Characteristic Organic Reactions (Cambridge (CIE) AS Chemistry)

Definitions & Terminology in Organic Chemistry

Homologous series

• A homologous series is a group of organic compounds that have the same functional group, the same general formula and the same chemical properties

The homologous series of alcohols

Ethanol and propanol belong to the same homologous series

Saturated & unsaturated hydrocarbons

• Saturated hydrocarbons are hydrocarbons which contain single bonds only resulting in the maximum number of hydrogen atoms in the molecule

• Unsaturated hydrocarbons are hydrocarbons which contain carbon-carbon double or triple bonds

Comparing the alkane and alkene homologous series

The diagram shows saturated hydrocarbons which contain single bonds only and unsaturated hydrocarbons which contain double/triple bonds as well

Homolytic & heterolytic fission

• Homolytic fission is breaking a covalent bond in such a way that each atom takes an electron from the bond to form two radicals

• Heterolytic fission is breaking a covalent bond in such a way that the more electronegative atom takes both the electrons from the bond to form a negative ion and leaves behind a positive ion

Examples of homolytic & heterolytic fission

In heterolytic fission, the most electronegative atom takes both electrons in the covalent bond. While in homolytic fission, each atom takes one electron from the covalent bond

Radical chain reactions

• A free radical is a species with one (or more than one) unpaired electrons

Free radicals

The diagram shows a free radical which has one unpaired electron

• A free radical reaction is a reaction involving free radicals and is a three-step reaction:

  • Initiation is the first step and involves breaking a covalent bond using energy from ultraviolet (UV) light from the sun to form two free radicals

  • The propagation step is the second step in which the formed radical can attack reactant molecules to form even more radicals

    • These in turn can again attack other molecules to form more free radicals and so on

  • In the termination step, two free radicals react together to form a product molecule

The steps of a free radical reaction mechanism

The main steps of a free radical reaction mechanism are initiation, propagation and termination

Nucleophiles & electrophiles

• A nucleophile is an electron-rich species that can donate a pair of electrons

  • ‘Nucleophile’ means ‘nucleus/positive charge loving’ as nucleophiles are attracted to positively charged species

  • Nucleophilic refers to reactions that involve a nucleophile

• An electrophile is an electron-deficient species that can accept a pair of electrons

  • ‘Electrophile’ means ‘electron/negative charge loving’ as electrophiles are attracted to negatively charged species

  • Electrophilic refers to reactions that involve an electrophile

Examples of a nucleophile and an electrophile 

A nucleophile ‘loves’ a positive charge and an electrophile ‘loves’ a negative charge

Types of reactions

• An addition reaction is an organic reaction in which two (or more) molecules combine to give a single product with no other products

• A substitution reaction is a reaction that involves replacing an atom or group of atoms with another atom or group of atoms

• An elimination reaction is a reaction in which a small molecule (such as H₂O or HCl) is removed from an organic molecule

• A hydrolysis reaction is a reaction in which a compound is broken down by water (it can also refer to the breakdown of a substance by dilute acids or alkalis)

• A condensation reaction is a reaction in which two organic molecules join together and in the process eliminate small molecules (such as H₂O or HCl)

The different types of reactions in organic chemistry

It is important to be able to describe and identify the different types of reaction in organic chemistry

Oxidation & reduction

• An oxidation reaction is a reaction in which oxygen is added, electrons are removed or the oxidation number of a substance is increased

  • In organic chemistry it often refers to the addition of oxygen or removal of hydrogen atoms to a substance

  • In equations for organic redox reactions, the symbol [O] can be used to represent one atom of oxygen from an oxidising agent

• A reduction reaction is a reaction in which oxygen is removed, electrons are added or the oxidation number of a substance is decreased

  • In organic chemistry it often refers to the removal of oxygen or addition of hydrogen atoms to a substance

  • In equations for organic redox reactions, the symbol [H] can be used to represent one atom of hydrogen from a reducing agent

Examples of oxidation and reduction

In organic chemistry oxidation is often the gain of oxygen or loss of hydrogen atoms and reduction is the gain of hydrogen and loss of oxygen atoms