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

Aldeidi e chetoni hanno una struttura simile. Entrambi possiedono un gruppo carbonilico, che è un doppio legame di carbonio con un ossigeno. Un'aldeide ha almeno un idrogeno collegato al carbonio carbonilico. Il secondo gruppo è un gruppo a base di idrogeno o carbonio. Al contrario, un chetone ha due gruppi a base di carbonio collegati al carbonio carbonilico. Alcune reazioni subite da aldeidi e chetoni possono essere utilizzate per distinguerli o identificare i loro gruppi funzionali in base a differenze visibili nell'esito della reazione.
Una di queste reazioni è il test DNPH, che viene utilizzato per determinare se un'aldeide o un chetone è aromatico. In questa reazione, la 2,4-dinitrofenilidrazina, o DNPH, attacca il carbonile di un'aldeide o di un chetone in una soluzione acida acquosa. Questa reazione di condensazione produce un idrazone, che precipita dalla soluzione acquosa.
Quando un chetone o un'aldeide non aromatici reagisce con DNPH, il precipitato è giallo. Tuttavia, i chetoni aromatici e le aldeidi danno un precipitato rosso-arancio. Il DNPH può essere utilizzato anche per distinguere alcoli ed esteri da aldeidi e chetoni poiché il DNPH non reagisce con alcoli o esteri. Chiamiamo questa mancanza di reazione un risultato negativo.
Un'altra reazione utile viene utilizzata come test dello iodoformio per i metilchetoni, che sono chetoni che hanno almeno un metile come gruppo funzionale. Quando un metilchetone viene miscelato con iodio in condizioni acquose alcaline, lo iodio sostituisce ogni metilidrogeno, creando un eccellente gruppo uscente. La sostituzione, seguita dal trasferimento di protoni, converte il gruppo uscente in iodoformio, che precipita dalla soluzione come un solido giallo pallido.
Questa reazione funziona anche per l'acetaldeide, che è un'aldeide con metile come gruppo R. La reazione si basa sulla reattività unica degli idrogeni in un gruppo alfa-metilico, quindi mescolando qualsiasi altro chetone o aldeide con lo iodio non si otterrà lo iodoformio giallo solido.
Infine, possiamo distinguere tra chetoni e aldeidi utilizzando il test di Tollens. Il diamminargento(1+), o reagente di Tollens, ossida le aldeidi in acidi carbossilici. Il reagente di Tollens viene ridotto ad argento elementare nel processo, che ricopre la parete interna della provetta o forma un precipitato nero. La maggior parte dei chetoni, tuttavia, non viene ossidata dal reagente di Tollens, quindi non si formerà argento solido.
In questo laboratorio, identificherai diverse aldeidi e chetoni sconosciuti eseguendo il test DNPH, il test di Tollens e il test dello iodoformio.

identificazione di aldeidi e chetoni sconosciuti

<h4>Aldeidi e chetoni</h4>
Le aldeidi e i chetoni hanno un gruppo carbonilico (C=O) come gruppo funzionale. Un chetone ha due gruppi alchilici o arilici attaccati al carbonio carbonilico (RCOR'). Il chetone più semplice è l'acetone, che ha due gruppi metilici attaccati al carbonio carbonilico (CH3COCH3).
Un'aldeide è simile a un chetone, tranne per il fatto che invece di due gruppi laterali collegati al carbonio carbonilico, hanno almeno un idrogeno (RCOH). L'aldeide più semplice è la formaldeide (HCOH), in quanto ha due idrogeni collegati al gruppo carbonilico. Tutte le altre aldeidi hanno un idrogeno legato al gruppo carbonilico, come la semplice molecola acetaldeide, che ha un idrogeno e un gruppo metilico (HCOCH3).
Il carbonio carbonilico sia nelle aldeidi che nei chetoni è elettrofilo, il che significa che ha un dipolo a causa dell'elettronegatività dell'atomo di ossigeno attaccato. Questo rende il carbonio carbonilico un bersaglio ideale per i nucleofili in una reazione di addizione nucleofila. Durante questa reazione, il nucleofilo, o donatore di elettroni, attacca il carbonile per formare l'intermedio tetraedrico. L'ossigeno caricato negativamente accetta uno ione idrogeno per formare un gruppo ossidrile.
Tipicamente, i nucleofili possiedono una carica negativa o una coppia solitaria su un eteroatomo, che può assumere diverse forme (OH-, RO-, CN-, R3, C-, RNH2, ROH). Per le ammine primarie (RNH2), la reazione non si ferma alla formazione dell'intermedio tetraedrico con un gruppo ossidrile. Piuttosto, si verifica una reazione di eliminazione che produce un gruppo funzionale di carbonio e azoto a doppio legame noto come immina. Comprendere le reazioni che aldeidi e chetoni possono subire fornisce un modo per distinguere tra questi tipi di composti organici simili.
<h4>Test DNPH</h4>
La formazione di immine da chetoni o aldeidi viene sfruttata attraverso l'uso del reagente 2,4-dinitrofenilidrazina (DNPH). In questa reazione di addizione-eliminazione, il gruppo amminico primario del DNPH attacca il carbonile di un'aldeide o chetone in un ambiente acido. La reazione di condensazione provoca la formazione di un idrazone, che precipita fuori dalla soluzione.
I precipitati gialli indicano chetoni o aldeidi non coniugati, mentre i precipitati rosso-arancio indicano sistemi coniugati. Questo test viene utilizzato per differenziare chetoni e aldeidi da alcoli ed esteri con i quali DNPH non reagisce e quindi non si forma alcun precipitato. I derivati chetonici o aldeidici sono materiali cristallini con punti di fusione molto ben definiti, che sono riportati in letteratura e possono essere utilizzati per identificare i composti specifici.
<h4>Test dell'aloformio</h4>
Un altro test è il test dell'aloformio, che viene utilizzato per determinare se un chetone è un metilchetone. I metilchetoni hanno un gruppo metilico collegato al carbonio carbonilico (RCOCH3). Il gruppo R può essere un idrogeno (metilaldeide), un gruppo alchilico o un gruppo arilico.
Il meccanismo di reazione per il test dell'aloformio avviene in condizioni di base. In primo luogo, il chetone subisce una tautomerizzazione cheto-enolo. Quindi l'enolato nucleofilo attacca lo iodio per dare uno ione iodio e un chetone alogenato. Questo si ripete per un totale di tre volte fino a quando tutti gli idrogeni del gruppo metilico chetonico vengono sostituiti da uno iodio. Successivamente, l'idrossido reagisce con il centro elettrofilo del carbonio carbonilico per formare l'intermedio tetraedrico con un ossigeno caricato negativamente, che è legato singolarmente al carbonile. Dopo la riformazione del doppio legame C-O, il gruppo trialometilico esce come gruppo uscente stabilizzato. Infine, l'acido carbossilico formato viene deprotonato dal gruppo di trialometile caricato negativamente per dare l'aloformio — trialometano — e un carbossilato. Se lo iodio è l'alogeno, il precipitato avrà un caratteristico colore giallo. Questa reazione si verificherà solo per i metilchetoni e darà un risultato negativo per qualsiasi altro composto contenente carbonile.
<h4>Test di Tollens</h4>
Il test di Tollens è una reazione che viene utilizzata per distinguere le aldeidi dai chetoni, poiché le aldeidi possono essere ossidate in un acido carbossilico mentre i chetoni no. Il reagente di Tollens, che è una miscela di nitrato d'argento e ammoniaca, ossida l'aldeide in un acido carbossilico. Lo ione argento Ag+ viene ridotto ad argento massiccio, Ag(s). L'argento massiccio forma una pellicola sulla parete interna della provetta, simile a uno specchio d'argento. I chetoni non reagiscono con il reagente di Tollen e quindi non provocano la formazione di uno specchio d'argento sulle pareti della provetta.
<h4>referenze</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduzione alla Chimica Organica. Upper Saddle River, NJ: Prentice Hall.</li>
	<li style="font-size:17px">Fuson, R.C., Bull, B.A. (1934). La reazione dell'aloformio. Recensioni chimiche. 15(3), 275-309.</li>
</ol>

Identificazione di aldeidi e chetoni sconosciuti

Aldeidi e chetoni
Le aldeidi e i chetoni hanno un gruppo carbonilico (C=O) come gruppo funzionale. Un chetone ha due gruppi alchilici o arilici attaccati al ...

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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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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...

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Video completi e incentrati sul curriculum sia per gli istruttori che per gli studenti nei corsi di laboratorio di Chimica introduttiva.

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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

Georgia Standards of Excellence

9-12

SC3.

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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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