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Aldehydes and Ketones

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Chemistry

If you leave a bottle of wine open for long enough, it eventually becomes sour and acidic. The pH drops and you have turned ethanol into ethanoic acid. This is an example of oxidising an alcohol. But before the alcohol turns into a carboxylic acid, it first oxidises into something else entirely: an aldehyde.

Aldehydes and ketones are organic molecules that contain the carbonyl functional group. This is a carbon atom joined to an oxygen atom by a double bond. We represent it as .

  • This article is an introduction to aldehydes and ketones in organic chemistry.
  • We will start by learning the general formulas of aldehydes and ketones before looking at their nomenclature.
  • We'll then explore some of their properties and see how they compare to other organic molecules.
  • Finally, we'll briefly look at some of the common reactions involving aldehydes and ketones.

What's the difference between aldehydes and ketones?

As we mentioned above, aldehyde and ketones are organic molecules Containing the carbonyl group, . In aldehydes, the carbon atom in the bond is attached to at least one hydrogen atom. This gives aldehydes the general formula . In contrast, the carbon atom in the bond in ketones is attached to two organic R groups. These may be the same or different. We represent ketones using the formula .

Aldehydes and ketones general formula StudySmarterThe general formulas for an aldehyde (left) and a ketone (right). Note that the carbonyl group in a ketone is never attached to a hydrogen atom. Instead, it is always bonded to two organic R groups. Anna Brewer StudySmarter Originals

Aldehydes and ketones are structural isomers of each other. These are molecules with the same molecular formulas but different structural formulas. More specifically, they are functional group isomers - isomers with different functional groups. For example, the molecule could either be propanal, with the structural formula , or propanone, with the structural formula .

The following table should help summarise the differences between aldehydes and ketones:

Aldehydes and ketones comparing aldehydes ketones table StudySmarterThe differences between aldehydes and ketones, with examples. Note the different numbers of hydrogen atoms bonded to the carbonyl group in each case. Anna Brewer StudySmarter Originals

Methanal, , is the only aldehyde without any R groups. Its only carbon atom is part of the carbonyl functional group and is bonded to two hydrogen atoms. All other aldehydes have one R group.

How do we name aldehydes and ketones?

Naming aldehydes and ketones is pretty straightforward. If you aren't familiar with classifying compounds we'd recommend first checking out Organic Compounds, but if you feel confident with nomenclature laws, you shouldn't find this too much of a challenge.

Naming aldehydes

Aldehydes use the suffix - al . We include the carbon atom in thebond when finding their root name, which you'll remember indicates the length of their longest carbon chain. However, we don't need to use a number to show where to find the group, as in aldehydes it is always located at one end of the molecule. Instead, we always assume the carbonyl group is located on carbon 1.

For example, this aldehyde has three carbon atoms in its longest chain and no other functional groups. We call it propanal.

Aldehydes and ketones propanal example StudySmarterAn example of an aldehyde. Anna Brewer, StudySmarters Originals

The following aldehydes are a bit more tricky. It also has a chain three carbon atoms long, but contains two side groups: a methyl group shown in green and a chlorine atom shown in blue. If we count the carbon atom of the carbonyl group as carbon 1, then the methyl group is attached to carbon 2 and the chlorine atom is attached to carbon 3. Remember that we list other side groups in alphabetical order. This molecule is therefore called 3-chloro-2-methylpropanal.

Aldehydes and ketones 3-chloro-2-methylpropanal StudySmarter3-chloro-2-methylpropanal. Anna Brewer, StudySmarter Originals.

Naming ketones

To name ketones, we use the suffix - one . Remember to include the carbonyl group when counting the number of carbon atoms in the molecule's longest carbon chain. You should also note that in longer ketones, we need to show the position of the carbonyl group using a number, making sure to give it the lowest number possible. This is an example of the 'lowest numbers' rule and could mean counting from the left side of the chain or from the right.

For example, this ketone has five carbons atoms in its backbone and a carbonyl group in either position 2 or 4, depending on which side of the molecule you count from. As 2 is lower than 4, we correctly classify it as pentan-2-one:

Aldehydes and ketones Pentane-2-one StudySmarterPentan-2-one, shown with the correct numbering of the carbon atoms in its backbone. Anna Brewer StudySmarter Originals

The next ketone has a carbon chain four atoms long. This gives it a root name of -but-. Taking the carbonyl group as carbon 2, we can see that it also has a bromine atom attached to carbon 3, and so it is therefore called 3-bromobutan-2-one.

Anna Brewer, StudySmarter Originals

What are the properties of aldehydes and ketones?

Take a look at the following table. It shows the electronegativities of several elements.

Aldehydes and Ketones electronegativity table StudySmarterElectronegativity of select elements. Anna Brewer, StudySmarter Originals

You'll notice that oxygen has a much higher electronegativity than carbon. In the carbonyl bond, oxygen attracts the shared bonding pair of electrons towards itself, becoming partially negatively charged and leaving the carbon atom partially positively charged. This makes the bond polar and creates a dipole moment , influencing the properties of aldehydes and ketones.

Aldehydes and ketones polar bond of methanal StudySmarterThe polar bond in an aldehyde, shown here in methanal. Anna Brewer, StudySmarter Originals

If you aren't sure about polar bonds, take a look at Polarity.

Let's explore some of these properties now.

Boiling point

Aldehydes and ketones have high melting and boiling points compared to alkanes with a similar molecular mass. This is because aldehydes and ketones experience permanent dipole-dipole forces between molecules due to their polar double bond, as explored above. However, their boiling points are not as high as similar alcohols. You may remember that alcohols can form hydrogen bonds between molecules as they have an oxygen atom bonded to a hydrogen atom. These hydrogen bonds are much stronger than permanent dipole-dipole forces and require more energy to overcome.

Like with alkanes, the boiling point of carbonyls increases as chain length increases. This is because larger molecules have more electrons and form stronger temporary dipoles. This increases the strength of van der Waals attraction between molecules.

For further information about van der Waals attraction, hydrogen bonding and permanent dipole-dipole forces, check out Intermolecular Forces.

Solubility

Generally, aldehydes and ketones are soluble in water.

Although aldehydes and ketones molecules can't form hydrogen bonds with each other, they can form hydrogen bonds with water. This makes them soluble in water. The oxygen atom in the carbonyl group has two lone pairs of electrons that are attracted to the densely charged hydrogen atoms in water molecules, releasing lots of energy. However, longer-chain aldehydes and ketones are less soluble in water than shorter-chain ones. Their long, non-polar hydrocarbon chains get in the way of the hydrogen bonds and interfere with the bonding.

Aldehydes and ketones propanone hydrogen bonding properties solubility StudySmarterHydrogen bonding between propanone and water. As a result, propanone dissolves well in aqueous solution. Anna Brewer, StudySmarter Originals

How do aldehydes and ketones react?

As we explored above, the bond in aldehydes and ketones is strongly polar. As a result, the partially positively charged carbon atom is easily attacked by nucleophiles.

A nucleophile is an electron pair donor.

Examples include the cyanide ion, . Nucleophiles are always negatively or partially negatively charged and contain a lone pair of electrons.

You might also have noticed that aldehydes and ketones are unsaturated - they contain a double bond. They therefore readily take part in addition reactions. In fact, most of their reactions are nucleophilic additions. For example, reacting cyanide with an aldehyde produces a hydroxynitrile, which is a molecule containing both nitrile and alcohol functional groups, and respectively.

Aldehydes and Ketones - Key takeaways

  • Aldehyde and ketones are organic molecules that contain the carbonyl group, . Aldehydes have the general formula and ketones have the general formula .

  • We name aldehydes using the suffix -al and ketones using the suffix -one .

  • Because of the polarity of the bond, aldehydes and ketones have high melting and boiling points compared to similar alkanes. They are also soluble in water.

  • Aldehydes and ketones are readily attacked by nucleophiles in nucleophilic addition reactions.

Aldehydes and Ketones

Both aldehydes and ketones contain the carbonyl functional group, C=O. However, in aldehydes this function group is bonded to at least one hydrogen atom and either zero or one R groups, whereas in ketones this functional group is bonded to two R groups.

Aldehydes and ketones both contain the carbonyl functional group, C=O. They are also structural isomers of each other.

Methanal is a common fungicide, germicide, and tanning agent, but other aldehydes are used as solvents and flavouring agents. For example, the molecule cinnamaldehyde, responsible for the main flavour in cinnamon, is an aldehyde. Ketones are often used as solvents and are found in some steroids.

The most widely known aldehyde is methanal, also known as formaldehyde. It is used as a fungicide, herbicide, and tanning agent. The simplest ketone is propanone, a molecule with a carbon chain that is just three carbon atoms long.

Aldehydes and ketones are organic molecules containing the carbonyl group, C=O.

Final Aldehydes and Ketones Quiz

Question

Explain the difference between aldehydes and ketones.


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Answer

Aldehydes contain a carbonyl group bonded to at least one hydrogen atom, whereas ketones contain a carbonyl group bonded to two organic R groups.

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Question

What suffix is used to name aldehydes?

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Answer

-al

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Question

What suffix is used to name ketones?

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Answer

-one

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Question

 Name the straight chain aldehyde with five carbon atoms.


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Answer

Pentanal

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Question

Name the two possible straight chain ketones with six carbon atoms.


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Answer

Hexan-2-one

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Question

The C=O bond is ______.

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Answer

Polar

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Question

Explain the polarity seen in the C=O bond.


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Answer

The C=O bond is polar because oxygen has a higher electronegativity than carbon. This means it attracts the bonding pair of electrons towards itself and becomes partially negatively charged, leaving carbon partially positively charged.

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Question

Aldehydes and ketones have ______ boiling points compared to similar alcohols.

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Answer

Lower

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Question

Aldehydes and ketones have ______ boiling points compared to similar alkanes.

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Answer

Higher

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Question

Complete the following sentence, justifying your answer: Aldehydes and ketones are ______ in water.


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Answer

Soluble

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Question

Name a common type of reaction involving aldehydes and ketones.

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Answer

Nucleophilic addition

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Question

Define nucleophile.


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Answer

A nucleophile is an electron pair donor. It contains a lone pair of electrons and a negative or partially negative charge.

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Question

Explain why we don’t use hydrogen cyanide directly in the lab.


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Answer

Hydrogen cyanide is a highly poisonous gas.

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Question

Define optical isomer.


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Answer

Optical isomers are non-superimposable mirror-image molecules that have the same molecular and structural formulas, but different spatial arrangements of bonds.

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Question

Define racemic mixture.


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Answer

A racemic mixture is a 50:50 mixture of a pair of optical isomers.

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Question

Explain why reacting butanone with potassium cyanide in acidic solution produces a racemic mixture.

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Answer

  • The cyanide ion can attack butanone from above or below the plane. 
  • This produces two non-superimposable mirror-image molecules, as the carbon atom from the C=O bond now forms a chiral centre, meaning it is attached to four different groups of atoms. 
  • The cyanide ion is equally likely to attack from above as from below, so a 50:50 racemic mixture is produced.

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Question

Reducing a ketone gives _______.


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Answer

A secondary alcohol.

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Reducing an aldehyde gives _____.

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Answer

A primary alcohol.

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Question

Oxidising an aldehyde gives ______.


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Answer

An alcohol.

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Question

Oxidising a ketone gives ____.

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Answer

No reaction.

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Question

Give the colour change when an aldehyde reacts with acidified potassium dichromate. 

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Answer

Orange to green.

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Question

Give the change seen when a ketone reacts with Tollens' reagent.

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Answer

Mixture turns from silver to colourless.

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Question

Give the colour change seen when an aldehyde reacts with Fehling's solution.

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Answer

Blue to red.

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Question

An unknown compound turns orange potassium dichromate green but has no effect on Tollens’ reagent. What can you infer about the compound?

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Answer

It is an alcohol.

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