A LevelChemistryOCRRevision Notes4. Core Organic ChemistryBasic ConceptsChemical Names & Formulae

Chemical Names & Formulae (OCR A Level Chemistry A): Revision Note

Exam code: H432

Richard Boole

Written by: Richard Boole

Reviewed by: Caroline Carroll

Updated on

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

Number of C atoms

Molecular formula of straight-chain alkane

Name of alkane

Stem used in naming

1

CH4

methane

meth-

2

C2H6

ethane

eth-

3

C3H8

propane

prop-

4

C4H10

butane

but-

5

C5H12

pentane

pent-

6

C6H14

hexane

hex-

7

C7H16

heptane

hept-

8

C8H18

octane

oct-

9

C9H20

nonane

non-

10

C10H22

decane

dec-

  • 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 a side chain: CH3CH2CH(CH3)CH2CH2CH3 has a longest chain of 6 carbons; numbering from the end that gives the lowest number makes it 3-methylhexane, not 4-methylhexane.

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

3,3,4-trimethylhexane: a six-carbon chain with two methyl groups on carbon 3 and one methyl group on carbon 4.

Naming Multiple Side Chains

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

4-ethyl-2-methylhexane: a six-carbon chain with a methyl group on carbon 2 and an ethyl group on carbon 4, with the substituents named in alphabetical order.

Naming Side Chains in Alphabetical Order

Examiner Tips and Tricks

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 Table

Functional group

Nomenclature

Alkene

-ene

Example: ethene

Displayed formula of ethene, H2C=CH2

Halogenoalkane

bromo-

chloro-

fluoro-

iodo-

Example: chloroethane

Displayed formula of chloroethane, CH3CH2Cl

Alcohol

-ol

Example: ethanol

Displayed formula of ethanol, CH3CH2OH

Aldehyde

-al

Example: ethanal

Displayed formula of ethanal, CH3CHO

Ketone

-one

Example: propanone

Displayed formula of propanone, CH3COCH3

Carboxylic acid

-oic acid

Example: ethanoic acid

Displayed formula of ethanoic acid, CH3COOH

Ester

alkyl -oate

Example: propyl ethanoate

Displayed formula of propyl ethanoate, CH3COOCH2CH2CH3

Amine

alkyl -amine

Example: ethylamine

Displayed formula of ethylamine, CH3CH2NH2

Nitrile

-nitrile

Example: ethanenitrile

Displayed formula of ethanenitrile, CH3CN

Worked Example

Naming organic molecules

Name the following molecules using correct systematic nomenclature: 

Six skeletal formulae to name: an alkene with two methyl groups on the C=C and a CH2C(CH3)3 group, a heptane chain with =CH2 on the middle carbon, propanal, an aldehyde with a methyl branch, butanoic acid and a chloroalkane.

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:

  • 2-propylpent-1-ene

    • The longest carbon chain that contains the C=C double bond is 5 carbons = pent

    • The double bond starts on carbon 1 = -1-ene

    • There is a propyl side chain on carbon 2 = 2-propyl

    • Note: the longest chain overall has 7 carbons, but the main chain must contain the C=C double bond, so the name is based on pentene, not heptane

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

  • A homologous series is a group of organic compounds that have:

    • The same functional group

    • The same general formula

    • Similar chemical properties (each successive member differs by CH2)

    • Gradually changing physical properties, e.g. boiling point, melting point and density

  • The general formula is a formula that represents a homologous series of compounds using letters and numbers

  • The general formulae of the homologous series are:

Homologous series

General formula

Alkene

CnH2n

Halogenoalkane

CnH2n+1X

Alcohol

CnH2n+1OH

Aldehyde

CnH2n+1CHO

Ketone

CnH2nO

Carboxylic acid

CnH2n+1COOH

Ester

CnH2nO2

Amine

CnH2n+1NH2

Nitrile

CnH2n+1CN

  • 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 formula that shows the carbon skeleton (the carbon atoms are implied at the ends and corners of the lines) and the functional groups; hydrogen atoms on the alkyl chain are not shown, but hydrogens that are part of a functional group, e.g. OH, are 

Overview of the Formulae of Organic Compounds Table

Table of nine homologous series (alkene, halogenoalkane, alcohol, aldehyde, ketone, carboxylic acid, ester, amine, nitrile) with general formula, structural formula, displayed formula, skeletal formula and an example name.

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:

Skeletal formulae of butan-1-ol, 2-methylpropan-1-ol and ethoxyethane.

Worked Example

Drawing displayed formulae of molecules Draw the displayed formula of the following molecules:

Three skeletal formulae to draw as displayed formulae: propene, but-3-enoic acid and methyl 3-chloropropanoate.

Answer

Displayed formulae, with every atom and bond drawn, of propene, but-3-enoic acid and methyl 3-chloropropanoate.

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Author
Reviewer
Richard Boole

Author: Richard Boole

Expertise: Curriculum Expert

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.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.