When an organic acid, R-COOH, is heated through an alcohol, R'-OH, in the presence of a strong mineral acid, the chief organic product is a member of the family of organic compounds identified as esters. The common reaction for the esterification of an organic acid by an alcohol is
R-COOH + HO-R' ↔ R-CO-OR' + H2O
In this common reaction, R and R' symbolize hydrocarbon chains that might be the similar or different. As a specific instance, suppose acetic acid, CH3COOH, is heated through ethyl alcohol, CH3CH2OH, in the occurrence of a mineral acid catalyst. The esterification reaction will be CH3-COOH + HO-CH2CH3 ↔ CH3 -COO-CH2 CH3 + H2O
The ester product of this reaction (CH3-COO-CH2 CH3) is named ethyl acetate, indicating the acid and alcohol from which it is prepared. Esterification is an equilibrium reaction that means that the reaction doesn't go to completion on its own. Frequently, though, the esters produced are very volatile and can be eliminated from the system by distillation. If the ester isn't extremely easily distilled, it might be possible instead to add a desiccant to the equilibrium system, thus removing water from the system and forcing the equilibrium to the right.
Unlike many organic chemical compounds, esters often have extremely pleasant, fruitlike odours. Many of the odours and flavourings of fruits and flowers are due to the existence of esters in the necessary oils of such materials. The table that follows lists several esters by pleasant fragrances, as well as indicating from what alcohol and which acid the ester might be organized.
A fruit or flower usually contains only a few drops of ester, giving an extremely subtle odour. Frequently, the ester is part of several complex mixtures of substances that taken as a whole have the aroma attributed to the material. When prepared in the laboratory in moderately huge amounts, the ester might seem to have a pronounced chemical odour, and it might be hard to identify the fruit or flower that has this aroma.
Table: Common Esters and their constituents
Preparation of esters
Hotplate; 50% sulfuric acid; assorted alcohols and organic acids, as offered via the instructor, for the preparation of fruit and flower aromas; methyl salicylate; 20% NaOH; disposable 4 mL plastic pipette by stem cut to 2.5 cm.
Several general esters, and the acids/alcohols from that they are synthesized, were specified in the table in the introduction to this chapter. Synthesize at least 2 of the esters, and note their aromas. Diverse students might synthesize different esters, as directed via the instructor, and evaluate the odours of the products.
To synthesize the esters, mix 3-4 drops (or approximately 0.1 g if the acid is a solid) of the appropriate acid through 3-4 drops of the indicated alcohol on a clean, dry watch glass. Add 1 drop of 50% sulfuric acid to the mixture on the watch glass (Caution!). Utilize the tip of a plastic pipette to stir the mixture on the watch glass, and then suck as much as possible of the mixture into the pipette. Place the pipette, tip upward, into the warm-water bath, and permit it to heat for approximately 5 minutes. Squirt the consequential ester from the pipette into a beaker of warm water, and watchfully waft the vapours toward our nose.
Remember that the odour of an ester is extremely concentrated. Several sniffs might be needed for us to identify the odour of the ester. Record which esters we prepared and their aromas.
Concept of aldehydes and ketones:
Aldehydes and ketones both enclose the C=O or carbonyl group. Aldehydes have at least one hydrogen bonded directly to the C=O whereas ketones always have 2 alkyl groups attached to the C=O. Aldehydes might be prepared through oxidation of 1° alcohols; potassium dichromate (K2Cr2O7) in acidic solution (H2SO4 (aq)) can sometimes be used as the oxidizing agent.
R-CH2-OH + K2Cr2O7 R-C=O + Cr3+
(1° alcohol) (aldehyde)
Though it is hard to prevent further oxidation of the aldehyde product to a carboxylic acid. Ketones might be prepared via oxidation of 2° alcohols. Again, acidic K2Cr2O7 might be utilized.
R-CH-OH + K2Cr2O7 R-C=O + Cr3+
(2° alcohol) (ketone)
Unlike through aldehydes (see above) further oxidation of the ketone product isn't feasible. The general and significant compound acetone (IUPAC name, 2-propanone) is the easiest ketone) Acetone is a commercial solvent and is utilized in paint thinners and nail polish removers. Acetone is easily prepared via the oxidation of 2-propanol through acidic dichromate, a reaction that we will carry out in this lab. The acetone product will be purified through distillation.
Since they enclose the polar carbonyl group, aldehydes and ketones are polar compounds. Though, they can't form hydrogen bonds one to another, as do alcohols. Consequently, the boiling points of aldehydes and ketones are less than those of alcohols of similar molecular weight, but greater than those of hydrocarbons of similar molecular weight. The solubility of aldehydes and ketones in H2O is significant if they enclose less than 5 carbons. This is since hydrogen bonds to the water molecules are formed. Acetaldehyde (ethanal, CH3CHO) and acetone are miscible through water in all proportions.
Aldehydes are simply oxidized a fact due to the occurrence of the hydrogen joined to the carbonyl group (this isn't present in ketones that are less easily oxidized). Oxidation of aldehydes yields carboxylic acids. Even air will oxidize an aldehyde.
R-C=O + O2 R-C=O
(Aldehyde) (From air) (Carboxylic acid)
Other weak oxidizing agents can bring about this reaction. One of such is Tollens reagent, a basic (OH-) solution of the silver complex ion, Ag (NH3) +. The reaction produces metallic silver (Ag0), which often shapes a shiny 'mirror' on the sides of the container.
R-C=O + Ag(NH3)2+ R-C=O + Ag0
(Tollens' reagent) (Silver mirror)
Tollens' reagent is utilized to detect the presence of aldehydes. A solution of Benedict's reagent can also oxidize aldehydes. This solution consists of a basic (OH-) solution of copper (II) citrate (whose complex composition cannot be represented by a simple formula):
R-C=O + copper (II) citrate R-C=O + Cu2O
(Benedict's reagent) (copper (I) oxide)
The conversion of the clear, blue copper (II) citrate to insoluble, reddish copper (I) oxide specifies a positive test. The reaction takes place not only through easy aldehydes but also by 'reducing sugars' these as glucose.
20 mL 70% 2-propanol (isopropyl alcohol) Distilled water
100 mL acidic dichromate (K2Cr2O7/H2SO4) solution
Distillation apparatus including thermometer
Prepare an ice/water bath; this might be conveniently done in a large (for example > 500 mL) beaker. Place 20 mL of 70% 2-propanol in a 250-mL beaker, and add 20 mL of distilled H2O. Stir to mix, and cool the beaker in an ice bath to about 10°C.
With the solution still in the ice bath, add, all at once, 100 mL of "acidic dichromate" solution (CAUTION: Corrosive!). In a few seconds, the mixture will turn dark, followed by a rather sudden rise in temperature to 50-60°C. Stir the mixture (still in the ice bath!) until its temperature has fallen to below 50°C. As we know that don't utilize the thermometer as a stirring rod.
Pour the mixture into a 250-mL (or larger) distilling flask using a funnel to prevent spilling any. Accumulate a distillation apparatus as revealed via our lab instructor. Utilize a graduated cylinder as the receiver.
Heat gently. After 10-15 minutes, the liquid should start to boil and drops of acetone initiate to gather in the receiver. Proof the temperature when the 1st drop appears. Carry on the distillation until at least 5 mL of acetone has collected. Record the temperature again, and then stop the distillation. Calculate the volume of acetone obtained.
Oxidation of Ethanol to Ethanal Using CuO.
In this experiment you will study how to prepare acetaldehyde from ethanol.
10 ml Ethanol C2H5OH
Test tube + holder on a stand
a. Add the ethanol to the test tube and place the test tube on the stand.
b. Heat the Cu wire in an open fire until it becomes black (CuO)
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