Alkenes

Alkenes are unsaturated hydrocarbons General formula is CnH2n. Alkenes contain a carbon- carbon double bond somewhere in their structure. Numbers need to be added to the name when positional isomers can occur. The arrangement of bonds around the >C=C< is planar and has the bond angle 120o

C=C double covalent bond electron density. consists of one sigma (σ) bond and one pi (π) bond. π bonds are exposed and have high electron density. They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles.

Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms. E-Z stereoisomers arise when: (a) There is restricted rotation around the C=C double bond. (b) There are two different groups/atoms attached both ends ont the double bond

Cis isomers Same end (Top or bottom)

Trans Diagnal

First determine the priority groups on both sides of the double bond Priority group: The atom with the bigger atomic number is classed as the priority atom If the priority atom is on the same side of the double bond it is labelled Z from the german If not its the E

Definition Electrophile: an electron pair acceptor The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions. Ad dition reaction: a reaction where two molecules react together to produce one

Change in functional group: alkene —> dihalogenoalkane Reagent: Bromine Conditions: Room temperature (not in UV light) Mechanism: Electrophilic addition Type of reagent: Electrophile, Br+

As the Br2 molecule approaches the alkene, the pi bond electrons repel the electron pair in the Br-Br bond. This induces a dipole. Br2 becomes polar and electrophilic (Brδ+). The intermediate formed, which has a positive charge on a carbon atom is called a :Br - carbocation

Change in functional group: alkene —>halogenoalkane Reagent: HCl or HBr Conditions: Room temperature Mechanism: Electrophilic addition Type of reagent: Electrophile, Hδ+

HBr is a polar molecule because Br is more electronegative than H. The H δ + is attracted to the electron-rich pi bond. This reaction can lead to two products when the alkene is unsymmetrical. If the alkene is unsymmetrical, addition of hydrogen bromide can lead to two isomeric products

Electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate. This carbocation intermediate is more stable because the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it.

Thee order of stability for carbocations is tertiary > secondary >primary

Change in functional group alkene —> alkyl hydrogensulfate Reagents: concentrated H2SO4 Conditions: room temperature Mechanism: Electrophilic addition Type of reagent: Electrophile, H2SO4 CH2=CH2 + H2SO4 —> CH3CH2OSO2OH

Change in functional group alkyl hydrogensulfate —> alcohol Reagents: water Conditions: warm mixture Type of reaction: hydrolysis CH3CH2OSO2OH + H2O —> CH3CH2OH + H2SO4

Industrially alkenes are converted to alcohols in one step rather than the two in the above sulfuric acid reaction. They are reacted with water in the presence of an acid catalyst. This reaction can be called hydration: a reaction where water is added to a molecule

High temperature 300 to 600°C High pressure 70 atm Catalyst: concentrated H3PO4 CH2=CH2 (g) + H2O (g) —> CH3CH2OH (l) The high pressures needed mean this cannot be done in the laboratory. It is preferred industrially, however, as there are no waste products and so has a high atom economy. It would also mean separation of products is easier (and cheaper) to carry out. See equilibrium chapter for more on the industrial conditions for this reaction.

Bromine water decolourises in the presence of a double bond. This can be used as a test for the presence of an double bond in a molecule. It can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils.

Addition polymers are formed from alkenes This is called addition polymerisation Poly(alkenes) like alkanes are unreactive due to the strong C-C and C-H bonds

Poly(chloroethene) is a polymer that is water proof, an electrical insulator and doesn’t react with acids. In its pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each other. In this un-plasticised form it is used make uPVC window frame coverings and guttering. If a plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer. In this form PVC is used to make insulation on electrical wires, and waterproof clothing.

Alkene