24.4 Chemical Properties of Aldehydes and Ketones

Learning Objectives

By the end of this section, you will be able to:

  • Explain the formation of aldehydes and ketones.
  • Describe the typical reactions that take place with aldehydes and ketones.

Organic functional groups can be converted into other functional groups through reactions.  A map of some of the more common reactions to convert functional groups can be found in Section 19.6 – General Reactions of Carbon in Infographic 19.6a.

Preparation of Aldehydes and Ketones

Aldehydes are commonly prepared by the oxidation of alcohols whose –OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol, as shown in Figure 24.4a.

 

A reaction is shown. An alcohol appears on the left and an aldehyde on the right of the reaction arrow. The alcohol is shown as C H subscript 3 C H subscript 2 C H subscript 2 O H, and the aldehyde is shown as C H subscript 3 C H subscript 2 C H O. The O H group at the right end of the alcohol structure and the C H O group at the right end of the aldehyde structure are in red.
Figure 24.4a. Primary alcohols can be oxidized to produce aldehydes (credit: Chemistry 2e (OpenStax), CC BY )

Alcohols that have their –OH groups in the middle of the chain are necessary to synthesize a ketone, which requires the carbonyl group to be bonded to two other carbon atoms, as shown in Figure 24.4b.

A reaction is shown. An alcohol appears on the left and a ketone on the right of the reaction arrow. The alcohol is shown as C H subscript 3 C H ( O H ) C H subscript 3 and the ketone is shown as C H subscript 3 C O C H subscript 3. The O H group in the alcohol structure and the C O group at the center of the ketone structure are in red.
Figure 24.4b. Secondary alcohols can be oxidized to produce ketones (credit: Chemistry 2e (OpenStax), CC BY).

An alcohol with its –OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. An alcohol with its –OH group attached to two other carbon atoms will form a ketone. If three carbons are attached to the carbon bonded to the –OH, the molecule will not have a C–H bond to be replaced, so it will not be susceptible to oxidation.

The oxidation of alcohols to aldehydes and ketones was previously discussed in Section 23.4 Reactions of Alcohols and displayed in Infographic 23.4a.

Oxidation of Aldehydes and Ketones

Aldehydes and ketones are much alike in many of their reactions, owing to the presence of the carbonyl functional group in both. They differ greatly, however, in one most important type of reaction: oxidation. Aldehydes are readily oxidized to carboxylic acids, whereas ketones resist oxidation, as shown in Figure 24.4c.

An aldehyde is oxidized to a carboxylic acid using permanganate (in the presence of sulfuric acid), dichromate, oxygen, or by mild oxidizing agents such as Cu2+ or Ag+ whereas a ketone shows no reaction under the same conditions.
Figure 24.4c. Oxidation is a chemical reaction that can be achieved using permanganate (in the presence of sulfuric acid), dichromate, oxygen, or by mild oxidizing agents such as Cu2+ or Ag+. Aldehydes are readily oxidized in this process to carboxylic acids, whereas ketone will resist this reaction. (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

The aldehydes are, in fact, among the most easily oxidized of organic compounds. They can easily be oxidized by oxygen (O2) in air to carboxylic acids:

\[2RCHO + O_2 \rightarrow 2RCOOH \label{14.10.1} \]

The ease of oxidation helps chemists identify aldehydes. A sufficiently mild oxidizing agent can distinguish aldehydes not only from ketones but also from alcohols. Tollens’ reagent, for example, is an alkaline solution of silver (Ag+) ion complexed with ammonia (NH3), which keeps the Ag+ ion in solution.

\[H_3N—Ag^+—NH_3 \label{14.10.2} \]

When Tollens’ reagent oxidizes an aldehyde, the Ag+ ion is reduced to free silver (Ag), as shown in Figure 24.4d.

 

A aldehyde group RCHO reacts with 2 Ag(NH subscript 3) subscript 2 superscript positive sign and 3 hydroxide ions to give COO superscript negative sign in addition to 2 silver in its solid state, 4 NH subscript 3 and 2 water.
Figure 24.4d. Reaction scheme of a typical aldehyde in the Tollens’ test, often referred to as the “silver mirror test.” This simple test allows chemists to differentiate aldehydes from ketones and alcohols. (Credits: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.)

Deposited on a clean glass surface, the silver produces a mirror, as shown in Figure 24.4e. Ordinary ketones do not react with Tollens’ reagent.

An image of the silvering process
Figure 24.4e. A reaction related to the Tollens’ reaction is often used to silver mirrors. These ornaments were silvered by such a reaction. Glucose, a simple sugar with an aldehyde functional group, is used as the reducing agent. (Credit: Photo by Krebs Glas Lauscha, CC BY 3.0)

Spotlight on Everyday Chemistry: Silver Mirrors

The Tollens’ reaction is used to identify the presence of an aldehyde and also used in the production of mirrors.  Infographic 24.4a. shows the details.

Infographic 24.4a.  Read more about “Making silver mirrors using chemistry” by Andy Brunning / Compound Interest, CC BY-NC-ND, or access a text-based summary of infographic 24.4a [New tab].

Other Oxidation Tests

The oxidation of aldehydes can be confirmed using the Tollens’ reagent (explained above) which produces a silver mirror finish on the reaction vessel.  Other tests that confirm the oxidation of an aldehyde use the Benedict’s reagent or the Fehling’s test.  Both of these require the presence of a copper ion in solution that changes colour when an aldehyde is present.

With the Benedict’s reagent, complexed copper (II) ions are reduced to copper (I) ions that form a brick-red precipitate (copper (I) oxide) (Figure 24.4f. and Figure 24.4g.).  The Fehling’s test contains copper (II) ions complexed with tartrate ions and results in the same changes as with the Benedict’s reagent.

Top: Oxidation of an aldehyde-based compound (an aldose) using the Tollens' reagent to produce carboxylate anion and silver. Bottom: Oxidation of an aldehyde-based compound (an aldose) using the Benedict's reagent to produce carboxylate anion and brick-red precipitate.
Figure 24.4f. Oxidation of an aldehyde-based compound using the Tollens’ reagent and the Benedict’s reagent.  Notice the change in colour of each reaction (credit: Intro Chem: GOB (V. 1.0)., CC BY-NC-SA 3.0.).

 

An image of a chemical reaction in a test tube: oxidation of aldehyde with Benedict's reagent.  Original blue reagent on top and resulting red precipitate on bottom. This indicates a positive result.
Figure 24.4g. Oxidation of aldehyde with Benedict’s reagent.  Original blue reagent on top and resulting red precipitate on bottom. This is a positive result (credit: Image by Kala Nag, CC BY-SA 4.0).

Although ketones resist oxidation by ordinary laboratory oxidizing agents, they undergo other chemical reactions such as reduction, addition, and combustion, as do aldehydes.

Reduction of Aldehydes and Ketones

The most general method for preparing alcohols, both in the laboratory and in living organisms, is by reduction of a carbonyl compound. Just as reduction of an alkene adds hydrogen to a carbon-carbon double bond to produce an alkene, the reduction of an aldehyde or ketone add hydrogen to the carbon-oxygen double bond to give an alcohol. All kinds of carbonyl compounds can be reduced, including aldehydes, ketones, carboxylic acids, and esters.

Reduction of a carbonyl group (where the oxygen is depicted in a pink colour) to an alcohol (the OH group is depicted in a pink colour). The reducing agent used is [H]
Figure 24.4h. Reduction of a carbonyl group to an alcohol (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Aldehydes are easily reduced to give primary alcohols, and ketones are reduced to give secondary alcohols.

Two reactions. On the left: An aldehyde is reduced to a primary alcohol. On the right: A ketone is reduced to a secondary alcohol
Figure 24.4i. An aldehyde is reduced to a primary alcohol. A ketone is reduced to a secondary alcohol (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Dozens of reagents are used in the laboratory to reduce aldehydes and ketones, depending on the circumstances, but sodium borohydride, NaBH4, is usually chosen because of its safety and ease of handling (Figure 24.4j.). Lithium aluminum hydride, LiAlH4, is another reducing agent often used but is much more reactive and much more dangerous than NaBH4 (Figure 24.4k.).

Reduction of butanal forming 1-butanol with reducing agents NaBH subscript 4, ethanol and a hydronium ion
Figure 24.4j. Reduction of butanal forming 1-butanol (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

 

Reduction of 2-cyclohexenone forming 2-cyclohexenol with LiAlH subscript 4, ether and a hydronium ion as the reducing reagents
Figure 24.4k. Reduction of 2-cyclohexenone forming 2-cyclohexenol (credit: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0).

Example 24.4a

What carbonyl compounds would you reduce to obtain the following alcohols?

Two structures. a) a 6 carbon chain with a methyl group at the 4th carbon and a hydroxy group at the 2nd carbon; b) a 5 carbon chain with a methyl group at the 4th carbon, a double bond at the 2nd carbon and a hydroxyl group at the 1st carbon.

Solution:

Identify the target alcohol as primary, secondary, or tertiary. A primary alcohol can be prepared by reduction of an aldehyde, an ester, or a carboxylic acid; a secondary alcohol can be prepared by reduction of a ketone; and a tertiary alcohol can’t be prepared by reduction.

(a) The target molecule is a secondary alcohol, which can be prepared only by reduction of a ketone. Either NaBH4 or LiAlH4 can be used.

A secondary alcohol is produced by reduction of a ketone

(b) The target molecule is a primary alcohol, which can be prepared by reduction of an aldehyde, an ester, or a carboxylic acid. LiAlH4 is needed for the ester and carboxylic acid reductions.

A primary alcohol is produced by reduction of an aldehyde, an ester, or a carboxylic acid

Source: Organic Chemistry (OpenStax), CC BY-NC-SA 4.0.

Exercise 24.4a

Draw the carbonyl compound that when reduced will form the following alcohols.

There are 4 alcohol structures: a) a primary alcohol substituted benzene; b) a secondary alcohol substituted benzene, c) an alcohol group attached to a benzene ring; and d) a 4 carbon chain with a methyl at the 3rd carbon and an alcohol group at the 1st carbon.

Check Your Answers:[1]

Source: Exercise 24.4a is adapted by Samantha Sullivan Sauer from Organic Chemistry (OpenStax) on Libre Texts, images created using Biovia Draw, CC BY-NC-SA.

Oxidation and reduction are paired reactions in that one reverses the results of the other.  Examining the oxidation states of carbon as it gets oxidized helps to understand the changes.  In Figure 24.4l., methane is oxidized step-by-step to methanol, then methanal, then methanoic acid, then carbon dioxide.  Carbon dioxide cannot be further oxidized. In Figure 24.4m., the functional groups of carbon are ranked based on the oxidation state of carbon with alkanes being the most reduced and carboxylic acid derivatives being the most oxidized.

Structures and their associated oxidation states of methane, methanol, methanal, methanoic acid and carbon dioxide.  Left to right shows an increase in oxidation state (more oxidized).  Right to left shows a decrease in oxidation state (more reduced)
Figure 24.4l. Oxidation states of methane, methanol, methanal, methanoic acid and carbon dioxide.  Left to right increase in oxidation state (more oxidized).  Right to left decrease in oxidation state (more reduced) (credit: Org Chem Bio Emphasis, CC BY-NC-SA 4.0).

 

Oxidation states of functional groups. Left to right: alkane, alcohol/thiol/amine/alkene, aldehyde/ketone/imine and carboxylic acid derivative. Left to right increase in oxidation state (more oxidized).  Right to left decrease in oxidation state (more reduced)
Figure 24.4m. Oxidation states of functional groups.  Left to right increase in oxidation state (more oxidized).  Right to left decrease in oxidation state (more reduced) (credit: Org Chem Bio Emphasis, CC BY-NC-SA 4.0).

Example 24.4b

Methane represents the completely reduced form of an organic molecule that contains one carbon atom. Sequentially replacing each of the carbon-hydrogen bonds with a carbon-oxygen bond would lead to an alcohol, then an aldehyde, then a carboxylic acid, and, finally, carbon dioxide:

[latex]\text{CH}_4\;{\longrightarrow}\;\text{CH}_3\text{OH}\;{\longrightarrow}\;\text{CH}_2\text{O}\;{\longrightarrow}\;\text{HCO}_2\text{H}\;{\longrightarrow}\;\text{CO}_2[/latex]

What are the oxidation numbers for the carbon atoms in the molecules shown here?

Solution:

In this example, we can calculate the oxidation number for the carbon atom in each case (note how this would become difficult for larger molecules with additional carbon atoms and hydrogen atoms, which is why organic chemists use the definition dealing with replacing C–H bonds with C–O bonds).

For CH4, the carbon atom carries a –4 oxidation number (the hydrogen atoms are assigned oxidation numbers of +1 and the carbon atom balances that by having an oxidation number of –4).

For the alcohol (in this case, methanol), the carbon atom has an oxidation number of –2 (the oxygen atom is assigned –2, the four hydrogen atoms each are assigned +1, and the carbon atom balances the sum by having an oxidation number of –2; note that compared to the carbon atom in CH4, this carbon atom has lost two electrons so it was oxidized).

For the aldehyde, the carbon atom’s oxidation number is 0 (–2 for the oxygen atom and +1 for each hydrogen atom already balances to 0, so the oxidation number for the carbon atom is 0).

For the carboxylic acid, the carbon atom’s oxidation number is +2 (two oxygen atoms each at –2 and two hydrogen atoms at +1).

For carbon dioxide, the carbon atom’s oxidation number is +4 (here, the carbon atom needs to balance the –4 sum from the two oxygen atoms).

Exercise 24.4b

Indicate whether the marked carbon atoms in the three molecules below are oxidized or reduced relative to the marked carbon atom in ethanol:

A molecular structure is shown. A C H subscript 3 group is bonded up and to the right to a C H subscript 2 group. Bonded to the C H subscript 2 group down and to the right is an O H group.
Ethanol

 

There is no need to calculate oxidation states in this case; instead, just compare the types of atoms bonded to the marked carbon atoms:
Three molecular structures are shown, each with a red central C atom. In a, a C H subscript 3 group is bonded to the lower left, an H atom is bonded above, and H subscript 2 appears to the right of the central C atom. In b, an O atom is double bonded above the central C atom, a C H subscript 3 group is bonded to the lower left, and an H atom is bonded to the lower right. In c, an O atom is double bonded above the central C atom, a C H subscript 3 group is bonded to the lower left, and an O H group is bonded to the lower right.

Check Your Answers: [2]

Exercise and image source: Chemistry (OpenStax), CC BY 4.0

Addition of Aldehydes and Ketones

Addition of Alcohol

One of the most important examples of an addition reaction in biochemistry is the addition of an alcohol to a ketone or aldehyde. When an alcohol adds to an aldehyde, the result is called a hemiacetal; when an alcohol adds to a ketone the resulting product is a hemiketal (Figure 24.4n). The prefix ‘hemi’ (half) is used in each term because addition of a second alcohol can occur resulting in species called acetals and ketals. The conversion of an alcohol and aldehyde (or ketone) to a hemiacetal (or hemiketal) is a reversible process.

Two reactions. Top: addition of an alcohol to an aldehyde. Bottom: additional of an alcohol to a ketone forming a hemiacetal and a hemiketal.
Figure 24.4n. Addition of an alcohol to an aldehyde and a ketone forming a hemiacetal and a hemiketal. (credit: Org Chem Bio Focus (Vol 2), CC BY-NC-SA)

The generalized mechanism for the process is shown in Figure 24.4o. Focus on the connection made between the aldehyde and the alcohol.

A carbonyl carbon of the aldehyde group connects to the oxygen from the alcohol group in a biochemical mechanism creating a hemiacetal formation
Figure 24.4o Biochemical mechanism of hemiacetal formation. (credit: Organic Chemistry with a Biological Focus (Vol 2), CC BY-NC-SA)

Addition of Water

Aldehydes and ketones, when in aqueous solution, exist in equilibrium with their hydrate form. A hydrate forms as the result of a water molecule adding to the carbonyl carbon of the aldehyde or ketone (Figure 24.4p.). Although you should be aware that aldehyde and ketone groups may exist to a considerable extent in their hydrated forms when in aqueous solution (depending upon their structure), they are usually drawn in their non-hydrated form for the sake of simplicity.

An aldehyde reacts with water, where the water is added to the aldehyde to form a hydrate.
Figure 24.4p. Addition of an aldehyde with water to form a hydrate. (credit: Organic Chemistry with a Biological Focus (Vol 2), CC BY-NC-SA)

Addition of HCN

Hydrogen cyanide (HCN) adds across the carbon-oxygen double bond in aldehydes and ketones to produce compounds known as hydroxynitriles or cyanohydrins. For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitrile (Figure 24.4q.). With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile (Figure 24.4r.).

The addition reaction showing HCN reacting with ethanoic acid to produce ethanal. The HCN adds to the carbonyl portion of the carboxylic acid.
Figure 24.4q. Addition of HCN to ethanal.(credit: Addition Reactions, CC BY-NC)
Addition of HCN to propanone showing the HCN adding to the carbonyl portion of propanone.
Figure 24.4r. Addition of HCN to propanone.(credit: Addition Reactions, CC BY-NC)

These are examples of nucleophilic addition. The carbon-oxygen double bond is highly polar, and the slightly positive carbon atom is attacked by the cyanide ion acting as a nucleophile (Figure 24.4s.).

CN ion attacking the carbonyl group with a blue arrow.
Figure 24.4s. CN ion attacking the carbonyl group. (credit: Addition Reactions, CC BY-NC)

This is considered to be a reversible addition.  Remember that HCN or hydrogen cyanide contains a carbon-nitrogen triple bond (HC≡N).  This triple bond is open itself to addition or reduction reactions.

RCH=O + H–C≡N reversible reaction arrow RCH(OH)CN (a cyanohydrin)

Spotlight on Everyday Chemistry: Millipede Defense

The cyanohydrin from benzaldehyde is named mandelonitrile (Figure 24.4t.).

Formation of mandelonitrile from the addition of hydrogen cyanide to benzaldehyde
Figure 24.4t. Formation of mandelonitrile from benzaldehyde and hydrogen cyanide. (credit: Image by Samantha Sullivan Sauer using Biovia Draw, CC BY-NC-SA 4.0)
An image of a millepede
Figure 24.4u. Millepede – Apheloria Viginiensis Corrugata (credit: Image by Marshal Hedin, CC BY 2.0)

The reversibility of cyanohydrin formation is put to use by the millipede Apheloria corrugata  (Figure 24.4u.) in a remarkable defense mechanism. This arthropod releases mandelonitrile from an inner storage gland into an outer chamber, where it is enzymatically broken down into benzaldehyde and hydrogen cyanide before being sprayed at an enemy.

Watch Aldehyde and Ketone Reactions – Hydrates, Acetals, & Imines: Crash Course Organic Chemistry #29 – YouTube (13 min). Note not all parts of the video are relevant to this text.

Video Source: Crash Course. (2021, June 9). Aldehyde and Ketone Reactions – Hydrates, Acetals, & Imines: Crash Course Organic Chemistry #29 – YouTube [Video]. YouTube.

Attribution & References

Except where otherwise noted, this page is adapted by Gregory A. Anderson and Samantha Sullivan Sauer from

  1. Only one potential answer is shown for each question.  There may be other possible answers. 4 ketone structures: a) a ketone substituted benzene;at the primary carbon chain b) a ketone substituted benzene at the secondary carbon chain; c) cyclohexanone; and d) 3-methylbutanone.

    ↵

    1. reduced (bond to oxygen atom replaced by bond to hydrogen atom);
    2. oxidized (one bond to hydrogen atom replaced by one bond to oxygen atom);
    3. oxidized (2 bonds to hydrogen atoms have been replaced by bonds to an oxygen atom)

    ↵

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Organic and Biochemistry Supplement to Enhanced Introductory College Chemistry Copyright © 2024 by Gregory Anderson; Caryn Fahey; Adrienne Richards; Samantha Sullivan Sauer; David Wegman; and Jen Booth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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