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Revise Functional Groups in Organic Molecules (A-Level Organic Chemistry)

Naming Aldehydes

(Chemical) Definition of Aldehydes:

Aldehydes are organic chemical compounds that include a -carbonyl group (i.e. an oxygen atom attached to a carbon atom by a double covalent bond) and a hydogen atom attached to the same carbon atom as is attached to the carbonyl group:

aldehyde group

That is, aldehydes are a class or category of organic chemical compounds that include a carbon atom attached to both an oxygen atom (by a double covalent bond), and also a hydrogen atom (by a single covalent bond).

Bearing in mind that carbon atoms form a total 4 single covalent bonds - or equivalent in combinations of double or triple bonds, a carbon atom attached to both an oxygen atom (by a double covalent bond) and a hydrogen atom (by a single covalent bond) can only form one other single covalent bond linking it to the rest of an organic molecule. It must therefore always be the first- or last - (which are equivalent positions) carbon atom in the chain of carbon atoms that form the organic molecule of which it is a part. This position of the -carbonyl group attached to the end- carbon in a carbon-chain is important because it distinguishes aldehydes from a similar category of organic compounds, called ketones.

Aldehyde molecules can vary in size up to very long molecules most of which consist of carbon atoms attached to each other and also to hydrogen atoms.

Names of Aldehydes in General

Aldehydes are named according to the same system as other organic compounds, with the suffix -al used to designate the presence in the molecule of a carbonyl group (that is, a carbon atom attached to an oxygen atom by via a double covalet bond) and a single hydrogen atom - rather than a second chain of carbon atoms - attached to a carbon atom at the end of a chain of carbon atoms.

The exception to this general rule is, of course, formaldehyde (also known as methanal), because in that case there is only one single carbon atom in the molecule, hence it is only "at the end of a chain of carbon atoms" to the extent than a single carbon atom may be understood as a "chain" of one atom - in comparison with ethanal being based on a chain of 2 carbon atoms, propanal being based on a chain of 3 carbon atoms, butanal being based on the chain of 4 carbon atoms, and so on - see the table of molecular structures below.

 

The first step to consider when working out the name of an aldehyde molecule is the number of carbon atoms forming a chain. If they are attached together in a linear (i.e. unbranched) configuration then the number of carbon atoms is indicated according to the same system as used for naming alkanes.

However, if the carbon atoms do not form a linear chain but include branches, the longest linear chain of carbon atoms within the molecule determines the base of the name of the compound, onto which is added information about the branches incl. their lengths in terms of the number of carbon atoms in each branch (i.e. methyl- usually indicates a branch consisting of just one carbon atom attached to the main chain, ethyl- indicates a branch of two carbon atoms in length, etc.) and their positions along the longest linear carbon chain (e.g. attached to the 2nd carbon, 3rd carbon, etc.).

The simplest linear aldehydes are named and their structures drawn in the following table.
More complex aldehyde molecules follow afterwards.

Names and Structures of simple Linear Aldehydes

The homologous series of linear aldehydes is shown below.

Name
Simple Structure

Other Names = Synonyns
(*Examples not complete lists)

and simple formula

showing bond types but not accurate bond angles

1

formaldehyde (CH2O)


Structure of formaldehyde

2

ethanal (C2H4O)


Structure of ethanal

3

propanal (C3H6O)


Structure of propanal

4

butanal (C4H8O)


Structure of butanal

5

pentanal (C5H10O)


Structure of Pentanal

6

hexanal (C6H12O)


Structure of hexanal

7

heptanal (C7H14O)


Structure of heptanal

8

octanal (C8H16O)


Structure of octanal

9

nonanal (C9H18O)


Structure of nonanal

10

decanal (C10H20O)


Structure of decanal

 

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