HS Chemistry - Carbonyl Functional Groups

Aldehydes

Overview of The Page

This page will cover:

• What are aldehydes?
• How are aldehydes named?
• What reactions do aldehydes participate in?
• How can the presence of aldehydes in a compound be tested for?

Carbonyl functional groups are functional groups that contain a C=O bond $i.e. there is at least one instance of a Carbon atom double-bonded to an Oxygen atom$. This does not mean, however, that all C=O bonds are carbonyl bonds. CO₂, for example, has the structural formula O=C=O, but the C=O bonds are not considered carbonyl because it is not an organic compound. Aldehydes are a type of carbonyl functional group, and this subpage will look at them.

Aldehydes and ketones are very similar. In aldehydes, the C=O is bonded to one Hydrogen atom and one other non-Hydrogen atom $i.e. it is at the end of the organic molecule's main chain$. In ketones, the C=O is bonded to two non-Hydrogen atoms $i.e. it is not at the end of the organic molecule's main chain$.

The formula for the aldehyde functional group is RCHO, where R represents the rest of the organic molecule. To form the name for an aldehyde, we use the same naming convention as we do for other organic molecules. The suffix ending for aldehydes is "-al", but the way it's added is different. Instead of removing the entire suffix, we take the suffix of the hydrocarbon and add "-al" to the end. Thus, if we have the following molecule:

Where there is both an alkene functional group and an aldehyde group, we take the alkene suffix $"\-ene"$ and add "-al" to the end after removing the "e" at the end $giving us "\-enal"$. Thus, the molecule shown above is 2-butenal, or but-2-enal.

Following that same logic, this:

Is butanal. Although it is preferable to keep all the Carbon atoms in the main chain in a single line if they are single-bonded to each other $and not double\-bonded to each other like in alkenes$:

As you can see, the bond angles for the aldehyde functional group are still retained, but the aldehyde functional group has been rotated when drawing the molecule's displayed formula so that the single-bonded Carbon atoms in the main chain are all on the same line. It is not necessary to keep the bond angles, however, and it is acceptable to draw the bonds at perpendicular angles to each other, like this:

It is important to note, however, that this does not change the bond angles between the atoms - it merely makes it easier to draw the molecule.

It should also be noted that an unintended limitation of our diagram above is that it may seem like the Oxygen atom is closer to the Carbon atom than the other Hydrogen atoms are to their Carbon atoms. That is not what the diagram is trying to depict.

The skeletal formula of butanal would be:

First, we have 3 points representing Carbon atoms, and then we have an Oxygen atom double bonded to the fourth point, which represents the fourth Carbon atom. However, to complete the skeletal formula, we then draw another line after the double-bonded Oxygen atom, with a Hydrogen atom at the end. This way, it's clear that there are 4 Carbon atoms in butanal, not 3.

The skeletal formula of but-2-enal would be:

With the Carbon-Carbon double bond between the second and third Carbon atoms.

You may be wondering why we didn't number the organic molecule butanal as butan-1-al. After all, we usually number the position of the functional group to indicate where it is in the organic molecule. However, since aldehydes must be present at the end of the main chain of an organic molecule, it is not necessary to note their position in the organic molecule with a number. Additionally, when numbering an organic molecule, numbering starts from the end with the aldehyde functional group.

This raises an important question. What if we have a displayed formula of an organic molecule that shows the aldehyde functional group as a side chain?

In this organic molecule, not only is the aldehyde functional group shown as a side chain, but the longest chain of Carbon atoms is the chain that doesn't include the aldehyde group:

It may seem that we have to pick the longest Carbon chain shown above, which doesn't include the aldehyde group, as our main chain.

However, whenever there is a type of carbonyl group present in an organic compound, it takes precedence over the Carbon-Carbon double bond functional groups and the Carbon-Carbon single bond functional groups. Since aldehydes are a type of carbonyl functional group, they take precedence.

What does it mean for the aldehyde functional group to take precedence? It means:

1. The main chain of the organic compound is the longest Carbon chain that includes the aldehyde, unless there is another functional group present that takes precedence over the aldehyde. Carboxylic acids, for instance, will take precedence over aldehydes.

2. When we number the Carbon atoms in the main chain to determine positions, the numbering must start at the aldehyde. The Carbon atom that is part of the aldehyde functional group must be labeled #1.

3. Even if the organic compound includes an alcohol or alkene or some other functional group, the compound will be classified as an aldehyde unless there is some other functional group present which takes precedence over the aldehyde.

Thus, the main chain for the organic molecule shown above is:

In this case, since Carbon-Carbon single bonds are rotatable, the molecule can be re-drawn as:

Now its clear that the propyl group is the side chain, and we can number and name the molecule accordingly. The main chain is numbered in blue, and the side chain is numbered in red:

The organic molecule is 3-propylhexanal.

Even if the side chain included an alkene functional group, like in the example below:

The main chain would still be the longest Carbon chain that includes the aldehyde, because the aldehyde functional group takes precedence over the alkene functional group.

However, if there are multiple possible main chains that include aldehyde functional groups, then there are a few criteria for determining which chain should be used as the main chain:

1. If there is a chain that includes two aldehyde groups $on both ends$, it will be used as the main chain over a chain that only has one aldehyde group.

2. If there are no possible main chains with two aldehyde groups, then the chain with one aldehyde group that has the most Carbon atoms will be used as the main chain.

When there are two aldehyde functional groups present in a molecule, the molecule is called a dialdehyde. Dialdehydes can be treated as aldehydes, except that when naming them, the suffix is "-dial" rather than "-al". Naming an organic compound with more than two aldehyde functional groups is beyond the scope of this page.

There are also organic molecules with both hydroxyl $OH$ groups and aldehyde groups. In these organic molecules, the aldehyde group takes precedence over the hydroxyl group, and the molecule is classified as an aldehyde. Thus, the suffix is "-al" $or "\-dial" if it is a dialdehyde$. The hydroxyl group is added to the prefix of the organic molecule with the prefix "n-hydroxy-", where n is a number noting the hydroxyl group's position. Again, since aldehydes take precedence over hydroxyl groups, the numbering starts from the end with the aldehyde group.

Thus, to name the following organic molecule:

We would number the main chain:

And the molecule would be named 6-hydroxyhexanal.

Since the "hydroxy" part is a prefix, it follows the same rules as other prefixes - namely, that we have to indicate how many hydroxyl groups there are, and their positions, with the same rules that we have for other prefixes.

Thus, the following molecule:

Would be named 5,6-dihydroxyhexanal. Organic molecules with both halogenoalkane and aldehyde groups are also treated the same way $and so are organic molecules with all three\!$.

Longer suffixes are used in cyclic aldehydes, which are like cycloalkanes but with the aldehyde group attached. When naming cyclic aldehydes, the suffix "-carbaldehyde" is used, with a prefix like "di" or "tri" appended to the beginning of the suffix to indicate the number of ketone groups present. The numbering starts from the aldehyde group $unless there is another functional group present that takes higher precedent$.

Thus, the following molecule:

Would be named 1,2,3-cyclobutanetricarbaldehyde. Aldehydes attached to arenes are named the same way as cyclic aldehydes.

Aldehydes can also be formed from, and participate in, certain reactions.

Aldehyde Reactions

• Forming Aldehydes Through Oxidation:

  • There are two ways in which aldehydes can be formed:

    1. Oxidizing an alkene using hot, concentrated acidified potassium permanganate $KMnO~4~$ solution. However, this requires that one of the Carbons in the double bond is bonded to one Hydrogen atom and one other non-Hydrogen atom $typically a Carbon atom$. The products will include an aldehyde.

    2. Oxidizing a primary alcohol using a hot, concentrated solution of acidified potassium permanganate $KMnO~4~$ or a hot, concentrated solution of acidified potassium dichromate $K~2~Cr~2~O~7~$. A primary alcohol must be used, or else an aldehyde will not be formed. The products will be an aldehyde and H₂O.

  • In both of these reactions, the alkene/primary alcohol must be gently heated in a distillation apparatus with the acidified KMnO₄ or K₂Cr₂O₇. The aldehyde must be distilled as it is formed in order to prevent the KMnO₄ or K₂Cr₂O₇ from further oxidizing the aldehyde into a carboxylic acid. Since the aldehyde product will have a lower boiling point than the alcohol reactant, the aldehyde can be distilled as it is forming by keeping the temperature of the reaction above the aldehyde's boiling point but below the alcohol's boiling point, and then collecting the aldehyde in a separate flask through a distillation apparatus.

  • In both of these reactions, if KMnO₄ is used as the oxidizing agent, the KMnO₄ solution will lose its purple color and become colorless. If K₂Cr₂O₇ is used as the oxidizing agent, the K₂Cr₂O₇ solution will turn from an orange color to a green color.

  • When writing the oxidation reaction, we usually just show the Oxygen atoms from the oxidizing agent, rather than the whole oxidizing agent. We represent those Oxygen atoms as [O]. For example, the oxidation of 1-butanol, a primary alcohol, can be represented as:

    CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + H₂O

    Here, the primary alcohol 1-butanol is oxidized to yield butanal and water.

• Reducing Aldehydes:

  • Given that aldehydes can be formed through oxidation, they can also be reduced back when reacted with a reducing agent. Reducing an aldehyde will yield a primary alcohol. The reaction method used depends on the reducing agent used:

    1. If sodium tetrahydridoborate $NaBH~4~, also known as sodium borohydride$ is the reducing agent, the aldehyde must be heated with a solution of NaBH₄ dissolved in water. The product will be a primary alcohol.

    2. If lithium tetrahydridoaluminate $LiAlH~4~, also known as lithium aluminum hydride$ is the reducing agent, the LiAlH₄ must be dissolved in a dry ether at room temperature before being added to the aldehyde. The product will be a primary alcohol.

  • When writing the reduction reaction, we usually just show the Hydrogen atoms from the reducing agent, rather than the whole reducing agent. We represent those Hydrogen atoms as [H]. For example, the reduction of butanal can be represented as:

    CH₃CH₂CH₂CHO + 2[H] → CH₃CH₂CH₂CH₂OH

    Here, the aldehyde butanal is reduced to the primary alcohol 1-butanol.

• Addition Reaction With Cyanide Compounds:

  • Aldehydes can undergo addition reactions with hydrogen cyanide $molecular formula: HCN; structural formula: H−C≡N$. However, since HCN is a very toxic gas, it is not added directly. Instead, HCN is produced in the reaction apparatus by reacting another cyanide, like KCN, with an acid, like sulfuric acid $only a very dilute acid is needed to turn the KCN into HCN$. The newly-produced HCN will dissolve into the solution. Some of the HCN molecules will dissociate into cyanide $CN^\-^$ ions and H⁺ ions. The cyanide will then begin to react with the aldehyde in the reaction apparatus.

    • Note that cyanides like KCN are still toxic, but because they are a solid, they are safer than HCN, which is a gas at STP $Standard Temperature and Pressure$.

  • The aldehyde functional group contains a C=O bond. A CN^- ion, which is a nucleophile, can break this double bond. The Carbon atom in the C=O bond is slightly positively charged, as Oxygen is more electronegative than Carbon. Therefore, the CN^- ion is attracted to the Carbon atom and bonds with it. This causes the Carbon atom to break its double bond with the Oxygen atom so that it can form a bond with the cyanide ion. Since the Oxygen atom had a slightly negative charge before the bond was broken, it is left with an extra electron after the bond is broken $heterolytic fission$.

  • After the bond is broken, the Oxygen atom is left with a negative charge. It then attracts a H⁺ ion towards it and bonds with it, forming a hydroxyl group. The H⁺ ion could come from a HCN molecule $which would undergo heterolytic fission and supply the H^\+^ ion$, from the water, or from the acid.

  • The resulting product is a molecule that belongs to a class of molecules known as 2-hydroxynitrile molecules; after undergoing the reaction, the Carbon atom that was previously part of the carbonyl group now has a hydroxyl group and a nitrile group bonded to it. The nitrile group is the −C≡N group that was formerly part of the hydrogen cyanide molecule. To fully name the molecule, we can put the rest of the product organic molecule's name between the "2-hydroxy" and "nitrile" portions $e.g. 2\-hydroxypropenenitrile, 2\-hydroxy\-4\-chlorohexanenitrile, etc.$. The reason for this is that "2-hydroxy" is the prefix, and "nitrile" is the suffix, that is common among all 2-hydroxynitrile molecules.

    • The addition reaction of pentanal with HCN can be used to demonstrate this:

      CH₃CH₂CH₂CH₂CHO + HCN → CH₃CH₂CH₂CH₂CHOHCN

      If we wanted to represent the molecules with their displayed formulae, we could show it as:

      

    • In the pentanal reactant, the Oxygen atom is attached to the fifth Carbon atom from the left, or the first Carbon atom from the right. In the product molecule, the Oxygen atom $now part of a hydroxyl group$ is still attached to the fifth Carbon atom from the left, but now there are six Carbon atoms in the main chain, so the hydroxyl group is attached to the second Carbon atom from the right, not the first. Thus, since our molecule contains a nitrile group, and nitrile groups take precedence over hydroxyl groups, our prefix will contain "2-hydroxy-". Since a nitrile group must always be present at the end of a main chain, as the Nitrogen cannot further bond onto other atoms, the suffix will be "nitrile" $it is not necessary to note the nitrile's position, as it will be at one of the ends of the main chain$.

    • Since an aldehyde group must be present at the end of the main chain, the only position available for the hydroxyl group at the end of the reaction is at the second Carbon atom. Therefore, in the case of aldehydes, the "2-" portion is sometimes dropped from the "2-hydroxynitrile". However, it is optional to drop the "2-" portion.
    • The rest of the molecule is named normally, in between the "2-hydroxy" prefix and the "nitrile" suffix. For example, in the molecule above, since we have 6 Carbon atoms in the main chain, and no other functional groups $apart from the nitrile group and the hydroxyl group at the 2^nd^ position$ are present, our molecule will be named 2-hydroxyhexanenitrile.
    • Overall: Pentanal + Hydrogen Cyanide → 2-hydroxyhexanenitrile

    
    

  • An extra Carbon atom has been added to the main chain through this reaction. This makes this kind of reaction very useful in organic chemistry, as chemists often need to extend the main chain of an organic molecule, and repeating a reaction like this allows them to do so. The nitrile can then:

    1. Undergo hydrolysis with a dilute solution of HCl under reflux to become a carboxylic acid. The water molecule and dissociated Hydrogen ion react with the nitrile to form a carboxylic acid and an ammonium ion. With 2-hydroxyhexanenitrile, the full reaction would be:

       CH₃CH₂CH₂CH₂CHOHCN + 2H2O + H⁺ → CH₃CH₂CH₂CH₂CHOHCOOH + NH₄⁺

       Most of the organic molecule is left unchanged. Only the functional group at the end of the organic molecule is different now.

    2. Be reduced by sodium and ethanol to an amine. The [H] atoms react with the nitrile to form an amine. With 2-hydroxyhexanenitrile, the full reaction would be:

       CH₃CH₂CH₂CH₂CHOHCN + 4[H] → CH₃CH₂CH₂CH₂CHOHCH₂NH₂

       Unlike the hydrolysis reaction, there is only one product molecule here. Again, most of the organic molecule is left unchanged. Only the functional group at the end of the organic molecule is different now.

       
       

  • When writing a reaction, to quickly determine the product's structural formula given the reactants, the Hydrogen atom will bond with the Oxygen atom in the aldehyde, and the CN will bond at the end of the main chain. The reaction between pentan-2-one and HCN is shown below, with the H and CN from the HCN colored in red to show where they bond:

    CH₃CH₂CH₂CH₂CHO + HCN → CH₃CH₂CH₂CH₂CHOHCN

    This rule holds for ketones too.

Testing for Aldehydes

• 2,4-DNPH Test:

  • If we have an organic molecule, but we don't know whether it is an aldehyde or ketone or some other molecule, we can test it by mixing it with a solution of 2,4-dinitrophenylhydrazine $2,4\-DNPH, aka Brady's reagent$ in a test tube and then shaking it. 2,4-DNPH solution normally has a lime-green color, but if an aldehyde or ketone is present, an orange precipitate will be formed. If a precipitate doesn't form, then the organic molecule isn't an aldehyde or ketone.
  • A 2,4-DNPH molecule contains a NH₂ at its end. When an aldehyde or ketone reacts with it, the Oxygen atom in the carbonyl $C=O$ group reacts with the H₂ to form H₂O. The aldehyde/ketone then bonds with the Nitrogen atom, and the products are H₂O and a large molecule called a 2,4-dinitrophenylhyrdazone molecule. This molecule is seen as an orange precipitate. Note that the molecule's ending is "-one", not "-ine". To show the type of 2,4-dinitrophenylhydrazone molecule that is formed, we append the name of the original aldehyde/ketone to the beginning; for example, using hexanal would give us hexanal 2,4-dinitrophenylhydrazone. The overall reaction is a condensation reaction.
  • But what if we don't know the type of aldehyde/ketone that we originally had? By measuring the melting point of the precipitate and comparing it with known values, we can determine the type of aldehyde/ketone we originally had. In order to find the melting point, however, we first need to purify the precipitate crystals by washing them and recrystallizing them with a suitable solvent, like a mixture of ethanol and water for example. Once the crystals are purified, we can measure their melting point and compare it with known values of other 2,4-dinitrophenylhydrazones to find out which one we have, and by extension, what aldehyde/ketone we originally had.

• Fehling's Solution:

  • If we know that the organic molecule we have is either an aldehyde or a ketone $which can be proven if the molecule produces an orange precipitate when reacted with 2,4\-DNPH solution$, we can use Fehling's solution to see if it is an aldehyde. Fehling's solution contains Copper $Cu$ ions, which will form an oxide, seen as a red$ish$ precipitate, if the organic molecule is an aldehyde.
  • However, Fehling's solution can't be stored directly, as the Copper compound in the solution will quickly break down. Therefore, it is formed when the test needs to be carried out by combining equal amounts of two stored solutions: Fehling's solution A, which has a blue color, and Fehling's solution B, which has no color. As would be expected, the product, Fehling's solution, has a light blue color.
  • To carry out the test, add the Fehling's solution to a test tube with the organic compound. If the organic molecule is an aldehyde, a red precipitate will form. If no precipitate forms, but the organic molecule reacted with 2,4-DNPH solution, it is a ketone. If it didn't react with 2,4-DNPH solution, it is neither an aldehyde nor a ketone.

• Tollens' Reagent:

  • If we know that the organic molecule we have is either an aldehyde or a ketone $which can be proven if the molecule produces an orange precipitate when reacted with 2,4\-DNPH solution$, we can use Tollens' reagent to see if it is an aldehyde. Tollens' reagent is a solution of silver nitrate $AgNO~3~$ in excess ammonia solution.
  • Excess ammonia solution is required to prevent the silver nitrate from reacting with the water, and to establish alkaline conditions $pH less than 7$ for the reaction to occur.
  • To carry out the test, add the Tollens' reagent to a test tube with the organic compound. If the organic molecule is an aldehyde, the silver $Ag^\+^$ ions will oxidize and form Ag₂O, seen as a mirror-like substance in the liquid. If no precipitate forms, but the organic molecule reacted with 2,4-DNPH solution, it is a ketone. If it didn't react with 2,4-DNPH solution, it is neither an aldehyde nor a ketone.