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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:

Molecular Formulae of Organic Molecules
Structural Formulae of Organic Molecules:
- Fully Displayed Formulae
- Simpler Displayed Formulae
- Sketched 3D Formulae
- Modelled 3D Formulae
- Skeletal Formulae

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:

(Read more about alkanes)

Ethane

Molecular Formula: C₂H₆

Full Displayed Formula of Ethane:

Skeletal Formula of Ethane:

displayed formula for molecular structure 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

Molecular Formula: C₃H₈

Full Displayed Formula of Propane:

Skeletal Formula of Propane:

displayed formula of the molecular structure of propane

Butane

Molecular Formula: C₄H₁₀

Full Displayed Formula of Butane:

Skeletal Formula of Butane:

displayed formula of the molecular structure of butane

Pentane

Molecular Formula: C₅H₁₂

Full Displayed Formula of Pentane:

Skeletal Formula of Pentane:

displayed formula of the molecular structure of pentane

Hexane

Molecular Formula: C₆H₁₄

Full Displayed Formula of Hexane:

Skeletal Formula of Hexane:

displayed formula of the molecular structure of hexane

Heptane

Molecular Formula: C₇H₁₆

Full Displayed Formula of Heptane:

Skeletal Formula of Heptane:

displayed formula of the molecular structure of heptane

Octane

Molecular Formula: C₈H₁₈

Full Displayed Formula of Octane:

Skeletal Formula of Octane:

displayed formula of the molecular structure of octane

Nonane

Molecular Formula: C₉H₂₀

Full Displayed Formula of Nonane:

Skeletal Formula of Nonane:

displayed formula of the molecular structure of nonane

Decane

Molecular Formula: C₁₀H₂₂

Full Displayed Formula of Decane:

Skeletal Formula of Decane:

displayed formula of the molecular structure 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

Molecular Formula: C₅H₁₂

Full Displayed Formula of Methylbutane:

Skeletal Formula of Methylbutane:

full displayed formula of methylbutane
or equivalents, e.g. may be drawn counting carbons from the right- or left- and with the methyl-group shown above- or below- the main alkane chain

or any of the following equivalents:

oror

Dimethylpropane

Molecular Formula: C₅H₁₂

Full Displayed Formula of Dimethylpropane:

Skeletal Formula of Dimethylpropane:

full displayed formula of dimethylpropane

3,3-Dimethylpentane

Molecular Formula: C₇H₁₆

Full Displayed Formula of 3,3-Dimethylpentane:

Skeletal Formula of 3,3-Dimethylpentane:

full displayed formula of 3,3-dimethylpentane
or equivalent, e.g. if drawn rotated by 90 degress the same molecule would be represented - but such organic molecules are usually drawn with the longest carbon-chain horizonal (as above).

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:

(Read more about naming amines)

Structure of Methanamine

Molecular Formula: CH₃NH₂

Full Displayed Formula of Methanamine:

Skeletal Formula of Methanamine:

full displayed formula of methanamine

Structure of Ethanamine

Molecular Formula:
CH₃CH₂NH₂

Full Displayed Formula of Ethanamine:

Skeletal Formula of Ethanamine:

full displayed formula of ethanamine

Structure of
Propan-1-amine

Molecular Formula:
CH₃CH₂CH₂NH₂

Full Displayed Formula of Propan-1-amine:

Skeletal Formula of Propan-1-amine:

full displayed formula of propan-1-amine

Structure of
Butan-1-amine

Molecular Formula:
CH₃CH₂CH₂CH₂NH₂

Full Displayed Formula of Butan-1-amine:

Skeletal Formula of Butan-1-amine:

full displayed formula of butan-1-amine

Structure of
Pentan-1-amine

Molecular Formula:
CH₃CH₂CH₂CH₂CH₂NH₂

Full Displayed Formula of Pentan-1-amine:

Skeletal Formula of Pentan-1-amine:

full displayed 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:

(Read more about naming carboxylic acids)

Structure of
Formic Acid

Molecular Formula:
CH₂O₂

Full Displayed Formula of Methanoic Aic:

Skeletal Formula of Methanoic Acid:

Structure of
Ethanoic Acid

Molecular Formula: C₂H₄O₂

Full Displayed Formula of Ethanoic Acid:

Skeletal Formula of Ethanoic Acid:

full displayed formula of ethanoic acid

Structure of
Propanoic Acid

Molecular Formula: C₃H₆O₂

Full Displayed Formula of Propanoic Acid:

Skeletal Formula of Propanoic Acid:

full displayed formula of propanoic acid

Structure of
Butanoic Acid

Molecular Formula: C₄H₈O₂

Full Displayed Formula of Butanoic Acid:

Skeletal Formula of Butanoic Acid:

full displayed formula of butanoic acid

Structure of
Pentanoic Acid

Molecular Formula: C₅H₁₀O₂

Full Displayed Formula of Pentanoic Acid:

Skeletal Formula of Pentanoic Acid:

full displayed 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. - You're invited to add comments about learning techniques etc. to our Chemistry Forum.

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

Molecular Formula: C₆H₁₂

Read more about cycloalkanes

Full Displayed Formula of Cyclohexane:

Skeletal Formula of Cyclohexane:

Fully Displayed Formula of Cyclohexane

Structure of Benzene

Molecular Formula: C₆H₆

Full Displayed Formula of Benzene:

Skeletal Formula of Benzene:

Fully Displayed Formula of Benzene

or Skeletal Formula of Benzene

Notice that the regular hexagons that form part (or all) of the skeletal formulae are approx. the same size as the carbon ring drawn out in the full displayed formulae on the left.
In the case of skeletal formulae, the presence of the appropriate number of hydrogen atoms attached to each carbon is assumed - UNLESS replaced by something else, e.g a carbon chain or functional group. For example, consider the structure and representations of ethylbenzene:

Structure of Ethylbenzene

Molecular Formula: C₈H₁₀

Full Displayed Formula of Ethylbenzene:

Skeletal Formula of Ethylbenzene:

or

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

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

	... and with the following fully displayed formulae:

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




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