Qualitative Analysis

Texas A&M University Corpus Christi

Organic Chemistry Lab 3411.104

Ryla Best/ 81

4/25/12

Introduction

The purpose of the experiment was to qualitatively determine unsaturation, alkyl

halide, alcohol, and carbonyl functional groups.

Background

The first step of identifying an unknown compound is uncovering its purity and

physical properties like physical state, color, odor, melting point, boiling point, and

various functional group tests.

The Bromine Test is tested on alkenes and alkynes for unsaturation. The bromine adds to

the carbon-carbon double bond of alkenes to produce dibromoalkanes or reacts with

alkynes to produce tetrabomoalkanes. A colorless product indicates that the test is

positive for unsaturation.

The silver nitrate test is tested on alkyl halides for determining if the carbon is primary,

secondary, or tertiary halide. Tertiary halides are more reactive than secondary halides

and secondary are more reactive than primary which determine the rate of precipitation.

The white color indicates AgCl, pale yellow indicates AgBr, and AgI indicates dark

yellow. Aryl halides like general vinyl or alkynyl are unreactive

The Lucus Test is tested on alcohols for the chloride replacing the hydroxyl group and to

determine if the carbon is primary, secondary, or tertiary. With the reagent, primary

alcohols give no reaction, secondary alcohols react more rapidly, and tertiary alcohols

react very rapidly. Formation of a second layer due to the alkyl chloride or emulsion

indicates a positive test.

2-4 DNPH Test

2-4 DNPH Test is tested on carbonyls to determine if it contains either an aldehyde or a

ketone. Arylhdrazines give a positive test indicated by forming precipitation.

Tollen’s Test

The Tollen’s Test tests carbonyls to distinguish between an aldehyde and a ketone. A

positive test indicates the presence of an aldehyde function by forming a silver mirror or

precipitate. Ketones have no reaction. The reagent oxidizes both aliphatic and aromatic

aldehydes to the corresponding carboxylic acids and reduces the silver ion.

SOP/Results

Bromide Test: 0.1mL of Br2 was dissolved in 20mL of CH2Cl2. 2 drops of the

alkenes listed in table 1 was dissolved in .5mL of CH2CL2.

Table 1: Bromide Test

Substrate Color Functional group

1-Hexene colorless Alkene

2-Hexene colorless Alkene

3,3- dimethyl-1-

butene

colorless Alkene

Silver Nitrate Test: 0.35g of AgNO3 was dissolved in 20mL of 95% EtOH. 1 drop

of the alkyl halide listed in Table 2 was added to 2mL of the test solution.

Table 2: Silver Nitrate Test

Substrate Color Functional group Carbon-carbon

bond

2-chloro-butane White cloudy Alkyl halides Secondary

2-Iodopropane Dark yellow Alkyl halides Secondary

1-bromobutane cloudy Alkyl halides Primary

Lucas Test: 29.8g of anhydrous ZnCl2 was dissolved in 20mL of 12M HCl. 5mL

of the Lucas reagent was added to 0.5mL of the alcohol listed in Table 3.

Table 3: Lucas Test

Substrate Color Functional group Carbon-carbon

bond

Ethanol Instant clear Alcohol Primary

2-propanol Slighty cloudy then

clear

Alcohol Secondary

Octanol emulsion Alcohol Tertiary

2,4 DNPH Test: 0.2g of 2,4 DNPH was prepared in concentrated sulfuric acid.

25/75 (1.5mL/5mL) of H2O/EtOH was mix well. 2 drops of carbonyl from Table 4 was

added to 2mL 95% EtOH.

Table 4: 2,4 DNPH Test

Substrate Observations MP (oC) Functional

Group

Benzaldehyde Orange, lots of

precipitate

238 Aldehyde

Butyraldehyde Yellow

precipitate,

pumpkin smell

92 Aldehyde

2-Furaldehyde Red precipitate 198 Ketone

2,4-Dimethyl-

3-pentanone

Minty smell, no

precipitate

84 Ketone

Tollen’s Test: 30mL of KOH was dissolved in 4.2mL of water. 3 drops of carbonyls from

Table 5 to 0.5mL of Tollen’s reagent.

Table 5: Tollen’s Test

Substrate Observations Functional Group

Benzaldehyde Yellow, brown precipitate

after heat

Aldehyde

Butyraldehyde Clear, white/silver mirror

after heat

Aldehyde

2-Furaldehyde Black, black after heat Ketone

2,4-Dimethyl-3-pentanone Clear, silver after heat Ketone

Discussion/Conclusion

For the bromide test, a hypothesis can be stated that all the alkenes will be

colorless. Alkenes are unsaturated because of the presence of double bond between the

atoms of carbon. Since the bromine adds to the carbon-carbon double bond to produce

dibromoalkanes, the result shows a colorless product which means it was unsaturated.

The reaction was an example of an addition reaction. For the silver nitrate test, a

hypothesis can be stated that the primary alkyl halide will have a slower reaction rate.

The white cloudy of 2-chloro-butane indicates AgCl in the compound. The dark yellow

color of 2-Iodopropane indicates AgI in the compound. The cloudiness of 1-bromobutane

should indicate AgBr but the product wasn’t pale yellow which may be due to the

primary carbon for its slow reaction rate or human error. If it was a vinyl or alkynyl, it

would have been unreactive. The reaction was an example of a Sn1 reaction which the

leaving group of Br leaves and forms a carbocation. For the Lucas test, a hypothesis can

be stated that primary alcohols, give no reactions. Ethanol became instantly clear, this

lead to be a primary alcohol. 2-propanol was slightly cloudy then became clear which

lead to being a secondary alcohol. Octanol was the only one that showed emulsion which

indicated positive for the chloride replacing the hydroxyl group that results in forming a

chloroalkane and being tertiary. Overall, our hypothesis of primary alcohols giving no

reaction was failed to be rejected. The reaction was an example of a Sn2 reaction which

the nucleophile (Cl) attaches to the electrophilic center and expels the leaving group

(H2O). For the 2,4 DNPH test, a hypothesis can be stated that 2,4-Dimethyl-3-pentanone

will be the only one that doesn’t form a precipitate because it’s not an aldehyde.

Benzaldehyde formed a lot of orange precipitate and had a melting point of 238oC which

was close to the expected melting point of 239-241oC. Butyraldehyde formed a yellow

precipitate, a pumpkin-like smell, and had a melting point of 92oC which was close to the

expected melting point of 99oC. 2-Furaldehyde formed a red precipitate and had a

melting point of 198oC which was close to the expected melting point of 196oC. 2,4-

Dimethyl-3-pentanone had a minty-like smell but formed no precipitate and had a

melting point of 84oC which was close to the expected melting point of 88oC.

Arylhdrazines give a positive test for forming a precipitate which means they contain

aldehydres. Since all the substrates formed a precipitate except Dimethyl-3-pentanone, it

can be concluded that Dimethyl-3-pentanone is a ketone instead of an aldehyde. For the

Tollen’s test, a hypothesis can be stated that Dimethyl-3-pentanone will have no reaction.

Benzaldehyde was yellow and formed a brown precipitate after heating. Butyraldehyde

was clear and formed a white/silver mirror after heat. 2-Furaldehyde was black and

remained black after heating. 2,4-Dimethyl-3-pentanone was clear and turned silver after

heating. The forming of a silver mirror or a precipitate indicates a positive test for the

presence of an aldehyde. Benzaldehyde and Butyraldehyde show distinct positive tests

for an aldehyde. Since 2-Furaldehyde had no change and 2,4-Dimethyl-3-pentanone

didn’t form a mirror or precipitate, it can be concluded that these are ketones. The reagent

oxidizes the aldehydes and reduces the silver ion.