Alkenes

All alkenes have a general formula of CnH2n (where n is the number of carbon atom, n=1 n=2 n=3). This general formula is different from alkane in terms of the number of hydrogen. Alkene molecules have two fewer hydrogen atoms than an alkane molecule with the same number of carbon atoms.

The functional group of alkenes is the -C=C- carbon-carbon double bond in its compound. It is because of the presence of the -C=C- that causes alkene molecules to have two fewer hydrogen atoms.

Alkenes, unlike alkanes, are also unsaturated as its functional group contains a reactive -C=C-.

An unsaturated compound is one that has one or more double bonds between carbon atoms within the molecule.

Physical properties of alkenes

Solubility

  • Only soluble in organic solvent such as alcohol.
  • Insoluble in water.
  • This is similar to alkanes

Density

  • All alkenes are less dense than water.
  • Density increases as the molecular size increases.

Melting and boiling point

  • Alkenes have low boiling and melting point as there are weak intermolecular forces of attraction between the molecules which require little heat energy to overcome.
  • As the size of alkenes increases, the intermolecular forces of attraction are stronger which require a large amount of heat energy to overcome.

Viscosity (resistance to flow)

  • As alkenes get larger, they contain higher percentage of carbon and are more difficult to burn and produce smokier flame.

Flammability

  • They burnt with a smokier flame than the corresponding alkanes as they contain a higher percentage of carbon (due to less hydrogen atom per molecule).

Chemical properties of alkenes

Only alkenes mainly undergo addition reaction, which is a reaction where small molecules are added to the double bond of the alkene molecule and forming only one product.

Alkanes do not undergo addition reaction, because it is fully saturated and do not contain -C=C- for molecules to add into it.

Types of chemical reactions that alkenes undergo

  1. Combustion
  2. Addition reaction
    1. Hydrogenation (addition of hydrogen into the alkene molecule)
    2. Hydration (addition of water molecule to the alkene molecule)
    3. Bromination (mainly used as a test to differentiate between alkene and alkanes)
    4. Addition polymerisation (where many alkene molecules react with itself under high temperature and pressure to form a large compound)

1. Combustion

Combustion of alkenes is the reaction of alkene with oxygen. This reaction is a highly exothermic reaction which releases a large amount of heat energy to the surroundings.

Just like combustion of alkanes, there are two types of combustion

  1. Complete combustion (reaction with excess oxygen)
    • Carbon dioxide and water is produced only.
  2. Incomplete combustion (reaction with insufficient oxygen)
    • Carbon monoxide/carbon and water is produced only.

Complete and incomplete combustion require oxygen but they produce different products.

Alkenes generally reacts with oxygen with a smokier flame compared to their corresponding alkanes that contain the same number of carbons. This is due to higher percentage of carbon present leading to incomplete combustion.

2a. Addition reaction: Hydrogenation

This is a reaction of alkene with hydrogen to produce alkane. Only one product, which is the alkane, is produced. Note: All addition reactions only produce one product only.

Conditions for hydrogenation

  1. 200°C
  2. Nickel catalyst

Note: A catalyst is a substance that provides an alternative and more efficient reaction pathway with lowered activation energy barrier.

Application: Hydrogenation is used to produce margarine from vegetable oil. Vegetable oil is a polyunsaturated compound with many -C=C-.

2b. Addition reaction: Hydration

This is a reaction of alkenes with steam to produce alcohol. A common misconception is that hydration is the reaction of alkenes with water. This is not true as water refers to the liquid state with steam refers to the gaseous state.

Conditions:

  1. Phosphoric(V) acid as catalyst (H3PO4)
  2. 300°C
  3. 60 atmospheres

2c. Addition reaction: Bromination

This is the reaction of reaction of alkenes with aqueous bromine/bromine water. Aqueous bromine is a reddish brown solution, hence when bromine water is added to alkene, the mixture will turn from reddish brown to colourless, also known as decolorisation.

This is due to the reactive -C=C- which allows the bromine molecules to add into. Also, the location of the -C=C- will determine which carbon atoms the bromine atom is attached to.

To differentiate alkanes and alkenes, aqueous bromine is added to the sample that we are analysing. Note: only alkenes can undergo addition reaction and thus reacts with aqueous bromine. There must be an absence of UV light so as to prevent (free-radical) substitution reaction to occur which allows alkanes to decolourise the aqueous bromine and result in a false positive test

FAQ: What happens when Br2(aq) is added to a sample of alkenes? [1m]

Answer template: The (name of alkene compound) will decolourise reddish-brown bromine solution to a colourless solution immediately.

2d. Addition reaction: Addition polymerisation

Addition polymerisation is a reaction when small short chain molecules react together to form one large long chain molecules only.

Conditions:

  1. 200°C
  2. 1000 atmospheres

The short chain molecules are called monomers and the long chain products are called the polymers.

Naming of polymer

The structure of the name of the polymer is: poly(name of monomer).

E.g. if the monomer is ethene, the polymer name will be poly(ethene).

Differences between alkanes and alkenes

  Alkane Alkene
Functional Group No functional group. Only single bonds throughout the molecule. -C=C- carbon-carbon double bond
General Formula CnH2n+2 CnH2n
Type of reaction 1.    Combustion

 

2.    Substitution

1. Combustion

 

2. Addition

•        hydrogenation

•        hydration

•        bromination

•        addition polymerisation

Reactivity Unreactive Reactive
Molecular structure Only contain single bonds Contain double bonds between carbon atom
Reaction with aqueous bromine (* distinguishing test for alkanes and alkenes) No reaction React and decolourise aqueous bromine

 

 

Combustion Less smokier flame Smokier flame

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