Alkenes

• Alkenes are unsaturated hydrocarbons with the general formula CnH2n
• There are 3 bonding regions around a C=C bond, so the bonding around it forms 120 degree bond angles. ( Trigonal planar shape )
• In alkenes, the carbon to carbon double bond consists of two types of covalent bond: a sigma bond and a pi bond.
• In a sigma bond, orbitals directly overlap one another from the two atoms.
• In a pi bond, the p orbitals have a sideways overlap of two lobes. One lobe is above and below the sigma bond.
• Sigma bonds by themselves are fully rotational.
• The pi bond in a C=C bond blocks the free rotational capabilities of the sigma bond.
• Because the C=C double bond doesn't have free rotation, alkenes have a form of isomerism called stereoisomerism.
• Alkenes are also highly reactive molecules. This is because the bond enthalpy of pi bonds are lower than that of sigma bonds due to their sideways overlap, meaning pi bonds require less energy to break and are likely to take part in a reaction.
• Because of the double bond in an alkene, consisting of a sigma and a pi bond, a region of high electron density is formed.
• This double bond, and region of high electron density, enables alkenes to react via electrophilic addition.
• The position of the pi bond above and below the molecule makes it easier to 'attack' an electron pair.
• In hydrogenation of an alkene, an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K causing an addition reaction producing an alkane as a product.
• In the halogenation of alkenes, alkenes undergo a rapid addition reaction with either chlorine or bromine at room temperature. Halogenation of propene would form a dihalide propane.
• Gaseous alkenes can react with gaseous hydrogen halides at room temperature to form haloalkanes. If the alkene is a liquid, then the hydrogen halide is bubbled through it.
• Alcohols are formed in the reaction between alkenes and steam in the presence of a phosphoric acid catalyst (H3PO4).
• An electrophile is an electron pair acceptor and because of this, they are usually positively charged molecules/ions.
• Electrophiles are attracted to areas of high electron density.
• Heterolytic fission is the splitting of a covalent bond where one atom takes both electrons forming two ions.
• The the electrophilic addition of a hydrogen halide, the halide is more electronegative forming a dipole with the hydrogen atom being slightly positive charged and therefore, acting as an electrophile.
• The hydrogen atom of the hydrogen halide attracts an electron pair and begins to form a covalent bond. Meanwhile heterolytic fission occurs in the hydrogen halide which produces a halide ion and a carbocation intermediate.
• The electron pair on the halide ion is attracted to the positive carbon atoms on the carbocation intermediate which reacts to then form a haloalkane.
• Addition reactions with symmetrical alkenes only produce one product and so have a 100% atom economy.
• Using an asymmetrical alkene, we could produce two products.
• In electrophilic addition of a halogen to an alkene, halogen molecules contain atoms of the same electronegativity, so there is no permanent dipole. When nearing an alkene molecule, the area of high electron density in the double bond can induce a dipole in the halogen molecule.
• A pair of electrons from the pi bond of the alkene is attracted to the positive region of the induced dipole in the halogen molecule which is acting as an electrophile. This causes heterolytic fission to occur forming a bromide anion and a carbocation intermediate.
• The electron pair on the bromide anion is attracted to the positive region of the carbocation intermediate and reacts to form a dihalide alkane.
• To test for an unsaturated molecule, add drops of bromine water and gently shake. If unsaturated, bromine would add across the double bond forming a colourless solution.