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Revise Alkanes (GCSE Organic Chemistry)

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How to draw Skeletal Formulae of Organic Molecules

Drawing Organic Molecules (in general)

There are several standard ways to represent the structures of organic molecules in various levels of detail. These include:

For more about drawing organic molecules generally see: How to Draw Organic Molecules.

What are "skeletal formulae" or "skeletal structures (of organic compounds)" ?

In organic chemistry, skeletal formulae are the most abbreviated diagrammatic descriptions of molecules in common use. They look very bare because in skeletal formulae the hydrogen atoms (attached directly to carbons) are removed, leaving just a "carbon skeleton" with functional groups attached to it.

Don't be fooled: The hydrogen atoms are present in the molecules but their presence is assumed - rather than drawn or stated - in the case of skeletal formulae.

This type of representation may take some getting used-to and is not always taught at school-level chemisty (so you may not need to know about use it, check your syllabus to be sure).

 

Simple Examples of Skeletal Formulae:

As for organic molecules in general, the simplest examples of skeletal formulae of organic molecules are those of linear alkanes.

Linear Alkanes:

Ethane

displayed formula for molecular structure of ethane skeetal formula of ethane


So, a "chain link" of two carbon atoms linked together by a single covalent bond (together with the hydrogen atoms attached to them) is represented by a single line when drawn in the form of a skeletal formula.

Further examples:

Propane

displayed formula of the molecular structure of propane

skeletal formula of propane

Butane

displayed formula of the molecular structure of butane

skeletal formula of butane

Pentane

displayed formula of the molecular structure of pentane

skeletal formula of pentane

Hexane

displayed formula of the molecular structure of hexane

skeletal formula of hexane

Heptane

displayed formula of the molecular structure of heptane

skeletal formula of heptane

Octane

displayed formula of the molecular structure of octane

skeletal formula of octane

Nonane

displayed formula of the molecular structure of nonane

skeletal formula of nonane

Decane

displayed formula of the molecular structure of decane

skeletal formula of decane


So, increasing lengths of alkane "chains" of carbon atoms linked together by single covalent bonds (with the appropriate number of hydrogen atoms attached to each) are represented by a series of single short straight lines representing each "link" in the "chain", and these lines are arranged at the angles shown in the right-hand column (above).

Notice that it is not necessary to write the symbols "C" or "H" for atoms that form part of simple linear chains, as opposed to other structures such as functional groups attached to alkane-chains.

Examples of the Skeletal Formulae of a few Branched Alkanes:

Methylbutane

skeletal formula of methylbutane

or any of the following equivalents:

oror

Dimethylpropane

full displayed formula of dimethylpropane skeletal formula of dimethylpropane

3,3-Dimethylpentane

skeletal formula of 3,3-dimethylpentane

or equivalent, e.g. rotating by 90 degrees would not change the meaning of (i.e. molecule represented by) this skeletal formula.


Alkanes are the simplest examples of skeletal formulae because they involve only simple carbon chains and no other functional groups. See also further examples on the page about cycloalkanes.

Further examples:

Linear Amines:

Structure of Methanamine

full displayed formula of methanamine skeletal formula of methanamine

Structure of Ethanamine

full displayed formula of ethanamine skeletal formula of ethanamine

Structure of
Propan-1-amine

full displayed formula of propan-1-amine skeletal formula of propan-1-amine

Structure of
Butan-1-amine

full displayed formula of butan-1-amine skeletal formula of butan-1-amine

Structure of
Pentan-1-amine

full displayed formula of pentan-1-amine skeletal formula of pentan-1-amine


The formulae of the first five linear amines (shown above) indicate how skeletal formulae are simplifications of the full displayed formulae of organic molecules.

One further set of examples is included below. These illustrate formulae of carboxylic acids, which are slightly more complicated than amines because carboxylic acids include a double covalent bond. Note how this is represented in the skeletal formulae by two parallel lines, just as it is also represented in the full displayed formulae by two parallel lines:

Linear Carboxylic Acids:

Structure of
Formic Acid

full displayed formula of methanoic acid skeletal formula of formic acid

Structure of
Ethanoic Acid

full displayed formula of ethanoic acid skeletal formula of ethanoic acid

Structure of
Propanoic Acid

full displayed formula of propanoic acid skeletal formula of propanoic acid

Structure of
Butanoic Acid

full displayed formula of butanoic acid skeletal formula of butanoic acid

Structure of
Pentanoic Acid

full displayed formula of pentanoic acid skeletal formula of pentanoic acid

The above sets of examples are included because some people find it easier to understand skeletal formulae of organic molecules by looking at series of similar molecules (see also homologous series) and noticing trends through the series, as well as comparisons between drawings of the full displayed formula and the skeletal formula of individual molecules.


Why
draw skeletal structures of organic molecules ?

Organic molecules can reach huge sizes. Sometimes it is necessary to describe the whole molecule, but not to draw every single atom and chemical bond in full.

Organic molecules are usually only represented using skeletal formulae (that is, by drawing their skeletal structures) in cases of molecules beyond a certain size and complexity. Therefore moderately advanced knowledge of organic chemistry is assumed.

The benefits of using the skeletal structures (which are sometimes referred to as "skeletal formulae") include:

  • Such simpler (abbreviated) diagrams are generally quicker and easier to draw - by hand, or electronically.
    Similarly,
  • such simpler (abbreviated) diagrams often take-up less space.
  • Important parts of the molecules are more obvious, i.e. have more prominence, in skeletal diagrams than in most fully displayed formulae.
  • This type of diagram is commonly used in the chemical industry so it is useful for students to be familiar with it.

 

When and where are skeletal formulae used ?

This type of representation of organic molecules is most frequently used in more advanced texts, research papers, and specialist areas. It is generally the most practical way to draw large and very complicated organic molecules. Even at lower levels of complexity (e.g. High School Chemistry and UK A-Level), skeletal formulae may be used - especially to describe structures involving carbon rings, such as cycohexane and benzene - see below.

 

Skeletal Formulae of Organic Molecules that have a "ring", rather than a "linear" or "branched" structure:

Cyclohexane and Benzene

Structure of Cyclohexane

Fully Displayed Formula of Cyclohexane

Structure of Benzene

 

Fully Displayed Formula of Benzene

or Skeletal Formula of Benzene

Structure of Ethylbenzene

 


or
Skeletal Formula of ethylbenzene

As can be seen from the example of ethylbenzene (above), units of alkane chains are represented by straight lines at alternating angles, as indicated with examples further up this page.



More Examples of Skeletal Formulae

Note that some sources such as books, websites, reports etc., emphasize the presence of atoms other than carbon and hydrogen using colours - as in the following examples of skeletal formulae of predominately linear carbon-chain molecules. This representation (with use of colours) may be helpful but isn't strictly necessary and obviously cannot be used everywhere as some journals are printed in black ink only.

 
Propanoic Acid
Hexanenitrile
Propyl Pentanoate
Skeletal Structure of Propanoic Acid
Skeletal Formula of hexanenitrile
Skeletal Structure of Propyl Pentanoate


Compare the above with the equivalent semi-displayed formulae (= "simplified displayed formulae") below:



... and with the following fully displayed formulae:

Molecular Structure of Propanoic Acid - Fully Displayed Formula Molecular Structure of hexanenitril - Fully Displayed Formula Molecular Structure of Propyl Pentanoate - Fully Displayed Structure


See also our main page about how to draw organic molecules.

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