Unknown Aldehyde Or Ketone Lab Report

Identifying an Unknown Aldehyde or Ketone

Introduction

The purpose of this lab is to identify an unknown aldehyde or ketone substance using chemical tests. The chemical tests used in this experiment are solubility, Schiff, Bisulfite, and Iodoform tests. Also, a 2,4-dinitrophenylhydrazone derivative synthesis reaction will be completed from which a melting point will be obtained. The chemical test results and the melting point analysis will be compared to the table of compounds given to find the unknown’s identity1.

Experimental

An unknown substance labeled Unknown 2 was obtained.
Solubility test:
Water (1 ml) was added to a test tube, and then the unknown (2-3 drops) were also added to the test tube. A pipet was used to blow bubbles
into the test tube in order to mix the solution. The solution turned cloudy white. Oil-like bubbles at the top of the solution were observed.
Schiff Test:
In a new test tube, the Schiff reagent (2 ml) was added. Then, the unknown (3-5 drops) was also added. A pipet was used to mix the solution by blowing bubbles into the test tube. The result of the Schiff test was that the solution immediately turned magenta, which means aldehydes are present.
Bisulfite test: In another clean test tube, methanol (2-3 drops) was added. Then, the bisulfite solution (1 ml) was also added, and then the unknown (5-10 drops) was added. The solution was mixed by using a pipet to blow bubbles into the test tube. Then, the solution was let sit with a cap on top. The test results were not immediately visible via a white precipitate. So, throughout the ten minute allotted bisulfite test period, the solution was continually mixed with the bubbles blown through the pipet, then capped and let sit. The result of the bisulfite test was that a white precipitate formed at the top of the solution. Iodoform Test:
In a 15 ml plastic conical vial, 1,2-dimethoxyethane (2 ml, 19.3 mmol) was added, and then the unknown (4 drops) was added. Sodium hydroxide (2 ml, 6 mmol) was added. The Iodoform solution (3 ml) was slowly added and then diluted with water (10 ml). The Iodoform test solution was first brown from the reagent, but quickly turned yellow once mixed into the solution. Once the water was added and mixed, the yellow color disappeared. The solution was a cloudy white color. No precipitate formed during the Iodoform test.
Synthesis reaction of a 2,4-dinitrophenylhydrazone derivative:
The 2,4-dinitrophenylhydrazone solution (5 ml) was poured into a 15 ml plastic conical vial.
The unknown (0.05 g) was added. The mixed solution was observed to be an orange color. A cap was placed onto the vial. A water bath was used to warm the mixture for five to ten minutes. In the first five minutes of the test, not a lot of precipitate had formed. So, the capped vial was tapped, and then let sit repeatedly until enough precipitate formed. Precipitate was observed forming more and more as the solution heated and time passed. The solution was let sit to cool to room temperature, and then placed into an ice bath for a couple minutes. Vacuum filtration was used to collect crystals while a lot of water was used to wash. A small piece of litmus paper was used to check the crystal’s pH. Vacuum filtration with water as a wash was utilized to wash the crystals until they turned basic. Then, the crystals were dried with filter papers. After fully dried, the precipitate was a dark orange color. The precipitate was weighed. (0.128 g) Now, in a new 15 ml plastic conical vial, the product was recrystallized from ethanol (9 ml, 154 mmol). The precipitate that formed was a cloudy orange color. The product was weighed and a melting point analysis completed. (0.049 g): mp
104-105oC.

Data

Results of the Chemical Tests

Solubility Test
Schiff Test
Bisulfite Test
Iodoform Test
Positive

✔
✔

Negative
✔

✔

Melting Point Analysis Results = 104-105 oC

The Three Compounds from Table 3.1 that are Insoluable and Contain Aldehydes
Compound
Formula
MW
Water Solubility
2,4-DNPH Derivative MP (oC)
Benzaldehyde
C6H5CHO
106.12
Insoluble
237
n-Heptanal n-C6H13CHO 114.18
Insoluble
108
Cinnamaldehyde
C6H5CH=CHCHO
132.15
Insoluble
255

Unknown 2’s Identity from the Test Results of Insoluable, Contains Aldehydes, and has a Melting Point around 104-105oC
Compound
Formula
MW
Water Solubility
2,4-DNPH Derivative MP (oC) n-Heptanal n-C6H13CHO
114.18
Insoluble
108

Product Weights after Recrystallizations

After first filtration
After last filtration, final product
Product Weight
0.128 g
0.049 g

Conclusion

The unknown obtained was Unknown 2.
For the solubility test, the solution was observed to turn cloudy white and oil-like bubbles formed at the top of the solution, so Unknown 2 is insoluable. The Schiff test results were that the solution immediately turned the color magenta signifying that aldehydes are present in Unknown 2. The bisulfite test first required the addition of methanol to make the solution soluble because our unknown was shown to be insoluable from the solubility test results. The bisulfite test results were that a white precipitate formed at the top, which signified that aldehydes, unhindered cyclic ketones, or unhindered methyl ketones are part of Unknown 2. The Iodoform test showed that methyl ketones are not present in Unknown 2 because no yellow solid iodoform precipitate formed. The 2,4-dinitrophenylhydrazone derivative’s melting point found was 104-105 oC. So from all these tests, Unknown 2 was found to be insoluable, contain aldehydes, not contain methyl ketones, and have a melting point around 104-105 oC. Looking at the table of compounds that the unknowns could be in Table 3.1, there are only three compounds that contain aldehydes and are insoluable, which are benzaldehyde, n-Heptanal, and cinnamaldehyde. Out of these three compounds, only n-Heptanal is close to the melting point found. The reason that n-Heptanal’s melting point of 108oC is slightly off from the experimental melting point range found of 104-105 oC could be from a small amount of
impurities in the product. In conclusion, Unknown 2’s identity is n-Heptanal.

Works Cited

Lindsay, Maria C., and Sean P. Hickey. “Aldehydes and Ketones: Identification of an Unknown.” Chem 3018 Organic Chemistry Lab 2. 2nd ed. Vol. 1. John Wiley and Sons, 2015. 31-36. Print.