National 5ChemistrySQARevision NotesNature’s ChemistryHomologous SeriesStructural Formulae

Structural Formulae (SQA National 5 Chemistry): Revision Note

Exam code: X813 75

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Full & shortened structural formulae

  • Organic chemistry uses different types of formulae to represent molecules:

  • Molecular formula

    • This shows the total number of each type of atom

      • e.g., butane is written as C4H10

    • It tells you what's in a molecule, but not how it's connected

  • Full structural formula

    • This is a diagram that shows every atom and every single bond

    • This is the most detailed picture

  • Shortened structural formula

    • This is a line of text that summarises the structure

    • It groups the atoms on each carbon together

  • Condensed structural formula

    • This is a common way of writing a formula that gives some structural information

    • It shows the functional group separately

      • e.g., ethanol is written as C2H5OH

Example of different formulae

  • Let's look at an example for the alkane, butane:

  • Its molecular formula is C4H10

    • This shows that butane contains 4 carbon atoms and 10 hydrogen atoms

  • To write its shortened structural formula:

    • We write each carbon and its attached hydrogens in order

      • The first carbon is bonded to 3 hydrogen atoms and 1 neighbouring carbon atom

      • The second carbon atom is bonded to 2 hydrogen atoms and 2 neighbouring carbon atoms

      • The third carbon atom is bonded to 2 hydrogen atoms and 2 neighbouring carbon atoms

      • The final carbon is bonded to 3 hydrogen atoms and 1 neighbouring carbon atom

    • So, the shortened structural formula is CH3CH2CH2CH3

  • To draw its full structural formula:

    • Draw out every atom and every bond

    • So, the full structural formula is:

Structural diagram of butane, showing four carbon atoms in a chain, each bonded to hydrogen atoms.

Examiner Tips and Tricks

The "Four Bond" rule

  • When drawing a full structural formula, always check your final drawing

  • Every carbon atom must have exactly four bonds

Examples from each homologous series

Alkanes

  • Alkanes are saturated hydrocarbons containing only C-C single bonds

  • Their shortened structural formula is the most common way to write them

  • They have no specific functional group to highlight

Name

Molecular formula

Full structural formula

Shortened structural formula

Methane

CH4

Structural diagram of ethane, featuring two carbon atoms single-bonded in the centre, each bonded to three hydrogen atoms.

CH4

Ethane

C2H6

Structural diagram of ethane, featuring two carbon atoms single-bonded in the centre, each bonded to three hydrogen atoms.

CH3CH3

Propane

C3H8

Structural diagram of propane with three carbon atoms single-bonded linearly and eight hydrogen atoms bonded to the carbons.

CH3CH2CH3

Alkenes

Name

Molecular formula

Full structural formula

Shortened structural formula

Ethene

C2H4

Ethene molecular structure showing two carbon atoms connected by a double bond, each bonded to two hydrogen atoms.

CH2CH2

Propene

C3H6

Structural formula of propene, showing three carbon atoms. The first carbon is double-bonded to the second, with hydrogen atoms attached.

CH2CHCH3

But-1-ene

C4H8

Structural formula of but-1-ene showing four carbon atoms with a double bond between the first two, and single bonds with hydrogen atoms.

CH2CHCH2CH3

Cycloalkanes

  • Cycloalkanes are saturated hydrocarbons where the carbon atoms form a ring

  • Their full structural formula is the most important representation

  • Shortened structural formulae are not usually required for cycloalkanes

Name

Molecular formula

Full structural formula

Cyclopropane

C3H6

Structural diagram of cyclopropane molecule with three carbon atoms connected in a ring, each bonded to hydrogen atoms denoted by 'H' around them.

Cyclobutane

C4H8

Structural diagram of cyclobutane displaying a square ring of four carbon atoms, each bonded to two hydrogens, illustrating its chemical structure.

Cyclopentane

C5H10

Chemical structure diagram of cyclopentane, showing a pentagonal ring of five carbon atoms, each bonded to two hydrogen atoms.

Worked Example

Draw the full structural formula for cyclohexane, C6H12

Answer:

  • From the name:

    • Cyclo- means it's a ring

    • hex- means 6 carbon atoms

    • -ane means all C-C single bonds

  • To draw the full structural formula:

    • Draw a hexagon of 6 carbon atoms

    • Add bonds and hydrogen atoms to each carbon until every carbon has a total of 4 bonds

Structural formula of cyclohexane, a six-carbon ring with each carbon atom bonded to two hydrogens, forming a hexagonal shape.

Alcohols

  • Alcohols are a homologous series containing the -OH (hydroxyl) functional group

  • Since this group is so important, their formulae are often written as a condensed formula to show it

Name

Molecular formula

Condensed structural formula

Full structural formula

Shortened structural formula

Methanol

CH4O

CH3OH

Chemical structure of methanol with a central carbon atom bonded to three hydrogen atoms and one oxygen atom, which is also bonded to a hydrogen.

CH3OH

Ethanol

C2H6O

C2H5OH

Chemical structure of ethanol: two carbon atoms single-bonded, with five hydrogens and one oxygen bonded to an additional hydrogen.

CH3CH2OH

Propan-1-ol

C3H8O

C3H7OH

Structural formula of propan-1-ol, showing three carbon atoms, seven hydrogen atoms, and one hydroxyl group (OH) connected in a linear arrangement.

CH3CH2CH2OH

Worked Example

Write the shortened structural formula for butan-1-ol

Structural formula of butan-1-ol shows four carbon atoms bonded to hydrogen atoms, one oxygen atom, and an additional hydrogen.

Answer:

  • From the name:

    • But- means a 4-carbon chain

    • -ol means an -OH group

    • -1- means the -OH is on the first carbon

  • To write the shortened structural formula:

    • Start with the first carbon and its group:

      • CH2OH

    • Add the two middle carbons:

      • CH2CH2

    • Add the carbon:

      • CH3

    • Combine them in order:

      • CH3CH2CH2CH2OH

Carboxylic acids

  • Carboxylic acids contain the -COOH (carboxyl) functional group

  • Like alcohols, their formulae are often written in a condensed way to highlight this important functional group

Name

Molecular formula

Condensed structural formula

Full structural formula

Shortened structural formula

Methanoic acid

CH2O2

HCOOH

Structural formula of methanoic acid, showing carbon bonded to hydrogen, carbonyl oxygen, and hydroxyl group, arranged in a simple diagram.

HCOOH

Ethanoic acid

C2H4O2

CH3COOH

Chemical structure of ethanoic acid showing two carbon atoms, three hydrogen atoms, and two oxygen atoms in a molecular diagram format.

CH3COOH

Propanoic acid

C3H6O2

C2H5COOH

Structural formula of propanoic acid showing three carbon atoms, six hydrogen atoms, and two oxygen atoms, forming a carboxyl group on the end.

CH3CH2COOH

Worked Example

Draw the full structural formula for butanoic acid, C3H7COOH

Answer:

  • From the name:

    • Butan- means 4 carbons in total

    • -oic acid means it has a -COOH group

  • To draw the full structural formula:

    • Draw the carboxyl group (-COOH) at the end of the chain

    • Draw the other 3 carbons in a chain leading to it

    • Add hydrogen atoms until every carbon has 4 bonds

Structural formula of butanoic acid, showing a four-carbon chain with the carboxylic acid group (COOH) at one end, and hydrogen atoms attached.

How to name organic compounds

  • The names of organic compounds follow a set of specific rules based on the

    • Carbon chain length

    • The functional group

  • You can find the detailed rules for naming each family in their own revision notes:

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    Author
    Reviewer
    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry Content Creator

    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.

    Philippa Platt

    Reviewer: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener