Aldehydes & Ketones: Reaction Mechanisms

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson we will look at reactions that aldehydes and ketones under-go. These reaction include oxidation into carboxylic acids, reduction into alcohols, and forming cyanohydrin.

Aldehydes and Ketones

What is the difference between honey and table sugar? On a chemical level their structures are very similar. A single molecule has the same number of hydrogen, carbon, and oxygen atoms. The major difference is that honey contains more ketones (in the form of fructose) while table sugar contains more aldehydes (in the form of glucose). We can see, based on this observation, that a ketone and aldehyde, while similar, can react in different ways.

Aldehyde structure (left) and ketone structure (right) only differ by what is connected to the carbon
structure

A ketone and aldehyde both contain a carbonyl (a carbon double bonded to an oxygen). Yet a ketone has that carbonyl attached to two R-groups (which is simply a chain of carbons). While an aldehyde has that carbonyl at the terminating carbon, so the carbon is attached on one side to an R-group but the other side only has a hydrogen. It might help you to remember his difference by noticing that the word 'aldehyde' has an 'h' in it, which you can think of as standing for hydrogen.

Oxidation to Carboxylic Acids

The hydrogen on the carbonyl is important to the oxidation reaction to make carboxylic acids. Therefore, we are able to oxidize an aldehyde but we cannot oxidize a ketone, as it has no hydrogen in its structure.

Aldehydes can be oxidized into carboxylic acids but ketones cannot
oxidize

Aldehydes can react with many different compounds in order to form carboxylic acids. These can be chromium reagents with a positive 6 informal charge - such as CrO3. If the R-group on the aldehyde has another alcohol on it, then the chromium reagents will also oxidize that alcohol group.

When the aldehydes react with one of these compounds, another oxygen gets added onto the carbonyl which forms a carboxylic acid.

Fehling's solution or Benedict's solution are also used by scientists to determine if a substance is a ketone or an aldehyde. They are both essentially copper ions in an alkaline solutions. If the substance turns blue, this indicates that carboxylic acids formed and tells us that the substance therefore contains aldehydes.

If we only want to oxidize the aldehyde and not the alcohol, then we need to use Tollens' reagent, a solution of silver nitrate and ammonia. The solution will take on a silvery sheen if there are aldehydes present; it will remain clear if not.

Tollens reagent will turn silvery (left) to indicate the presence of an aldehyde, or remain clear if not (right).
tollens reagent

Reduction of Aldehydes and Ketones

Aldehydes and ketones can both be reduced into alcohols, which is at least one hydroxyl group (-OH) attached to a carbon atom.

  • An aldehyde will form a primary alcohol (since the carbon which is bonded to the OH is only bonded to one other carbon).
  • A ketone will form a secondary alcohol (since the carbon which is bonded to the OH is bonded to two other carbons).

Both aldehydes (top) and ketones (bottom) can be reduced into alcohols
reduce

There are many different reagents that can be used to reduce aldehydes and ketones. The most common are: sodium borohydride with methanol, lithium-aluminum hydride then water, or hydrogen with Pd-C.

The reaction mechanism for reducing aldehydes and ketones are exactly the same. Let's look at an example of an aldehyde being reduced lithium-aluminum hydride then water.


reduce mech


Step #1: the hydrogen on the aluminum attacks the carbonyl. This carbonyl can be easily attacked since the negative charge can go onto the electrophilic oxygen.

Step #2: The oxygen removes a hydrogen from water which then forms the alcohol.

So it is a 2-step process which the hydrogen attacks the carbonyl then the oxygen gets a hydrogen to make an alcohol.

Making Cyanohydrin

There are many compounds that can react with the carbonyl carbon. Since the oxygen is pulling electrons away from the carbon, that carbon wants electron rich compounds to attack it.

One example of an electron rich compound is a cyanide ion. The carbon on this ion has a negative charge so it wants to attack the electron deficient carbonyl. This forms a new carbon-carbon bond making a cyanohydrin. In this mechanism we see that there are 2 steps: the cyanide ion attacks the carbonyl, then the oxygen takes a hydrogen from a cyanide.


cyanide


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