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

酯是一类具有水果或花香气味的有机分子。酯的结构是一个羰基，其一侧连接一个烷基或芳基，另一侧的氧原子则与另一个烷基或芳基相连。
根据烷基或芳基的不同，酯类可以具有多种不同的特性。例如，甘油与三个脂肪酸（即带有长烷基链的羧酸）反应会生成甘油三酯，其分子中含有三个酯基。脂肪酸的长烷基链使甘油三酯具有很高的分子量。相比之下，简单的酯类分子量较低，且官能团较小。
水果和花朵的气味与香味归因于简单的酯类化合物。即使是简单酯类结构上的微小变化，也会显著影响其香味。例如，将该酯类中的羟基变为氨基，其气味就会从冬青油味转变为葡萄味。同样，丙酸丙酯具有梨的气味，而丁酸丁酯（其碳链仅多一个碳原子）则具有苹果的气味。
制备酯的一种常见方法是费歇尔酯化反应，即羧酸和醇在酸性催化剂存在下反应生成酯和水。其中，R基团和羰基来自羧酸，而烷氧基或芳氧基连同R'则来自醇。该酯化反应是可逆的。当羧酸与醇以1:1的比例混合时，反应通常达到平衡，酯的产率最高约为70%。
然而，根据勒夏特列原理，我们可以使酯的产率超过上述水平。勒夏特列原理指出，处于化学平衡状态的系统，若受到浓度、压力、温度或体积等条件变化的影响，将重新调整至一个新的平衡状态，以抵消这种变化。
因此，如果增加可逆反应中某一反应物的浓度，平衡将向降低该反应物浓度的方向移动。这将导致在平衡时酯化产物的产率更高。因此，我们可以通过使用3:1或1:3的羧酸与醇的摩尔比来提高酯化反应的产率。
本实验将使用过量的醇和硫酸作为催化剂进行费歇尔酯化反应。在该反应中，硫酸通过质子化羰基的氧原子，增强羧酸的反应活性。醇作为亲核试剂进攻羰基碳原子，形成一个中间体。随后，醇中的氢原子转移至邻近的羟基。接着，羰基重新形成，并脱去一分子水。最后，去质子化生成中性的酯。
在本实验中，你将使用一种未知的醇和羧酸，以 3:1 的摩尔比进行 Fischer 酯化反应。然后通过酯的气味鉴定该酯，测定产率，并根据酯的结构确定两种未知反应物。

酯化反应

<h4>酯</h4>
酯的结构是在羰基的一侧连接一个烷基或芳基（R），另一侧通过氧原子连接另一个烷基或芳基（R’），其通式表示为：RCOOR’。酯通常由羧酸与醇之间的酯化反应制得，其中羧酸的羟基被烷基或芳基取代。分子量较低且R和R’基团较小的简单酯类具有水果和花朵的气味与香味。
最简单的羧酸是甲酸（HCOOH），最简单的醇是甲醇。因此，由这两种分子通过酯化反应生成的最简单酯是甲酸甲酯（HCOOCH3）。甲酸甲酯被视为甲酸的衍生物，因为其 OH 基团被 OCH3 基团取代。甲酸甲酯与甲酸之间的结构差异决定了它们在性质上的不同。这是因为分子的性质在很大程度上取决于 R 和 R’ 基团。
水果和花朵中含有的酯类因其结构相对其他酯类更为简单，被称为简单酯。这些酯类中的烷基和芳基对其散发的香味有显著影响；因此，增加一个碳原子或不同的官能团都可能改变酯类产生的气味。例如，乙酸丙酯（C5H10O2）具有梨的香味。然而，乙酸丁酯（C6H12O2）仅在其碳链上多一个碳原子，却具有苹果的气味。
长链酯是指具有长烷基链的酯，被称为脂肪酸。脂肪酸是一类重要的生物化合物，在某些生物过程中具有必需作用。脂肪酸也是生物体内一种能量储存形式。当三个脂肪酸分子与甘油发生反应时，会形成三酰甘油——这是脂肪酸最常见的存在形式。三酰甘油具有很高的分子量，并且可能具有高度支链结构。其功能和性质取决于构成其结构的烷基链的长度和类型。三酰甘油最常见的工业用途之一是用于肥皂和洗涤剂的生产。
<h4>酯化反应</h4>
酯通过酯化反应生成，其中简单酯可通过费舍尔酯化反应制得。该反应将羧酸和醇转化为酯，同时生成水作为副产物。费舍尔酯化反应是可逆反应，反应速率很慢。通常加入酸催化剂（如硫酸）以加快反应速率，同时该催化剂还起到脱水剂的作用。
Fisher酯化反应的作用机制始于羰基氧进攻硫酸并夺取其羟基上的质子，从而使羟基去质子化。这导致羰基氧带正电荷。为了更好地稳定这一正电荷，羰基双键中的一对电子会转移至羰基氧上，同时形成一个缺电子的碳中心。接下来，醇的羟基作为亲核试剂进攻该缺电子的碳（即亲电中心），形成一个中间体。随后，醇分子中带正电荷的羟基被羧酸的羟基去质子化，从而使醇的氧原子得以稳定。在最后一步中，羰基氧被共轭碱去质子化，同时孤对电子移向羰基中心，而质子化的羧基则以水分子的形式离去。
由于酯化反应是可逆的，羧酸与醇以1:1的比例混合时，反应达到平衡后酯的产率约为70%。然而，与任何平衡过程一样，通过改变反应物的浓度、压力、温度或体积，可以使反应向某一方向进行。这种改变会导致反应重新调整至新的平衡状态，以抵消外界变化的影响，这正是勒夏特列原理的基础。
当某一反应物的浓度增加时，平衡会向降低该反应物浓度的方向移动。因此，通过增加一种反应物的浓度，平衡将生成更多的产物，从而提高酯化产物的产率。
有机化学实验室中常用的一种方法是提供过量的其中一种试剂，通常是醇。如前所述，费歇尔酯化反应可使用硫酸作为催化剂。然而，硫酸的另一个作用是作为脱水剂，将水分子从反应体系中移除。根据勒夏特列原理，当反应产物之一（在此为水）被移走时，反应会向生成更多产物的方向进行，从而提高所形成酯的产率。
<h4>参考文献</h4>
<ol>
	<li style="font-size:17px">Streitwieser, A., Heathcock, C.H., Kosower, E.M. (1998). 有机化学导论。Upper Saddle River, NJ: Prentice Hall.</li>
</ol>

酯  
酯的结构是一个羰基，其一侧连接一个烷基或芳基（R），另一侧的氧原子与另一个烷基或芳基（R'）相连……

Education

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JoVE Lab Manual

Chemistry

化学

酯化作用

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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Learn how carboxylic acids and alcohols form esters, why some smell fruity, and how Fischer esterification uses sulfuric acid to boost yield.

Ester Formation and Everyday Fragrances | JoVE K12

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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 ...

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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