Lecture 5

Aliphatic compounds (2)

1. Alkenes (continued)

2. Alkynes

3. Dienes

1. Alkenes (continued)

Addition of Bromine

Alkenes react rapidly with Br₂ in 1,1,1-trichloroethane at room temperature and in the absence of light

The behaviour of alkenes towards Br₂ in CH₃CCl₃ is a useful test for the presence of carbon-carbon multiple bonds

Addition of Bromine Water

In an aqueous solution of Br₂, the following equilibrium exists

Br₂ + H₂O → HBr + HOBr Bromic(I) acid

The bromine atom bears a partial positive charge while the oxygen atom bears a partial negative charge ∵ oxygen is more electronegative than bromine

When bromic(I) acid reacts with alkenes, bromohydrin is formed

Addition of Sulphuric(VI) Acid

Alkenes react with cold and concentrated H2SO4 to form alkyl hydrogensulphates

The large bulky –OSO₃H group makes the alkyl hydrogensulphate very unstable. Two possible further reactions take place:

1. Regeneration of alkenes

2. Production of alcohols

Catalytic Hydrogenation

In the presence of metal catalysts (e.g. Pt, Pd or Ni), H₂ is added to each atom of C = C double bond to form an alkane

• Hydrogenation is useful in analyzing unsaturated hydrocarbons

• The number of double or triple bonds present in the unsaturated hydrocarbon molecule can be deduced by the number of moles of hydrogen reacted

• Catalytic hydrogenation is used to convert liquid vegetable oil to semi-solid fats in making margarine and solid cooking fats

Oxidation

As a result of reaction of alkenes and diluted KMnO₄ (Vagner reaction) diols are formed (decolorization of diluted KMnO₄ is the qualitive reaction of multiple bond)

When alkenes react with powerful oxidizing agents such as ozone or KMnO₄, under the high temperature, double bond is broken and aldehydes, ketones, and carboxylic acids are formed.

Ozonolysis

Ozonolysis is a widely used method for locating the double bond of an alkene

(unstable)

The unstable ozonide is reduced directly by treatment with Zn and H₂O

Overall process of ozonolysis:

Polymerization

Polymers: Compounds that consist of very large molecules made up of many repeating units

Monomer: Each repeating unit

Polymerization: The reaction by which monomers are joined together

Addition polymerization: alkene monomers are joined together without the elimination of small molecules

Addition polymer: The polymer produced by addition polymerization

Poly(ethene)

Monomer: ethene

Depending on the conditions, two kinds of poly(ethene) are formed

Low density poly(ethene) (LDPE):

• Molecular mass: 50 000 to 3 000 000

• Light, flexible and low melting temperature

• Uses: make soft items like wash bottles, plastic bags and food wraps

High density poly(ethene) (HDPE):

• Molecular mass: up to 3 000 000

• Tougher and higher melting temperature

• Uses: make more rigid items like milk bottles and water buckets

Poly(propene)

Properties: more rigid than HDPE, high mechanical strength, strong resistance to abrasion

Uses: make moulded furniture; make crates, kitchenware, food containers; make ropes and hard-wearing carpets

Poly(phenylethene) (or Polystyrene)

Preparation of monomer (phenylethene):

Formation of poly(phenylethene):

Poly(phenylethene):

Properties: Transparent, brittle and chemically inert

Uses: Make toys, specimen containers and cassette cases