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AS Chemistry - Things You Need To Know

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an example of a couple of formulas for ethane

an example of a couple of formulas for ethane


  • General formula: the simlest algebraic formula of a member of a homologous series.
  • Displayed formula: a formula that shows the relative positioning of all the atoms that are in a molecule.
  • Molecular formula: the actual number of atoms of each element in a molecule.
  • Structual formula: the minimal detail for arrangement of atoms in a molecule.
  • Skeletal formula: simplified orgnic formula with hydrogen atoms removed from alkyl chains leaving a carbon skeleton.
Number of carbon atoms in longest chainStemAlkyl group































Naming Compounds

  • With over 10,000,000 organic compounds it is very important that each has its own unique name.
  • Organic nomenclature is the system used for naming compounds.
  • Nomenclature was developed by the International Union ofPure and Applied Chemistry.
  • The stem indicates the number of carbon atoms in the longest chain of the molecule.
  • The prefix is added before the stem as a part of the name.
  • The suffix is added after the stem.
  • If you remove a hydrogen atom from an alkane you're left with an alkyl group.

Naming Alkanes

Alkanes are the simplest molecules to name because all of their names end with the suffix 'ane'!


For the molecule to the right:

  • Identify the longest carbon chain.
  • Identify any side chains.
  • Identify whether it is an alkane or not.

There are 4 carbons in the longest chain, using the table above you can then see that the stem of the name would be 'but'.

There are no side chains and it is an alkane, therefore the name of the molecule is 'Butane'.


Example 2:

  • The molecule to the right has five carbons in the longest chain - this means that it has the stem 'pent'.
  • There is a side chain and it is a 'methyl' alkyl group, therefore 'methyl' is added to the suffix as a prefix.
  • The methyl group is on the second carbon atom into the chain.
  • The compound is an alkane so it has the suffix 'ane'.
  • From this you can deduce that the name of the compound is 2-methylpentane.

Reactions of organic reagents

Bond Fission

During an organic chemical reaction, bonds are broken in a process called bond fission.

A covalent bond can either be broken by heterolytic or homolytic fission.

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  • Heterolytic fission is when one of the bonded atoms of the molecule takes both of the shared pair of electrons.
  • The atom that takes both of the electrons becomes negatively charged.
  • The atom that doesn't have the shared pair of electrons becomes positive.
  • You are left with two ions.
  • Homolytic fission is when each bonded atom takes one electron from the shared pair and the unpaired electron is called a radical.
  • Two of the same type of species are produced.
  • When drawing diagrams of homolytic fission a dot is often written next to the two species to represent the unpaired electron (radical)

Nucleophiles and Electrophiles

A nucleophile is a reactant that will attack an a carbon atom that is electron-deficient and donate an electron pair.

Nucleophiles are often negative ions that have lone pairs of electrons

Electrophiles are reactants that attack high density electron areas and accept pairs of electrons.

Electrophiles are often positive ions.

Addition Reactions

In an addition reaction, two reactants will combine together (or react!) to make one product.

An example of this is when an unsaturated molecule bonds to another molecule, this creates a double bond and a saturated molecule.

For example, during the reaction of chlorine and ethene, the reactants combine to form 1,2-dichloroethene.


Substitution Reactions

In this kind of reaction an atom, or group of atoms, will be replaced by different atoms/group of atoms.

An example of this is the reaction between chlorooethane and hydroxide ions, F is replaced by OH and the products formed are ethanol and a F- ion.

Elimination Reactions

One reatant in an elimination reaction will form two products and the previously saturated molecule will become unsaturated.

In the reaction of ethanol using an acid catalyst the water is eliminated from the ethanol molecule to form ethene (which is an unsaturated molecule).



For each question, choose the best answer. The answer key is below.

  1. What is a nucleophile?
    • A reactant that will attack an electron-deficient carbon atom.
    • A reactant that will attack a high density electron area.
    • An unpaired electron.
  2. In an elimination reaction, how many products are made?
    • 1
    • 2
    • none
  3. When naming organic compounds, what stem is given to a carbon chain with 5 carbon atoms?
    • Hept
    • But
    • Pent
  4. How many products are made in an addition reaction?
    • 1
    • 2
    • none

Answer Key

  1. A reactant that will attack an electron-deficient carbon atom.
  2. 2
  3. Pent
  4. 1


Jo on October 20, 2020:

I second the first comment. Fix your ethane skeletal structure please.

Aidan on May 08, 2016:

In the top image, the skeletal formula for ethane is wrong. Ethane has only 2 carbon atoms, which should be represented by the 2 ends of one line in a skeletal formula.

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