Name: carbonyl tests for aldehydes and ketones
Uploaded: 2020-03-26T07:22:29.000Z
Duration: 3 min 9 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.8

SC.912.P.8.13

identification-of-unknown-aldehydes-and-ketones

Identification of Unknown Aldehydes and Ketones

Les aldéhydes et les cétones ont une structure similaire. Les deux possèdent un groupe carbonyle, qui est un carbone doublement lié à un oxygène. Un aldéhyde a au moins un hydrogène connecté au carbone carbonyle. Le deuxième groupe est soit un groupe à base d’hydrogène, soit un groupe à base de carbone. En revanche, une cétone a deux groupes à base de carbone liés au carbone carbonyle. Certaines réactions subies par les aldéhydes et les cétones peuvent être utilisées pour les distinguer ou identifier leurs groupes fonctionnels en fonction des différences visibles dans le résultat de la réaction.
L’une de ces réactions est le test DNPH, qui est utilisé pour déterminer si un aldéhyde ou une cétone est aromatique. Dans cette réaction, la 2,4-dinitrophénylhydrazine, ou DNPH, attaque le carbonyle d’un aldéhyde ou d’une cétone dans une solution acide aqueuse. Cette réaction de condensation produit une hydrazone, qui précipite à partir de la solution aqueuse.
Lorsqu’une cétone ou un aldéhyde non aromatique réagit avec le DNPH, le précipité est jaune. Cependant, les cétones et les aldéhydes aromatiques donnent un précipité rouge-orange. Le DNPH peut également être utilisé pour distinguer les alcools et les esters des aldéhydes et des cétones, car le DNPH ne réagit pas avec les alcools ou les esters. Nous appelons cette absence de réaction un résultat négatif.
Une autre réaction utile est utilisée comme test iodoforme pour les méthylcétones, qui sont des cétones qui ont au moins un méthyle comme groupe fonctionnel. Lorsqu’une méthylcétone est mélangée à de l’iode dans des conditions alcalines aqueuses, l’iode remplace chaque méthylhydrogène, créant ainsi un excellent groupe de départ. La substitution, suivie d’un transfert de protons, convertit le groupe partant en iodoforme, qui précipite de la solution sous la forme d’un solide jaune pâle.
Cette réaction fonctionne également pour l’acétaldéhyde, qui est un aldéhyde dont le groupe R est le méthyle. La réaction repose sur la réactivité unique des hydrogènes d’un groupe alpha-méthyle, de sorte que le mélange de toute autre cétone ou aldéhyde avec de l’iode ne donnera pas d’iodoforme jaune solide.
Enfin, on peut distinguer les cétones et les aldéhydes à l’aide du test de Tollens. Le diamminesilver(1+), ou réactif de Tollens, oxyde les aldéhydes en acides carboxyliques. Le réactif de Tollens est réduit en argent élémentaire dans le processus, qui recouvre la paroi interne du tube à essai ou forme un précipité noir. La plupart des cétones, cependant, ne sont pas oxydées par le réactif de Tollens, de sorte qu'aucun argent solide ne se forme.
Dans ce laboratoire, vous identifierez plusieurs aldéhydes et cétones inconnus en effectuant le test DNPH, le test Tollens et le test iodoforme.

identification d’aldéhydes et de cétones inconnus

<h4>Aldéhydes et cétones</h4>
Les aldéhydes et les cétones ont un groupe carbonyle (C=O) comme groupe fonctionnel. Une cétone a deux groupes alkyle ou aryle attachés au carbone carbonyle (RCOR). La cétone la plus simple est l’acétone, qui a deux groupes méthyle attachés au carbone carbonyle (CH3COCH3).
Un aldéhyde est similaire à une cétone, sauf qu’au lieu de deux groupes latéraux connectés au carbone carbonyle, ils ont au moins un hydrogène (RCOH). L’aldéhyde le plus simple est le formaldéhyde (HCOH), car il possède deux hydrogènes connectés au groupe carbonyle. Tous les autres aldéhydes ont une liaison hydrogène au groupe carbonyle, comme la molécule simple acétaldéhyde, qui a un groupe hydrogène et un groupe méthyle (HCOCH3).
Le carbone carbonyle dans les aldéhydes et les cétones est électrophile, ce qui signifie qu’il a un dipôle en raison de l’électronégativité de l’atome d’oxygène attaché. Cela fait du carbone carbonyle une cible idéale pour les nucléophiles dans une réaction d’addition nucléophile. Au cours de cette réaction, le nucléophile, ou donneur d’électrons, attaque le carbonyle pour former l’intermédiaire tétraédrique. L’oxygène chargé négativement accepte un ion hydrogène pour former un groupe hydroxyle.
Typiquement, les nucléophiles possèdent une charge négative ou un doublet non liant sur un hétéroatome, qui peut prendre plusieurs formes (OH-, RO-, CN-, R3C-, RNH2, ROH). Pour les amines primaires (RNH2), la réaction ne s’arrête pas à la formation de l’intermédiaire tétraédrique avec un groupe hydroxyle. Au lieu de cela, une réaction d’élimination se produit qui produit un groupe fonctionnel carbone et azote à double liaison connu sous le nom d’imine. Comprendre les réactions que les aldéhydes et les cétones peuvent subir permet de différencier ces types de composés organiques similaires.
<h4>Test DNPH</h4>
La formation d’imines à partir de cétones ou d’aldéhydes est exploitée par l’utilisation du réactif 2,4-dinitrophénylhydrazine (DNPH). Dans cette réaction d’addition-élimination, le groupe amino primaire du DNPH attaque le carbonyle d’un aldéhyde ou d’une cétone dans un environnement acide. La réaction de condensation entraîne la formation d’une hydrazone, qui précipite hors de la solution.
Les précipités jaunes indiquent des cétones ou des aldéhydes non conjugués, tandis que les précipités rouge-orange indiquent des systèmes conjugués. Ce test est utilisé pour différencier les cétones et les aldéhydes des alcools et des esters avec lesquels le DNPH ne réagit pas, et donc aucun précipité n’est formé. Les dérivés cétoniques ou aldéhydes sont des matériaux cristallins avec des points de fusion très bien définis, qui sont rapportés dans la littérature et peuvent être utilisés pour identifier les composés spécifiques.
<h4>Essai d’haloforme</h4>
Un autre test est le test de l’haloforme, qui est utilisé pour déterminer si une cétone est une méthylcétone. Les méthylcétones ont un groupe méthyle lié au carbone carbonyle (RCOCH3). Le groupe R peut être soit un groupe hydrogène (méthylaldéhyde), soit un groupe alkyle, soit un groupe aryle.
Le mécanisme de réaction pour l’essai de l’haloforme se produit dans des conditions de base. Tout d’abord, la cétone subit une tautomérisation céto-énol. Ensuite, l’énolate nucléophile attaque l’iode pour donner un ion iode et une cétone halogénée. Cela se répète trois fois au total jusqu’à ce que tous les hydrogènes du groupe méthyle cétonique soient remplacés par de l’iode. Ensuite, l’hydroxyde réagit avec le centre de carbone carbonyle électrophile pour former l’intermédiaire tétraédrique avec un oxygène chargé négativement, qui est lié simplement au carbonyle. Lors de la reformation de la double liaison C-O, le groupe trihalométhyle sort en tant que groupe de départ stabilisé. Enfin, l’acide carboxylique formé est déprotoné par le groupe trihalométhyle chargé négativement pour donner l’haloforme - le trihalométhane - et un carboxylate. Si l’iode est l’halogène, le précipité aura une couleur jaune caractéristique. Cette réaction ne se produira que pour les méthylcétones, et elle donnera un résultat négatif pour tout autre composé contenant du carbonyle.
<h4>Test de Tollens</h4>
Le test de Tollens est une réaction utilisée pour distinguer les aldéhydes des cétones, car les aldéhydes peuvent être oxydés en un acide carboxylique alors que les cétones ne le peuvent pas. Le réactif de Tollens, qui est un mélange de nitrate d’argent et d’ammoniac, oxyde l’aldéhyde en un acide carboxylique. L’ion argent Ag+ est réduit en argent massif, Ag(s). L’argent massif forme un film sur la paroi interne du tube à essai, ressemblant à un miroir argenté. Les cétones ne réagissent pas avec le réactif de Tollen et n’entraînent donc pas la formation d’un miroir argenté sur les parois du tube à essai.
<h4>références</h4>
<ol>
	<li style="font-size :17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction à la chimie organique. Upper Saddle River, NJ : Prentice Hall.</li>
	<li style="font-size :17px">Fuson, R.C., Bull, B.A. (1934). La réaction haloforme. Revues chimiques. 15(3), 275-309.</li>
</ol>

Identification d’aldéhydes et de cétones inconnus

Aldéhydes et cétones
Les aldéhydes et les cétones ont un groupe carbonyle (C=O) comme groupe fonctionnel. Une cétone a deux groupes alkyle ou aryle attachés au ...

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carbonyl tests for aldehydes and ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group.

The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom pulls electron density toward itself. This makes the carbonyl carbon a target for nucleophiles, or electron donors, in a nucleophilic addition reaction. In that reaction, the nucleophile attacks the carbonyl and forms a tetrahedral intermediate. The negatively charged oxygen then accepts a hydrogen ion to form a hydroxyl group. Common nucleophiles can include OH-, RO-, CN-, R3C-, RNH2, and ROH. With primary amines, the reaction continues past the intermediate and can form an imine, which has a carbon-nitrogen double bond.

These reaction patterns help chemists tell similar carbonyl compounds apart. One useful test is the 2,4-dinitrophenylhydrazine, or DNPH, test. In acidic solution, DNPH reacts with an aldehyde or ketone by an addition-elimination process. The result is a hydrazone precipitate. Yellow precipitates usually point to non-conjugated aldehydes or ketones, while red-orange precipitates suggest conjugated systems. Alcohols and esters do not react with DNPH, so no precipitate forms. The crystalline derivatives also have well-defined melting points that can help identify the specific compound.

Another test is the haloform test, which checks whether a ketone is a methyl ketone. A methyl ketone has a methyl group attached to the carbonyl carbon, written as RCOCH3. In basic conditions, the ketone first undergoes keto-enol tautomerization. Then the enolate reacts with iodine until the methyl hydrogens are replaced. After three substitutions, hydroxide reacts again, and the trihalomethyl group leaves. The reaction gives a haloform, which is a trihalomethane, along with a carboxylate. When iodine is used, the precipitate is yellow. This test is negative for carbonyl compounds that are not methyl ketones.

The Tollens test distinguishes aldehydes from ketones. Tollens reagent is a mixture of silver nitrate and ammonia. It oxidizes an aldehyde to a carboxylic acid, while silver ion, Ag+, is reduced to solid silver, Ag(s). The silver coats the inside of the test tube and forms a silver mirror. Ketones do not react with Tollens reagent, so they do not produce this mirror.

Carbonyl Tests for Aldehydes and Ketones

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Learn how DNPH, haloform, and Tollens tests help identify aldehydes and ketones by spotting precipitates, methyl ketones, and the silver mirror.

Carbonyl Tests for Aldehydes and Ketones | JoVE K12

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Heating a crystalline solid increases the average energy of its atoms, molecules, or ions, and the solid gets hotter. At some point, the added energy ...

phase-transitions-sublimation-and-deposition

Phase Transitions: Sublimation and Deposition

Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation. At room temperature and ...

Heating and Cooling Curves

When a substance&#8212;isolated from its environment&#8212;is subjected to heat changes, corresponding changes in temperature and phase of the substance ...

Phase Diagrams

A phase diagram combines plots of pressure versus temperature for the liquid-gas, solid-liquid, and solid-gas phase-transition equilibria of a substance. ...

SC.912.P.8.2

The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

The Wave Nature of Light

The nature of light has been a subject of inquiry since antiquity. In the seventeenth century, Isaac Newton performed experiments with lenses and prisms ...

The Electromagnetic Spectrum

The electromagnetic spectrum consists of all the types of electromagnetic radiation arranged according to their frequency and wavelength. Each of the ...

Interference and Diffraction

Interference is a characteristic phenomenon exhibited by waves. When two electromagnetic waves interact with their peaks and troughs coinciding, a ...

Photoelectric Effect

When light of a particular wavelength strikes a metal surface, electrons are emitted. This is called the photoelectric effect. The minimum frequency of ...

The Bohr Model

Following the work of Ernest Rutherford and his colleagues in the early twentieth century, the picture of atoms consisting of tiny dense nuclei surrounded ...

Emission Spectra

When solids, liquids, or condensed gases are heated sufficiently, they radiate some of the excess energy as light. Photons produced in this manner have a ...

The de Broglie Wavelength

In the macroscopic world, objects that are large enough to be seen by the naked eye follow the rules of classical physics. A billiard ball moving on a ...

The Uncertainty Principle

Werner Heisenberg considered the limits of how accurately one can measure properties of an electron or other microscopic particles. He determined that ...

The Quantum-Mechanical Model of an Atom

Shortly after de Broglie published his ideas that the electron in a hydrogen atom could be better thought of as being a circular standing wave instead of ...

Explore the scientific theory of atoms (also known as atomic theory) by describing changes in the atomic model over time and why those changes were necessitated by experimental evidence.

SC.912.P.8.3

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Ions and Ionic Charges

In ordinary chemical reactions, the nucleus &mdash; which contains the protons and neutrons of each atom and thus identifies the element &mdash; remains ...

The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

Atoms &mdash; and the protons, neutrons, and electrons that compose them &mdash; are extremely small. For example, a carbon atom weighs less than 2 ...

Atomic Number and Mass Number

The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element. Its value determines the identity of ...

Explore the scientific theory of atoms (also known as atomic theory) by describing the structure of atoms in terms of protons, neutrons and electrons, and differentiate among these particles in terms of their mass, electrical charges and locations within the atom.

SC.912.P.8.4

Periodic Classification of the Elements

The periodic table arranges atoms based on increasing atomic number so that elements with the same chemical properties recur periodically. When their ...

Relate properties of atoms and their position in the periodic table to the arrangement of their electrons.

SC.912.P.8.5

A. A working definition of matter is that it takes up space, has mass, and has measurable properties. Matter is comprised of atomic, subatomic, and elementary particles.

B. Electrons are key to defining chemical and some physical properties, reactivity, and molecular structures. Repeating (periodic) patterns of physical and chemical properties occur among elements that define groups of elements with similar properties. The periodic table displays the repeating patterns, which are related to the atom's outermost electrons. Atoms bond with each other to form compounds.

C. In a chemical reaction, one or more reactants are transformed into one or more new products. Many factors shape the nature of products and the rates of reaction. 

D. Carbon-based compounds are building-blocks of known life forms on earth and numerous useful natural and synthetic products.

Carbonyl Tests for Aldehydes and Ketones

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