ALKANES

• Alkanes are saturated hydrocarbons consisting of single carbon-carbon bonds and single carbon-hydrogen bonds only

  
• The general formula for an alkane is CnH2n+2, except for alkanes in a ring structure where it is CnH2n
• Methane (CH4), the simplest alkane, has an H-C-H bond angle of 109.5°
• Different ways to represent the structure of alkanes:

  • Molecular formula: shows the number of each type of atom
  • Displayed formula: displays all the atoms and bonds
  • Structural formula: shows the positions of the atoms
  • Skeletal formula: shows the backbone of the molecule
• Naming alkanes: The name of an organic compound is related to the number of carbon atoms in the compound, with specific prefixes for each number of carbons
• Isomers of alkanes can possess chain isomerism, where the hydrocarbon chain is arranged differently while maintaining the same numbers and types of atoms
• How to name branched alkanes:

  • Find the longest continuous chain of carbons and number them
  • Identify additional groups attached to the longest chain
  • Name the molecule by putting the number of the group, then the group name, then the chain name
• Properties of alkanes: Alkanes do not dissolve in water due to being non-polar, interacting with water molecules held together by H-bonds differently
• Fractional distillation of crude oil:

  • Used to separate different molecules of crude oil for various uses
  • The process involves heating the mixture in a distillation flask, where vapors rise up a column, condense, and are collected based on boiling points
• Industrial cracking:

  • Cracking breaks longer hydrocarbons into shorter alkanes and alkenes
  • Thermal cracking conditions: Temp 700-1200K, Pressure 7000kPa, leading to the formation of alkenes due to the high proportion of alkenes created
  • Catalytic cracking conditions: Temp <720, Pressure >1atm, using a zeolite catalyst to produce a mixture of alkanes and alkenes mainly of short chains
• Combustion of alkanes:

  • Alkanes are good fuels but their combustion releases carbon dioxide and impurities like sulphur dioxide, contributing to environmental issues
  • Products obtained from combustion are separated by fractional distillation, and they decolourise bromine water to prove the presence of unsaturated compounds
• Alkanes are generally unreactive but are very flammable and will react with halogens under the right conditions

  
• Complete combustion of methane equation: CH4(g) + 2O2(g) → CO2(g) + H2O(l), releasing -890 kJmol-1 of energy
• Burning fossil fuels pollutes the environment by releasing carbon dioxide, carbon monoxide, nitrous oxides, sulphur dioxide, carbon particulates, unburnt hydrocarbons, and water vapor
• Incomplete combustion of hydrocarbons in limited oxygen supply leads to the formation of carbon monoxide and solid carbon (soot)
• Pollutants from combustion of hydrocarbons include carbon monoxide (toxic gas), nitrous oxides (may form nitric acid), sulphur dioxide (may form sulphuric acid), carbon particulates (trigger asthma and cancer), unburnt hydrocarbons (significant greenhouse gases), and carbon dioxide (greenhouse gas)
• Reducing pollutants like sulphur dioxide can be done by reacting sulphur impurities with oxygen to form calcium sulfate using calcium carbonate
• Vehicles are fitted with catalytic converters to reduce nitrous oxide and carbon monoxide emissions
• Global warming is caused by the greenhouse effect where gases trap energy from the sun, mainly due to human production of carbon dioxide
• Halogenoalkanes are alkanes with one or more halogen groups, formed by putting a mixture of an alkane and a halogen into bright light, causing a substitution reaction
• The reaction mechanism for the free radical substitution of methane involves initiation, propagation, and termination steps
• Crude oil is a mixture of different hydrocarbons that can be separated by fractional distillation due to their different boiling points
• Fractional distillation process:

  1. The mixture is vaporized and fed into the fractionating column.

  2. Vapors rise, cool, and condense.

  3. Products with short carbon chains (lower boiling points) are collected at the top, while those with long carbon chains (higher boiling points) are collected at the bottom
• Compounds collected from the fractionating column are further broken down via cracking
• Cracking breaks down longer carbon chains into smaller, more useful molecules by breaking carbon-carbon bonds under harsh conditions
• Thermal cracking produces a high proportion of alkanes and alkenes at high temperatures and pressures, while catalytic cracking produces aromatic compounds with carbon rings at lower temperatures with a zeolite catalyst
• Alkanes are good fuels as they release a lot of energy when burned, undergoing complete combustion with sufficient oxygen to produce carbon dioxide and water
• Incomplete combustion of alkanes due to insufficient oxygen produces carbon monoxide alongside water, which is toxic and odorless
• Catalytic converters use a rhodium catalyst to convert harmful products like carbon monoxide into more stable products such as CO2 or H2O
• Incomplete combustion can produce carbon particulates, small fragments of unburned hydrocarbon, which can cause serious respiratory problems if not removed
• Sulfur impurities in alkanes can lead to acidification of water in the atmosphere, which can be removed via flue gas desulfurization using calcium oxide and gypsum
• Pollutants from incomplete combustion and sulfur impurities can contribute to global warming, acid rain, and health issues if not treated or removed
• Alkanes react with halogens in the presence of UV light to produce halogenoalkanes through a series of reactions involving free radicals
• Chlorination of alkanes involves initiation (halogen breakdown), propagation (hydrogen replacement), and termination (radicals join to form stable products) steps