Chemistry AS - Chapter 05 - Introduction to Organic Chemistry

Revision cards for Chapter 5 - Introduction to Organic Chemistry


Carbon Compounds

Carbon can form rings and very long chains which may be branched because:

  • a carbon atom has four electrons in its outer shell which can form 4 covalent bonds.
  • Carbon-Carbon bonds are relatively strong and non-polar.

Carbon-Hyrdrogen bond is also strong and hydrocarbon chains form the skeleton of most organic compounds.

In all stable carbon compounds, carbon forms four covalent bonds and has eight electrons in its outer shell. It can do this by forming bonds in different ways.

  • By forming four single bonds as in methane (CH4)
  • By forming two single bonds and one double bond as in ethane (C2H4)
  • By forming one single bond and one triple bond as in ethyne (C2H2)
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Carbon Compounds

Empirical Formula shows the simplest ratio of the atoms of each element present in a compound. See Chapter 2.

Molecular formula shows the actual number of atoms of each element in the molecule. It is found from the empirical formula, R.M.M of the empirical formula and the R.M.M of the molecule.

Eg. Empirical formula of CH3 and this group of atoms has a R.M.M of 15.0. The R.M.M of CH3 is 30.0 which is 2 x 15.0. So there are 2 units of the empirical formula in every molecule of CH3.

Therefore it is (CH3)2 or C2H6.

We use other types of formulae in organic chemistry as the molecules are more varied.

Displayed formula - shows every atom and every bond in the molecule.

Structural formula - shows the unique arrangement of atoms in a molecule in a simplified form without showing all the bonds.

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Carbon Compounds

Displayed formula:

For ethene (C2H4): (

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Carbon Compounds

Structural Formula:

( is written as CH3CH3

Branches in carbon chains are shown in brackets.

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Nomenclature - Naming Organic Compounds

A systematic name has a root that tells us the longest unbranched hydrocarbon chain/ring. The syllable after the root tells us whether there are any double bonds.

-ane means there are no double bonds. (eg ethane - 2 carbon atoms, no double bond).

-ene means there is a double bond. (eg ethene - 2 carbon atoms, 1 double bond).

Prefixes and suffixes describe the changes that have been made to the root molecule. Prefixes are added to the beginning of the root.

Side chains are shown by a prefix, whose name tells us the number of carbons.

Functional groups are shwon by a suffix which tell us what kind of hydrocarbon it is.

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Nomenclature - Naming Organic Compounds


  • 1 - meth
  • 2 - eth
  • 3 - prop
  • 4 - but
  • 5 - pent
  • 6 - hex


  • Methyl - CH3-
  • Propyl - C3H7-
  • Ethyl - C2H5-
  • Butyl - C4H9-
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Nomenclature - Naming Organic Compounds

Most organic compounds are made up of a hydrocarbon chain that has one or more reactive groups attached to it. These reactive groups are called functional groups.

They react the same way whatever length of the hydrocarbon chain.

Alkanes - Has no double bonds

Alkenes - Has a double bond

Haloalkanes - Has a halogen attached to it

Alcohols - Has an OH group attached to it

Aldehydes- Has an CHO group

Ketones - Has an CO group

Carboxylic Acids - Has a COOH group

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Nomenclature - Naming Organic Compounds

With longer chains, we need to say where a side chain or a functional group is located on the main chain. Methylpentane could refer to 2-methylpentane (with the methyl group on the second carbon atom) or 3-methylpentane (with the methyl group on the third carbon atom).

We use a number (locant) to tell us the position of any branching in a chain and the position of any functional group. 

Structural isomers have the same molecular formula but different structural formulae.

We always use the smallest possible number to locate the functional group.

We may have more than one functional group. We arrange the functional groups in alphabetical order rather than numerical order. 

We can add prefixes as well as functional groups such as di-, tri- tetra to mean two, three and four respectively.

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Nomenclature - Naming Organic Compounds

Homologous series is a family of organic compounds with the same functional group but different carbon chain length.

Members of a homologous series have the same general formula.

Length of the carbon chain has little effect on the chemical reactivity of the functional group.

The length of the carbon chain affects physical properties like melting point, boiling point and solubility. Melting/Boiling points increase slightly as the number of carbon atoms increase in the chain. Intermolecular forces increase.

Small molecules are gases, larger ones are liquids and solids.

Chain branching reduces melting points as molecules pack together less well.

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Isomers are molecules that have the same molecular formula but whose atoms are arranged differently.

Structural isomers have the same molecular formula but different structural formulae. Structural isomers can have:

  • same functional groups attached to the main chain at different points (positional isomerism).
  • functional groups that are different (functional group isomerism)
  • a different arrangement of the hydrocarbon chain, such as branching. (chain isomerism).

Positional Isomerism: the functional group is attached to the main chain at different points. C3H7Cl could represent CH3CH2CH2Cl (1-chloropropane) or CH3CHClCH3 (2-chloropropane).

Functional Group Isomerism: there are different functional groups. C2H6O could represent ethanol or methoxymethane.

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Chain isomerism: The hydrocarbon chain is arranged differently. C4H9OH could represent butan-1-ol or 2-methylpropan-1-ol

These are chain-branching isomers. The existence of isomers make the task of identifying an unknown organic compound more difficult. This is because there may be a number of compounds with different structures that all have the same molecular formula.

We use analytical methods that tell us about the structure.

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Great help, thanks!

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