In this lesson, we will learn:
- To identify and name aldehydes and ketones in organic molecules.
- How the bonding of aldehydes and ketones affect their properties.
- Chemical tests to identify and distinguish aldehydes and ketones.
- Several key reactions of the aldehyde and ketone functional groups.
- Aldehydes and ketones are two of the most common functional groups found in organic molecules. Both contain the common C=O carbon-oxygen double bond . This C=O bond is known as a carbonyl group which is the basis for a few other organic functional groups.
Aldehydes and ketones do have one crucial difference that affects their reactivity, however:
See the images below:
- A ketone is an organic functional group containing a C=O carbon-oxygen double bond where the carbon atom’s other bonds are both to carbon atoms. Practically, this means that ketones are carbonyl groups found in the middle of a carbon chain.
- The naming suffix for a ketone is -one. For example, a three-carbon chain with a ketone group (it can only be a ketone on the second carbon) would be called propanone. A ketone on the second carbon in a five-carbon chain would be called pentan-2-one.
- An aldehyde is an organic functional group containing a C=O carbon-oxygen double bond where the carbon atom is bonded to at least one hydrogen atom. Practically, this means that aldehydes are ketones at the end of carbon chains!
- The naming suffix for an aldehyde is -al. For example, a three-carbon chain with an aldehyde group (it can only be an aldehyde on the first, chain-end carbon) would be called propanal. An aldehyde on a 6 carbon-chain would be called hexanal.
- You normally don’t need to number aldehydes because they are high order functional groups and are always on the end of carbon chains; they’ll be the 1-position unless a carboxylic acid is also present.
- When attached to a ring, the naming suffix is -aldehyde or -carbaldehyde. For example, an aldehyde attached to a benzene ring is the compound benzaldehyde.
- Like a lot of organic molecules, the properties of aldehydes and ketones are shaped by their (lack of) ability to form intermolecular bonds.
- Ketones and aldehydes do not form intermolecular hydrogen bonds between other ketones and aldehydes. The C=O double bond creates a O atom and a C atom, but there are no partially-charged hydrogen atoms produced so hydrogen-bonding does not exist in pure ketone/aldehyde samples.
This is unlike alcohols, so ketones and aldehydes have lower melting/boiling points than their alcohol analogues – an alcohol of the same carbon chain length.
- Ketones and aldehydes form hydrogen bonds with water molecules and therefore are soluble in water.
- The are many well-known reactions of aldehydes and ketones, involving reduction, oxidation and distinguishing between them.
- There are multiple ways to test for aldehydes and ketones by oxidising them:
- Tollens’ reagent is a test that distinguishes between aldehydes and ketones. It is prepared with silver nitrate solution with some sodium hydroxide, followed by excess ammonia.
Tollens’ reagent gives a positive test for aldehydes: a silver mirror in the test tube. See the equation below:
RCHO + 2 [Ag(NH3)2]+ + 3OH- → 2Ag + 4NH3 + RCOO- + 2H2O
The aldehyde is converted to a carboxylate (a deprotonated carboxylic acid group) and elemental silver is produced. That’s why the observation is called a ‘silver mirror’ which blankets the interior walls of the test tube.
A ketone will not react with Tollens’ reagent. Producing the carboxylate means a fairly weak C-H bond must be broken in the aldehyde to form a C-O bond. This is fine, but a strong C-C bond must be broken in the ketone. This does not happen, so there is no observed change for ketones; you won’t get the silver mirror.
- Benedict’s solution is a copper (II) complex that also reacts with aldehydes but not ketones .
The active reagent is made with sodium hydroxide and the Cu2+ complex ion reacts in a similar way to Tollens’ reagent.
RCHO + 2 Cu2+ + 5 OH- → RCOO- + Cu2O + 3 H2O
This test with an aldehyde produces a dark red copper precipitate from the blue solution.
As with Tollens’ reagent, because a carboxylate is formed, the ketone will not react, so no observable change is seen.
- Another oxidation reaction of the aldehyde (that doesn’t work with a ketone) like the tests above is with acidified potassium dichromate (VI). The dichromate, Cr2O72- is an excellent oxidising agent that gets reduced to Cr3+.
This is therefore a redox reaction with the equations below:
3RCHO + Cr2O72- + 8H+ → 3RCOOH + 2Cr3+ + 4H2O
The dichromate reacting to produce chromium (III) ions causes a colour change from orange to green for aldehydes, but no reaction for ketones.
- Benedict’s solution and Tollens’ reagent will identify whether you have an aldehyde or a ketone. There is a general test for the carbonyl group, meaning both aldehydes and ketones will react.
Brady’s reagent, also known as DNPH or 2,4-dinitrophenylhydrazine, will produce a yellow/red precipitate in the presence of a carbonyl group.
It is a condensation reaction – H2O is lost as the two molecules join together, and the C=O is effectively just replaced with a C=N, where the N was the NH2 group on the DNPH molecule.
The product made is known as a hydrazone, which can be crystallised and its melting point determined. The exact hydrazone you make (and its melting point) depends on what aldehyde or ketone it was made with, so you can use melting point data to find out which aldehyde or ketone was there in the first place.
- The carbonyl groups in aldehydes and ketones can be reduced to alcohols by a strong reducing agent such as LiAlH4, or lithium aluminium hydride.
This reaction is a nucleophilic addition of a hydride to the carbonyl carbon. This means two things:
This nucleophilic addition reduces the carbonyl groups:
- “Addition” is a reaction where a molecule is ‘added’ to a double bond, which gets opened up to become a single bond. The previously double-bonded atom now has one extra attachment, the molecule that got ‘added’.
- “Nucleophilic” means the reaction is driven by a nucleophile: an electron-rich molecule attracted to positive (whole or partial) charges. Usually negatively charged or at least containing a lone-pair, the nucleophile is what gets added to the other molecule. Here, the nucleophile is a hydride (H-) group.
For example, the reaction of propanal with LiAlH4 is shown in the image below. [H] represents the reducing agent:
- LiAlH4 will reduce a ketone to a secondary alcohol.
- LiAlH4 will reduce aldehydes to primary alcohols.
- Alcohols can be produced from aldehydes and ketones by the reaction with KCN in a nucleophilic addition reaction.
This is similar to the reaction with LiAlH4, except the cyanide ion (CN- ) is the nucleophile instead of the hydride group.
See the image below for the reaction mechanism:
This reaction has a few important features:
- The CN- group has been added to the molecule at the carbon end (forming a C-C bond). This increases the carbon chain length by one, and with a ketone you will have an alkyl branch now because the CN group is a higher order group.
- The addition of the CN- group often creates a chiral centre. Like we saw in Chirality and optical isomers, this is where all four attachments to carbon are different. Like we also learned in that lesson, we can use polarimetry data to study the mechanism.
Cyanohydrins formed this way are racemic mixtures. This is because when the cyanide nucleophile attacks the carbonyl group, it has a 50:50 chance of attacking above or below the plane. The polarimetry data should have net zero polarisation.
- The reaction with iodine in basic conditions is an unusual reaction that produces a pale-yellow precipitate of triiodomethane, CHI3, or iodoform. This works for ketones and ethanal, the two-carbon chain aldehyde.
The reaction has two steps:
- The first step creates the -CI3
- The second step breaks the C-CI3
The product formed is a carboxylate, which is a deprotonated carboxylic acid. These are the focus of next lesson.
A table summary of aldehydes and ketones is below:
Oxidise 1° alcohol (aldehyde) or 2° alcohol
1. Lower BP than alcohols (no pure H-bonds)
2. Water soluble, drops with size
1. Tollens’ reagent (silver mirror for aldehyde)
2. Brady’s reagent (orange/yellow ppt for both)
Forms cyanohydrins, nucleophilic addition
Chirality is evidence for mechanism
Forms 1° alcohols (aldehydes), 2° alcohols