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

Les esters sont une classe de molécules organiques qui peuvent avoir un arôme fruité ou fleuri. La structure d’un ester est un carbonyle avec un groupe alkyle ou aryle d’un côté et un oxygène lié à un autre groupe alkyle ou aryle de l’autre côté.
Selon les groupes alkyle ou aryle, l’ester peut prendre de nombreuses caractéristiques différentes. Par exemple, la réaction du glycérol et de trois acides gras, qui sont des acides carboxyliques avec de longues chaînes alkyles, aboutit à un triglycéride, qui a trois groupes esters. Les longues chaînes alkyles des acides gras confèrent aux triglycérides un poids moléculaire très élevé. En revanche, les esters simples ont un faible poids moléculaire et de petits groupes fonctionnels.
Les odeurs et les saveurs des fruits et des fleurs sont attribuées à de simples esters. Même de petits changements dans la structure d’un simple ester affectent grandement son parfum. Par exemple, le changement d’un groupe hydroxyle en une amine change l’odeur de cet ester de gaulthérie à raisin. De même, l’acétate de propyle sent la poire, tandis que l’acétate de butyle, qui n’a qu’un seul carbone de plus dans sa chaîne, sent la pomme.
Une façon courante de fabriquer un ester est l’estérification de Fischer, où un acide carboxylique et un alcool réagissent en présence d’un catalyseur acide pour former l’ester et l’eau. Le groupe R et le carbonyle proviennent de l'acide carboxylique, et le groupe alcoxy ou aryloxy avec le R' provient de l'alcool. Cette réaction d’estérification est réversible. Avec un mélange 1 à 1 de l’acide carboxylique et de l’alcool, il a tendance à atteindre l’équilibre avec un rendement d’environ 70 % de l’ester au mieux.
Cependant, le principe du Chatelier nous permet d'augmenter le rendement de l'ester au-delà. Le principe de Le Chatelier stipule que tout système à l'équilibre chimique qui est soumis à un changement de concentration, de pression, de température ou de volume s'adaptera à un nouvel équilibre qui contrecarre le changement.
Ainsi, si nous augmentons la concentration de l’un des réactifs dans une réaction réversible, l’équilibre se déplacera dans la direction qui diminue sa concentration. Il en résulte un rendement plus élevé du produit ester à l’équilibre. Ainsi, nous pouvons améliorer le rendement de l’estérification en utilisant un rapport molaire de 3:1 ou 1:3 entre l’acide carboxylique et l’alcool.
Dans cette expérience, une estérification de Fischer sera réalisée en utilisant de l’alcool en excès et de l’acide sulfurique comme catalyseur. Dans cette réaction, la réactivité de l’acide carboxylique est renforcée par l’acide sulfurique, qui protone l’oxygène du carbonyle. L’alcool est un nucléophile qui attaque le carbone du carbonyle pour former un intermédiaire. Ensuite, l’hydrogène de l’alcool est transféré à un hydroxyle voisin. Le carbonyle se reformage ensuite, éliminant une molécule d’eau. Enfin, la déprotonation aboutit à un ester neutre.
Dans ce laboratoire, vous allez effectuer une réaction d'estérification de Fischer avec un alcool inconnu et de l'acide carboxylique dans un rapport molaire de 3:1. Vous identifierez ensuite l'ester à l'aide de son odeur, déterminerez le rendement et identifierez les deux réactifs inconnus en fonction de la structure de l'ester.

estérification

<h4>Esters</h4>
La structure d’un ester est un carbonyle avec un groupe alkyle ou aryle (R) d’un côté, et un oxygène lié à un autre groupe alkyle ou aryle (R') de l’autre côté, représenté par la formule générale : RCOOR'. Les esters peuvent être couramment obtenus par une réaction d’estérification entre un acide carboxylique et un alcool. Le groupe hydroxyle de l’acide carboxylique est remplacé par un groupe alkyle ou aryle. Les esters simples, qui ont un faible poids moléculaire et de petits groupes fonctionnels R et R', ont des odeurs et des saveurs de fruits et de fleurs.
L’acide carboxylique le plus simple est l’acide formique (HCOOH), l’alcool le plus simple étant le méthanol. Ainsi, l’ester le plus simple formé à partir de l’estérification de ces deux molécules est le formiate de méthyle (HCOOCH3). Le formiate de méthyle est considéré comme un dérivé de l’acide formique car le groupe OH est remplacé par le groupe OCH3. Les différences structurelles entre le formiate de méthyle et l’acide formique dictent la différence de leurs propriétés. En effet, les propriétés de la molécule dépendent fortement des groupes R et R'.
Les esters que l’on trouve dans les fruits et les fleurs sont connus sous le nom d’esters simples en raison de la faible complexité de leur structure par rapport aux autres esters. Les groupes alkyle et aryle présents sur ces esters influencent grandement le parfum qu’ils émettent ; Ainsi, l’ajout d’un carbone ou d’un groupe fonctionnel différent peut impacter l’odeur produite par l’ester. Par exemple, l’acétate de propyle (C5H10O2) a l’odeur de la poire. Cependant, l’acétate de butyle (C6H12O2), qui n’a qu’un seul carbone de plus dans sa chaîne, sent la pomme.
Les esters à longue chaîne, c’est-à-dire ceux qui ont de longues chaînes alkyles, sont appelés acides gras. Les acides gras sont des composés biologiques importants qui sont essentiels à certains processus biologiques. Les acides gras sont également une forme biologique de stockage d’énergie. Lorsque trois acides gras réagissent avec le glycérol, ils forment un triglycéride - la forme la plus courante d’acides gras. Les triglycérides ont un poids moléculaire très élevé et peuvent être très ramifiés. Leur fonction et leurs propriétés dépendent de la longueur et des types de chaînes alkyles incorporées dans leur structure. L’une des utilisations industrielles les plus courantes des triglycérides est la production de savons et de détergents.
<h4>Réaction d’estérification</h4>
Les esters sont formés à partir d’une réaction d’estérification, les esters simples étant formés par estérification de Fisher. Cette réaction convertit un acide carboxylique et un alcool en un ester avec de l’eau comme sous-produit. L’estérification de Fisher est une réaction réversible qui se déroule très lentement. Un catalyseur acide, généralement sous forme d’acide sulfurique, est ajouté pour augmenter la vitesse de la réaction tout en agissant comme un agent déshydratant.
Le mécanisme d’action d’une estérification de Fisher commence par l’oxygène carbonyle attaquant l’acide sulfurique et déprotonant son groupe OH. Cela conduit à un oxygène carbonyle chargé positivement. Pour mieux stabiliser la charge positive, une paire d’électrons de la double liaison carbonyle est poussée vers l’oxygène carbonyle tout en créant simultanément un centre de carbone pauvre en électrons. Ensuite, le groupe hydroxyle de l’alcool agit comme un nucléophile et attaque le carbone pauvre en électrons – l’électrophile – formant un intermédiaire. Ensuite, le groupe hydroxyle chargé positivement de l’alcool est déprotoné par le groupe hydroxyle de l’acide carboxylique, stabilisant l’atome d’oxygène de l’alcool. Dans la dernière étape, l’oxygène carbonyle est déprotoné par la base conjuguée, et la paire d’électrons libres est déplacée vers le centre carbonyle tandis que le groupe carboxyle protoné part sous la forme d’une molécule d’eau.
Comme la réaction d’estérification est réversible, un mélange 1:1 d’acide carboxylique et d’alcool atteindra l’équilibre avec un rendement d’environ 70 % de l’ester. Cependant, comme pour tout processus d’équilibre, la réaction peut être entraînée dans une direction en modifiant la concentration, la pression, la température ou le volume des réactifs. Ce changement amène la réaction à s’ajuster à un nouvel équilibre pour contrer le changement, ce qui est la base du principe de Le Chatelier.
Lorsque la concentration de l’un des réactifs est augmentée, l’équilibre se déplace dans la direction qui diminuera sa concentration. Ainsi, en augmentant la concentration d’un réactif, l’équilibre produit une plus grande quantité du produit, ce qui se traduit par un rendement plus élevé du produit ester.
Une méthode couramment utilisée dans les laboratoires de chimie organique consiste à fournir un excès de l’un des réactifs, généralement l’alcool. Comme mentionné précédemment, l’estérification de Fisher peut utiliser l’acide sulfurique comme catalyseur. Cependant, l’une des autres fonctions de l’acide sulfurique est d’agir comme un agent déshydratant, en séquestrant les molécules d’eau loin de la réaction. L’application du principe de Le Chatelier démontre que lorsque l’un des produits est retiré – dans ce cas, l’eau – la réaction est poussée à produire plus de produit. Cela augmente le rendement de l’ester formé.
<h4>référence</h4>
<ol>
	<li style="font-size :17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). Introduction à la chimie organique. Upper Saddle River, NJ : Prentice Hall.</li>
</ol>

Estérification

Esters
La structure d’un ester est un carbonyle avec un groupe alkyle ou aryle (R) d’un côté, et un oxygène lié à un autre groupe alkyle ou aryle (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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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 ...

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.

Ester Formation and Everyday Fragrances

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