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

Aldehyde und Ketone haben eine ähnliche Struktur. Beide besitzen eine Carbonylgruppe, bei der es sich um einen Kohlenstoff handelt, der doppelt an einen Sauerstoff gebunden ist. Ein Aldehyd hat mindestens einen Wasserstoff, der mit dem Carbonylkohlenstoff verbunden ist. Die zweite Gruppe ist entweder eine Wasserstoff- oder eine kohlenstoffbasierte Gruppe. Im Gegensatz dazu hat ein Keton zwei kohlenstoffbasierte Gruppen, die mit dem Carbonylkohlenstoff verbunden sind. Bestimmte Reaktionen, die Aldehyde und Ketone durchlaufen, können verwendet werden, um sie zu unterscheiden oder ihre funktionellen Gruppen auf der Grundlage sichtbarer Unterschiede im Ergebnis der Reaktion zu identifizieren.
Eine solche Reaktion ist der DNPH-Test, mit dem festgestellt wird, ob ein Aldehyd oder Keton aromatisch ist. Bei dieser Reaktion greift 2,4-Dinitrophenylhydrazin oder DNPH das Carbonyl eines Aldehyds oder Ketons in einer wässrigen sauren Lösung an. Bei dieser Kondensationsreaktion entsteht ein Hydrazon, das aus der wässrigen Lösung ausfällt.
Wenn ein nicht-aromatisches Keton oder Aldehyd mit DNPH reagiert, ist der Niederschlag gelb. Aromatische Ketone und Aldehyde ergeben jedoch einen rot-orangefarbenen Niederschlag. DNPH kann auch zur Unterscheidung von Alkoholen und Estern von Aldehyden und Ketonen verwendet werden, da DNPH nicht mit Alkoholen oder Estern reagiert. Wir nennen diese fehlende Reaktion ein negatives Ergebnis.
Eine weitere nützliche Reaktion ist der Jodoform-Test für Methylketone, bei denen es sich um Ketone handelt, die mindestens ein Methyl als funktionelle Gruppe haben. Wenn ein Methylketon unter wässrigen alkalischen Bedingungen mit Jod gemischt wird, ersetzt Jod jeden Methylwasserstoff, wodurch eine ausgezeichnete Abgangsgruppe entsteht. Die Substitution, gefolgt von einem Protonentransfer, wandelt die Abgangsgruppe in Jodoform um, das aus der Lösung als blassgelber Feststoff ausfällt.
Diese Reaktion funktioniert auch bei Acetaldehyd, einem Aldehyd mit Methyl als R-Gruppe. Die Reaktion beruht auf der einzigartigen Reaktivität der Wasserstoffatome in einer Alpha-Methylgruppe, so dass das Mischen von anderem Keton oder Aldehyd mit Jod kein festes gelbes Jodform ergibt.
Schließlich können wir mit dem Tollens-Test zwischen Ketonen und Aldehyden unterscheiden. Diamminesilver(1+) oder Tollens-Reagenz oxidiert Aldehyde zu Carbonsäuren. Das Tollens-Reagenz wird dabei zu elementarem Silber reduziert, das entweder die Innenwand des Reagenzglases bedeckt oder einen schwarzen Niederschlag bildet. Die meisten Ketone werden jedoch nicht durch das Tollens-Reagenz oxidiert, so dass sich kein festes Silber bildet.
In diesem Labor identifizieren Sie mehrere unbekannte Aldehyde und Ketone, indem Sie den DNPH-Test, den Tollens-Test und den Jodoform-Test durchführen.

identifizierung unbekannter aldehyde und ketone

<h4>Aldehyde und Ketone</h4>
Aldehyde und Ketone haben eine Carbonylgruppe (C=O) als funktionelle Gruppe. Ein Keton hat zwei Alkyl- oder Arylgruppen, die an den Carbonylkohlenstoff (RCOR') gebunden sind. Das einfachste Keton ist Aceton, das zwei Methylgruppen hat, die an den Carbonylkohlenstoff (CH3, COCH3) gebunden sind.
Ein Aldehyd ähnelt einem Keton, nur dass sie anstelle von zwei Seitengruppen, die mit dem Carbonylkohlenstoff verbunden sind, mindestens einen Wasserstoff (RCOH) enthalten. Der einfachste Aldehyd ist Formaldehyd (HCOH), da er zwei Wasserstoffatome hat, die mit der Carbonylgruppe verbunden sind. Alle anderen Aldehyde haben einen Wasserstoff, der an die Carbonylgruppe gebunden ist, wie das einfache Molekül Acetaldehyd, das eine Wasserstoff- und eine Methylgruppe (HCOCH3) hat.
Der Carbonylkohlenstoff in Aldehyden und Ketonen ist elektrophil, was bedeutet, dass er aufgrund der Elektronegativität des angehängten Sauerstoffatoms einen Dipol hat. Dies macht den Carbonylkohlenstoff zu einem idealen Ziel für Nukleophile in einer nukleophilen Additionsreaktion. Während dieser Reaktion greift das Nukleophil oder der Elektronendonor das Carbonyl an, um das tetraedrische Zwischenprodukt zu bilden. Der negativ geladene Sauerstoff nimmt ein Wasserstoffion auf, um eine Hydroxylgruppe zu bilden.
Typischerweise besitzen Nukleophile eine negative Ladung oder ein einsames Paar auf einem Heteroatom, das verschiedene Formen annehmen kann (OH-, RO-, CN-,R3C-, RNH2, ROH). Bei primären Aminen (RNH2) hört die Reaktion nicht bei der Bildung des tetraedrischen Zwischenprodukts mit einer Hydroxylgruppe auf. Vielmehr kommt es zu einer Eliminationsreaktion, bei der eine doppelt gebundene funktionelle Gruppe aus Kohlenstoff und Stickstoff entsteht, die als Imin bekannt ist. Das Verständnis der Reaktionen, die Aldehyde und Ketone durchlaufen können, bietet eine Möglichkeit, zwischen diesen ähnlichen Arten organischer Verbindungen zu unterscheiden.
<h4>DNPH-Prüfung</h4>
Die Bildung von Iminen aus Ketonen oder Aldehyden wird durch die Verwendung des Reagenzes 2,4-Dinitrophenylhydrazin (DNPH) ausgenutzt. Bei dieser Additions-Eliminationsreaktion greift die primäre Aminogruppe des DNPH das Carbonyl eines Aldehyds oder Ketons in einem sauren Milieu an. Die Kondensationsreaktion führt zur Bildung eines Hydrazons, das aus der Lösung ausfällt.
Gelbe Ausfällungen weisen auf nicht konjugierte Ketone oder Aldehyde hin, während rot-orange Ausfällungen auf konjugierte Systeme hinweisen. Dieser Test dient der Unterscheidung von Ketonen und Aldehyden von Alkoholen und Estern, mit denen DNPH nicht reagiert und somit kein Niederschlag gebildet wird. Bei den Keton- oder Aldehydderivaten handelt es sich um kristalline Materialien mit sehr gut definierten Schmelzpunkten, die in der Literatur beschrieben werden und zur Identifizierung der spezifischen Verbindungen verwendet werden können.
<h4>Haloform-Test</h4>
Ein weiterer Test ist der Haloform-Test, mit dem festgestellt wird, ob es sich bei einem Keton um ein Methylketon handelt. Methylketone haben eine Methylgruppe, die mit dem Carbonylkohlenstoff (RCOCH3) verbunden ist. Die R-Gruppe kann entweder ein Wasserstoff (Methylaldehyd), eine Alkylgruppe oder eine Arylgruppe sein.
Der Reaktionsmechanismus für den Haloform-Test findet unter grundlegenden Bedingungen statt. Zunächst erfährt das Keton eine Keto-Enol-Tautomerisierung. Dann greift das nukleophile Enolat das Jod an, um ein Jodaron und ein halogeniertes Keton zu erhalten. Dies wiederholt sich insgesamt dreimal, bis alle Wasserstoffatome der Ketonmethylgruppe durch ein Jod ersetzt sind. Als nächstes reagiert das Hydroxid mit dem elektrophilen Carbonylkohlenstoffzentrum und bildet das tetraedrische Zwischenprodukt mit einem negativ geladenen Sauerstoff, der einfach an das Carbonyl gebunden ist. Bei der Neubildung der C-O-Doppelbindung verlässt die Trihalomethylgruppe als stabilisierte Abgangsgruppe. Schließlich wird die gebildete Carbonsäure durch die negativ geladene Trihalomethylgruppe deprotoniert, um das Haloform – Trihalomethan – und ein Carboxylat zu erhalten. Wenn Jod das Halogen ist, hat der Niederschlag eine charakteristische gelbe Farbe. Diese Reaktion tritt nur bei Methylketonen auf und führt zu einem negativen Ergebnis für alle anderen carbonylhaltigen Verbindungen.
<h4>Tollens-Test</h4>
Der Tollens-Test ist eine Reaktion, die zur Unterscheidung von Aldehyden von Ketonen verwendet wird, da Aldehyde zu einer Carbonsäure oxidiert werden können, Ketone jedoch nicht. Das Tollens-Reagenz, ein Gemisch aus Silbernitrat und Ammoniak, oxidiert den Aldehyd zu einer Carbonsäure. Das Silberion Ag+ wird zu festem Silber, Ag(s), reduziert. Das feste Silber bildet an der Innenwand des Reagenzglases einen Film, der einem silbernen Spiegel ähnelt. Ketone reagieren nicht mit dem Tollen-Reagenz und führen daher nicht zur Bildung eines Silberspiegels an den Wänden des Reagenzglases.
<h4>Referenzen</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Einführung in die Organische Chemie. Upper Saddle River, NJ: Prentice Hall.</li>
	<li style="font-size:17px">Fuson, R.C., Bull, B.A. (1934). Die Haloform-Reaktion. Chemische Bewertungen. 15(3), 275-309.</li>
</ol>

Identifizierung unbekannter Aldehyde und Ketone

Aldehyde und Ketone
Aldehyde und Ketone haben eine Carbonylgruppe (C=O) als funktionelle Gruppe. Ein Keton hat zwei Alkyl- oder Arylgruppen, die an die ...

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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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Clausius-Clapeyron Equation

The equilibrium between a liquid and its vapor depends on the temperature of the system; a rise in temperature causes a corresponding rise in the vapor ...

Phase Transitions: Melting and Freezing

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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