Properties of Alkanones and Alkanals (ketones and aldehydes) Chemistry Tutorial

Key Concepts

• Alkanones and alkanals together are referred to as carbonyl compounds because they are organic molecules which contain the carbonyl, C=O, functional group:

  ⚛ Alkanals: C=O occurs at the end of a carbon chain

  The general structure of an alkanal is R-CHO (R is an alkane chain or a hydrogen atom)

  ⚛ Alkanones: C=O does not occur at the end of a carbon chain

  The general structure of an alkanone is R-CO-R' (R and R' are both alkane chains)

• Alkanals (R-CHO) belong to a class of organic molecules known as aldehydes.
• Alkanones (R-CO-R') belong to a class of organic molecules known as ketones.
• Physical properties of alkanones and alkanals are similar and are related to the polarity of the C=O functional group and the length of the carbon chain:

  ⚛ Melting points and boiling points are greater than for alkanes of comparable alkane chain length but less than for the corresponding alkanol.

  ⚛ Melting points and boiling points increase as the length of the carbon chain increases.

  ⚛ Short chain alkanals and alkanones are soluble in water, but solubility decreases as the length of the carbon chain increases.

• Chemical properties of alkanones and alkanals:

  ⚛ Oxidation (addition of oxygen using an oxidising agent):

  (a) Alkanals can be oxidised to alkanoic acids

  (b) Alkanones can not be readily oxidised.

  ⚛ Reduction (addition of hydrogen using a reducing agent):

  (a) Alkanals can be reduced to primary alkanols.

  (b) Alkanones can be reduced to secondary alkanols.

• Carbonyl compounds (aldehydes and ketones) are:

  ⚛ important in synthetic chemistry (that is, are used to make other industrial and/or commercial compounds)

  ⚛ found in nature as odour and flavour agents

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Carbonyl Functional Group

Carbon, a group 14 element, has 4 valence electons (4 electrons in the highest energy level).

Oxygen, a group 16 element, has 6 valence electrons (6 electrons in the highest energy level).

An oxygen atom can share 2 of its unpaired valence electrons with an atom of carbon:

A covalent double bond exists between the oxygen atom and the carbon atom (C=O). This is known as the carbonyl functional group.
The oxygen atom now has a share in 8 valence electrons, while the carbon atom has a share in 6 valence electrons.
The carbon atom can now bond to hydrogen atoms or other carbon atoms.

If the carbon atom covalently bonds to a hydrogen and to a carbon atom it will form an alkanal.

If the carbon atom covalently bonds to two carbon atoms it will form an alkanone.

Oxygen is a more electronegative atom than carbon.
The Pauling electronegativity of carbon is 2.5 while it is 3.5 for oxygen.
Oxygen has a greater ability to attract the electrons in the covalent bonds to itself than does carbon.
This results in a polar bond in which the oxygen atom acquires a partial negative charge (δ−) and the carbon atom a partial positive charge (δ+):

>C^δ+=O^δ−

The polarity of the C=O (carbonyl) functional group is used to explain many of the properties of alkanals and alkanones.

Physical Properties of Alkanones and Akananls

The physical properties of alkanals and alkanones of similar molar mass (molecular mass) are similar due to the polarity of the carbonyl functional group (C=O).

Melting Points and Boiling Points of Alkanones and Alkanals

Consider the melting point and boiling point of the alkanals and alkanones in the table below:

The alkanal containing 3 carbon atoms, propanal, has a boiling point of 49°C which is only slightly lower than the boiling point for the comparable alkanone with 3 carbon atoms, acetone or propanone, with a boiling point of 56°C.
Similarly, the 4 carbon atom alkanal, butanal, has a boiling point of 76°C which is only slightly lower than the boiling point for the 4 carbon atom alkanone, butanone, with a boiling point of 80°C.

In the pure substance, the polar C=O functional group of one alkanal molecule is attracted by dipole-dipole interactions to the polar C=O functional group of another alkanal molecule:

Similarly, for a pure alkanone, the partial positive charge on the carbon atom of one carbonyl functional group is attracted to the partial negative charge on the oxygen atom of another carbonyl functional group resulting in a dipole-dipole interaction:

These dipole-dipole interactions are stronger than the weak intermolecular forces (London forces or dispersion forces) acting between non-polar alkane molecules, so the melting points and boiling points of similar chain-length alkanes are much lower than the comparable alkanal or alkanone.

On the other hand, these dipole-dipole interactions between alkanal molecules in the pure substance, or between alkanone molecules in the pure substance, are not as strong the hydrogen bonds that exist between alkanol molecules of similar chain lengths, hence it requires more energy to melt or boil alkanols so their melting points and boiling points are higher than for the comparable alkanal or alkanone.
The graph below compares the boiling points of straight-chain alkanes, alkanals and primary alkanols with chain lengths of 1 to 5 carbon atoms:

From the graph we note that:

• For the same number of carbon atoms:

  ⚛ the alkane has the lowest boiling point

  ⚛ the alkanol has the highest boiling point

  ⚛ the boiling point of the alkanal lies between the boiling point of the alkane and that of the alkanol

• The boiling point of a homologous series (such as the alkanals) increases as the length of the non-polar hydrocarbon chain increases due to the increasingly significant effect of the weak intermolecular forces (London forces or dispersion forces) acting between these longer hydrocarbon chains.

  ⚛ as the number of carbon atoms in the alkane chain increases, the boiling point increases.

  ⚛ as the number of carbon atoms in the alkanal chain increases, the boiling point increases.

  ⚛ as the number of carbon atoms in the alkanol chain increases, the boiling point increases.

Solubility of Alkanals and Alkanones in Water

Short-chain alkanals and alkanones are soluble in water, but their solubility decreases as the length of the carbon chain increases.
The table below lists the solubility in water of some alkanals and alkanones:

The carbonyl functional group is polar, C^δ+=O^δ-

Water molecules are also polar:

So, the partial negative charge on the oxygen atom of the carbonyl functional group of an alkanal or alkanone will be attracted to the partial positive charge on the hydrogen atom of a water molecule, allowing the carbonyl compound to dissolve in water:

However, as the length of the carbon chain (R) of an alkanal or alkanone increases, the weak intermolecular forces (London forces or dispersion forces) acting between the non-polar hydrocarbon chains becomes increasingly significant so that the attraction between alkanal (or alkanone) molecules and water molecules decreases and they become less soluble in water.

Chemical Properties of Alkanones and Alkanals

The active site on a molecule of an alkanal or an alkanone is the carbonyl functional group (C=O).

While carbonyl compounds can undergo many reactions⁽¹⁾, we are only going to discuss oxidation reactions and reduction reactions in the following paragraphs.

Oxidation of Alkanals and Alkanones

In general, alkanals are easy to oxidise while alkanones are difficult to oxidise.

Common oxidising agents used to oxidise alkanals include:

• dichromate (Cr₂O₇^2-_(aq))

  The orange dichromate ion is reduced to the green chromium(3+) ion (chromium(III) ion) in the process

• permanganate (MnO₄^-_(aq))

  The purple permanganate ion is reduced to the colourless manganese(2+) ion (manganese(II) ion) in the process

For example, acidified potassium dichromate solution can be used to oxidise acetaldehyde (ethanal) to acetic acid (ethanoic acid):

We should observe a change in the dichromate solution from orange to green as it oxidises the colourless acetaldehyde (ethanol) to colourless acetic acid (ethanoic acid).

Even weak oxidising agents such as silver(1+), Ag⁺, present in Tollen's Reagent, can be used to oxidise an alkanal to an alkanoic acid. In this test for aldehydes, the presence of an alkanal causes the Ag⁺ to be reduced to solid silver, Ag_(s), which will form a "silver mirror" if the test is carried out in a glass test tube or flask.

Copper(2+) will also cause an alkanal to be oxidised to an alkanoic acid. In this process, Cu²⁺ is reduced to Cu₂O_(s) and the blue solution produces a brick-red precipitate. Both Fehling's solution and Benedict's solution use this reduction of Cu²⁺ to Cu⁺ to test for the presence of an aldehyde.

In summary, the common tests to distinguish between an alkanal (aldehyde) and an alkanone (ketone) are:

Reduction of Alkanones and Alkanals

Both alkanals and alkanones can undergo reduction using hydrogen gas and a catalyst, or a metal hydride reducing reagent.
In effect we are adding a hydrogen atom (H) to the carbon of the carbonyl functional group, and, to the oxygen present in the carbonyl functional group (C=O) to produce a new functional group, the hydroxyl functional group (OH).
When we do this to an alkanal, R-CHO, the hydroxyl group will be present at the end of the carbon chain and hence a primary alkanol is produced, R-CH₂OH.
When we do this to an alkanone, R-CO-R', the hydroxyl group will be present, not at the end of a chain, but somewhere between the ends of the chain, R-CH(OH)-R'. The hydroxyl group will be present on a carbon atom which is itself covalently bonded to 2 other carbon atoms, therefore this will be a secondary alkanol.

• The reduction of an alkanal produces a primary alkanol.
• The reduction of an alkanone produces a secondary alkanol.

For example, using a platinum catalyst with hydrogen gas under pressure, we can convert butanal to butan-1-ol, and we can convert butanone to butan-2-ol using a nickel catalyst as shown below: