Introduction

Hydroxyl group is used to describe the functional group –OH when it is a substituent in an organic compound. Hydroxyl groups are especially important in biological chemistry because of their tendency to form hydrogen bonds both as donor and acceptor. This property is also related to their ability to increase hydrophilicity and water solubility. The hydroxyl group is especially predominant in the family of molecules known as carbohydrates. Hydroxyl group is the characteristic functional group of alcohols and phenols.

Figure 1 Structure of a Hydroxyl-containing compound

Alcohols are characterized by one or more hydroxyl (−OH) groups attached to a carbon atom of an alkyl group (hydrocarbon chain). Alcohols may be considered as organic derivatives of water (H2O) in which one of the hydrogen atoms has been replaced by an alkyl group, typically represented by R in organic structures. Because of hydrogen bonding, alcohols tend to have higher boiling points than comparable hydrocarbons and ethers Alcohols, like water, can show either acidic or basic properties at the O-H group. With a pKa of around 16-19 they are generally slightly weaker acids than water, but they are still able to react with strong bases such as sodium hydride or reactive metals such as sodium. The salts that result are called alkoxides. Alcohols have an odor that is often described as “biting” and as “hanging” in the nasal passages.

Classification Tests for Hydroxyl- and Carbonyl- Containing Compounds

Reyes, K.O., Rivera, G.I., Samonte, A.A., Soliven, R.Y.*, Sotto, K.C.2CMT, Faculty of Pharmacy, UST

Abstract

Hydroxyl- or carbonyl- containing samples were given to the group for analysis. Hydroxyl group refers to a functional group containing OH- when it is a substituent in an organic compound whereas carbonyl group refers to a divalent chemical unit consisting of a carbon and an oxygen atom connected by a double bond. Hydroxyl group is the characteristic functional group of alcohols and phenols while carbonyl group is the characteristic functional group of aldehydes and ketones. The samples were analyzed through tests involving solubility of alcohols in water, Lucas Test, Chromic Acid Test (Jones Oxidation), 2,4-Dinitrophenylhydrazone (2,4-DNP) Test, Fehling’s Test, Tollens’ Silver Mirror Test, and Iodoform Test. Lucas Test differentiates primary, secondary, and tertiary alcohols. Chromic Acid Test is a test for oxidizables or any compounds that possess reducing property 2,4-DNP Test is a test for aldehydes and ketones. Fehling’s Test and Tollens’ Silver Mirror Test are tests for aldehydes. Iodoform test is a test for methyl carbinol and methyl carbonyl groups. When solubility of ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol and benzyl alcohol was tested, only benzyl alcohol exhibited insolubility. Upon subjecting n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol to Lucas Test, tert-butyl alcohol formed two layers, sec-butyl alcohol demonstrated slight turbidity and n-butyl alcohol produced a clear solution. N-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-butraldehyde, benzaldehyde, acetone, and acetophenone underwent Chromic Acid Test (Jones Oxidation). N-butyl alcohol, sec-butyl alcohol, n-butraldehyde, and benzaldehyde showed a positive result of green or blue-green solution. Fehling’s Test, 2,4-dinitrophenylhydrazone (2,4-DNP) test, and Tollens’ Silver Mirror Test were performed on acetone, acetaldehyde, n-butraldehyde, benzaldehyde, and acetophenone. All compounds produced a positive result of either a yellow or an orange precipitate when subjected to 2,4-DNP test. Acetaldehyde and n-butraldehyde are positive to Fehling’s test forming a crude red precipitate and benzaldehyde demonstrated turbidity. Acetone and acetophenone produced a negative result of a clear blue solution under the same test. The samples produced a positive result of silver mirror except for acetone which produced a grayish-black solution and acetophenone which formed a gel-like precipitate. Lastly, Iodoform Test was performed on acetaldehyde, acetone, acetophenone, benzaldehyde, and isopropyl alcohol. All of the compounds exhibited positive result forming a yellow precipitate excluding benzaldehyde which formed a white suspended precipitate instead.

There are three classifications of alcohols by the carbon to which the hydroxyl group is attached. Primary alcohols are those in which the hydroxyl group is attached to the carbon with only one carbon attached. Secondary alcohols are compounds in which the OH- is attached to a carbon which has two other carbons attached. Tertiary alcohols are compounds in which a hydroxyl group is attached to a carbon with three attached carbons.

Phenols are aromatic compounds in which a hydroxide group is directly bonded to an aromatic ring system. They are very weak acids, and like alcohols, form ethers and esters. The main phenols are phenol itself, cresol, resorcinol, pyrogallol, and picric acid. Phenol itself (C6H5OH), also known as carbolic acid, is a white, hygroscopic crystalline solid, isolable from coal tar, but made by acid hydrolysis of cumene hydroperoxide, or by fusion of sodium benzenesulfonate with sodium hydroxide. Formerly used as an antiseptic, phenol has more latterly been used to make bakelite and other resins, plastics, dyes, detergents, and drugs.

The hydroxyl- containing compounds used in the experiment are ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, isopropyl alcohol, and benzyl alcohol.

Ethanol, C2H5OH, (also known as ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol) is the second member of the aliphatic alcohol series. It is a clear, colorless, volatile, and flammable liquid which is completely miscible with water and organic solvents. It burns with a smokeless blue flame that is not always visible in normal light. Ethanol has widespread use as a solvent of substances intended for human contact or consumption, including scents, flavorings, colorings, and medicines. In chemistry, it is both an essential solvent and a feedstock for the synthesis of other products.

Figure 2 Structure of Ethanol

N-butyl alcohol (also known as n-butanol, 1-Butanol or 1-butyl alcohol) is a four carbon straight chain alcohol. It is a volatile, clear liquid with a strong alcoholic odor, and is miscible with water. It is a highly refractive compound which corrodes some plastics, and rubbers. It is miscible with many organic solvents, and incompatible with strong oxidizers. N-butanol is used as a direct solvent and as an intermediate in the manufacture of other organic chemicals.

Figure 3 Structure of n-butyl alcohol

Sec-butyl alcohol with the formula C H 3CH(OH)CH2CH3 (also known as sec-butanol, 2-butyl alcohol, or 2-butanol) is a flammable, colorless liquid that is soluble in 12 parts water and completely miscible with polar organic solvent such as ethers and other alcohols.

Figure 4 Structure of sec-butyl alcohol

Tert-Butanol, C4H10O is a colorless liquid or white solid, depending on the ambient temperature. It is the simplest tertiary alcohol. and one of the four isomers of butanol. tert-Butanol is a clear liquid with a camphor-like odor. It is very soluble in water and miscible with ethanol and diethyl ether. It is unique among the isomers of butanol because it tends to be a solid at room temperature, with a melting point slightly above 25 °C. As a tertiary alcohol, tert-butanol is more stable to oxidation and less reactive than

the other isomers of butanol. tert-Butanol is used as a solvent, as a denaturant for ethanol, as an ingredient in paint removers, as an octane booster for gasoline, as an oxygenate gasoline additive, and as an intermediate in the synthesis of other chemical commodities, other flavors and perfumes.

Figure 5 Structure of tert-butyl alcohol

Isopropyl alcohol (also propan-2-ol, 2-propanol is a common name for a chemical compound with the molecular formula C3H8O. It is a colorless, flammable chemical compound with a strong odor. It is the simplest example of a secondary alcohol, where the alcohol carbon is attached to two other carbons. Being a secondary alcohol, isopropyl alcohol can be oxidized to acetone, which is the corresponding ketone. Isopropyl alcohol dissolves a wide range of non-polar compounds. It is also relatively non-toxic and evaporates quickly. Thus it is used widely as a solvent and as a cleaning fluid, especially for dissolving lipophilic contaminants such as oil.

Figure 7 Structure of Isopropyl alcohol

Benzyl alcohol, C6H5CH2OH, is a colorless liquid with a mild pleasant aromatic odor. It is a useful solvent due to its polarity, low toxicity, and low vapor pressure. Benzyl alcohol is partially soluble in water (4 g/100 mL) and completely miscible in alcohols and diethyl ether. Like most alcohols, it reacts with carboxylic acids to form esters. Benzyl alcohol is used as a general solvent for inks, paints, lacquers, and epoxy resin

coatings. It is also a precursor to a variety of esters, used in the soap, perfume, and flavor industries. It is often added to intravenous medication solutions as a preservative due to its bacteriostatic and antipruritic properties.

Figure 6 Structure of Benzyl alcohol

Carbonyl group is a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom connected by a double bond. The group is a constituent of carboxylic acids, esters, anhydrides, acyl halides, amides, and quinones, and it is the characteristic functional group of aldehydes and ketones. Carboxylic acid (and their derivatives), aldehydes, ketones, and quinones are also known collectively as carbonyl compounds. Aldehydes and ketones contain carbonyl groups attached to alkyl or aryl groups and a hydrogen atom or both. These groups have little effect on the electron distribution in the carbonyl group; thus, the properties of aldehydes and ketones are determined by the behaviour of the carbonyl group. In carboxylic acids and their derivatives, the carbonyl group is attached to one of the halogen atoms or to groups containing atoms such as oxygen, nitrogen, or sulfur. These atoms do affect the carbonyl group, forming a new functional group with distinctive properties.

Figure 7 Structure of a Carbonyl-containing compound

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, called an aldehyde group, consists of a carbon atom bonded to a hydrogen atom with a single

covalent bond and an oxygen atom with a double bond. Thus the chemical formula for an aldehyde functional group is -CH=O, and the general formula for an aldehyde is R-CH=O. The aldehyde group is occasionally called the formyl or methanoyl group. The word aldehyde is a combination of parts of the words alcohol and dehydrogenated, because the first aldehyde was prepared by removing two hydrogen atoms (dehydrogenation) from ethanol. Molecules that contain an aldehyde group can be converted to alcohols by the addition of two hydrogen atoms to the central carbon oxygen double bond (reduction). Organic acids are the result of the introduction of one oxygen atom to the carbonyl group (oxidation). Aldehydes are very easy to detect by smell. Some are very fragrant, and others have a smell resembling that of rotten fruit.

Figure 8 Structure of Aldehyde

Ketone features a carbonyl group (C=O) bonded to two other carbon atoms. They differ from aldehydes in that the carbonyl is placed between two carbons rather than at the end of a carbon skeleton. They are also distinct from other functional groups, such as carboxylic acids, esters and amides, which have a carbonyl group bonded to a hetero atom. Ketone compounds have important physiological properties. They are found in several sugars and in compounds for medicinal use, including natural and synthetic steroid hormones.

Figure 9 Structure of Ketone

Some of the carbonyl-containing compounds used in the experiment are

benzaldehyde, n-butraldehyde, acetaldehyde, acetone and acetophenone.

Benzaldehyde, C6H5CHO (also known as benzenecarbonal) is a colorless liquid aldehyde with a characteristic almond odor. It boils at 180°C, is soluble in ethanol, but is insoluble in water. It is formed by partial oxidation of benzyl alcohol, and on oxidation forms benzoic acid. It is called oil of bitter almond, since it is formed when amygdalin, a glucoside present in the kernels of bitter almonds and in apricot pits, is hydrolyzed, e.g., by crushing the kernels or pits and boiling them in water; glucose and hydrogen cyanide (a poisonous gas) are also formed. It is also prepared by oxidation of toluene or benzyl chloride or by treating benzal chloride with an alkali, e.g., sodium hydroxide. Benzaldehyde is used in the preparation of certain aniline dyes and of other products, including perfumes and flavorings.

Figure 10 Structure of Benzaldehyde

Acetaldehyde, CH3CHO (also known as ethanol) is a colorless liquid aldehyde, sometimes simply called aldehyde. It is soluble in water and ethanol. Acetaldehyde is made commercially by the oxidation of ethylene with a palladium catalyst. It is used as a reducing agent (e.g., for silvering mirrors), in the manufacture of synthetic resins and dyestuffs, and as a preservative.

Figure 11 Structure of Acetaldehyde

N-butyraldehyde (also known as butanal) is an organic compound with the formula CH3(CH2)2CHO. This compound is

the aldehyde derivative of butane. It is a colourless flammable liquid that smells like sweaty feet. It is miscible with most organic solvents. n-Butyraldehyde is used as an intermediate in the manufacturing plasticizers, alcohols, solvents and polymers (such as 2-Ethylhexanol, n-butanol, trimethylolpropane, n-butyric acid, polyvinyl butyral, methyl amyl ketone). It is also used as an intermediate to make pharmaceuticals, agrochemicals, antioxidants, rubber accelerators, textile auxiliaries, perfumery and flavors.

Figure 12 Structure of n-Butyraldehyde

Acetone (also known as propanone) is the organic compound with the formula (CH3)2CO. This colorless, mobile, flammable liquid with a characteristic sweetish smell is the simplest example of the ketones. Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory.

Figure 13 Structure of Acetone

Acetophenone is the organic compound with the formula C6H5C(O)CH3. It is the simplest aromatic ketone. This colourless, viscous liquid is a precursor to useful resins and fragrances. Acetophenone can be obtained by a variety of methods. In industry, acetophenone is recovered as a by-product of the oxidation of ethylbenzene, which mainly gives ethylbenzene hydroperoxide for use in the production of propylene oxide

Figure 14 Structure of Acetophenone

The hydroxyl- and carbonyl- containing compounds were analyzed by utilization of different tests such as testing the solubility of alcohols in water, Lucas Test, Chromic Acid Test (Jones Oxidation), 2,4-Dinitrophenylhydrazone Test, Fehling’s Test, Tollens’ Silver Mirror Test, and Iodoform Test.

Most organic compounds are not soluble in water with the exception of low molecular-weight amines and oxygen-containing compounds like alcohols, carboxylic acids, aldehydes, and ketones. Low molecular-weight compounds are generally limited to those with fewer than five carbon atoms.

Lucas Test often provides classification information for alcohols, as well as a probe for the existence of the hydroxyl group. Substrates that easily give rise to cationic character at the carbon bearing the hydroxyl group undergo this test readily; primary alcohols do not give a positive result. Since the Lucas Test depends on the appearance of the alkyl chloride as a second liquid phase, it is normally applicable only to alcohols that are soluble in the reagent. This limits the test in general to monofunctional alcohols lower than hexyl and certain polyfunctional molecules.

Chromic Acid Test (Jones Oxidation) detects the presence of a hydroxyl substituent that is on a carbon bearing at least one hydrogen, and therefore oxidizable.

2,4-Dinitrophenylhydrazone Test can be used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group.

Fehling’s Test and Tollens’ Silver Mirror Test are used to detect aldehydes. However, Fehling's solution can only be used to test for aliphatic aldehydes,

whereas Tollens' reagent can be used to test for both aliphatic and aromatic aldehydes.

Iodoform Test is a test for methyl carbinol (secondary alcohol with adjacent methyl group) and methyl carbonyl group.

Methodology

A. Solubility of Alcohols in Water

Five test tubes were labelled accordingly and ten drops each of ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, and benzyl alcohol were placed into the test tubes by the use of a Pasteur pipette. 1-ml of water was then added dropwise to the tube containing alcohol and the mixture was shaken thoroughly after each addition. If cloudiness resulted, 0.25-ml of water at a time was added continuously with vigorous shaking until a homogeneous dispersion results. The total volume of water added was noted. If cloudiness resulted after the addition of 2.0-ml of water, the alcohol is said to be soluble in water. The results were noted down.

B. Lucas Test

This test was performed on n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol.

Lucas reagent was prepared by dissolving 16 g of anhydrous zinc chloride in 10-ml of concentrated hydrochloric acid. The mixture was then allowed to cool.

50-mg or 2-3 drops of the sample was added to 1-ml of the reagent in a small vial or test tube and the mixture was shaken vigorously for a few seconds. The mixture was allowed to stand at room temperature. The rate of formation of the cloudy suspension or the formation of two layers were observed.

C. Chromic Acid Test (Jones Oxidation)

This test was performed on n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-butyraldehyde, benzaldehyde, acetone, and acetophenone.

1 drop of liquid or a small amount of the solid sample was dissolved in 1-ml of acetone in a small vial or test tube. 2 drops of 10% aqueous Potassium chromate solution and 5 drops of 6M sulphuric acid were added into the mixture.

D. 2,4-dinitrophenylhydrazone (or 2,4-DNP Test)

This test was performed on acetone, acetaldehyde, n-butyraldehyde, benzaldehyde, and acetophenone.

The reagent was prepared by slowly adding a solution of 3 g of 2,4-dinitrophenylhydrazine in 15-ml of concentrated sulphuric acid, while stirring to a mixture of 20-ml of water and 70-ml of 95% ethanol. The solution was then stirred and filtered.

A drop of a liquid sample was placed into a small sample. 5 drops of 95% ethanol was added and well shaken. Afterwards, 3 drops of 2,4-DNP was added and if no yellow or orange precipitate formed, the solution was allowed ro stand for at least 15 minutes.

E. Fehling’s Test

This test was performed on acetaldehyde, n-butyraldehyde, acetone, benzaldehyde, and acetophenone.

Fehling’s reagent was prepared by mixing equal amounts of Fehling’s A and Fehling’s B. Fehling’s A was prepared by dissolving 7 g of hydrated copper (II) sulfate in 100-ml of water. Fehling’s B was prepared by mixing 35 g of Potassium sodium tartrate and 10 g of Sodium hydroxide in 100-ml water.

1-ml of freshly prepared Fehling’s reagent was placed into each test tube. 3

drops of the sample to be tested was added in to the tube. The tubes were then placed in a beaker of boiling water and changes within 10-15 minutes were observed.

F. Tollens’ Silver Mirror Test

This test was performed on acetaldehyde, benzaldehyde, acetone, n-butyraldehyde, and acetophenone.

The reagent was prepared by adding 2 drops of 5% Sodium hydroxide solution to 2-ml of 5% Silver nitrate solution and mixing thoroughly. Next, only enough 2% ammonium hydroxide (concentrated ammonium hydroxide is 28%) was added drop by drop and with stirring to dissolve the precipitate. Adding excess ammonia will cause discrepancies on the result of the test.

Four test tubes with 1-ml of freshly prepared Tollens’ reagent were prepared. Two drops each of the samples were then added. The mixture was shaken and allowed to stand for 10 minutes. If no reaction has occurred, the test tube was placed in a beaker of warm water (35-50 oC) for 5 minutes. Observations were recorded.

It was noted that if Tollens’ reagent is left unused for a period of time, it may form explosive silver. This was avoided by neutralizing unused reagent with a little nitric acid and discarded afterwards.

G. Iodoform Test

This test was performed on acetaldehyde, acetone, acetophenone, benzaldehyde, and isopropyl alcohol.

2 drops of each sample was placed into its own small vial or test tube. 20 drops of fresh chlorine bleach (5% Sodium hypochlorite) was slowly added while shaking to each test tube and then, mixed. The formation of a yellow participate was noted.

Results and Discussion

Table 1 Solubility of Alcohol in WaterAlcohol Condensed

Structural FormulaAmount of Water (in ml) needed to produce a homogeneous dispersion

Solubility in Water

Ethanol CH3CH2OH 1 ml MiscibleN-butyl alcohol

CH3CH2CH2CH2OH 2 ml Miscible

Sec-butyl alcohol

1 ml Miscible

Tert-butyl alcohol

1 ml Miscible

Benzyl alcohol

N/A Immiscible

Table 1 shows alcohols such as ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, and benzyl alcohol and their solubility in water.

Ethanol, n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol are all miscible with water with the exception of benzyl alcohol which exhibited insolubility.

The table shows that all alcohols are soluble in water except under C6. There are different factors affecting solubility. One of which is number of carbon atom wherein the higher the number of carbon atoms, the more insoluble the alcohol is in water. Another factor is the branching of carbon chain in which the more branching present, the more soluble (with the same number of carbons) it is. Lastly, the presence of polar functional groups (-OH, -NH2, -CO2H) also tends to affect alcohol solubility in water. A compound with polar functional group is more soluble in water.

As stated, all alcohols are soluble in water except under C6. Hence, ethanol, n-butyl alcohol, sec-butyl alcohol, and tert-butyl alcohol are all miscible with water. Ethanol has two carbon atoms, while the other three all have four carbons since they are all derivatives of the alcohol, butanol. Benzyl alcohol is immiscible with water because it is an aromatic alcohol.

Ethanol is the most soluble alcohol followed by tert-butyl alcohol, sec-butyl

alcohol, and n-butyl alcohol. Ethanol exhibits fastest solubility because it has only two carbon atoms as compared to the butanol derivatives having four carbon atoms. Tert-butyl alcohol is the most soluble among the butanol derivatives because it has the most branching substituents present.

Table 2 Reaction of Sample Compounds to Lucas Test

Substance Condensed Structural Formula

Reaction Inference

n-butyl alcohol

CH3CH2CH2CH2OH Clear solution

Miscible

Sec-butyl alcohol

Clear solution

Miscible

Tert-butyl alcohol

Turbid solution and formation of two layers

Immiscible

Table 2 shows the reaction of butanol derivatives to Lucas Test. N-butyl alcohol and sec-butyl alcohol yielded a clear solution when subjected to Lucas Test whereas tert-butyl alcohol resulted to a cloudy immiscible suspension which eventually formed two layers.

Lucas Test differentiates primary, secondary, and tertiary alcohols. Reagents used include anhydrous ZnCl2 and HCl. Positive result is based on turbidity (alkyl chloride formation) and the rate of the reaction was observed. Tertiary alcohols form the second layer in less than a minute. Secondary alcohols require 5-10 minutes before formation of second layer while primary alcohols are usually unreactive. Based on Table 2, tert-butyl alcohol immediately formed two layers; hence, it is known to be a tertiary alcohol. Sec-butyl alcohol when subjected to Lucas test resulted to a clear solution although theoretically, a secondary alcohol dissolves to give a clear solution (provided R does not have too many carbon atoms in the chain.), then form chlorides (cloudy solution) within five minutes. N-butyl alcohol was unreactive and is considered to be the primary alcohol. Generally, the order of reactivity of the alcohols toward Lucas reagent is 3°>2°>1° because the

reaction rate is much faster when the carbocation intermediate is more stabilized by a greater number of electron donating alkyl group bonded to the positive carbon atom. This means that the greater the alkyl groups present in a compound, the faster its reaction would be with the Lucas solution.

The reaction of alcohols with halogen acids is a displacement reaction in which the reactive species is the conjugate acid of the alcohol R-OH2

+, and as might be expected, is analogous to the replacement reactions of organic halides and related compounds with silver nitrate and iodide ion. The effects of structure on reactivity in these reactions are closely related. Thus, primary alcohols do not react perceptibly with hydrochloric acid even in the presence of zinc chloride at ordinary temperatures; chloride ion is too poor a nucleophilic agent to effect a concerted displacement reaction, on the one hand, and the primary carbonium ion is too unstable to serve as an intermediate in the carbonium mechanism, on the other. Hydrogen bromide and Hydrogen iodide, which have anions with nucleophilic reactivity increasing in that order, are increasingly reactive toward primary alcohols. These are nucleophilicity orders to be expected in hydroxylic solvents.

Tertiary alcohols react with concentrated hydrochloric acid so rapidly that the alkyl halide is visible within a few minutes at room temperature, at first as a milky suspension and then as an oily layer. The acidity of the medium is increased by the addition of the anhydrous zinc chloride (a strong Lewis acid), and the reaction rate is increased further. This reaction is not a nucleophilic displacement comparable to that undergone by primary alcohols but rather proceeds by way of a carbonium ion intermediate. The high reactivity of tertiary alcohols is a consequence of the

relatively great stability of the intermediate carbonium ion. Allyl alcohol, although a primary alcohol, yields a carbonium ion that is relatively stable because its charge is distributed equally on the two terminal carbon atoms. CH2=CH-CH2OH→[CH2=CHCH2

+↔+CHCH=CH2]As might be expected, it reacts rapidly with Lucas reagent with the evolution of heat. Allyl chloride may be caused to separate by dilution of the mixture with ice water.

Secondary alcohols are intermediate in reactivity between primary and tertiary alcohols. Although they are not appreciably affected by concentrated hydrochloric acid alone, they react with it fairly rapidly in the presence of anhydrous zinc chloride; a cloudy appearance of the mixture is observed within 5 minutes, and in about 10 minutes, a distinct layer is usually visible.

Table 3 Reaction of Sample Compounds to Chromic Acid Test

Substance Condensed Structural Formula

Reaction

n-butyl alcohol CH3CH2CH2CH2OH Clear blue solution

Sec-butyl alcohol Clear blue solution

Tert-butyl alcohol Clear yellow solution

n-butyraldehyde Clear blue solution

Benzaldehyde Clear green solution

Acetone Clear yellow solution

acetophenone Clear yellow solution

Chromic Acid Test/Dichromate Test/Jones Test is a test for oxidizables or

any compounds that possess reducing property (has an alpha acidic hydrogen. Reagent used includes 10% Potassium chromate and 6 M sulfuric acid.

Table 3 shows the reaction of n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, n-butyraldehyde, benzaldehyde, acetone, and acetophenone to Chromic Acid test. N-butyl alcohol. sec-butyl alcohol, benzaldehyde, and n-butyraldehyde resulted to a clear blue or green solution whereas tert-butyl alcohol, acetone, and acetophenone resulted to a clear yellow solution.

Primary, secondary alcohols and aldehydes give a positive visible result. Positive result exhibits a green or blue-green solution; hence, n-butyl alcohol, sec-butyl alcohol, and n-butyraldehyde all formed either blue or green solutions. Chromic acid test involves redox reaction. Primary, secondary alcohols and aldehydes undergo oxidation and chromium undergoes reduction (from Cr+6

to Cr+3). Primary, secondary alcohols and aldehydes will reduce the orange-red chromic acid/sulfuric acid reagent to an opaque green or blue suspension of Cr(III) salts in 2-5 seconds. A primary alcohol reacts with chromic acid to yield aldehyde, which is further oxidized tocarboxylic acid. A secondary alcohol reacts with chromic acid to yield ketone, which does not oxidize further. A tertiary alcohol is usually unreactive.

Table 4 Reaction of Sample Compounds to 2,4-DNP Test

Substance Condensed Structural Formula

Reaction

Acetaldehyde Yellow precipitate

n-butyraldehyde Orange solution

Benzaldehyde Yellow precipitate

Acetone Yellow precipitate

acetophenone Orange precipitate

2,4-Dinitrophenylhydrazone (2,4-DNP) test is a test for carbonyl groups It gives a positive result for aldehydes and ketones. Its mechanism is condensation or addition and elimination. The test involves nucleophilic addition of NH2 to C=O and elimination of H2O. Reagents used include 2,4-dinitrophenylhydrazine, ethanol, and H2SO4. Positive result is the formation of a red-orange precipitate (conjugated carbonyl compounds) or yellow precipitate (non-conjugated carbonyl compounds).

Table 4 shows the reaction of acetaldehyde, n-butyraldehyde, benzaldehyde, acetone, and acetophenone to 2,4-DNP test. All the samples exhibited positive result because they all formed either a yellow or an orange precipitate. Hence, 2,4-DNP test proved that the samples are carbonyl-containing compounds and are either aldehydes or ketones.

The reaction of 2,4-DNPH with aldehydes and ketones in an acidic solution is a dependable and sensitive test. Most aldehydes and ketones yield dinitrophenylhydrazones that are insoluble solids. The precipitate may be oily at first and become crystalline on standing. A number of ketones, however, give dinitrophenylhydrazones that are oils. A further difficulty with the test is that certain allyl alcohol derivatives may be oxidized by the reagent to aldehydes and ketones, which then give a positive result.

If the dinitrophenylhydrazone appears to be formed in very small amount, it may be desirable to carry out the reaction on the scale employed for the preparation of a derivative and to make an estimate of the yield. The melting point of the solid should be checked to be sure it is different from that of 2,4-dinitrophenylhydrazine (MP 198oC). If necessary, this hydrazone derivative can be recrystallized from a solvent such as ethanol. Solvents containing reactive carbonyl groups should not be used, as they may result to formation of another hydrazone.

The color of a 2,4-dinitrophenylhydrazone may give an indication as to the structure of the aldehyde or ketone from which it is derived. Dinitrophenylhydrazones of aldehydes or ketones in which the carbonyl group is not conjugated with another functional group are yellow. Conjugation with a carbon-carbon double bond or with a benzene ring shifts the absorption maximum towards the visible and is easily detected by an examination of the ultraviolet spectrum. However, this shift is also responsible for a change in color from yellow to orange-red. In general, then, a yellow dinitrophenylhydrazone may be assumed to be unconjugated. However, an orange or red color should be interpreted with caution, since it may be due to contamination by an impurity.

Table 5 Reaction of Sample Compounds to Fehling’s Test

Substance Condensed Structural Formula

Reaction

Acetaldehyde Crude red precipitate

n-butyraldehyde Crude red precipitate

Benzaldehyde Turbid solution; oily layer

Acetone Clear blue solution

acetophenone Clear blue solution

Fehling’s Test is a test for aldehydes. Reagents include CuSO4, NaOH ( Cu2+ in alkaline solution). Positive result is the formation of crude-red precipitate (Cu2O/cuprous oxide).

As shown in Table 5, acetaldehyde, n-butyraldehyde, and benzaldehyde exhibited positive result. Acetaldehyde, in particular turned from blue to muddy green then formed a crude red precipitate upon heating. These three sample compounds which exhibited positive result to Fehling’s test are all aldehydes.

Fehling’s test involves redox reaction wherein aldehyde is oxidized to carboxylic acid and ketones do not undergo oxidation. Copper is reduced (from Cu2+ to Cu+).

Table 6 Reaction of Sample Compounds to Tollens’ Silver Mirror Test

Substance Condensed Structural Formula

Reaction

Acetaldehyde Silver Mirror

n-butyraldehyde Silver Mirror

Benzaldehyde Silver Mirror

Acetone Clear grayish-black solution

acetophenone Gel-like precipitate

Tollens’ Silver Mirror test is a test for aldehydes. The preparation of Tollens reagent is based on the formation of a silver diamine complex that is water soluble in basic solution. As shown in

Table 6, acetaldehyde, n-butyraldehyde, and benzaldehyde exhibited positive result of formation of silver mirror whereas acetone and acetophenone do not. Acetone resulted to a clear grayish-black solution while acetophenoe formed a gel-like precipitate. They are both negative for Tollens’ Silver Mirror test. The test proved that acetaldehyde, n-butyraldehyde, and benzaldehyde are aldehydes.

Tollens’ Silver Mirror test involves reduction-oxidation reaction wherein aldehyde is oxidized to carboxylic acid and ketones do not undergo oxidation except alpha-hydroxyketone. Silver is reduced from Ag+ to Ag0. Formic acid, hydroxylamine, acyloins, diphenylamine, and other aromatic amines, as well as a-naphthol and certain other phenols will give a positive result. a-Alkoxy and a-dialkylamino ketones have also been found to reduce ammoniacal silver nitrate. In addition, the stable hydrate of trifluoroacetaldehyde gives a positive result.

The test often results in a smooth deposit of silver metal on the inner surface of the test tube, hence the name “silver mirror” test. In some cases, however, the metal forms merely as a granular gray or black precipitate, especially if the glass is not scrupulously clean. The reaction is autocatalyzed by the silver metal and often involves an induction period of a few minutes.

Table 7 Reaction of Sample Compounds to Iodoform Test

Substance Condensed Structural Formula

Reaction

Acetaldehyde Yellow precipitate

Benzaldehyde White suspended precipitate

Acetone Yellow precipitate

acetophenone Yellow precipitate

Isopropyl alcohol Yellow precipitate

Iodoform Test is a test for methyl carbinol (secondary alcohol with adjacent methyl group) and methyl carbonyl groups. Reagents include 10% KI and NaClO. Positive result is exhibited by the formation of yellow crystals or precipitate. Table 7 shows that among the sample compounds tested, acetaldehyde, acetone, acetophenone, and isopropyl alcohol exhibited positive result. Compounds with a methyl group next to a carbonyl group give a positive result with the iodoform (tri-iodomethane) test. Ethanol and secondary alcohols with a methyl group attached to the same carbon as the –OH group will also give a positive iodoform test. This is because the iodine oxidizes the alcohols to a carbonyl compound with a methyl group next to the carbonyl group.

When a - methyl carbonyl compounds react with iodine in the presence of a base, the hydrogen atoms on the carbon adjacent to the carbonyl group (a  hydrogens) are subsituted by iodine to form tri iodo methyl carbonyl compounds which react with OH - to produce iodoform and carboxylic acid  (2):

References

BOOKSShriner, Ralph Lloyd. (1980). Systematic Identification of Organic Compound: A Laboratory Manual Sixth Edition. John Wiley & Sons, Inc. New York: Van Hoffmann Press

Rodewald, L., Roberts, R. (1979). Modern Experimental Organic Chemistry. New York: Holt, Rinehart and Winston

Shriner, Fuson, Curtin. (1964). Systematic Identification of Organic Compound: A Laboratory Manual Fifth Edition. John Wiley & Sons, Inc. New York: Van Hoffmann Press

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