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

Aldeídos e cetonas têm uma estrutura semelhante. Ambos possuem um grupo carbonila, que é um carbono duplamente ligado a um oxigênio. Um aldeído tem pelo menos um hidrogênio conectado ao carbono carbonílico. O segundo grupo é um grupo baseado em hidrogênio ou carbono. Em contraste, uma cetona tem dois grupos à base de carbono conectados ao carbono carbonílico. Certas reações sofridas por aldeídos e cetonas podem ser usadas para distingui-los ou identificar seus grupos funcionais com base em diferenças visíveis no resultado da reação.
Uma dessas reações é o teste DNPH, que é usado para determinar se um aldeído ou cetona é aromático. Nesta reação, a 2,4-dinitrofenilhidrazina, ou DNPH, ataca a carbonila de um aldeído ou cetona em uma solução ácida aquosa. Essa reação de condensação produz uma hidrazona, que precipita a partir da solução aquosa.
Quando uma cetona ou aldeído não aromático reage com DNPH, o precipitado é amarelo. No entanto, cetonas e aldeídos aromáticos dão um precipitado vermelho-alaranjado. O DNPH também pode ser usado para distinguir álcoois e ésteres de aldeídos e cetonas, uma vez que o DNPH não reage com álcoois ou ésteres. Chamamos essa falta de reação de resultado negativo.
Outra reação útil é usada como teste de iodofórmio para metilcetonas, que são cetonas que têm pelo menos um metil como grupo funcional. Quando uma metilcetona é misturada com iodo em condições alcalinas aquosas, o iodo substitui cada metil-hidrogênio, formando um excelente grupo de saída. A substituição, seguida pela transferência de prótons, converte o grupo de saída em iodofórmio, que precipita da solução como um sólido amarelo pálido.
Essa reação também funciona para o acetaldeído, que é um aldeído com metila como seu grupo R. A reação depende da reatividade única dos hidrogênios em um grupo alfa-metil, portanto, misturar qualquer outra cetona ou aldeído com iodo não produzirá iodofórmio amarelo sólido.
Finalmente, podemos distinguir entre cetonas e aldeídos usando o teste de Tollens. Diamminesilver (1+), ou reagente de Tollens, oxida aldeídos em ácidos carboxílicos. O reagente de Tollens é reduzido a prata elementar no processo, que reveste a parede interna do tubo de ensaio ou forma um precipitado preto. A maioria das cetonas, no entanto, não é oxidada pelo reagente de Tollens, portanto, nenhuma prata sólida se formará.
Neste laboratório, você identificará vários aldeídos e cetonas desconhecidos realizando o teste DNPH, o teste de Tollens e o teste de iodofórmio.

identificação de aldeídos e cetonas desconhecidos

<h4>Aldeídos e cetonas</h4>
Aldeídos e cetonas têm um grupo carbonila (C = O) como um grupo funcional. Uma cetona tem dois grupos alquil ou arila ligados ao carbono carbonílico (RCOR'). A cetona mais simples é a acetona, que possui dois grupos metil ligados ao carbono carbonílico (CH3COCH3).
Um aldeído é semelhante a uma cetona, exceto que, em vez de dois grupos laterais conectados ao carbono carbonílico, eles têm pelo menos um hidrogênio (RCOH). O aldeído mais simples é o formaldeído (HCOH), pois possui dois hidrogênios conectados ao grupo carbonila. Todos os outros aldeídos têm um hidrogênio ligado ao grupo carbonila, como a molécula simples acetaldeído, que possui um grupo hidrogênio e um grupo metil (HCOCH3).
O carbono carbonílico em aldeídos e cetonas é eletrofílico, o que significa que possui um dipolo devido à eletronegatividade do átomo de oxigênio ligado. Isso torna o carbono carbonílico um alvo ideal para nucleófilos em uma reação de adição nucleofílica. Durante essa reação, o nucleófilo, ou doador de elétrons, ataca a carbonila para formar o intermediário tetraédrico. O oxigênio carregado negativamente aceita um íon hidrogênio para formar um grupo hidroxila.
Normalmente, os nucleófilos possuem uma carga negativa ou par solitário em um heteroátomo, que pode assumir várias formas (OH-, RO-, CN-, R3C-, RNH2, ROH). Para aminas primárias (RNH2), a reação não para na formação do intermediário tetraédrico com um grupo hidroxila. Em vez disso, ocorre uma reação de eliminação que produz um grupo funcional de carbono e nitrogênio de ligação dupla conhecido como imina. Compreender as reações que os aldeídos e as cetonas podem sofrer fornece uma maneira de diferenciar entre esses tipos semelhantes de compostos orgânicos.
<h4>Teste DNPH</h4>
A formação de iminas a partir de cetonas ou aldeídos é explorada através do uso do reagente 2,4-dinitrofenilhidrazina (DNPH). Nesta reação de adição-eliminação, o grupo amino primário do DNPH ataca a carbonila de um aldeído ou cetona em um ambiente ácido. A reação de condensação resulta na formação de uma hidrazona, que precipita a partir da solução.
Precipitados amarelos indicam cetonas ou aldeídos não conjugados, enquanto precipitados vermelho-laranja indicam sistemas conjugados. Este teste é usado para diferenciar cetonas e aldeídos de álcoois e ésteres com os quais o DNPH não reage e, portanto, nenhum precipitado é formado. Os derivados de cetona ou aldeído são materiais cristalinos com pontos de fusão muito bem definidos, que são relatados na literatura e podem ser usados para identificar os compostos específicos.
<h4>Teste de halofórmio</h4>
Outro teste é o teste de halofórmio, que é usado para determinar se uma cetona é uma metilcetona. As metilcetonas têm um grupo metil conectado ao carbono carbonílico (RCOCH3). O grupo R pode ser um hidrogênio (aldeído metil), um grupo alquila ou um grupo arila.
O mecanismo de reação para o teste de halofórmio ocorre em condições básicas. Primeiro, a cetona sofre uma tautomerização ceto-enol. Em seguida, o enolato nucleofílico ataca o iodo para dar um íon iodo e uma cetona halogenada. Isso se repete um total de três vezes até que todos os hidrogênios do grupo cetona metil sejam substituídos por um iodo. Em seguida, o hidróxido reage com o centro eletrofílico do carbono carbonílico para formar o intermediário tetraédrico com um oxigênio carregado negativamente, que é ligado à carbonila. Após a reforma da ligação dupla CO, o grupo trihalometil sai como um grupo de saída estabilizado. Por fim, o ácido carboxílico formado é desprotonado pelo grupo trihalometil carregado negativamente para dar o halofórmio - trihalometano - e um carboxilato. Se o iodo for o halogênio, o precipitado terá uma cor amarela característica. Essa reação ocorrerá apenas para metilcetonas e dará um resultado negativo para qualquer outro composto contendo carbonila.
<h4>Teste de Tollens</h4>
O teste de Tollens é uma reação usada para distinguir aldeídos de cetonas, pois os aldeídos podem ser oxidados em ácido carboxílico, enquanto as cetonas não. O reagente de Tollens, que é uma mistura de nitrato de prata e amônia, oxida o aldeído a um ácido carboxílico. O íon prata Ag+ é reduzido a prata sólida, Ag(s). A prata maciça forma uma película na parede interna do tubo de ensaio, semelhante a um espelho de prata. As cetonas não reagem com o reagente de Tollen e, portanto, não resultam na formação de um espelho de prata nas paredes do tubo de ensaio.
<h4>Referências</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introdução à Química Orgânica. Rio Saddle Superior, NJ: Prentice Hall.</li>
	<li style="font-size:17px">Fuson, R.C., Bull, B.A. (1934). A reação haloforme. Revisões químicas. 15(3), 275-309.</li>
</ol>

Identificação de aldeídos e cetonas desconhecidos

Aldeídos e cetonas
Aldeídos e cetonas têm um grupo carbonila (C = O) como um grupo funcional. Uma cetona tem dois grupos alquil ou arila ligados ao ...

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JoVE Lab Manual

Chemistry

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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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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 ...

Electron Configurations

Electron configurations and orbital diagrams can be determined by applying the Aufbau principle (each added electron occupies the subshell of lowest ...

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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