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Summary OCR A Level Chemistry Notes Module 4
Covers the full OCR A level specification with notes from past paper mark schemes, textbooks and examples.
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Module 44.1.1
Naming and representing the formulae of organic compounds
IUPAC rules of nomenclature can be used to systematically name organic compounds. Systematic
nomenclature is used to avoid ambiguity
Chemical formulae (Definitions not required)
- General formula: The simplest algebraic formula of a member of a homologous series. The general
formula of a homologous series can be used to predict the formula of any member of the series:
Aliphatic Alkanes: CnH2n+2
Alicyclic Alkanes: CnH2n
Alkenes: CnH2n (Cyclic alkenes and alkenes with more than one double bond
(polyunsaturated) do not obey the general formula however branched and aliphatic
alkenes do)
Alicyclic alkenes CnH2n-2 (Alicyclic alkenes with more than one double bond
(polyunsaturated) do not obey the general formula)
Alkyl groups: CnH2n+1
Haloalkanes: CnH2n+1Xy (X = F, Cl, Br, I)
Alcohols: CnH2n+1OH
Carboxylic acids: CnH2nO2
Aldehydes: Cn-1CH2nO
Ketones: CnH2nO
- Molecular Formula: The actual number and type of atoms of each element present in a molecule. The
molecular formula of a compound must be simplified and not written in a notation for the type of
compound, but rather split into the number of atoms per element
- Empirical formula: The simplest whole number ratio of atoms of each element present in a compound
- Structural formula: The minimal detail that shows the arrangements of atoms in a molecule. In structural
formulae, the carboxyl group will be represented COOH and the ester group as COO. Branches go in
brackets.
- Displayed formula: The relative positioning of atoms and the bonds between them E.g. for Ethanol
(separate the O from the H for the OH group)
- Skeletal formula: The simplified organic formula, shown by removing hydrogen atoms from alkyl chains,
leaving just a carbon skeleton and associated functional groups
Functional groups
Homologous series: A series of organic compounds having the same functional group but with each
successive member differing by CH2
R may be used to represent Alkyl groups, but also other fragments of organic compounds not involved in
reactions
Functional group: (definition not required) a group of atoms responsible for the characteristic reactions of a
compound
All organic compounds are either aliphatic or aromatic, however some aliphatic compounds are also alicyclic
Aliphatic: (definition not required) A compound containing carbon and hydrogen atoms joined together in
straight chains, branched chains, or non-aromatic rings
Alicyclic: (definition not required) An aliphatic compound arranged in non-aromatic rings with or without
side chains. Addition reaction of alicyclic alkanes gives straight chained compounds
Aromatic: (definition not required) A compound containing a benzene ring
Saturated: Single carbon-carbon bonds only
Unsaturated: The presence of multiple carbon-carbon bonds, including C=C, C≡C and aromatic rings
The terms saturated and unsaturated will be used to indicate the presence of multiple carbon–carbon bonds as
distinct from the wider term ‘degree of saturation’ used also for any multiple bonds and cyclic compounds.
Saturated and unsaturated simply refers to whether there are double bonds present (the latter) or not (the
former), whereas degree of saturation is a measure of how many double bonds are present in a molecule
Isomerism
, Structural isomers are compounds with the same molecular formula but different structural formulae
Reaction Mechanisms
The different types of covalent bond fission:
- Homolytic fission: The covalent bond breaks and each bonding atom receives
one electron from the bonded pair, forming two radicals
- Heterolytic fission: The covalent bond breaks and one bonding atom receives
both electrons from the bonded pair
The breaking of the covalent bond forms two ions and curly arrows need to be
drawn which should start from the middle of a bond, a negative charge
(therefore must originate from the ion) or the middle of a lone pair of electrons
and end at an atom. Any relevant dipoles must be included
- Radical: A species with an unpaired electron. A dot (•) is used to represent
species that are radicals in mechanisms and these dots are required in all
instances where there is a single unpaired electron (e.g. Cl• and CH 3•. Dots are
not required for species that are diradicals (e.g. O)
- A curly arrow is used to describe the movement of an electron pair showing either heterolytic fission or
formation of a covalent bond
4.1.2
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