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

Aldehydy i ketony mają podobną strukturę. Oba posiadają grupę karbonylową, która jest podwójnym wiązaniem węgla z tlenem. Aldehyd ma co najmniej jeden wodór połączony z węglem karbonylowym. Druga grupa to grupa oparta na wodorze lub węglu. Natomiast keton ma dwie grupy oparte na węglu połączone z węglem karbonylowym. Niektóre reakcje zachodzące pod wpływem aldehydów i ketonów można wykorzystać do ich rozróżnienia lub identyfikacji ich grup funkcyjnych na podstawie widocznych różnic w wyniku reakcji.
Jedną z takich reakcji jest test DNPH, który służy do określenia, czy aldehyd lub keton jest aromatyczny. W tej reakcji 2,4-dinitrofenylohydrazyna lub DNPH atakuje karbonyl aldehydu lub ketonu w wodnym roztworze kwasowym. W wyniku tej reakcji kondensacji powstaje hydrazon, który wytrąca się z roztworu wodnego.
Kiedy niearomatyczny keton lub aldehyd reaguje z DNPH, osad jest żółty. Jednak ketony aromatyczne i aldehydy dają czerwono-pomarańczowy osad. DNPH może być również stosowany do odróżniania alkoholi i estrów od aldehydów i ketonów, ponieważ DNPH nie reaguje z alkoholami ani estrami. Ten brak reakcji nazywamy wynikiem negatywnym.
Inną przydatną reakcją jest test jodoformowy dla ketonów metylowych, które są ketonami, które mają co najmniej jeden metyl jako grupę funkcyjną. Kiedy keton metylowy miesza się z jodem w wodnych warunkach alkalicznych, jod zastępuje każdy wodór metylowy, tworząc doskonałą grupę opuszczającą. Substytucja, po której następuje transfer protonu, przekształca opuszczającą grupę w jodoform, który wytrąca się z roztworu w postaci bladożółtego ciała stałego.
Ta reakcja działa również w przypadku aldehydu octowego, który jest aldehydem z metylem jako grupą R. Reakcja opiera się na unikalnej reaktywności wodorów w grupie alfa-metylowej, więc zmieszanie jakiegokolwiek innego ketonu lub aldehydu z jodem nie spowoduje powstania stałego żółtego jodoformu.
Wreszcie możemy rozróżnić ketony i aldehydy za pomocą testu Tollensa. Srebro diaminowe (1+), czyli odczynnik Tollensa, utlenia aldehydy do kwasów karboksylowych. Odczynnik Tollensa jest w tym procesie redukowany do pierwiastkowego srebra, które albo pokrywa wewnętrzną ściankę probówki, albo tworzy osad. Większość ketonów nie jest jednak utleniana przez odczynnik Tollensa, więc nie powstanie żadne stałe srebro.
W tym laboratorium zidentyfikujesz kilka nieznanych aldehydów i ketonów, wykonując test DNPH, test Tollensa i test jodoformowy.

identyfikacja nieznanych aldehydów i ketonów

<h4>Aldehydy i ketony</h4>
Aldehydy i ketony mają grupę karbonylową (C = O) jako grupę funkcyjną. Keton ma dwie grupy alkilowe lub arylowe przyłączone do węgla karbonylowego (RCOR'). Najprostszym ketonem jest aceton, który ma dwie grupy metylowe przyłączone do węgla karbonylowego (CH3, COCH, 3).
Aldehyd jest podobny do ketonu, z tą różnicą, że zamiast dwóch grup bocznych połączonych z węglem karbonylowym, mają co najmniej jeden wodór (RCOH). Najprostszym aldehydem jest formaldehyd (HCOH), ponieważ ma dwa wodory połączone z grupą karbonylową. Wszystkie inne aldehydy mają jeden wodór związany z grupą karbonylową, podobnie jak prosta cząsteczka aldehydu octowego, która ma jeden wodór i jedną grupę metylową (HCOCH3).
Węgiel karbonylowy zarówno w aldehydach, jak i ketonach jest elektrofilowy, co oznacza, że ma dipol ze względu na elektroujemność przyłączonego atomu tlenu. To sprawia, że węgiel karbonylowy jest idealnym celem dla nukleofilów w reakcji addycji nukleofilowej. Podczas tej reakcji nukleofil lub donor elektronów atakuje karbonyl, tworząc czworościenny produkt pośredni. Ujemnie naładowany tlen przyjmuje jon wodorowy, tworząc grupę hydroksylową.
Zazwyczaj nukleofile posiadają ładunek ujemny lub samotną parę na heteroatomie, która może przybierać różne formy (OH-, RO-, CN-,R3C-, RNH2, ROH). W przypadku amin pierwszorzędowych (RNH2) reakcja nie zatrzymuje się na utworzeniu czworościennego produktu pośredniego z grupą hydroksylową. Zamiast tego zachodzi reakcja eliminacji, w wyniku której powstaje podwójnie związana grupa funkcyjna węgla i azotu, znana jako imina. Zrozumienie reakcji, jakie mogą przechodzić aldehydy i ketony, zapewnia sposób na rozróżnienie między tymi podobnymi typami związków organicznych.
<h4>Test DNPH</h4>
Tworzenie imin z ketonów lub aldehydów wykorzystuje się za pomocą odczynnika 2,4-dinitrofenylohydrazyny (DNPH). W tej reakcji addycji-eliminacji pierwszorzędowa grupa aminowa DNPH atakuje karbonyl aldehydu lub ketonu w kwaśnym środowisku. W wyniku reakcji kondensacji powstaje hydrazon, który wytrąca się z roztworu.
Osady, które są żółte, wskazują na niesprzężone ketony lub aldehydy, podczas gdy czerwono-pomarańczowe osady wskazują na układy sprzężone. Test ten służy do różnicowania ketonów i aldehydów od alkoholi i estrów, z którymi DNPH nie reaguje, a tym samym nie tworzy się osad. Pochodne ketonów lub aldehydów są materiałami krystalicznymi o bardzo dobrze zdefiniowanych temperaturach topnienia, które są opisywane w literaturze i mogą być wykorzystane do identyfikacji konkretnych związków.
<h4>Test haloformu</h4>
Innym testem jest test haloformowy, który służy do określenia, czy keton jest ketonem metylowym. Ketony metylowe mają grupę metylową połączoną z węglem karbonylowym (RCOCH3). Grupa R może być wodorem (aldehydem metylowym), grupą alkilową lub grupą arylową.
Mechanizm reakcji w teście haloformowym zachodzi w warunkach podstawowych. Po pierwsze, keton ulega tautomeryzacji keto-enolowej. Następnie enolan nukleofilowy atakuje jod, dając jon jodu i halogenowany keton. Powtarza się to w sumie trzy razy, aż wszystkie wodory grupy metylowej ketonowej zostaną zastąpione jodem. Następnie wodorotlenek reaguje z elektrofilowym centrum węgla karbonylowego, tworząc czworościenny półprodukt z ujemnie naładowanym tlenem, który jest pojedynczo związany z karbonylem. Po reformowaniu podwójnego wiązania C-O grupa trihalometylowa opuszcza się jako ustabilizowana grupa opuszczająca. Wreszcie, utworzony kwas karboksylowy jest deprotonowany przez ujemnie naładowaną grupę trihalometylu, aby uzyskać haloform - trihalometan - i karboksylan. Jeśli halogenem jest jod, osad będzie miał charakterystyczny żółty kolor. Ta reakcja zajdzie tylko w przypadku ketonów metylowych i da wynik ujemny dla każdego innego związku zawierającego karbonyl.
<h4>Test Tollensa</h4>
Test Tollensa to reakcja, która służy do odróżnienia aldehydów od ketonów, ponieważ aldehydy mogą zostać utlenione do kwasu karboksylowego, podczas gdy ketony nie. Odczynnik Tollensa, który jest mieszaniną azotanu srebra i amoniaku, utlenia aldehyd do kwasu karboksylowego. Jon srebra Ag+ jest redukowany do stałego srebra, Ag(s). Solidne srebro tworzy film na wewnętrznej ściance probówki, przypominający srebrne lustro. Ketony nie reagują z odczynnikiem Tollena, a zatem nie powodują powstawania srebrnego lustra na ściankach probówki.
<h4>Odwołania</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Wprowadzenie do chemii organicznej. Upper Saddle River, NJ: Prentice Hall.</li>
	<li style="font-size:17px">Fuson, R.C., Bull, B.A. (1934). Reakcja haloformu. Recenzje chemiczne. 15(3), 275-309.</li>
</ol>

Identyfikacja nieznanych aldehydów i ketonów

Aldehydy i ketony
Aldehydy i ketony mają grupę karbonylową (C = O) jako grupę funkcyjną. Keton ma dwie grupy alkilowe lub arylowe przyłączone do ...

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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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Chemistry is the study of matter and the changes it undergoes. Matter is anything that has mass and occupies space. Matter is all around us; the air, ...

States of Matter

Solids, liquids, and gases are the three states of matter commonly found on Earth. A solid is rigid and possesses a definite shape. A liquid flows and ...

States of Matter and Phase Changes

The internal energy of a substance&#8212;the total kinetic energy of all its molecules and the potential energy of their associated forces&#8212;depends ...

molecular-comparison-of-gases-liquids-and-solids

Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Differentiate between physical and chemical properties and physical and chemical changes of matter.

SC.912.P.8.1

physical-and-chemical-properties-of-matter

Physical and Chemical Properties of Matter

The characteristics that enable us to distinguish one substance from another are called properties.

Physical Properties of Matter

A physical property is ...

Phase Transitions

Whether solid, liquid, or gas, a substance&#39;s state depends on the order and arrangement of its particles (atoms, molecules, or ions). Particles in the ...

phase-transitions-vaporization-and-condensation

Phase Transitions: Vaporization and Condensation

The physical form of a substance changes on changing its temperature. For example, raising the temperature of a liquid causes the liquid to vaporize ...

Vapor Pressure

When a liquid vaporizes in a closed container, gas molecules cannot escape. As these gas phase molecules move randomly about, they will occasionally ...

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 ...

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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