18.1 to 18.3 Organic Chemistry

Organic compounds names, structures, and types

• Organic chemistry is the chemistry of carbon compounds
• Carbon forms a vast number of compounds because it can form strong covalent bonds with itself
• This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known
• Carbon always forms four covalent bonds which can be single, double or triple bonds
• A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule
• Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series
  • Every time a carbon atom is added to the chain, two hydrogen atoms are also added
  • Organic molecules are classified by the dominant functional group on the molecule
• Organic compounds can be represented in a number of ways

Types of formulae

1- General Formula

• This type of formula tells you the composition of any member of a whole homologous series of organic compound
• For example, all of the alkanes have the general formula CnH2n+2
• This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkane

2- Displayed Formula

• This shows the spatial arrangement of all the atoms and bonds in a molecule

• Molecular Formulae

  • This shows the actual number of each atom in a molecule, one element at a time

  

Structural Formula

• This gives enough information to make the structure clear, but most of the actual covalent bonds are omitted

• Only important bonds are shown, such as double and triple bonds

  • Identical groups can be bracketed together

  

  

Naming Compounds

• The names of organic compounds have two parts: the prefix (or stem) and the end part (or suffix)

• The prefix tells you how many carbon atoms are present in the longest continuous chain in the compound

• The suffix tells you what functional group is on the compound

  

Families of Organic Compounds

Structures

%%Structures and Formulae%%

Alkanes

• Alkanes are a group of saturated hydrocarbons
• The term saturated means that they only have single carbon-carbon bonds, there are no double bonds
  • The general formula of the alkanes is CnH2n+2
• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases
  • Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller molecules and can react with halogens in the presence of light
• Methane is an alkane and is the major component of natural gas
• Methane undergoes complete combustion forming carbon dioxide and water:
  • CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)

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Alkene

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• Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds (C=C)
• Their general formula is CnH2n
• The presence of the double bond, C=C, means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group
• Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds

This makes them much more reactive than alkanes

Manufacture of Alkenes and Hydrocarbons

• Although there is use for each fraction obtained from the fractional distillation of crude oil, the amount of longer chain hydrocarbons produced is far greater than needed
  • These long chain hydrocarbon molecules are further processed to produce other products
• A process called catalytic cracking is used to convert longer-chain molecules into short-chain and more useful hydrocarbons
  • Shorter chain alkanes, alkenes and hydrogen are produced from the cracking of longer chain alkanes
• Alkenes can be used to make polymers and the hydrogen used to make ammonia
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen
• Cracking involves heating the hydrocarbon molecules to around 600 – 700°C to vaporise them
  • The vapours then pass over a hot powdered catalyst of alumina or silica
• This process breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes
• Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure

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Alcohols

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Carboxylic Acid

Production of Esters

• Esters are compounds with an -COOR functional group and are characterised by their sweet and fruity smells

• They are prepared from the condensation reaction between a carboxylic acid and alcohol with concentrated H2SO4 as catalyst

  • This is also called esterification

• The first part of the ester’s name comes from the alcohol and the second part of the name comes from the carboxylic acid

  • E.g. Propanol and ethanoic acid will give the ester propyl ethanoate

  

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Esters

18.2 FUELS

Common Fossil Fuels

• A fuel is a substance which when burned, releases heat energy
• This heat can be transferred into electricity, which we use in our daily lives
• Most common fossil fuels include coal, natural gas and hydrocarbons such as methane and propane which are obtained from crude oil
• Hydrocarbons are made from hydrogen and carbon atoms only
• The main constituent of natural gas is methane, CH4

Petroleum

Petroleum and Fractional Distillation

• Petroleum is also called crude oil and is a complex mixture of hydrocarbons which also contains natural gas

• It is a thick, sticky, black liquid that is found under porous rock (under the ground and under the sea)

  

• Petroleum itself as a mixture isn't very useful but each component part of the mixture, called a fraction, is useful and each fraction has different applications

• The fractions in petroleum are separated from each other in a process called fractional distillation

• The molecules in each fraction have similar properties and boiling points, which depend on the number of carbon atoms in the chain

• The boiling point and viscosity of each fraction increases as the carbon chain gets longer

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  • Fractional distillation is carried out in a fractionating column
  • The fractionating column is hot at the bottom and cool at the top

• Crude oil enters the fractionating column and is heated so vapours rise

• Vapours of hydrocarbons with very high boiling points will immediately turn into liquid and are tapped off at the bottom of the column

• Vapours of hydrocarbons with low boiling points will rise up the column and condense at the top to be tapped off

• The different fractions condense at different heights according to their boiling points and are tapped off as liquids.

• The fractions containing smaller hydrocarbons are collected at the top of the fractionating column as gases

• The fractions containing bigger hydrocarbons are collected at the lower sections of the fractionating column

Properties of Fractions

• Viscosity

• This refers to the ease of flow of a liquid.

• High viscosity liquids are thick and flow less easily.

• The number of carbon atoms increases, the attraction between the hydrocarbon molecules also increases which results in the liquid becoming more viscous with the increasing length of the hydrocarbon chain.

• The liquid flows less easily with increasing molecular mass

  Colour

  • As carbon chain length increases the colour of the liquid gets darker as it gets thicker and more viscous

• Melting point/boiling point

  • As the molecules get larger, the intermolecular attraction becomes greater.
  • More heat is needed to separate the molecules.
  • With increasing molecular size there is an increase in boiling point

Volatility

• Volatility refers to the tendency of a substance to vaporise.
• With increasing molecular size hydrocarbon liquids become less volatile.
• This is because the attraction between the molecules increases with increasing molecular size

Uses of Fractions

• Refinery gas: heating and cooking
• Gasoline: fuel for cars (petrol)
• Naphtha: raw product for producing chemicals
• Kerosene: for making jet fuel (paraffin)
• Diesel: fuel for diesel engines (gas oil)
• Fuel oil: fuel for ships and for home heating
• Lubricating oil: for lubricants, polishes, waxes
• Bitumen: for surfacing roads

Trends in Properties

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18.3 Homologous Series

• This is a series or family of organic compounds that have similar features and chemical properties due to them having the same functional group

• The functional group is a group of atoms which are bonded in a specific arrangement that is responsible for the characteristic reactions of each member of a homologous series

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Characteristics, Formula & Structural Isomerism

Characteristics of Homologous Series

• All members of a homologous series have:
  • The same general formula
  • Same functional group
  • Similar chemical properties
  • Gradation in their physical properties, such as melting and boiling point
  • The difference in the molecular formula between one member and the next is CH2

General Formula

• This type of formula tells you the composition of any member of a whole homologous series of organic compound
• For example, all of the alkanes have the general formula CnH2n+2
• This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkanes

Structural isomers

• Structural isomers are compounds that have the same molecular formula but different structural formulae

• This is due to the different arrangement of their atoms in space

• There are two types: chain and position

• In chain isomerism the structure of the carbon chain differs

• In position isomerism, the position of the functional group differs

  

  

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