Alkanes and alkenes

• Crude oil is a mixture of different hydrocarbons, which are formed from the remains of ancient plants and animals.
• Crude oil can be separated into fractions of hydrocarbons of similar chain length as the longer the chain, the higher the boiling point.
• In fractional distillation, the temperature of the fractionating column decreases further up so fractions can be collected as they condense at various points up the column.
• Shorter chain hydrocarbons are more useful than longer chain hydrocarbons.
• Longer chain hydrocarbons are broken down (cracked) into smaller molecules.
• In thermal cracking, temperatures of 1200k and pressures of 7000kPa are used, and a high proportion of alkanes and alkenes are produced.
• In catalytic cracking, temperatures of 720K, atmospheric pressure and a zeolite catalyst are used, and a high proportion of aromatic compounds are produced.
• Alkanes make good fuel as they release a lot of energy when burned.
• In complete combustion of alkanes, carbon dioxide and water is produced.
• In incomplete combustion either carbon monoxide or carbon is formed instead of carbon dioxide, as this happens when there is not sufficient oxygen.
• Carbon monoxide is toxic, carbon particulates cause respiratory issues, nitrogen oxides cause respiratory issues, sulfur dioxide causes acid rain.
• Nitrogen oxides and carbon monoxides can be removed via a catalytic converter which uses a rhodium catalyst along with a honeycomb structure to increase surface area and converts them into less damaging products.
• Sulfur impurities can be removed via flue gas desulfurisation, which uses calcium oxide and gypsum.
• Alkanes react with halogens in the presence of UV light to produce halogenoalkanes with free radical intermediates in a mechanism called free radical substitution.
• In the first step - inititation - of free radical substitution, the halogen is broken down into two halogen free radicals.
• In the second step - propagation - of free radical substitution, a hydrogen from the alkane reacts with the radical producing HCL and an alkane radical. The alkane radical reacts with the halogen producing a halogenoalkane and a halogen radical (the halogen radical is reformed as a catalyst).
• In the last step - termination - of free radical substitution, any two radicals react together to form either an alkane, a halogenoalkane or a halogen, depending on which radicals react.
• Halogenoalkanes contain polar bonds as the halogens are more electronegative than carbon atoms.
• Nucleophiles are species attracted to the nucleus or a positive region of an atom.
• In nucleophilic substitution, a nucleophile attacks the carbon atom bonded to the halogen in a halogenoalkane. Electrons are then transferred to the halogen and it is replaced by the nucleophile, developing a negative charge.
• The greater the Mr of the halogen in a halogenoalkane, the lower the bond enthalpy and so it can be broken more easily, increasing the rate of reaction.
• Nucleophilic substitutions can only occur in primary and secondary halogenoalkanes
• In high temperatures and under alcoholic conditions, elimination occurs in secondary and tertiary halogenoalkanes.
• In elimination, a nucleophile acts as a base and accepts a proton, removing a hydrogen from the carbon adjacent to the carbon with the halogen on in a halogenoalkane. The electrons are transfered to the bond between the carbons and the halide is eliminated, forming a double bond and producing an alkene.
• Ozone in the atmosphere absorbs UV radiation.
• Chloro-fluoro carbons (CFCs) are used as refrigerants and solvents but can break carbon-halogen bonds forming halogen free radicals that catalyse ozone depletion.
• In ozone depletion, ozone (O3) reacts with a halogen radical to form oxygen and an halogen oxide radical. The halogen oxide radical also reacts with ozone to form oxygen and the halogen radical (which is reformed as a catalyst). Overall, two ozone molecules form three oxygen molecules.
• Alkenes are unsaturated hydrocarbons, meaning they contain a carbon - carbon double bond.
• The double bond in an alkene consists of a regular covalent (sigma) bond and a pi bond, and is an area of high electron density making it suspectable to attack from electrophiles.
• Alkenes turn bromine water from orange - brown to colourless.
• Alkenes can undergo electrophilic addition about the double bond, which can be used to from alkyl hydrogen sulphates or halogenoalkanes.
• When alkenes undergo electrophilic substitution, electrons move from the double bond to the partially positive side of the electrophile, forming a carbocation adjacent, and then the now negatively charged rest of the electrophile reacts with it.
• A carbocation is a positively charged carbon atom, with primary being the least stable and tertiary being the most, and therefore most likely to form.
• Multiple products can form in some reactions, but the major product is always the most stable.