Name: alcohol tests for primary, secondary, and tertiary
Uploaded: 2020-03-26T07:22:29.000Z
Duration: 3 min 32 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.8

SC.912.P.8.13

Identifying Alcohols

Os álcoois são uma classe de moléculas orgânicas que possuem pelo menos um grupo funcional hidroxila conectado a um átomo de carbono. Metanol e etanol são dois álcoois familiares. Ambos são álcoois alifáticos, o que significa que são derivados de um hidrocarboneto e não contêm um grupo benzeno.
Existem três classificações de álcoois alifáticos. O primeiro é um álcool primário, onde o grupo hidroxila está conectado a um átomo de carbono que está conectado a um grupo à base de carbono, ou alquil. Em um álcool secundário, o grupo hidroxila está conectado a um átomo de carbono que possui dois grupos alquila. Finalmente, em um álcool terciário, o grupo hidroxila está conectado a um átomo de carbono com três grupos alquila.
Os álcoois também podem ser aromáticos, nos quais um grupo hidroxila é conectado diretamente ao carbono de um anel de benzeno. A forma mais simples de um álcool aromático é o fenol. Os compostos que contêm fenol são conhecidos como derivados de fenol ou fenóis.
Certas reações sofridas por álcoois podem ser usadas para distingui-los com base em diferenças visíveis no resultado da reação. Uma dessas reações é o teste do cloreto férrico, que testa a presença de fenóis. Quando o cloreto férrico é adicionado a um fenol, forma-se um complexo ferro-três fenol roxo. Isso não é observado quando é adicionado a um álcool alifático.
Outra reação útil é o Teste de Jones, que é usado para identificar álcoois alifáticos primários e secundários. O reagente de Jones, uma mistura de ácido sulfúrico e trióxido de cromo em água, é um forte agente oxidante. Ele reage com álcoois primários para formar aldeídos - que então formam ácidos carboxílicos - e reage com álcoois secundários para formar cetonas. A adição do reagente de Jones aos álcoois primários e secundários faz com que a solução mude de cor de laranja para verde escuro. Os álcoois terciários não reagem com o reagente de Jones porque são resistentes à oxidação.
Finalmente, podemos distinguir álcoois alifáticos com o teste de Lucas. O reagente de Lucas, que é uma mistura de cloreto de zinco e ácido clorídrico, reage com álcoois secundários e terciários por meio de umareação de substituiçãonucleofílica SN 1. O cloreto de zinco coordena com o hidroxilo oxigênio para gerar um excelente grupo de saída. Uma vez eliminado, um carbocátion carregado positivamente permanece. Em seguida, um íon cloro ataca o carbocátion para formar um cloreto de alquila, que é insolúvel em água e parece turvo.
Os álcoois terciários reagem imediatamente com o reagente de Lucas para formar uma camada oleosa à temperatura ambiente. Os álcoois secundários dão um resultado positivo após alguns segundos a alguns minutos. Os álcoois primários dão um resultado negativo, a menos que sejam aquecidos.
Neste laboratório, você usará o teste de cloreto férrico, o teste de Jones e o teste de Lucas para identificar um álcool desconhecido.

identificação de álcoois

<h4>Álcoois</h4>
Os álcoois são compostos orgânicos que estão entre os mais reconhecíveis e familiares, pois têm amplas aplicações e usos na vida cotidiana. Os álcoois são moléculas orgânicas contendo um grupo funcional hidroxila conectado a um grupo alquil ou arila (ROH). Se o hidroxilo carbono tiver apenas um único grupo R, ele é conhecido como álcool primário. Se tiver dois grupos R, é um álcool secundário, e se tiver três grupos R, é um álcool terciário. Como muitos outros compostos orgânicos, os álcoois também podem ser aromáticos por conter um anel de benzeno. O álcool aromático mais simples é o fenol.
A identificação de um álcool em química orgânica pode ser alcançada explorando as diferentes propriedades dos tipos de álcoois. Embora vários métodos de análise de instrumentação, como ressonância magnética nuclear (RMN), possam ser usados, outros testes qualitativos também podem ser utilizados. Ao acoplar esses testes, a identidade de um álcool pode ser determinada de maneira semelhante à forma como os aldeídos e as cetonas podem ser identificados.
<h4>Teste de cloreto férrico</h4>
Um método para diferenciar entre um álcool alifático e um álcool aromático é usando cloreto de ferro (III). O composto de cloreto de ferro dá à solução uma aparência vermelho-laranja. Na presença de um álcool aromático, como o fenol, os átomos de cloreto são substituídos pelo álcool aromático, alterando a propriedade de coordenação do átomo de ferro central. Isso muda a cor da solução para uma cor roxa. Os álcoois alifáticos não reagem com o cloreto de ferro (III) e, portanto, a solução permanece vermelho-alaranjada.
<h4>Teste de Jones</h4>
O teste de Jones utiliza trióxido de cromo na presença de ácido sulfúrico para atuar como um poderoso agente oxidante. Na presença do reagente de Jones, um álcool primário é primeiro convertido em um aldeído e depois em um ácido carboxílico, enquanto um álcool secundário será oxidado em uma cetona. O estado de oxidação do cromo é a chave para este teste. O cromo está no estado de oxidação +6 no reagente de Jones. Os complexos Cr (VI) no reagente conferem-lhe sua cor avermelhada brilhante e laranja.
No processo da reação, o cromo é reduzido de Cr(VI) para o estado de oxidação +3 - Cr(III). Primeiro, o álcool e o ácido crômico formam um éster de cromato. Em seguida, uma base (H2O) cliva a ligação CH do álcool, formando o grupo carbonila enquanto reduz Cr (VI) a Cr (IV). O carbono do álcool sofre uma oxidação de 2 elétrons e o Cr (IV) sofre uma redução de 2 elétrons, então esta é uma etapa de redução-oxidação.
Cr (IV) participa de outras etapas de oxidação e é eventualmente reduzido a Cr (III). O Cr(III) está frequentemente presente como íons hexaaquacromo(III) — [Cr(H2O)6]3+ — e complexos de Cr(III), onde as moléculas de H2O são substituídas por um ou mais íons sulfato — [Cr(H2O)5(SO4)]+. Esses complexos dão ao Cr (III) a cor verde característica.
Os álcoois terciários não reagem com o cromo e, portanto, nenhum precipitado é feito, mantendo a solução laranja. Portanto, o teste de Jones pode ajudar a diferenciar álcoois primários e secundários de álcoois terciários.
<h4>Teste de Lucas</h4>
O teste de Lucas utiliza cloreto de zinco (II) na presença de ácido clorídrico como reagente. Na presença de um álcool, o reagente Lucas irá halogenar o álcool, formando um produto insolúvel em soluções aquosas. A taxa de reação para isso é altamente dependente da formação de um carbocátion causado pela perda da hidroxila como água. Um carbocátion formado por um álcool primário é instável e, portanto, não ocorrerá prontamente. Isso resulta em nenhuma reação observável à temperatura ambiente. Um álcool terciário formará um carbocátion estável, resultando em uma reação rápida após a adição do reagente Lucas. Isso resulta em uma segunda fase aparecendo na mistura de reação porque o produto halogenado final é insolúvel em água. Os álcoois secundários formarão um carbocátion menos estável do que os álcoois terciários, mas a reação ocorrerá à temperatura ambiente em questão de minutos. Portanto, o Teste de Lucas pode ajudar a diferenciar entre álcoois primários, secundários e terciários.
<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>
</ol>

Identificação de álcoois

Álcoois
Os álcoois são compostos orgânicos que estão entre os mais reconhecíveis e familiares, pois têm amplas aplicações e usos no dia a dia ...

Education

JoVE Lab Manual

Chemistry

Química

alcohol tests for primary, secondary, and tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary.

Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical behavior instead of relying only on instrument-based methods such as NMR. By combining several qualitative tests, the identity of an alcohol can be determined in a way similar to how aldehydes and ketones are identified.

The ferric chloride test helps tell aromatic alcohols from aliphatic alcohols. Iron(III) chloride starts as a red-orange solution. If an aromatic alcohol such as phenol is present, the chloride atoms are replaced and the solution turns purple. Aliphatic alcohols do not react in this test, so the color stays red-orange.

The Jones test uses chromium trioxide with sulfuric acid as a strong oxidizing reagent. A primary alcohol is oxidized first to an aldehyde and then to a carboxylic acid. A secondary alcohol is oxidized to a ketone. Chromium starts in the +6 oxidation state, which gives the reagent its reddish-orange color, and is reduced to Cr(III), which is green. Tertiary alcohols do not react, so the solution remains orange. This makes the Jones test useful for separating primary and secondary alcohols from tertiary alcohols.

The Lucas test uses zinc(II) chloride in hydrochloric acid. It halogenates the alcohol and produces an insoluble product in water. The test depends on carbocation formation when the hydroxyl group leaves as water. Primary alcohols do not show a visible reaction at room temperature because their carbocations are unstable. Tertiary alcohols react quickly because their carbocations are more stable, and a second layer appears. Secondary alcohols react more slowly and usually take a few minutes. This test helps distinguish primary, secondary, and tertiary alcohols.

Alcohol Tests for Primary, Secondary, and Tertiary

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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 ...

Electronic Structure of Atoms


An atom comprises protons and neutrons, which are contained inside the dense, central&#160;core called the nucleus, with electrons present around the ...

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.

Alcohol Tests for Primary, Secondary, and Tertiary

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