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4.6Unit 4. Rates, Equilibria and Further Organic Chemistry

Organic Chemistry: Carbonyls — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 11 parts Lesson 6 of 56 1 quiz

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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Lesson summary

4.6 Organic Chemistry: Carbonyls — structure, tests, reduction and nucleophilic addition — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Name aldehydes and ketones, explain their physical properties, distinguish them with oxidation tests, and describe reduction, HCN addition, 2,4-DNPH an…

  • Tollens' reagent
  • Fehling's solution
  • Iodoform test
  • Carbonyl group
  • Hydroxynitrile
  • Intermolecular forces

Part 1 of 9Aldehydes and ketones

Aldehydes and ketones contain the carbonyl group, C=O.
Aldehyde
  • C=O at the end of the chain, RCHO; the carbonyl carbon is always carbon 1, so no locant is used.
  • The simplest aldehyde is methanal, HCHO.
  • Ethanal is CH3CHO.
Ketone
  • C=O within the chain, RCOR'; it needs an alkyl group on each side.
  • The simplest ketone is propanone, CH3COCH3.
Carbonyls are made by oxidising alcohols: primary alcohols give aldehydes (distilled off) and secondary alcohols give ketones.

Part 2 of 9Physical properties

Physical properties
The C=O bond is polar (Cδ+, Oδ−), so molecules have permanent dipole–dipole forces as well as London forces.
There is no O-H or N-H, so carbonyls cannot hydrogen bond with each other.
  • Boiling points are higher than alkanes but lower than the corresponding alcohols.
Small carbonyls dissolve in water.
  • The lone pairs on oxygen form hydrogen bonds with δ+ hydrogen in water.
  • Larger molecules have long hydrocarbon chains that disrupt water's hydrogen bonding without forming bonds of their own, so they do not dissolve.

Part 3 of 9Oxidation of carbonyls

Aldehydes are oxidised to carboxylic acids; ketones resist oxidation because they have no readily available H atom.
Acidified K2Cr2O7
  • Aldehydeorange to green. Ketone: no change.
Tollens' reagent
  • Tollens' reagent ammoniacal silver nitrate, containing [Ag(NH3)2]+; warm gently with the sample.
  • An aldehyde gives a silver mirror as Ag+ is reduced to Ag.
  • A ketone gives no reaction.
Fehling's solution
  • Cu2+ in NaOH; Benedict's solution has Cu2+ in sodium carbonate.
  • An aldehyde turns blue to a brick-red precipitate of copper(I) oxide, Cu2O.
  • A ketone stays blue.
Examples: CH3CHO + [O] → CH3COOH.

Part 4 of 9Reduction of carbonyls

Reduction of carbonyls
LiAlH4 in dry ether supplies hydride ions, H−, a nucleophile.
Aldehydes are reduced to primary alcohols and ketones to secondary alcohols.
This is nucleophilic addition: CH3COCH3 + 2[H] → CH3CH(OH)CH3.

Part 5 of 9Nucleophilic addition

  • Oxygen is more electronegative than carbon, so the carbonyl carbon is δ+ and attacked by nucleophiles.
1
The nucleophile attacks the δ+ carbon.
2
The C=O π bond breaks and the electrons move onto oxygen.
3
A negatively charged intermediate forms.
4
The O− takes H+ (from HCN, water or acid) to give the product.

Part 6 of 9Addition of HCN

1
CN− adds to the C=O, then the alkoxide is protonated, forming a hydroxynitrile such as 2-hydroxypropanenitrile.
2
The chain is lengthened by one carbon atom, which is useful in synthesis.
3
The nitrile is the priority group and carbon 1; the OH is named with the prefix hydroxy-.
4
HCN is toxic, so KCN with dilute acid is used in a fume cupboard.
5
The planar carbonyl is attacked equally from either face, so a chiral product is a racemic mixture.

Part 7 of 92,4-DNPH and the iodoform test

2,4-dinitrophenylhydrazine (2,4-DNPH) detects the C=O group in aldehydes and ketones.
  • It is a condensation reaction that eliminates a small molecule.
  • It gives a deep orange precipitate that can be recrystallised.
  • Its melting point is compared with literature values to identify the specific carbonyl.
  • Carboxylic acids and esters do not react.
Iodoform test
  • Iodoform test warm with alkaline iodine.
  • A yellow precipitate of CHI3 forms with a CH3CO- group (methyl ketones and ethanal).
  • CH3COCH3 + 3I2 + 4OH− → CH3COO− + CHI3 + 3I− + 3H2O.
  • Alcohols with R-CH(OH)-CH3, such as ethanol and hexan-2-ol, also give it because they are first oxidised to carbonyls.
Iodination of propanone in acid replaces one hydrogen: CH3COCH3 + I2 → CH3COCH2I + H+ + I−.
  • The iodine colour fades, so the reaction is followed by colorimetry to find the orders.

Part 8 of 9Distinguishing aldehydes and ketones

TestAldehydeKetone
2,4-DNPHOrange precipitateOrange precipitate
Tollens' (warm)Silver mirrorNo change
Fehling's / Benedict's (warm)Blue to brick-red precipitateStays blue
Acidified K2Cr2O7Orange to greenStays orange
  • Tollens' reagent is the most commonly used test to identify an unknown.

Part 9 of 9More from the specification

More from the specification
Functional group
  • Functional group the C=O group in aldehydes and ketones is polar and attacked by nucleophiles
Acidified potassium dichromate
  • Acidified potassium dichromate warmed with an aldehyde it changes from orange to green, a positive result, whereas a ketone gives no change
Brady reagent
  • 2,4-dinitrophenylhydrazine gives an orange precipitate with carbonyl compounds, whose melting point identifies the compound
Reduction
  • Reduction sodium tetrahydridoborate reduces aldehydes to primary alcohols and ketones to secondary alcohols
Positive result
  • The observation that shows a functional group is present.
Compound
  • Two or more elements chemically combined in a fixed ratio.
Functional group
  • The atom or group of atoms that determines the reactions of a series.
Intermolecular forces
  • Weak attractions between separate molecules, overcome on melting or boiling.

Key terms

TermMeaning
Carbonyl groupThe C=O functional group in aldehydes and ketones.
AldehydeA carbonyl compound with C=O at the end of the chain, bonded to at least one H.
KetoneA carbonyl compound with C=O bonded to two carbon atoms.
Nucleophilic additionA reaction in which a nucleophile attacks the δ+ carbon and the C=O becomes a single bond.
Tollens' reagentAmmoniacal silver nitrate, reduced to a silver mirror by aldehydes.
Fehling's solutionAlkaline copper(II) solution reduced to red Cu2O by aldehydes.
HydroxynitrileA compound with OH and CN on adjacent carbons, formed by adding HCN to a carbonyl.
2,4-DNPHA reagent that gives an orange precipitate with aldehydes and ketones.

Avoid these mistakes

1
Saying carbonyls hydrogen bond with each other — they have no O-H, only dipole–dipole and London forces.
2
Saying 2,4-DNPH distinguishes aldehydes from ketones — both give an orange precipitate.
3
Saying ketones are oxidised by Tollens' — they give no reaction.
4
Using HCN without noting its toxicity — use KCN with dilute acid in a fume cupboard.
5
Saying the iodoform test only identifies ketones — ethanal and some alcohols also give it.

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