Synthesis and Properties of Alkenes Chemistry Tutorial

Key Concepts

• Alkenes were once known as olefins.
• Alkenes are hydrocarbons, molecules containing only carbon and hydrogen atoms.

• Alkenes are hydrocarbons containing a double bond between two carbon atoms (C=C).
      ⚛ Alkenes are therefore unsaturated hydrocarbons.
• Alkenes can be produced, or synthesised, by elimination reactions involving alkanols or haloalkanes.

Physical Properties of Alkenes:

• Alkenes are typically colourless.
• Alkenes with 2, 3 or 4 carbon atoms in the chain are gases at room temperature and pressure.
      ⚛ Alkenes with more than 4 carbon atoms in the chain are usually liquids with low boiling points at room temperature and pressure.
• Alkenes are insoluble in water.

Chemical Properties of Alkenes:

• alkenes will undergo complete combustion in excess oxygen and incomplete combustion if the supply of oxygen is limited: combustion reactions
• alkenes undergo addition reactions with:

  ⚛ hydrogen (H₂) : hydrogenation reactions

  ⚛ halogens (X₂) : halogenation reactions

  ⚛ hydrogen halides (HX) : hydrohalogenation reactions

  ⚛ water (H₂O) : hydration reactions

• alkenes can be oxidised : oxidation reactions
• alkenes can be polymerised : addition polymerisation reactions

Laboratory Tests for Alkenes:

• alkenes decolourise reddish-brown solutions of bromine
      ⚛ (bromination of alkene is a halogenation reaction)
• alkenes decolourise dilute purple solutions of potassium permanganate
      ⚛ (alkene is oxidised by potassium permanganate)

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Synthesis of Alkenes

Alkenes can be prepared by elimination reactions in which a small molecule such as water is eliminated from a reactant molecule such as an alkanol or haloalkane (alkyl halide).
Two elimination reactions commonly used to synthesise alkenes are

• acid-catalysed dehydration of alkanols (alcohols)
  (elimination of water from an alkanol)
• dehydrohalegenation of haloalkanes
  (elimination of water and the halogen from the haloalkane)

(i) Acid-catalysed Dehydration of Alkanols (alcohols)

(ii) Dehydrohalegnation of Haloalkanes

The conditions of the reaction are extremely important!

In order to eliminate water, a solution of potassium hydroxide in alcohol is used (referred to as alcoholic KOH).
If aqueous KOH were used, the result would be a substitution reaction producing an alcohol and a salt!

Physical Properties of Alkenes

The physical properties of alkenes such as boiling point and solubility are related to the non-polar nature of alkene molecules.

(i) Boiling Points

Alkenes are non-polar molecules.
Only weak intermolecular forces (dispersion or London forces) act between the molecules.
Since little energy is required to disrupt these weak intermolecular forces, alkenes are expected to have low melting and boiling points.

The data in the table below compares the number of carbon atoms in an alkene chain with its boiling point:

Notice that the boiling points of these alk-1-enes are low.

As the number of carbon atoms in the series of 1-alkenes (alk-1-enes) increases, the boiling point also increases.

The boiling points of these alk-1-enes are graphed below:

A CH₂ "unit" is being added to each successive molecule in the alk-1-ene series, so the molecular mass of the molecules is increasing as the number of carbon atoms in the chain increases.

Alkene boiling points increase with increasing molecular mass.

As the number of carbon atoms in the carbon chains increases, the long carbon chains are increasingly attracted to each other by weak intermolecular forces (dispersion or london forces) so more energy is required to separate the molecules and the boiling points of the alkenes increase.

(ii) Solubility

• Alkenes are soluble in non-polar solvents.
  Non-polar alkene molecules are attracted to other non-polar molecules by weak intermolecular forces (weak Van der Waals Forces, Dispersion Forces, London Forces), so non-polar alkene molecules will dissolve in non-polar solvents.
• Alkenes are insoluble in polar solvents like water.
  The molecules in a polar solvent such as water are strongly attracted to each other as a result of the attraction of partial negative and partial positive charges within each molecule:
  

  δ+H	-	Oδ-	-	Hδ+
  		. . . . .					Red dotted lines (...) represent the intermolecular attraction between the partial negative charge on the oxygen atom of one water molecule and the partial positive charge on the hydrogen atom of a different water molecule. This type of intermolecular attraction is known as a hydrogen bond.
  		δ+H	-	Oδ-	-	Hδ+

  
  When a non-polar alkene is added to a polar solvent:

  (a) the alkene molecules are strongly attracted to each other but are only extremely weakly attracted to the water molecules

  (b) the water molecules are strongly attracted to each other but are only extremely weakly attracted to the alkene molecules

  
  so the alkene does not dissolve in the polar solvent.

Chemical Reactions of Alkenes

The reactive site in alkene molecules is the carbon-carbon double bond (C=C).

In chemical reactions this double bond either :

• opens out to leave a carbon-carbon single bond (C-C)

  or

• it breaks completely to separate the molecule into two smaller fragments

(i) Combustion of Alkenes

Complete combustion of alkenes (combustion in exess oxygen) produces carbon dioxide and water.

Incomplete combustion of alkenes (combustion in insufficient oxygen) produces water, and, carbon monoxide and/or carbon.
One example of an incomplete combustion reaction is given below:

Note that there are other possible incomplete combustion reactions, for example, the products may include carbon as well as carbon monoxide and water.

(ii) Addition Reactions of Alkenes

In addition reactions, atoms are added across the carbon-carbon double bond (C=C) of the alkene to produce an alkane or a substituted alkane.

The table below shows some addition reactions for alkenes:

(iii) Oxidation of Alkenes

Under mild oxidising conditions such as cold, dilute aqueous potassium permanganate, alkenes are oxidised to alkanediols (diols).
These reactions are also known as hydroxylation reactions because hydroxyl (OH) groups add across the alkene's carbon-carbon double bond (C=C).

Under strong oxidising conditions, such as hot, concentrated potassium permanganate solution, the double bond in the alkene breaks completely.
The products of the reaction can include carbon dioxide and water, carboxylic acids, ketones, and will depend on the location of the carbon-carbon double bond (C=C).

(iv) Polymerisation of Alkenes

Using an alkene as the monomer, polymerisation occurs when the carbon-carbon double bond (C=C) in the alkene opens out to form new single bonds with the neighbouring alkene monomers.
This type of polymerisation is known as addition polymerisation.

Some examples of the polymerisation of alkenes are given below: