Aldehydes and Ketones

Carbonyl compounds are among the most important functional groups in organic chemistry. They contain the C=O (carbonyl) group and are divided into two main families: aldehydes and ketones. Understanding their chemistry is essential for Edexcel A-Level and underpins much of the organic synthesis you will encounter.

The Carbonyl Group

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom (C=O). The key difference between aldehydes and ketones lies in what is attached to the carbonyl carbon:

In aldehydes, the carbonyl carbon is bonded to at least one hydrogen atom. The carbonyl group is always at the end of the carbon chain. The general formula is RCHO.
In ketones, the carbonyl carbon is bonded to two carbon-containing groups (alkyl or aryl). The carbonyl group is within the carbon chain. The general formula is RCOR'.

The C=O bond is polar because oxygen is more electronegative than carbon (electronegativity values: O = 3.4, C = 2.5). This means the carbon atom carries a partial positive charge (delta+) and the oxygen carries a partial negative charge (delta-). This polarity is the reason carbonyl compounds undergo nucleophilic addition reactions.

Why the Carbonyl is So Reactive

The delta+ character on the carbonyl carbon is greater than on the carbon in a C-O single bond (as in an alcohol) for two reasons:

The pi bond holds the electron density closer to the oxygen than a sigma bond alone would.
There is no adjacent atom with lone pairs to delocalise into the C=O bond (contrast this with carboxylic acids, where the -OH lone pairs partially delocalise into the C=O, stabilising it and reducing the delta+ character).

This combination makes the carbonyl carbon in aldehydes and ketones an excellent electrophilic site for nucleophilic attack.

Naming Aldehydes and Ketones

Aldehydes are named using the suffix -al. The longest chain must include the carbonyl carbon, and numbering always starts from the carbonyl end:

Methanal (HCHO) -- one carbon
Ethanal (CH3CHO) -- two carbons
Propanal (CH3CH2CHO) -- three carbons

Ketones are named using the suffix -one, with a number indicating the position of the carbonyl group:

Propanone (CH3COCH3) -- the simplest ketone
Butanone (CH3COCH2CH3) -- four carbons
Pentan-2-one and pentan-3-one -- positional isomers

Physical Properties

Carbonyl compounds cannot form hydrogen bonds with themselves (unlike alcohols), because they lack an O-H or N-H bond. However, they can form hydrogen bonds with water through the lone pairs on the oxygen atom. This means:

Small aldehydes and ketones are soluble in water
Boiling points are higher than alkanes of similar molecular mass (due to permanent dipole-dipole interactions) but lower than alcohols (which have stronger hydrogen bonding)
Solubility in water decreases as the hydrocarbon chain length increases

Compound	Type	Mr	Boiling Point (degrees C)	Soluble in Water?
Propanal	Aldehyde	58	49	Yes
Propanone	Ketone	58	56	Yes (miscible)
Propan-1-ol	Alcohol	60	97	Yes
Butane	Alkane	58	-1	No

The table confirms the pattern: alcohol > carbonyl > alkane for boiling points at similar molecular mass, and the small carbonyls are water-soluble.

Nucleophilic Addition Reactions

The delta+ carbon of the carbonyl group is susceptible to attack by nucleophiles -- species with a lone pair of electrons that they can donate. The general mechanism is nucleophilic addition:

The nucleophile attacks the delta+ carbonyl carbon, forming a new bond
The C=O pi bond breaks and both electrons move onto the oxygen, creating an alkoxide intermediate (with a negative charge on oxygen)
The alkoxide is protonated (gains H+) to form the final product

Why Aldehydes Are More Reactive Than Ketones Toward Nucleophiles

This is a commonly examined comparison. Two factors contribute:

Factor	Aldehyde	Ketone
Steric effect	One H and one R group -- less steric hindrance around the carbonyl carbon	Two R groups -- more steric hindrance blocks the approaching nucleophile
Electronic (inductive) effect	One alkyl group pushes electron density toward C=O (small reduction of delta+)	Two alkyl groups push more electron density toward C=O (greater reduction of delta+)

Both factors make the aldehyde carbonyl carbon more delta+ and more accessible, so nucleophiles attack it more readily.

Reaction with HCN (Hydrogen Cyanide)

When an aldehyde or ketone reacts with HCN (in the presence of a trace of base such as KCN), the cyanide ion (CN-) acts as the nucleophile:

Step-by-step mechanism:

The KCN provides CN- ions in solution. CN- is the active nucleophile (HCN itself is a poor nucleophile because the lone pair on carbon is tied up in the triple bond; the ion is far more reactive).
The lone pair on the carbon of CN- attacks the delta+ carbonyl carbon, forming a new C-C bond.
Simultaneously, the C=O pi bond breaks and both electrons move onto oxygen, forming an alkoxide intermediate (tetrahedral, with O carrying a negative charge).
The alkoxide is protonated by HCN (which acts as the acid), regenerating CN- and forming the hydroxynitrile (cyanohydrin) product.

This reaction is important because it extends the carbon chain by one carbon, which is useful in synthesis. The nitrile group can subsequently be hydrolysed to a carboxylic acid or reduced to an amine.

Worked example -- Ethanal + HCN:

Ethanal (CH3CHO, 2C) + HCN --> 2-hydroxypropanenitrile (CH3CH(OH)CN, 3C)
The product has one more carbon than the starting aldehyde.
Hydrolysis of the nitrile group gives 2-hydroxypropanoic acid (lactic acid).

Safety note: HCN is extremely toxic (lethal dose ~1 mg/kg body mass), so in practice the reaction is carried out using KCN in acidified solution rather than HCN gas directly.

Reduction with NaBH4 (Sodium Borohydride)

Sodium borohydride is a mild reducing agent that reduces carbonyl compounds to alcohols:

Aldehydes are reduced to primary alcohols
Ketones are reduced to secondary alcohols

Detailed mechanism:

The hydride ion (H-) from NaBH4 acts as the nucleophile. It donates its pair of electrons to the delta+ carbonyl carbon.
The C=O pi bond breaks -- both electrons move onto oxygen, forming an alkoxide ion.
The alkoxide is protonated by the aqueous solvent (water) to form the alcohol product.

NaBH4 is dissolved in water or aqueous ethanol as the solvent. It is selective -- it reduces C=O but does not reduce C=C double bonds. This selectivity makes it a valuable reagent in organic synthesis when you need to reduce a carbonyl while leaving a double bond intact.

Comparison of reducing agents:

Reagent	Solvent	Reduces Aldehydes?	Reduces Ketones?	Reduces Carboxylic Acids?	Reduces C=C?
NaBH4	Water / aqueous ethanol	Yes	Yes	No	No
LiAlH4	Dry ether	Yes	Yes	Yes	No
H2/Ni	Gas phase	Yes	Yes	No	Yes

Oxidation of Aldehydes

Aldehydes can be oxidised to carboxylic acids. This is a key difference from ketones, which resist oxidation under normal conditions. The common oxidising agent is acidified potassium dichromate (K2Cr2O7/H2SO4):

RCHO --> RCOOH

During this reaction, the orange dichromate solution turns green (Cr3+ ions). Ketones show no colour change because they cannot be further oxidised without breaking a C-C bond.

Why ketones resist oxidation: To oxidise a ketone, you would need to break a C-C bond (since the carbonyl carbon is bonded to two carbon groups, not to hydrogen). Breaking C-C bonds requires far more energy than the mild oxidising conditions provide.

Distinguishing Aldehydes from Ketones

Because aldehydes are more easily oxidised than ketones, two classic tests exploit this difference:

Tollens' Reagent (Silver Mirror Test)

Tollens' reagent contains silver(I) ions in aqueous ammonia, [Ag(NH3)2]+. When warmed gently with an aldehyde:

The aldehyde is oxidised to a carboxylate ion
Ag+ ions are reduced to metallic silver, which deposits as a silver mirror on the inside of the test tube
Ketones give no reaction with Tollens' reagent

Ionic equation: RCHO + 2[Ag(NH3)2]+ + 2OH- --> RCOO- + 2Ag + 4NH3 + H2O

Fehling's Solution (Copper Test)

Fehling's solution contains Cu2+ ions complexed with tartrate, giving a deep blue colour. When warmed with an aldehyde:

The aldehyde is oxidised to a carboxylate ion
Cu2+ ions are reduced to Cu+, forming a brick-red precipitate of copper(I) oxide (Cu2O)
Ketones give no reaction with Fehling's solution

Both tests are positive for aldehydes and negative for ketones, providing a reliable way to distinguish between the two.

Diagnostic Decision Flowchart

flowchart TD
    A[Unknown Carbonyl Compound] --> B{Add 2,4-DNP reagent}
    B -->|Yellow/orange precipitate| C[Confirmed: Aldehyde or Ketone]
    B -->|No precipitate| D[Not an aldehyde or ketone]
    C --> E{Add Tollens' reagent, warm}
    E -->|Silver mirror forms| F[ALDEHYDE]
    E -->|No silver mirror| G[KETONE]
    F --> H{Oxidise with K2Cr2O7 / H2SO4 reflux}
    H --> I[Carboxylic acid product]
    G --> J{Reduce with NaBH4}
    J --> K[Secondary alcohol product]

Exam tip: 2,4-DNP (2,4-dinitrophenylhydrazine, also called Brady's reagent) is used to confirm the presence of a carbonyl group -- it produces a yellow or orange precipitate with both aldehydes and ketones. It does not distinguish between them. Follow up with Tollens' or Fehling's to identify which type of carbonyl is present.

Common Exam Mistakes

Confusing the mechanism name. The mechanism for HCN with a carbonyl is nucleophilic addition, not nucleophilic substitution. Nothing leaves -- the nucleophile simply adds to the molecule. Substitution requires a leaving group.
Drawing curly arrows from H of HCN. The nucleophile is CN-, not HCN. The curly arrow must start from the lone pair on the carbon of CN- and point to the delta+ carbonyl carbon.
Saying NaBH4 reduces everything. NaBH4 is a mild reducing agent. It reduces aldehydes and ketones but not carboxylic acids. LiAlH4 is needed for carboxylic acids.
Forgetting to state the solvent. NaBH4 works in water or aqueous ethanol. LiAlH4 must use dry ether (it reacts violently with water).
Confusing Tollens' observations. The silver mirror forms on the glass surface of the test tube, not in solution. If the test tube is not clean, you may get a grey precipitate instead of a mirror -- still a positive result.

Summary of Key Reactions

Reaction	Reagent / Conditions	Aldehyde Product	Ketone Product
Reduction	NaBH4 in water	Primary alcohol	Secondary alcohol
Oxidation	K2Cr2O7/H2SO4, reflux	Carboxylic acid	No reaction
HCN addition	HCN + KCN catalyst	Hydroxynitrile (chain +1C)	Hydroxynitrile (chain +1C)
Tollens' test	[Ag(NH3)2]+, warm	Silver mirror (positive)	No reaction (negative)
Fehling's test	Cu2+/tartrate, warm	Brick-red precipitate	No reaction (negative)
2,4-DNP test	2,4-dinitrophenylhydrazine	Yellow/orange ppt	Yellow/orange ppt

Understanding carbonyl chemistry is fundamental to the rest of organic chemistry at A-Level. The nucleophilic addition mechanism recurs throughout the specification, and the ability to distinguish aldehydes from ketones using chemical tests is a common exam question.