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4.1Organic Chemistry

Introduction — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 18 Unit quiz

📚 Edexcel Linear - OL (Chemistry) Hub / Topical Revision
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Lesson summary

Introduction to Organic Chemistry and Formulae — EDEXCEL LINEAR – OL (CHEMISTRY) notes: Use molecular, structural and displayed formulae, identify functional groups and explain why carbon forms so many compounds.Carbon has four electrons in its outer shell, so each carbon atom fo…

  • four covalent bonds
  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Homologous series

Section 1 of 8Why carbon forms so many compounds

Carbon has four electrons in its outer shell, so each carbon atom forms four covalent bonds.

Why carbon forms so many compounds
Carbon atoms can bond to each other in chains (straight or branched) and rings, and can form single, double or triple bonds,
  • so enormous numbers of compounds exist.
Organic compounds are generally covalent, with low melting and boiling points, and do not conduct electricity.
Most are insoluble in water, burn well and are found in fuels, plastics, medicines and living things.

Section 2 of 8Types of formula

FormulaWhat it showsEthanol example
EmpiricalSimplest whole-number ratio of atomsC2H6O
MolecularActual number of atoms of each elementC2H6O
StructuralGroups of atoms in orderCH3CH2OH
DisplayedEvery atom and every bondH–C(H)(H)–C(H)(H)–O–H

Examplebutane has molecular formula C4H10, empirical formula C2H5 and structural formula CH3CH2CH2CH3.

Section 3 of 8Functional groups and homologous series

A functional group is the atom or group of atoms that determines the characteristic reactions of a compound. Members of a homologous series share a general formula, have similar chemical properties and show a gradual change in physical properties.

Functional groupSeriesSuffix / prefixExample
C=CAlkenes-eneEthene
–OHAlcohols-olPropan-1-ol
–COOHCarboxylic acids-oic acidEthanoic acid
–COO–Esters-yl ...-oateMethyl ethanoate
–Cl, –BrHalogenoalkaneschloro-, bromo-Chloromethane

Section 4 of 8Drawing and naming steps

1
Identify the longest carbon chain and use the stem (meth, eth, prop, but, pent, hex).
2
Identify the functional group and add the suffix.
3
Number the chain from the end that gives the lowest number to the functional group, if needed.
4
Name any branches (methyl, ethyl) and halogen substituents as prefixes.

Example

  • CH3CH(OH)CH3 is propan-2-ol;
    • CH3CH2CH2OH is propan-1-ol;
    • they are position isomers with the same molecular formula C3H8O.

Section 5 of 8Using the ideas: a worked classification exercise

Classify each compoundCH3CH2CH3 (alkane, saturated hydrocarbon), CH3CH=CH2 (alkene, unsaturated hydrocarbon), CH3CH2OH (alcohol, not a hydrocarbon because it has oxygen), CH3COOH (carboxylic acid), CH3Br (halogenoalkane).

Using the ideas: a worked classification exercise
A set of compounds in a table with their boiling points shows the trend within a series: methane −162 °C, ethane −89 °C, propane −42 °C, butane −1 °C, pentane 36 °C.
Each additional CH2 increases the boiling point because the larger molecules have stronger intermolecular forces.

The other sharp contrast is between the physical properties of the alcohols, which are liquids at room temperature and more soluble in water than alkanes of similar size because the –OH group forms attractions with water.

Using the ideas: a worked classification exercise
Molecular models help: carbon is drawn with four bonds, oxygen with two, hydrogen with one, and halogens with one.
Any structure that breaks these valencies is wrong.
Learn the functional group table until you can complete it from memory,
  • since it supports every later topic in organic chemistry,
  • including naming, reactions and polymers.

Finally, remember how the data in the lesson link: the empirical formula can be calculated from percentage composition, the molecular formula from the relative molecular mass, and the structure from the functional group tests, such as bromine water for alkenes and effervescence with carbonates for carboxylic acids.

Section 6 of 8Worked examples

  1. Give the molecular, empirical and structural formulae of ethane. [3]3 marks

  2. Name CH3CH2COOH. [1]1 mark

  3. Explain what a functional group is. [2]2 marks

  4. Draw two isomers with formula C3H8O. [2]2 marks

Section 7 of 8Key terms

TermMeaning
Functional groupGroup that determines reactions
Homologous seriesFamily with same functional group
Displayed formulaShows all atoms and bonds
Empirical formulaSimplest ratio of atoms
Molecular formulaActual number of each atom
Structural formulaShows groups in the arrangement

Section 8 of 8Exam technique

1
Writing the molecular formula where the empirical formula is required.
2
Leaving out bonds in a displayed formula.
3
Forgetting that carbon always forms four bonds.
4
Numbering from the wrong end.

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