Edexcel IGCSE Chemistry · Spec 4.1-4.4
Introduction to Organic Chemistry
An introduction to organic chemistry, homologous series and functional groups.
Chemistry revision video
Introduction to Organic Chemistry
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Explained
Organic chemistry, and why families make it manageable
Organic chemistry is the chemistry of carbon compounds, and there are more of them than of everything else put together. Carbon forms four bonds, and it bonds to itself readily, so it builds chains, branches and rings of any length.
That would be unlearnable if every compound behaved differently. It is learnable because they sort themselves into families whose members all behave the same way, and the whole structure of this topic is built on that.
Hydrocarbons
A hydrocarbon is a compound containing hydrogen and carbon only.
Only is the word being tested. Ethanol contains carbon and hydrogen, and also oxygen, so it is not a hydrocarbon. Alkanes and alkenes are hydrocarbons; alcohols, carboxylic acids and esters are not.
Homologous series
A homologous series is a family of compounds with the same general formula, in which each member differs from the next by CH2, and which share similar chemical properties.
Those three features are usually three marks, so give all three.
Similar chemical properties is the useful one. Every alkene decolourises bromine water, so testing one of them tells you about all of them. What does change gradually down a series is the physical properties: boiling point rises with chain length, because larger molecules have stronger intermolecular forces between them.
The series in this course are the alkanes with general formula CnH2n plus 2, the alkenes with CnH2n, the alcohols with CnH2n plus 1 OH, and the carboxylic acids with CnH2n plus 1 COOH.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what a functional group is, for one mark, and prints its answer as a definition with two halves: an atom or group of atoms in a compound, or part of a molecule, that determines its chemical properties or reactions.
The second half is where the mark is. Naming the group is not defining it, and an answer that says the functional group is the OH in an alcohol has given an example rather than a definition. What makes the OH a functional group is that it decides how the whole molecule reacts.
It adds the abbreviation for or words to that effect, so the wording is open. The idea is not.
The same paper asks candidates to draw a compound whose name is given but which is not in the specification, a three carbon ring, and awards a mark for the structure. That is worth noticing, because it shows what the naming system is for. You are not expected to have met cyclopropane. You are expected to read the name, take cyclo to mean a ring, prop to mean three carbons, and ane to mean single bonds, and draw what the name describes.
Elsewhere the same question set asks for the condition needed for a bromination reaction and credits ultraviolet radiation, ultraviolet light or UV, while instructing the examiner to ignore sunlight. Sunlight contains ultraviolet, so the answer is nearly right, and it names a source rather than the thing that does the work.
The five kinds of formula
The empirical formula gives the simplest whole number ratio of atoms. For ethane, CH3.
The molecular formula gives the actual number of each atom. For ethane, C2H6.
The general formula gives the pattern for the whole series. For alkanes, CnH2n plus 2.
The structural formula shows how the atoms are arranged, in a condensed form. For ethanol, CH3CH2OH.
The displayed formula shows every atom and every bond drawn out. It must show all the bonds, including the ones to hydrogen, and a partly drawn structure earns nothing.
Naming
The stem gives the number of carbons: meth for one, eth for two, prop for three, but for four, pent for five, hex for six.
The ending gives the family: ane for an alkane, ene for an alkene, ol for an alcohol, oic acid for a carboxylic acid.
A number in the name says where the interesting part is, counting from whichever end gives the smaller number. Propan-1-ol has the OH on the end carbon; propan-2-ol has it in the middle.
Put together, the name is a set of instructions. But-2-ene means four carbons, a double bond, starting at the second carbon, and from that you can draw the molecule without ever having seen it before.
Spec 4.1-4.4
What you need to know
- Say what a hydrocarbon is
- Explain what a homologous series is
- Recognise displayed formulae and how compounds are named
Active recall
Quick check
Answer each question before opening the answer.
What is a homologous series?
A family of compounds with the same general formula and similar chemical properties, where each member differs by CH2.
What is a functional group?
The atom or group of atoms that gives a molecule its characteristic chemical reactions.
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