Introducing Cycloalkanes

Definition of Cycloalkanes:

Cycloalkanes are a particular type (or category) of alkanes.

Reminder: Alkanes are hydrocarbon chemical compounds, meaning that they consist only of the elements carbon (C) and hydrogen (H). Alkanes are saturated hydrocarbons, meaning that they contain only single (not double or triple) covalent bonds.

Each carbon atom forms 4 bonds (either C-H or C-C bonds).
Each hydrogen atom is connected to a single carbon atom, by a H-C bond.

Cycloalkanes are distinguished from other types of alkanes i.e. linear alkanes and branched alkanes by their structure.

That is, cycloalkanes include a part consisting of at least three carbon atoms linked together by single covalent bonds in the form of a ring, or loop. See diagrams below.

General Formula:

Simple cycloalkanes, that is those without any additional branches attached (see later), have the general formula:

CnH2n

where n represents the number of carbon atoms in the cyclalkane (must be a whole number greater than 2).

Have you seen this general formula anywhere else? It is also the general formula of linear alkenes. This means that cycloalkanes are isomers of the corresponding linear alkene, and vice-versa, for molecules containing 3 or more carbon atoms.

Names and Structures of simple Cycloalkanes

The first eight members of the homologous series of simple cycloalkanes are shown below.

Notice that the full displayed structures of cycloalkanes become increasingly awkward to draw as the size of the moleclues increase. You may therefore be tempted to group parts of a molecule together in order to represent it more easily, quickly and clearly. There are standard ways to do this and it is important to follow the conventions so that your drawings are understood - see the examples in the table.

  • Cyclopropane

    Molecular Formula: C3H6

    Full Displayed Structure:
    Displayed Structure of cyclopropane
    More concise Structure:
    concise structure of cyclopropane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclopropane
    (regular triangle, 3-sided regular polygon)
    Alternative names:
    • trimethylene
    • trimethylene (cyclic)
  • Cyclobutane

    Molecular Formula: C4H8

    Full Displayed Structure:
    Displayed structure of cyclobutane
    More concise Structure:
    concise structure of cyclobutane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclobutane
    (regular square, 4-sided regular polygon)
    Alternative names:
    • tetramethylene
  • Cyclopentpane

    Molecular Formula: C5H10

    Full Displayed Structure:
    Displayed structure of cyclopentane
    More concise Structure:
    concise structure of cyclopentane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclopentane
    (regular pentagon, 5-sided regular polygon)
    Alternative names:
    • pentamethylene
  • Cyclohexane

    Molecular Formula: C6H12

    Full Displayed Structure:
    Displayed structure of cyclohexane
    More concise Structure:
    concise structure of cyclohexane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclohexane
    (regular hexagon, 6-sided regular polygon)
    Alternative names:
    • hexamethylene
    • hexanaphthene
    • hexahydrobenzene
    • benzenehexahydride
    • polycyclohexane
    • hexahydro-benzene
  • Cycloheptane

    Molecular Formula: C7H14

    Full Displayed Structure:
    Displayed structure of cycloheptane
    More concise Structure:
    concise structure of cycloheptane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cycloheptane
    (regular heptagon, 7-sided regular polygon)
    Alternative names:
    • heptamethylene
    • suberane
  • Cyclooctane

    Molecular Formula: C8H16

    Full Displayed Structure:
    Displayed structure of cyclooctane
    More concise Structure:
    concise structure of cyclooctane
    Skeletal (Displayed) Structure:
    Skeletal Formula of cyclooctane
    (regular octagon, 8-sided regular polygon)
    Alternative names:
    • octamethylene
    • cyclooctan
  • Cyclononane

    Molecular Formula: C9H18

    Full Displayed Structure:
    Displayed structure of cyclononane
    More concise Structure:
    concise structure of cyclononane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclononane
    (regular enneagon, 9-sided regular polygon)
  • Cyclodecane

    Molecular Formula: C10H20

    Full Displayed Structure:
    Displayed Structure of cyclodecane
    More concise Structure:
    concise structure of cyclodecane
    Skeletal (Displayed) Structure:
    Diagram of the Skeletal Formula of cyclodecane

More about Drawing Molecules of Cycloalkanes:

Representing molecular structures clearly:

Taking the example of cyclohexane, consider the advantages and disadvantages of some possible ways of representing the structure of cyclohexane:

Possibility (1)

Advantages:

  • Correct
  • Complete:
    Every atom and bond is drawn

Disadvantages:

  • This level of detail, including every atom and bond, can cause diagrams of larger molecules to look messy.
  • Using more detailed representation than necessary also requires more time and space.

Possibility (2)

Advantages:

  • Correct
  • May be quicker and easier to draw than (1.), above, and also makes the detail clearer to a beginner than (3.), below.

Disadvantages:

  • Not every bond is represented by a line, hence some knowledge is assumed.

Possibility (3)

Diagram of the Skeletal Formula of cyclohexane

(regular hexagon; 6-sided regular polygon)

Advantages:

  • Simplest to draw.
  • Quickest to draw.
  • Requires least space to draw.
  • Appropriate for complicated organic structures.

Disadvantages:

  • Not used for simple structures.
  • Do not use this short-hand unless and until it has been covered by your course - otherwise you might lose marks for not including sufficient detail in your answer(s).
Which type of molecular structure should I draw ?

The most appropriate representation to use depends on the context and reason for drawing the molecular structure.

If you are drawing structures of cycloalkanes as part of your studies, coursework or in an exam you should choose which type of structure to draw based on:

  • Specific instructions given by your teacher or lecturer or in the question you have been asked.
  • Which type of structures have been taught and used by teachers or lecturers during this part of your course.
  • Which type of structures / diagrams appear elsewhere in the printed or electronic documents you are working from, e.g. textbooks or exam paper.
  • The number of marks allocated for the question (or part of the question), if you are given that information.
  • The overall complexity of the molecule; the more complicated the molecule, the more difficult it is to represent it clearly using by a full displayed structure that includes every single covalent bond.

Some of the chemistry pages on this website indicate only full displayed structures, especially when presenting introductory-level information about compounds / molecules and types of organic compounds / molecules. Other pages show skeletal molecular structures, especially when describing more complicated molecules and / or discussing and explaining their properties in terms of specific functional groups, which are often indicated more clearly by use of diagrams of skeletal structures.

Shapes of simple Cycloalkanes:

The above diagrams of the molecular structures of cycloalkanes may give the impression that these molecules have shapes similar to flat regular polygons. That is not true. It is always important to remember that molecular structures are drawn to represent the atoms present and the bonds between them as clearly as possible (often, as in this case) in two-dimensions. In reality, these molecules are three-dimensional structures.

More Complicated Cycloalkanes

The cycloalkanes listed and drawn in the main table (above) are described as 'simple' cycloalkanes because they do not include any additional branches or functional groups. When studying organic chemistry it is generally easier to begin by learning about different types of compounds by considering simple examples first. However, many important compounds and their molecules include parts of more than one type and consist of a combination of linear chains of carbon atoms, rings of carbon atoms (as in the case of cycloalkanes), and long or short branches extending from linear carbon chain and/or carbon rings.

We plan to add further pages about naming (complex) cycloalkanes, examples of cycloalkanes and reactions of cycloalkanes.

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