Definitions for AS OCR chemistry A

Key definitions for organic chemistry

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Functional Group

The part of the organic molecule responsible for its chemical reactions.

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Homologous Series

A series of organic compounds with the same functional group but with each successive member differing by CH2.

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Alkyl group

An alkane with a hydrogen atom removed. For example, CH3, C2H5 etc.

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General Formula

The simplest algebraic formula of a member of a homologous series. For example, alkanes have the general formula of CnH2n+2, alkenes- CnH2n, alcohols- CnH2n+1OH.

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Displayed Formula

Shows the relative positioning of all the atoms in a molecule and the bonds between them.

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Structural Formula

The minimal detail for the arrangement of atoms in a molecule.

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Skeletal Formula

  • Simplified organic formula with hydrogen atoms removed from the alkyl chains
  • leaving just the carbon skeleton and functional groups.
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Structural Isomers

Molecules with the same molecular formula but with different structural arrangements of atoms.

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Same structural formula but with a different arrangement of atoms in space.

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E/Z isomerism

  • Different groups are attached to each carbon of a carbon-carbon double bond.
  • These may be arranged differently in space because of the restricted rotation of the carbon-carbon double bond.
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cis-trans isomerism

  • A special type of E/Z isomerism.
  • There is a hydrogen and a non-hydrogen group on each C of the carbon-carbon double bond.
  • The Cis isomer (Z) has hydrogens on the same side.
  • The Trans (E) has hydrogens on different sides.
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Homolytic Fission

The breaking of a covalent bond with one of the bonded electrons going to each atom to form two radicals.

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Heterolyic Fission

The breaking of a covalent bond with both of the bonded electrons going to one of the atoms forming a cation and an anion.

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An atom that is attracted to an electron deficient atom where it donates a pair of electrons to form a new covalent bond.

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A species with an unpaired electron.

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An atom that is attracted to an electron rich atom where it accepts a pair of electrons to form a new covalent bond.

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Addition Reaction

A reactant is added across an unsaturated molecule to make a saturated molecule. In other words...

  • double bonds to single bonds
  • 2 reactants to 1 product
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Substitution Reaction

An atom is replaced with a different atom.

  • 2 reactants to 2 products
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Elimination Reaction

Removal of a molecule from a saturated to make an unsaturated.

  • Single bonds to double bonds
  • 1 reactant to 2 products
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Fractional Distillation

Separation of components in a liquid mixture into fractions which differ in boiling points.

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Breaking down a long chained saturated hydrocarbon to form a mixture of short chained alkanes and alkenes.

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Radical Substitution

A radical replaces a different atom.

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A sequence of steps showing the path taken by electrons in a reaction.

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The FIRST step in radical substiution in which the free radicals are generated by UV radiation.

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The LAST step in radical substitution when two radicals combine to form a molecule.

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Pi bond

  • The reactive part of the double bond.
  • It is formed above and below the plane of bonded atoms by overlap of p-orbitals.
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An organic ion where the carbon atom has a positive charge.

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Curly Arrow

Shows the movement of an electron pair in the breaking and formation of a covalent bond.

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A long molecular chain built up from many small monomer units.

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Addition Polymerisation

An unsaturated alkene (monomer) adds on to a polymer chain one at a time to form a long saturated molecular chain.

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Addition Polymer

A very long molecular chain formed by repeated addition reactions of many unsaturated alkene molecules (monomers).

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Repeat Unit

  • A specific arrangement of atoms that occur in the structure over and over again.
  • The repeat units will be in brackets outside of which is the symbol n.
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