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A LevelChemistryOCRRevision Notes4. Core Organic Chemistry4.1 Basic Concepts4.1.1 Chemical Names & Formulae

Chemical Names & Formulae (OCR A Level Chemistry)

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Systematic Nomenclature

  • Systematic nomenclature can be used to name organic compounds and therefore make it easier to refer to them
  • The alkanes provide the basis of the naming system and the stem of each name indicates how many carbon atoms are in the longest chain in one molecule of the compound

Nomenclature of Organic Compounds Table

An Introduction to AS Level Organic Chemistry Table 1_Nomenclature of Aliphatic Compounds, downloadable AS & A Level Chemistry revision notes

  • If there are any side-chains or functional groups present, then the position of these groups is indicated by numbering the carbon atoms in the longest chain starting at the end that gives the lowest possible numbers in the name
  • The hydrocarbon side-chain is shown in brackets in the structural formula

CH3CH(CH3)CH2CH3

  • The side-chain is named by adding ‘-yl’ to the normal alkane stem
  • This type of group is called an alkyl group

 

Naming Side Chains

  • If there are more than one of the same alkyl side-chain or functional groups, di- (for two), tri- (for three) or tetra- (for four) is added in front of its name
  • The adjacent numbers have a comma between them
  • Numbers are separated from words by a hyphen

 

Naming Multiple Side Chains

  • If there is more than one type of alkyl side-chain, they are listed in alphabetic order

Naming Side Chains in Alphabetical Order

Examiner Tip

An aliphatic compound is straight or branched-chain and also includes cyclic organic compounds that do not contain a benzene ring.

Functional Groups & their Nomenclature Tablenew-3-1-an-introduction-to-as-level-organic-chemistry-functional-group-nomenclature-1

Worked example

Naming organic molecules

Name the following molecules using correct systematic nomenclature:  

Answer 1:

  • 2,5,5-trimethylhex-2-ene
    • The longest carbon chain is 6 carbons = hex
    • There is an alkene group which is a higher priority group than the methyl side chains
      • So, the carbons are numbered to make the alkene have the lowest possible number = -2-ene
    • This means that the methyl groups are on carbons 2, 5 and 5 = 2,5,5-trimethyl

Answer 2:

  • 4-Methyleneheptane
    • The longest carbon chain is 7 carbons = hept
      • The bonds within the longest chain are all single bonds = ane
    • There is an alkene side chain on carbon 4 = 4-Methylene
    • Note: There is an argument for this to be called 2-propylpent-1-ene as the alkene group typically is a high priority, however, this is not the correct systematic nomenclature

Answer 3:

  • Propanal
    • The longest carbon chain is 3 carbons = prop
    • There is an aldehyde group = al
      • By convention, the aldehyde group is attached to carbon-1
    • The bonds within the main chain are all single bonds = an

Answer 4:

  • 3-methylbutanal
    • The longest carbon chain is 4 carbons = but
    • There is an aldehyde group = al
      • By convention, the aldehyde group is attached to carbon-1
    • The bonds within the main chain are all single bonds = an
    • There is a methyl side chain attached to carbon-3 = 3-methyl 

Answer 5:

  • Butanoic acid
    • The longest carbon chain is 4 carbons = but
    • There is a carboxylic acid group = -oic acid
      • By convention, the carboxylic acid is attached to carbon-1
    • The bonds within the main chain are all single bonds = an

Answer 6:

  • 2-chlorobutane
    • The longest carbon chain is 4 carbons = but
    • The bonds within the main chain are all single bonds = an
    • There is a chlorine atom attached to carbon-2 = 2-chloro

Organic Formulae

  • The general formula is a formula that represents a homologous series of compounds using letters and numbers
    • E.g. the general formula of alkanes is CnH2n+2
    • 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 structural formula is a formula that shows how the atoms are bonded to each carbon atom in a molecule
  • The displayed formula is a 2D representation of an organic molecule showing all its atoms (by their symbols) and their bonds (by single, double or triple bonds)
  • The skeletal formula is a simplified displayed formula with all the carbon and hydrogen atoms removed, with the exception of hydrogens that are part of a functional groups, e.g. OH 

Overview of the Formulae of Organic Compounds Table

 

Worked example

Drawing skeletal formulae of molecules

Draw the skeletal formula of the following molecules:

  1. CH3(CH2)3OH
  2. (CH3)2CHCH2OH
  3. CH3CH2OCH2CH3

Answer:

An Introduction to AS Level Organic Chemistry Answer Worked example - Drawing skeletal formulae of molecules, downloadable AS & A Level Chemistry revision notes

Worked example

Drawing displayed formulae of molecules

Draw the displayed formula of the following molecules: 

Answer

An Introduction to AS Level Organic Chemistry Answer Worked example - Drawing displayed formulae of molecules, downloadable AS & A Level Chemistry revision notes

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    Richard

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    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.