ChemistryChapter 19: Carbonyl Compounds
Tollens and Fehling (aldehyde Tests)
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
Learning objectives
- Explain tollens and fehling (aldehyde tests) in simple chemical language.
- Connect 1. tollens with 2. fehling using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
Important terms
- TOLLENS AND FEHLING (ALDEHYDE TESTS)
- Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
- 1. TOLLENS
- Clean test tube, warm.
- 2. FEHLING
- Blue to brick red.
Overview
Silver, then copper:
Tollens
Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link.
Examples Ethanal yes · Propanone no · Mirror named
Fehling
Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test.
Examples Brick red · Ketone no · Heat
WHAT — TOLLENS AND FEHLING (ALDEHYDE TESTS)
Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
WHY — Why this topic matters
TOLLENS AND FEHLING (ALDEHYDE TESTS) is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
TOLLENS: Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link. Examples: Ethanal yes; Propanone no; Mirror named. FEHLING: Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test. Examples: Brick red; Ketone no; Heat. Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
In simple words
ID tests. No silvering SOP, no clinical dosing.
Key points
- aldehyde: mirror or red ppt
- ketone: no
- Ag+ -> Ag
- Cu2+ -> Cu2O
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Tollens and Fehling (aldehyde Tests)
Sticky Notes · Quick Revision
Core Idea
- Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced). Ketones do not. Fehling / Benedict: Cu2+ blue to brick-red Cu2O with aldehydes (heat). Ketones no. Methanal and ethanal strong positives. Meaning: aldehydes oxidise easily; ketones do not under these mild tests. These are identification tests, not silvering-plant methods, not medical glucose protocols (clinic is different). Tollens silver mirror on aldehyde; Fehling blue Cu2+ to red Cu2O — ketone no change. Tollens distinguishes aldehyde from ketone; iodoform targets methyl carbonyl or ethanol specifically.
1. TOLLENS
- Clean test tube, warm. Mirror or black Ag. Ketone stays colourless. Reducing sugar biology link named, not a blood-sugar kit. Benedict similar to Fehling for reducing sugars link.
- Ethanal yes
- Propanone no
- Mirror named
2. FEHLING
- Blue to brick red. Same tell: aldehyde oxidised to acid (carboxylate in alkali). Heat required. Combine with iodoform to pin a structure. Oxidation of aldehyde to carboxylate drives test.
- Brick red
- Ketone no
- Heat
Exam Example
- Exam Tollens:
- aldehyde: silver mirror
- ketone: no mirror
- aldehyde is oxidised; Ag+ is reduced
Common Misconceptions
- Do not confuse 1. TOLLENS with 2. FEHLING; state the difference before giving an example.
- Writing only “aldehyde: mirror or red ppt” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain tollens and fehling (aldehyde tests), compare its two main parts, and include the correct chemical evidence.
- ID tests. No silvering SOP, no clinical dosing.
60-second revision
- Tollens: ammoniacal silver, aldehydes give a silver mirror (Ag+ reduced).
- 1. TOLLENS: Clean test tube, warm.
- 2. FEHLING: Blue to brick red.
- Exam reminder: Exam Tollens: aldehyde: silver mirror ketone: no mirror aldehyde is oxidised; Ag+ is reduced
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