Introducing Cycloalkanes

Definition of cycloalkanes:

General Formula:

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

C_nH_2n

where n represents the number of carbon atoms in the cyclalkane (and n 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 homologous series of simple cycloalkanes is shown below for carbon rings of three to ten carbon atoms.

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 as intended - see the examples in the table.

Number carbons in ring	Name	Full Displayed Structure	More concise structure	Skeletal (Displayed) Structure
	simple formula, and alternative names	showing bond types but not accurate bond angles
3	cyclopropane Molecular Formula : C3H6 trimethylene trimethylene (cyclic)			(regular triangle, 3-sided regular polygon)
4	cyclobutane Molecular Formula : C4H8 tetramethylene			(square; 4-sided regular polygon)
5	cyclopentane Molecular Formula : C5H10 pentamethylene			(pentagon; 5-sided regular polygon)
6	cyclohexane Molecular Formula : C6H12 hexamethylene hexanaphthene hexahydrobenzene benzenehexahydride polycyclohexane hexahydro-benzene			(hexagon; 6-sided regular polygon)
7	cycloheptane Molecular Formula : C7H14 heptamethylene suberane			(7-sided regular polygon)
8	cyclooctane Molecular Formula : C8H16 octamethylene cyclooctan			(8-sided regular polygon)
9	cyclononane Molecular Formula : C9H18			(9-sided regular polygon)
10	cyclodecane Molecular Formula : C10H20
Why stop at ten carbon atoms (in the ring) ? It doesn't. There are many more cycoalkanes.The above are examples of simple cycloalkanes.
Important Note: *The synonyms indicated for compounds listed above are just some examples of alternative names found online and believed to have been used to refer to the substance indicated. They have not all been verified and may include common (non-scientific) names, trade names specific to particular suppliers, and perhaps errors. The purpose of these is to give a general indication of the range of names by which cycloalkanes are known - both generally, and specifically.

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:

	Diagram of Molecular Structure	Advantages	Disadvantages
1.		Correct Complete: Every atom and bond is drawn	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.
2.		Correct May be quicker and easier to draw than (1.), above, and also makes more detail obvious to a beginner than (3.), below.	Not every bond is represented by a line, hence some knowledge is assumed.
3.	(regular hexagon; 6-sided regular polygon)	Simplest to draw. Quickest to draw. Requires least space to draw. Appropriate for complicated organic structures.	Not used for simple structures. Do not use this short-hand unless and until it has been covered by your course - otherwise you may 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 that particular 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/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) concerned, 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 "pure" examples first. However, in real practical chemistry 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.

Note: This is one of many pages about types of organic compounds in our Organic Chemistry Section. Similar and related pages describe hydrocarbons, alkanes, haloalkanes, alkenes, alkynes, cycloalkanes, aromatic hydrocarbons (arenes), alcohols, esters, ethers, aldehydes, ketones, carboxylic acids, acid chlorides, acid anhydrides, azo dyes, amines, amides, nitriles, amino acids (chemistry), peptides, proteins (chemistry), polypeptides and others. If you need further information ask your chemistry tutor.