organic chemistry

Organic Chemistry

Organic chemistry

Organic chemistry deals with compounds of carbon. Organic compounds are important to all living organisms. They contain carbon covalently bonded to hydrogen and usually other elements such as nitrogen and sulphur.

There are millions of organic compounds that can be grouped into families of with similar chemical properties. The name of an organic compound tells you:
which family (called homologous series) it belongs to and how many carbon atoms it comtains.

Compounds in the same family have the same functional group. A functional group is the part of a molecule that largely dictates how the molecule will react. A homologous series is a family of compounds that have the same functional group.

Hydrocarbons

Flame

Globally Harmonized System of Classification and Labelling of Chemicals (GHS) pictogram for flammable substances

A hydrocarbon is a compound that contains hydrogen and carbon only. Examples of families of hydrocarbons are alkanes and alkenes.

Alkanes

Alkanes have the general formula CnH2n+2, where n is the number of carbon atoms. All the bonds between the carbon atoms are single bonds. Since all their carbon – carbon bonds are single bonds, the alkanes are called saturated. A saturated compound is a compound in which all the carbon atoms are exercising full valency i.e. the carbon will have 4 single bonds.
This makes alkanes quite unreactive.

Alkanes do burn well in a good supply of oxygen, forming carbon
dioxide and water vapour, and giving out plenty of heat. So they are
used as fuels. Methane burns the most easily.


CH4 ( g) + 2O2 ( g) → CO2 ( g) + 2H2O (l) + heat energy


If there is not enough oxygen, the alkanes undergo incomplete
combustion
, giving poisonous carbon monoxide and less heat energy. Incomplete combustion is a wastage of fuel.


2CH4 ( g) + 3O2 ( g) → 2CO ( g) + 4H2O (l) + less heat energy


Examples of alkanes
PrefixNumber of carbon atomsNameFormulaDisplayed formula
meth-1methaneCH4Methane
eth-2 ethaneC2H6Ethane
prop-3 propaneC3H8Propane
but-4 butaneC4H10Butane
pent-5 pentaneC5H12Pentane

Biogas

Biogas bus
Biogas bus in the city of Linkoping, Sweden

Biogas is a mixture of methane with small amounts of carbon dioxide. It is made from the bacterial fermentation of biological waste.

Word equation:


Organic waste → methane + carbon dioxide


Biogas reactor
Biogas digester
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The anaerobic bacteria acts on organic waste (human and animal waste). Anaerobic means absence of oxygen. This type of bacteria only works in an environment lacking oxygen. The waste product of this process called sludge is later removed and used as an organic fertilizer.

Alkenes

Alkanes have the general formula CnH2n, where n is the number of carbon atoms. At least one of the bonds between the carbon atoms is a double bond. The double bond is their functional group, and largely dictates their reactions. Because they contain C=C double bonds, they are called unsaturated. An alkene with one carbon atom doesnt exist.

Alkenes are much more reactive than alkanes, because the double bond can break, to add on other atoms.

Examples of alkenes
PrefixNumber of carbon atomsNameFormulaDisplayed formula
eth-2 etheneC2H4Ethene
prop-3 propeneC3H6Propene
but-4 buteneC4H8Butene
pent-5 penteneC5H10Pentene

Question

The diagrams below show the structures of three hydrocarbon molecules, A, B, and C.

A. Ethene
B. Methane
C. Propane

  1. Name hydrocarbons A and B …….. [2]
  2. Describe the changes, if any, that are observed when bromine water is added separately to samples of hydrocarbons A and B ………. [2]
  3. Deduce the balanced equation for the complete combustion of hydrocarbon C ………. [2]

    Solution

  1. A ethene, B methane, C propane
  2. A decolourises bromine water and B has no effect on bromine water.
  3. C3H8 + 5O2 → 3CO2 + 4H2O


Question

The diagram below shows the structure of a hydrocarbon.
Ethene
(i) Define the term hydrocarbon. [1]
(ii) Name the hydrocarbon shown above [1]
(iii) Explain why the hydrocarbon is said to be unsaturated. [1]
(iv) Name the homologous series to which the hydrocarbon belongs. [1]
(v) State any two uses of the hydrocarbon. [2]

    Solution

  1. A hydrocarbon is a compound of hydrogen and carbon only.
  2. Ethene
  3. Because the carbon atoms are not execising their full combining power due to the presence of a double bond.
  4. Alkenes
  5. Manufacture of plastics and as a fuel

Alcohols

Alcohols are not hydrocarbons because they contain oxygen.
Alcohols have the general formula CnH2n+1OH where n is the number
of carbon atoms. Their OH functional group means they will all react in a similar way.

Examples of alcohols
PrefixNumber of carbon atomsNameFormulaDisplayed formula
meth-1 methanolCH3OHMethanol
eth-2 ethanolC2H5OHEthanol
prop-3 propanolC3H7OHPropanol
but-4 butanolC4H9OHButanol
pent-5 pentanolC5H11OHPentanol

Ethanol

Ethanol is the alcohol in alcoholic drinks. It is a good solvent for many substances that do not dissolve in water. It evaporates easily (it is volatile). That makes it a suitable solvent to use in glues, printing inks and perfumes.

Greenhouse gases, and global warming

Solar impulse
A clean source of energy.
In 2016, Solar Impulse 2 was the first solar-powered aircraft to complete a circumnavigation of the world.

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Burning of carbon based fuels release large quantities of carbon dioxide gas into the atmosphere. Carbon dioxide and methane are both greenhouse gases. This means they absorb heat in the atmosphere, and prevent it from escaping into space. Deforestation is also reducing the amount of carbon dioxide being absorbed by plants.

Wind power
A clean source of energy.
Wind turbines in a rapeseed field in Sandesneben, Germany.
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There are several other greenhouse gases, but the levels of carbon dioxide and methane are rising alarmingly. The rise in carbon dioxide being caused by the use of fossil fuels such as petrol (octane), diesel, coal etc. The level of methane is rising because there is an increase in animal farming, and rice farming, around the world – and more and more landfill sites.

Global warming is the rise in average temperatures all around the world. The main cause of which is the rising amount of greenhouse gases in the atmosphere. As average temperatures rise, climates around the world change too.

Planet solar
A clean source of energy.
PlanetSolar, the world’s largest solar-powered boat and the first ever solar electric vehicle to circumnavigate the globe (in 2012)
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