4.1.1 (b) Types of Organic Formulae

Syllabus

(b) interpretation and use of the terms:

(i) general formula (simplest algebraic formula of a homologous series) e.g. for an alkane: C_nH_2n+2

(ii) structural formula (the minimal detail showing the arrangement of atoms in a molecule) e.g. for Butane: CH₃CH₂CH₂CH₃ or CH₃(CH₂)₂CH₃

(iii) displayed formula (the relative positioning of atoms and the bonds between them)

(iv) skeletal formula (simplified organic formula shown by removing Hydrogen atoms from alkyl chains, leaving just a Carbon skeleton and associated functional groups)

{Definitions of types of formula not required.}

{In structural formulae, the carboxyl group will be represented as COOH and the ester group as COO.}

(d) use of the general formula of a homologous series to predict the formula of any member of the series

What does this mean?

Examiners will expect you to know the structure of some organic substances, and to be able to work out the structure of a lot of others.

They will ask you to write them in a number of ways and, unfair as it may seem, if you write the correct structure the wrong way you are likely to lose marks.

So it is important that you can distinguish between the types of formulae and know what is required for full marks when writing or drawing one

An examiner, yesterday

Displayed Formulae

Displayed formulae must show every atom and every bond.

This is the displayed formula for Ethanol.

It would still be correct if we moved the OH group to the first Carbon atom.

Or placed it on one of the other bonds (up or down).

This second example is still Ethanol but would be marked incorrect as a displayed formula even though you'll see it in text-books and your teacher will probably draw it frequently.

There's no bond between the O and the H so it is not a displayed formula.

Molecular formulae and general formulae.

Molecular formulae are the most commonly used formulae.

For Ethanol the molecular formula is C₂H₆O.

This is not what we commonly write because we tend to show the OH separately to indicate that we are talking about an alcohol rather than some other isomer of C₂H₆O , such of methoxy methane.

But a moelcular formula only contains the symbol for an element once.

So Ethanoic acid, which is commonly written CH₃COOH to show that it is a carboxylic acid, has a moecular formula of C₂H₄O₂.

Strictly speaking we should apply the same logic to general formuale.

So, you should know the general formula of alkanes from GCSE is C_nH_2n+2 and that for Alkenes it is C_nH_{2n+2 .}

_{What about alcohols?}

_{We should write} C_nH_2n+2O rather than C_nH_2n+1OH

Although it would be a harsh examiner that picked you up on this.

_{Similarly Carboxyllic acids should be} C_nH_2nO_{2 rather than} C_nH_2n-1O₂H or C_nH_2n-1OOH

If you're asked to work out the general formula for an unfamiliar homologous series you should remember that everything is based on Alkanes so all general formula will begin with CnH2n and you simply need to calculate how many to add or subtract from the n H atoms and remember to include any extra atoms/

eg Aldehydes

_{Looking at Methanal (CH2O) - we can suggest CnH2nO became there are twice as many H's as C's and only 1 O atom.}

_{Checking with Ethanal (}C₂H₄O) this still applies as it does to propanal (C₃H₆O) and even a branched aldehyde 2-methylbutanal (C₅H₁₀O)

Given a general formula, such as C_nH_2nO₂ for esters, all we need to do is count the Carbon atoms and double the number to calculate the number of Hydrogens. There will always be 2 Oxygens because the general formula ends with O₂ rather than O_2n or any other n-containing form.

So there are 5 Carbon atoms in ethyl propanoate making it C₅H₁₀O₂

_{Propyl methanoate has only 4 Carbon atoms} C₄H₈O₂

_{Methyl butanoate has 5 Carbon atoms} C₅H₁₀O₂

_{Propyl ethanoate also has 5 Carbon atoms} C₅H₁₀O₂