Name: ester formation and everyday fragrances
Uploaded: 2020-03-26T07:22:29.000Z
Duration: 3 min 48 s
Channel: JoVE

K12 Standards

Florida Department of Education

High School

Physical Science

SC.912.P.8

SC.912.P.8.13

Esterification

Os ésteres são uma classe de moléculas orgânicas que podem ter um aroma frutado ou floral. A estrutura de um éster é uma carbonila com um grupo alquila ou arila de um lado e um oxigênio ligado a outro grupo alquila ou arila do outro lado.
Dependendo dos grupos alquil ou arila, o éster pode assumir muitas características diferentes. Por exemplo, a reação de glicerol e três ácidos graxos, que são ácidos carboxílicos com longas cadeias alquílicas, resulta em um triglicerídeo, que possui três grupos éster. As longas cadeias alquílicas dos ácidos graxos conferem aos triglicerídeos um peso molecular muito alto. Em contraste, os ésteres simples têm baixo peso molecular e pequenos grupos funcionais.
Os cheiros e sabores de frutas e flores são atribuídos a ésteres simples. Mesmo pequenas mudanças na estrutura de um éster simples afetam muito sua fragrância. Por exemplo, mudar um grupo hidroxila para uma amina muda o aroma desse éster de gaultéria para uva. Da mesma forma, o acetato de propila cheira a peras, enquanto o acetato de butila, que tem apenas mais um carbono em sua cadeia, cheira a maçãs.
Uma maneira comum de fazer um éster é a esterificação de Fischer, onde um ácido carboxílico e um álcool reagem na presença de um catalisador ácido para formar o éster e a água. O grupo R e a carbonila vêm do ácido carboxílico, e o grupo alcoxi ou ariloxi com o R' vem do álcool. Esta reação de esterificação é reversível. Com uma mistura de 1 para 1 do ácido carboxílico e do álcool, tende a atingir o equilíbrio com cerca de 70% de rendimento do éster, na melhor das hipóteses.
No entanto, o princípio de Le Chatelier nos permite aumentar o rendimento do éster além disso. O princípio de Le Chatelier afirma que qualquer sistema em equilíbrio químico que esteja sujeito a uma mudança na concentração, pressão, temperatura ou volume se ajustará a um novo equilíbrio que neutralize a mudança.
Assim, se aumentarmos a concentração de um dos reagentes em uma reação reversível, o equilíbrio mudará na direção que diminui sua concentração. Isso resulta em um maior rendimento do produto éster em equilíbrio. Assim, podemos melhorar o rendimento da esterificação usando uma proporção molar de 3:1 ou 1:3 de ácido carboxílico para álcool.
Neste experimento, será realizada uma esterificação de Fischer utilizando álcool em excesso e ácido sulfúrico como catalisador. Nesta reação, a reatividade do ácido carboxílico é aumentada pelo ácido sulfúrico, que protona o oxigênio da carbonila. O álcool é um nucleófilo que ataca o carbono da carbonila para formar um intermediário. Em seguida, o hidrogênio do álcool é transferido para uma hidroxila próxima. A carbonila então se reforma, eliminando uma molécula de água. Finalmente, a desprotonação resulta em um éster neutro.
Neste laboratório, você realizará uma reação de esterificação de Fischer com um álcool desconhecido e ácido carboxílico em uma proporção molar de 3:1. Em seguida, você identificará o éster usando seu odor, determinará o rendimento e identificará os dois reagentes desconhecidos com base na estrutura do éster.

esterificação

<h4>Ésteres</h4>
A estrutura de um éster é uma carbonila com um grupo alquil ou arilo (R) de um lado e um oxigênio ligado a outro grupo alquil ou arila (R') do outro lado, representado pela fórmula geral: RCOOR'. Os ésteres podem ser comumente derivados por uma reação de esterificação entre um ácido carboxílico e um álcool. O grupo hidroxila do ácido carboxílico é substituído por um grupo alquila ou arila. Os ésteres simples, que possuem baixo peso molecular e pequenos grupos funcionais R e R', possuem cheiros e sabores de frutas e flores.
O ácido carboxílico mais simples é o ácido fórmico (HCOOH), sendo o álcool mais simples o metanol. Assim, o éster mais simples formado a partir da esterificação dessas duas moléculas é o formato de metila (HCOOCH3). O formato de metila é considerado um derivado do ácido fórmico porque o grupo OH é substituído pelo grupo OCH3. As diferenças estruturais entre o formato de metila e o ácido fórmico ditam a diferença em suas propriedades. Isso ocorre porque as propriedades da molécula dependem muito dos grupos R e R'.
Os ésteres encontrados em frutas e flores são conhecidos como ésteres simples devido à baixa complexidade de sua estrutura em comparação com outros ésteres. Os grupos alquil e arila encontrados nesses ésteres influenciam muito a fragrância que emitem; Assim, a adição de um carbono ou de um grupo funcional diferente pode afetar o aroma produzido pelo éster. Por exemplo, o acetato de propila (C5H10O2) tem cheiro de peras. No entanto, o acetato de butila (C6H12O2), que tem apenas mais um carbono em sua cadeia, cheira a maçãs.
Os ésteres de cadeia longa, ou seja, aqueles com longas cadeias alquílicas, são chamados de ácidos graxos. Os ácidos graxos são compostos biológicos importantes que são essenciais para certos processos biológicos. Os ácidos graxos também são uma forma biológica de armazenamento de energia. Quando três ácidos graxos reagem com o glicerol, eles formam um triglicerídeo - a forma mais comum de ácidos graxos. Os triglicerídeos têm um peso molecular muito alto e podem ser altamente ramificados. Sua função e propriedades dependem do comprimento e dos tipos de cadeias alquílicas incorporadas em sua estrutura. Um dos usos industriais mais comuns dos triglicerídeos é na produção de sabões e detergentes.
<h4>Reação de esterificação</h4>
Os ésteres são formados a partir de uma reação de esterificação, com ésteres simples sendo formados por esterificação de Fisher. Essa reação converte um ácido carboxílico e álcool em um éster com água como subproduto. A esterificação de Fisher é uma reação reversível que ocorre muito lentamente. Um catalisador ácido, normalmente na forma de ácido sulfúrico, é adicionado para aumentar a taxa da reação e, ao mesmo tempo, atuar como um agente desidratante.
O mecanismo de ação para uma esterificação de Fisher começa pelo oxigênio carbonílico atacando o ácido sulfúrico e desprotonando seu grupo OH. Isso leva a um oxigênio carbonílico carregado positivamente. Para estabilizar melhor a carga positiva, um par de elétrons da ligação dupla carbonila é empurrado para o oxigênio carbonílico enquanto cria simultaneamente um centro de carbono pobre em elétrons. Em seguida, o grupo hidroxi do álcool atua como um nucleófilo e ataca o carbono pobre em elétrons - o eletrófilo - formando um intermediário. Em seguida, o grupo hidroxila carregado positivamente do álcool é desprotonado pelo grupo hidroxila do ácido carboxílico, estabilizando o átomo de oxigênio do álcool. Na última etapa, o oxigênio carbonílico é desprotonado pela base conjugada e o par de elétrons livres é movido em direção ao centro carbonila enquanto o grupo carboxila protonado sai como uma molécula de água.
Como a reação de esterificação é reversível, uma mistura 1:1 do ácido carboxílico e do álcool atingirá o equilíbrio com cerca de 70% de rendimento do éster. No entanto, como em qualquer processo de equilíbrio, a reação pode ser conduzida em uma direção, alterando a concentração, pressão, temperatura ou volume dos reagentes. Essa mudança faz com que a reação se ajuste a um novo equilíbrio para neutralizar a mudança, que é a base do princípio de Le Chatelier.
Quando a concentração de um dos reagentes é aumentada, o equilíbrio muda na direção que diminuirá sua concentração. Assim, ao aumentar a concentração de um reagente, o equilíbrio produz mais do produto, resultando em um maior rendimento do produto éster.
Um método comum utilizado em laboratórios de química orgânica é fornecer um excesso de um dos reagentes, normalmente o álcool. Como mencionado anteriormente, a esterificação de Fisher pode utilizar ácido sulfúrico como catalisador. No entanto, uma das outras funções do ácido sulfúrico é atuar como um agente desidratante, sequestrando as moléculas de água da reação. A aplicação do princípio de Le Chatelier demonstra que, à medida que um dos produtos é removido - neste caso, a água - a reação é empurrada para fazer mais produto. Isso aumenta o rendimento do éster formado.
<h4>referência</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>

Esterificação

Ésteres
A estrutura de um éster é uma carbonila com um grupo alquila ou arila (R) de um lado e um oxigênio ligado a outro grupo alquila ou arila (R ') ...

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ester formation and everyday fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group.

Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl formate, the simplest ester from these two molecules. Methyl formate is a derivative of formic acid because the OH group is replaced by OCH3. Small changes in the R and R' groups can also change the properties and scent of an ester.

This is why similar esters can smell different. Propyl acetate has a pear-like scent, while butyl acetate smells like apples, even though the molecules differ by only one carbon. Long-chain esters are called fatty acids. They are important in biology and also serve as a form of energy storage.

When three fatty acids react with glycerol, they form a triglyceride. Triglycerides are the most common form of fatty acids and have a high molecular weight. They can also be highly branched. Their function and properties depend on the length and type of alkyl chains in their structure. Triglycerides are widely used in industry, including the production of soaps and detergents.

Simple esters are often made by Fischer esterification, which is a slow and reversible reaction. It converts a carboxylic acid and an alcohol into an ester and water. Sulfuric acid is commonly used as an acid catalyst. It speeds up the reaction and also acts as a dehydrating agent.

The reaction mechanism starts when the carbonyl oxygen interacts with sulfuric acid and becomes positively charged. The carbonyl double bond then shifts electron density toward the oxygen, leaving the carbonyl carbon more electron-poor. Next, the alcohol acts as a nucleophile and attacks that carbon, forming an intermediate. The alcohol oxygen is then deprotonated, and the carbonyl oxygen is finally deprotonated as water leaves.

Because Fischer esterification is reversible, a 1:1 mixture of the acid and alcohol reaches equilibrium at about 70% ester yield. The reaction can be pushed toward more ester by changing concentration, pressure, temperature, or volume. This follows Le Chatelier's principle, which says a system shifts to counter a change. In the lab, using excess alcohol is a common way to increase yield. Sulfuric acid also helps by removing water, and taking away this product drives the reaction to make more ester.

Ester Formation and Everyday Fragrances

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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 ...

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 ...

Electron Behavior

Electrons are negatively charged subatomic particles that are attracted to an orbit around the positively-charged nucleus of an atom. They reside in ...

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.

Ester Formation and Everyday Fragrances

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