.1c Hydrocarbons - Alkenes reactions part 2

• Chemistry
• Secondary Education
• Age 12,13

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

Expectations

• Hydrogen halides
• Addition of hydrogen halides
• Oxidation by cold and hot KMNO4 solution
• Polymerisation
• Cracking of long-chain hydrocarbons
• Environmental consequences of using hydrocarbons
• Next Chapter: Halogenoalkanes
• Physical properties of halogenoalkanes
• Reactions of halogenoalkanes

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

Common hydrogen halides 

Addition of hydrogen halide, HX

• Reagent: hydrogen halide, HX(g)
  
• Condition: room temperature
• Product: halogenoalkanes (alkanes with halogens)

Activity 1 - Addition of hydrogen halide to a symmetrical alkene

• Work in pairs and explain the reaction mechanism below: ethene + hydrogen bromide.

Addition of hydrogen halide to an unsymmetrical alkene.

unsymmetrical alkene

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

What conditions and reagents are necessary to oxidise alkenes by cold and dilute acidified KMNO4?

• Reagent: cold and dilute potassium manganate (VII) solution
  
• Condition: room temperature
• Product: alcohols (diols)

Oxidation of alkenes by cold and dilute KMNO4 solution

• Reagent: cold and dilute potassium manganate (VII) solution
• Condition: room temperature
• Product: alcohols (diols)

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

What conditions and reagents are necessary to oxidise alkenes by hot and concentrated acidified KMNO4?

• Reagent: hot and concentrated potassium manganate (VII) solution
  
• Condition: room temperature
• Product: carbon dioxide/ carboxylic acids/ ketones

Activity 2 - Oxidation by hot and concentrated KMNO4

Work in pairs and explain the reaction mechanism: 

• (a) four alkyl groups in ethene + 2[O] or oxygen from the oxidising agent
• (b) three alkyl groups and one hydrogen in ethene + 3[O] or oxygen from the oxidising agent
• (c) two alkyl groups and two hydrogen in ethene + 4[O] or oxygen from the oxidising agent

Plenary discussion

• four alkyl groups in ethene + 2[O] 

Plenary discussion

• three alkyl groups and one hydrogen in ethene + 3[O] 

Plenary discussion

• two alkyl groups and two hydrogen in ethene + 4[O] 

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

Recall

• Describe what a polymer is.
• Name an example.

Interdisciplinary integration: BIOLOGY

Examples of natural polymers

Polymerisation reactions

• (a) polymerisation of propene
• (b) polymerisation of chloroethene
• (c) polymerisation of ethene (with the required conditions and catalyst)

Polymerisation of propene

Polymerisation of chloroethene

Polymerisation of ethene (with the required conditions and catalyst)

• Pressure: a few atmospheres
• Temperature: about 60oC
• Catalyst: Ziegler-Natta catalysts or other metal compounds

REACTIONS OF ALKENES

Organic Chemistry

27 February 2024

What is the primary source of hydrocarbons?

Catalytic cracking

• Temperature: about 500oC
  
• Catalyst: zeolite (Aluminosilicate)

Cracking of long-chain hydrocarbons

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

HALOGENOALKANES

Organic Chemistry

27 February 2024

Describe a halogenoalkane.

Halogenoalkanes (Alkyl halides)

• Compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine)

Classifications

Halogenoalkanes (Alkyl halides)

Physical properties of halogenoalkanes

• boiling point =increases as the molecular size increases
• primary halogenoalkanes = higher boiling points than secondary and tertiary halogenoalkanes of the same molecular weight
• insoluble in water but soluble in organic solvents

REACTIONS OF HALOGENOALKANES

Organic Chemistry

27 February 2024

Chemical reactivity of halogenoalkanes

• Which carbon-halogen bond is the easiest to break?
• Which carbon-halogen bond is the hardest to break?
• Which halogenoalkane is the least reactive?

carbon-halogen bond

bond energy

Activity 4 - Nucleophile or Electrophile?

Nucleophilic substitution

• substitution of an atom by a nucleophile
• possible due to the polarity of the carbon-halogen bond
• two mechanisms, the SN1 or SN2 mechanism.

Two Mechanisms of Nucleophilic Substitution

• Primary halogenoalkanes will use SN2.
• Tertiary halogenoalkans will use SN1.
• Secondary halogenoalkanes will use both.

REACTIONS OF HALOGENOALKANES

Organic Chemistry

27 February 2024

Activity 5 - Nucleophilic Substitution

Work in groups of 3 and explain the SN1 and SN2 mechanisms.