Carbonyl-Containing Compounds

Carbonyl-containing compounds

The carbonyl group consists of a carbon atom that is joined to an oxygen by a double bond (see the figure below).

A compound containing a carbonyl group.

In the figure above, \(\color{green}{\textbf{R'}}\) and \(\color{purple}{\textbf{R}}\) are used to represent the rest of the atoms in the molecule. For example \(\color{purple}{\textbf{R}}\) could represent an alkyl chain, or a hydrogen atom.

Aldehydes and ketones

(a) An aldehyde and (b) a ketone.

If the functional group is on the end of the carbon chain, the organic compound is called an aldehyde (see figure (a) above). Being at the end of the chain means that \(\color{green}{\textbf{R'}}\) or \(\color{purple}{\textbf{R}}\) represents a hydrogen atom. The simplest aldehyde is methanal. The aldehyde containing 4 carbon atoms, butanal, is illustrated in the figure below. In this example \(\color{purple}{\textbf{R}}\) represents \(\text{H}\) and \(\color{green}{\textbf{R'}}\) represents \(\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\).

Tip:

Note that the condensed structural formula for an aldehyde ends in \(\text{CHO}\) not \(\text{COH}\). This is because COH could be confused with the hydroxyl (\(-\text{OH}\)) group of an alochol.

The (a) structural, (b) condensed structural and (c) molecular formula representations of butanal. (d) An atomic model of butanal.

Some uses of aldehydes include:

in resins (over \(\text{6}\) million tons of formaldehyde are produced per year)
in the production of plasticisers and alcohols used in detergents
in perfumes and flavourants

If the carbonyl group is in the middle of the carbon chain, the compound is called a ketone. Being in the middle of the chain means that \(\color{green}{\textbf{R'}}\) and \(\color{purple}{\textbf{R}}\) cannot represent \(\text{H}\). The simplest ketone is propanone (also known as acetone, the compound in nail varnish remover), which contains three carbon atoms. The ketone containing 4 carbon atoms, butanone, is illustrated in the figure below.

The (a) structural, (b) condensed structural and (c) molecular formula representations of butanone. (d) An atomic model of butanone.

Fact:

The molecular formulae representations for butanal and butanone are identical (\(\text{C}_{4}\text{H}_{8}\text{O}\)). This is why structural and condensed structural representations are necessary.

Some uses of ketones include:

as solvents
in the production of polymers
in the production of pharmaceuticals

The general formula for both the aldehydes and ketones can be written as: \(\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n}}\textbf{O}}\). This means that they cannot be told apart from their general formula alone. There are more complex general formulas that allow aldehydes and ketones to be distinguished, but they are not covered in this book.

Carboxylic acids

Carboxylic acids are organic acids that are characterised by having a carboxyl group, written as \(-\text{COOH}\). In a carboxyl group a carbon atom is double-bonded to an oxygen atom (carbonyl group), and it is also bonded to a hydroxyl group (\(\color{purple}{\textbf{R}}\)). The simplest carboxylic acid, methanoic acid, is shown in the figure below and ethanoic acid is shown in the second figure below.

The (a) structural, (b) condensed structural and (c) molecular formula representations of methanoic acid.

The (a) structural, (b) condensed structural and (c) molecular formula representations of ethanoic acid. (d) An atomic model of ethanoic acid.

Carboxylic acids are widespread in nature. Methanoic acid (also known as formic acid) has the formula \(\text{HCOOH}\) and is found in insect stings. Ethanoic acid \((\text{CH}_{3}\text{COOH})\), or acetic acid, is the main component of vinegar. More complex organic acids also have a variety of different functions. Benzoic acid for example, is used as a food preservative. Carboxylic acids have the general formula: \(\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n+1}}\textbf{COOH}}\).

Ethanoic acid can be produced through the oxidation of ethanol upon exposure to the oxygen in air. This is why wine that is left too long can taste acidic. Wine can easily go sour if exposed to the oxygen molecules (\(\text{O}_{2}\)) in the air, especially if the weather is warm.

The oxidation of wine.

Fact:

A certain type of ant, called formicine ants, manufacture and secrete formic acid, which is used to defend themselves against other organisms that might try to eat them.

The oxidation of ethanol to ethanoic acid can also be seen in the reaction of ethanol with potassium dichromate:

\(2(\text{Cr}_{2}\text{O}_{7})^{2-}(\text{aq}) + 3\text{C}_{2}\text{H}_{5}\text{OH}(\text{aq}) + 16\text{H}^{+}(\text{aq})\) \(\to\) \(4\text{Cr}^{3+}(\text{aq}) + 3\text{CH}_{3}\text{COOH}(\text{aq}) + 11\text{H}_{2}\text{O}(\text{l})\)

The colour change that occurs is shown in the image below and the following video:

The different colours of potassium dichromate (left) and potassium dichromate and ethanol (right). Screenshot taken from video by beggar098 on YouTube.

Optional Video: Addition of Ethanol and Potassium Dichromate

Optional Case Study: Breathalysers

Read the following extract taken from HowStuffWorks (12/08/13):

The Breathalyzer device contains:

A system to sample the breath of the suspect
Two glass vials containing the chemical reaction mixture
A system of photocells connected to a meter to measure the color change associated with the chemical reaction

To measure alcohol, a suspect breathes into the device. The breath sample is bubbled in one vial through a mixture of sulfuric acid, potassium dichromate, silver nitrate and water. The principle of the measurement is based on the following chemical reaction:

\(2\text{K}_{2}\text{Cr}_{2}\text{O}_{7}(\text{aq}) + 3\text{CH}_{3}\text{CH}_{2}\text{OH}(\text{aq}) + 8\text{H}_{2}\text{SO}_{4}(\text{aq})\) \(\to\)

\(2\text{Cr}_{2}(\text{SO}_{4})_{3}(\text{aq}) + 2\text{K}_{2}\text{SO}_{4} + 3\text{CH}_{3}\text{COOH}(\text{aq}) + 11\text{H}_{2}\text{O}(\text{l})\)

In this reaction:

The sulfuric acid removes the alcohol from the air into a liquid solution.
The alcohol reacts with potassium dichromate to produce: chromium sulfate, potassium sulfate, acetic acid, water

The silver nitrate is a catalyst, a substance that makes a reaction go faster without participating in it. The sulfuric acid, in addition to removing the alcohol from the air, also might provide the acidic condition needed for this reaction.

During this reaction, the reddish-orange dichromate ion changes color to the green chromium ion when it reacts with the alcohol; the degree of the color change is directly related to the level of alcohol in the expelled air. To determine the amount of alcohol in that air, the reacted mixture is compared to a vial of unreacted mixture in the photocell system, which produces an electric current that causes the needle in the meter to move from its resting place. The operator then rotates a knob to bring the needle back to the resting place and reads the level of alcohol from the knob - the more the operator must turn the knob to return it to rest, the greater the level of alcohol.

Break into groups of three or four. Research breathalysers and then report your information to the class.

Make sure to cover the following areas:

The effect of alcohol on the body
The effect of alcohol on reaction times
The origins of the breathalyser
The term mouth alcohol and its effect on breathalyser tests.

Esters

Tip:

Esters will be dealt with in much greater detail later.

When an alcohol reacts with a carboxylic acid, an ester is formed. Most esters have a characteristic smell. In the reaction a molecule of water is removed from the two compounds and a new bond is formed between what remains of the alcohol and the carboxylic acid. A catalyst is required in this reaction, in this case it must be an inorganic acid (e.g. \(\text{H}_{2}\text{SO}_{4}\)). An example is shown in the figure below.

The formation of an ester and water from an alcohol and carboxylic acid.

Fact:

The esterification process is reversible with large quantities of water (although it can be slow). In an acidic environment the reaction speeds up. Reversible reactions are covered in greater detail in Chemistry 109.

The esterification process with methanol and methanoic acid is shown with atomic models in the figure below. Esters have the general formula: \(\color{red}{\textbf{C}_{\textbf{n}}\textbf{H}_{\textbf{2n}}\textbf{O}_{\textbf{2}}}\). This general formula can also be applied to carboxylic acids, but the more complex general formula for esters alone is not covered in this tutorial.

The esterification process of methanol and methanoic acid to methyl methanoate and water, shown with three-dimensional model kits.

Some common uses for esters are:

in cosmetics and beauty products because they typically have a fruity smell, making them good as artificial flavourants and scents
in nail varnish remover and model plane glue
as solvents for non-water soluble compounds (e.g. oils, resins) because the ester of a specific carboxylic acid will be less water soluble than the carboxylic acid
as plasticisers because esters can make a compound less brittle, and more flexible

Carbonyl-Containing Compounds