Aldehydes and ketones - Chemistry (9701) - Cambridge A-Level

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🔬 Chemistry 9701 Study Notes: Aldehydes and Ketones (Carbonyl Compounds)

👋 Introduction: The Carbonyl Crew!

Welcome to the exciting world of Carbonyl Compounds! This chapter focuses specifically on Aldehydes and Ketones—two crucial families in organic chemistry. These molecules are everywhere, from the sweet smell of almonds to the compounds used to remove nail polish.

Understanding this section is vital because it introduces the key reaction type for this functional group: Nucleophilic Addition. Don't worry if the reaction mechanisms look complicated at first; we will break them down step-by-step!

1. Defining Aldehydes and Ketones

Both aldehydes and ketones contain the same core functional group: the carbonyl group, \( \mathbf{C=O} \).

1.1 The Carbonyl Functional Group (\( \mathbf{C=O} \))

The carbonyl group consists of a carbon atom doubly bonded to an oxygen atom.
The carbon atom in the carbonyl group is \( \mathbf{sp^2} \)-hybridised, meaning the bond angle around the carbon is approximately \( \mathbf{120^\circ} \) and the shape is trigonal planar.
This planar geometry is important because it allows nucleophiles (electron-rich species) to easily attack the carbon atom from above or below the plane.

1.2 Distinguishing Aldehydes and Ketones

The difference between these two classes lies in what is attached to the central carbonyl carbon:

(A) Aldehydes

An aldehyde has at least one hydrogen atom bonded directly to the carbonyl carbon.

Functional Group: \(\mathbf{RCHO}\)
The simplest aldehyde is methanal (\(\text{HCHO}\)), where R is a hydrogen atom.
Naming: The names end in -al (e.g., ethanal, propanal).

(B) Ketones

A ketone has two alkyl groups (R and R') bonded to the carbonyl carbon.

Functional Group: \(\mathbf{RCOR'}\)
Naming: The names end in -one (e.g., propanone, butanone).

💡 Memory Aid: Think of a KETONE as being "sandwiched" by two bulky R-groups, while an ALDEHYDE has a smaller, lighter H-atom on one side.

Key Takeaway: The Carbonyl group is planar (\(120^\circ\)) and the crucial difference is whether the carbonyl carbon is bonded to H (Aldehyde, -al) or two R groups (Ketone, -one).

2. Preparation of Aldehydes and Ketones

Aldehydes and ketones are typically made by the oxidation of alcohols. The type of product depends entirely on the class of alcohol used (primary, secondary, or tertiary) and the reaction conditions.

2.1 Synthesis via Oxidation of Alcohols

We use acidified potassium dichromate(VI) (\(\text{K}_2\text{Cr}_2\text{O}_7\)) or acidified potassium manganate(VII) (\(\text{KMnO}_4\)) as strong oxidising agents.

(A) Making Aldehydes (from Primary Alcohols)

Oxidation of a Primary Alcohol (\(\text{RCH}_2\text{OH}\)) yields an aldehyde. If the oxidation is allowed to continue, the aldehyde will be further oxidised to a carboxylic acid.

The Challenge: We must stop the reaction at the aldehyde stage.
Conditions: Use acidified \(\text{K}_2\text{Cr}_2\text{O}_7\) or \(\text{KMnO}_4\) and use distillation.
Why Distillation? The aldehyde product has a lower boiling point than the starting alcohol and the final carboxylic acid. By using distillation, the aldehyde is removed from the reaction mixture as soon as it forms, preventing further oxidation.

Reaction: \(\text{Primary Alcohol} + [\text{O}] \xrightarrow{\text{Distillation}} \text{Aldehyde}\)

(B) Making Ketones (from Secondary Alcohols)

Oxidation of a Secondary Alcohol (\(\text{RCH}(\text{OH})\text{R'}\)) yields a ketone. Ketones are much harder to oxidise than aldehydes, so controlling the reaction is simpler.

The Outcome: Ketones cannot be easily oxidised further under these conditions.
Conditions: Use acidified \(\text{K}_2\text{Cr}_2\text{O}_7\) or \(\text{KMnO}_4\) and heat (usually using distillation, or sometimes reflux, as the ketone is stable).

Reaction: \(\text{Secondary Alcohol} + [\text{O}] \xrightarrow{\text{Heat}} \text{Ketone}\)

🛑 Common Mistake Alert!

If you see "Reflux" used with a primary alcohol and an oxidising agent, the product will be the Carboxylic Acid, not the aldehyde. The keyword for making an aldehyde is Distillation.

Key Takeaway: Aldehydes require distillation of primary alcohols; ketones are made by oxidising secondary alcohols (and are stable to further oxidation).

3. Reactions: Nucleophilic Addition

3.1 Understanding the Carbonyl Group Polarity

The central feature of carbonyl chemistry is the high reactivity of the \(\text{C=O}\) double bond.

Oxygen is far more electronegative than carbon. This causes the electron pair in the \(\text{C=O}\) bond to be pulled strongly towards the oxygen atom.

Oxygen gains a slight negative charge (\(\mathbf{\delta^-}\)).
The carbonyl carbon gains a substantial slight positive charge (\(\mathbf{\delta^+}\)).

Because the carbon atom is electron-deficient (\(\delta^+\)), it is extremely vulnerable to attack by nucleophiles (species that are electron-rich and attracted to positive centres).

The reaction involves breaking the \( \mathbf{\pi} \) bond and forming two new single bonds, leading to an overall Nucleophilic Addition reaction.

3.2 Reduction to Alcohols

Aldehydes and ketones can be reduced back to their respective alcohols. This is the reverse of their preparation.

Reduction is the addition of hydrogen (\([\text{H}]\)).

Reagents: Strong reducing agents such as sodium tetrahydridoborate (\(\mathbf{\text{NaBH}_4}\)) or lithium tetrahydridoaluminate (\(\mathbf{\text{LiAlH}_4}\)).
Product of Aldehyde Reduction: Primary alcohol.
Product of Ketone Reduction: Secondary alcohol.

Example (Ethanal reduction):
\(\text{CH}_3\text{CHO} + 2[\text{H}] \xrightarrow{\text{NaBH}_4} \text{CH}_3\text{CH}_2\text{OH}\) (Ethanol, a primary alcohol)

Example (Propanone reduction):
\(\text{CH}_3\text{COCH}_3 + 2[\text{H}] \xrightarrow{\text{NaBH}_4} \text{CH}_3\text{CH}(\text{OH})\text{CH}_3\) (Propan-2-ol, a secondary alcohol)

3.3 Reaction with Hydrogen Cyanide (HCN)

This reaction is a key nucleophilic addition that increases the carbon chain length.

Reagents: Hydrogen cyanide (\(\text{HCN}\)), using potassium cyanide (\(\mathbf{\text{KCN}}\)) as a catalyst, and heat.
Product: A hydroxynitrile (contains both a hydroxyl \((-\text{OH})\) group and a nitrile \((-\text{C}\equiv \text{N})\) group).

Example (Ethanal forming 2-hydroxypropanenitrile):
\(\text{CH}_3\text{CHO} + \text{HCN} \xrightarrow{\text{KCN, heat}} \text{CH}_3\text{CH}(\text{OH})\text{CN}\)

Example (Propanone forming 2-hydroxy-2-methylpropanenitrile):
\(\text{CH}_3\text{COCH}_3 + \text{HCN} \xrightarrow{\text{KCN, heat}} \text{CH}_3\text{C}(\text{OH})(\text{CN})\text{CH}_3\)

🌟 Did You Know? Optical Isomerism

When an aldehyde (except methanal) reacts with \(\text{HCN}\), the resulting hydroxynitrile usually contains a chiral centre (a carbon atom bonded to four different groups). Since the nucleophile (\(\text{CN}^-\)) can attack from either side of the planar carbonyl group, the product is a racemic mixture (equal amounts of both optical isomers).

3.4 The Mechanism of Nucleophilic Addition (HCN)

This reaction proceeds via a Nucleophilic Addition mechanism. The actual nucleophile is the cyanide ion, \(\mathbf{\text{CN}^-}\), provided by the catalyst, \(\text{KCN}\).

Step 1: Nucleophilic Attack

The nucleophile, the cyanide ion (\(\text{CN}^-\)), is attracted to the electron-deficient carbonyl carbon (\(\delta^+\)). A curly arrow shows the lone pair on the carbon of \(\text{CN}^-\) attacking the carbonyl carbon. Simultaneously, the \( \pi \) electrons in the \(\text{C=O}\) bond move entirely onto the oxygen atom.
This forms an intermediate ion called an alkoxide ion.

Step 2: Protonation

The negatively charged oxygen atom in the alkoxide ion rapidly picks up a proton (\(\text{H}^+\)) from the solution (or from an \(\text{HCN}\) molecule), forming the neutral hydroxynitrile product.
This step regenerates the \(\text{CN}^-\) catalyst if the proton came from \(\text{HCN}\).

(Remember: The syllabus requires you to use curly arrows to represent the movement of electron pairs in the mechanism.)

Key Takeaway: Carbonyl compounds undergo nucleophilic addition due to the \(\delta^+\) carbon. They are reduced to alcohols by \(\text{NaBH}_4\) or reacted with \(\text{HCN}/\text{KCN}\) to form hydroxynitriles via a two-step nucleophilic addition mechanism.

4. Analysis and Identification Tests

Since aldehydes and ketones have similar structures, specific chemical tests are needed to: 1) prove a carbonyl group is present, and 2) determine if it is an aldehyde or a ketone.

4.1 Test 1: Detecting the Carbonyl Group (\(\mathbf{C=O}\))

This test confirms the presence of *any* aldehyde or ketone.

Reagent: 2,4-dinitrophenylhydrazine (2,4-DNPH) reagent (also known as Brady's reagent).
Observation: The formation of a bright yellow, orange, or red crystalline precipitate.
Conclusion: A carbonyl compound (aldehyde or ketone) is present.

Why is this useful?

This test is often used as the first step in organic analysis. The crystalline product formed (a 2,4-dinitrophenylhydrazone) can be purified and its melting point measured. Comparing the melting point to literature values helps identify the original aldehyde or ketone.

4.2 Test 2: Distinguishing Aldehydes from Ketones (Ease of Oxidation)

Aldehydes are easily oxidised to carboxylic acids, while ketones resist mild oxidation. We exploit this difference using mild oxidising agents, known as Tollens' and Fehling's reagents.

Both reagents use metal ions that are reduced by the aldehyde, causing a visible colour change. The aldehyde itself is oxidised (loss of [H]) to form the carboxylate ion, \(\text{RCO}_2^-\).

(A) Tollens' Reagent (Silver Mirror Test)

Reagent: Aqueous ammoniacal silver nitrate solution (contains \([\text{Ag}(\text{NH}_3)_2]^+\) ions).
Aldehyde Result: A silver mirror forms on the inner surface of the test tube (due to the reduction of \(\text{Ag}^+\) to \(\text{Ag}\)).
Ketone Result: No reaction (solution remains clear).

(B) Fehling's or Benedict's Reagent

Reagent: Alkaline solution containing copper(II) ions (\(\text{Cu}^{2+}\)).
Aldehyde Result: The blue solution turns into a brick-red precipitate (due to the reduction of \(\text{Cu}^{2+}\) to \(\text{Cu}_2\text{O}\)).
Ketone Result: No reaction (solution remains blue).

Compound Type	Tollens' Reagent	Fehling's Reagent
Aldehyde (\(\text{RCHO}\))	Silver mirror formed (Ag is reduced)	Brick-red precipitate formed (\(\text{Cu}_2\text{O}\))
Ketone (\(\text{RCOR'}\))	No reaction	No reaction

4.3 Test 3: The Tri-iodomethane (Iodoform) Test

This test is highly specific. It does not test for a carbonyl group in general, but for the presence of a methyl carbonyl group (\(\mathbf{\text{CH}_3\text{CO}-}\)) or the \(\mathbf{\text{CH}_3\text{CH}(\text{OH})-}\) group (found in secondary alcohols like ethanol, which are first oxidised to the methyl carbonyl).

Reagents: Alkaline aqueous iodine (\(\text{I}_2(\text{aq})\) in \(\text{NaOH}(\text{aq})\)).
Observation: Formation of a distinctive pale yellow precipitate of tri-iodomethane (\(\mathbf{\text{CHI}_3}\)).

If a compound contains the \(\text{CH}_3\text{CO}-\) group, such as ethanal (\(\text{CH}_3\text{CHO}\)) or propanone (\(\text{CH}_3\text{COCH}_3\)), it will give a positive result.

The reaction involves the substitution of the three alpha-hydrogens on the methyl group by iodine, followed by cleavage of the resulting bond, yielding the tri-iodomethane precipitate (\(\text{CHI}_3\)) and a carboxylate ion (\(\text{RCO}_2^-\)).

Key Takeaway: Use 2,4-DNPH to confirm a carbonyl group. Use Tollens' or Fehling's to distinguish aldehydes from ketones. Use the Iodoform test to confirm the specific \(\text{CH}_3\text{CO}-\) group.

🎉 Congratulations! You have covered all the required content on aldehydes and ketones for the 9701 syllabus! Keep practising those mechanisms and distinguishing tests!