Carbonyl Compounds Aldehyde and Ketones

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Carbonyl Compound: Aldehydes and Ketones Roque, Jason R. Frias, Abigail Pauline F. De Ramos, Jason Jalou Bachelor of Science in Biology – Major in Human Biology College of Science De La Salle University – Dasmariñas Dasmariñas, Cavite, Philippines

ABSTRACT

In this experiment the solubility and reactions of different aldehydes and ketones were tested and seen. 5 drops of acetone, acetaldehyde, benzaldehyde and cyclohexanone were prepared in separate test tubes. The samples were added 5 drops of water and sodium hydroxide each test tube in different set ups. cyclohexanone and acetone were found to be soluble in water and none of any compounds were soluble in sodium hydroxide. In 2,4-DNP tests, 1ml of 95% ethanol and 1mL 2,4-DNP were added in each sample. All compounds resulted to a bright yellow or orange precipitate that showed the carbon-oxygen double bond in aldehyde and ketone. In Tollen's test, 2ml Tollen's reagent was added in each sample and was kept for 10 minutes. In this test the oxidation will determine the presence of aldehyde and ketone. Only acetaldehyde and benzaldehyde were oxidized. In Iodoform test, 2ml of distilled water was added to the sample, placed inside the water bath with the temperature of 60 for several minutes then the Iodoform reagent was added. This test was done to determine the presence of CH3CO in aldehyde and ketone. Only aldehyde and acetone were positive in this test. Another test to determine the oxidation reaction was the Jone's test where 4 drops of chromic acid solution were added and stand for 10 minutes. Different tests were done to determine the presence of aldehyde and ketone.

INTRODUCTION Aldehydes and ketones have same chemistry polarity. Its bonding affects its reactivity. Carbonyl group is a divalent chemical unit consisting of a carbon (C) and an oxygen (O) atom connected by a double bond. Both are simple carbonyl compounds which contains the carbonyl group, -C=O. it has no other reactive group that attaches to the carbon atom itself. (3) Aldehydes and ketones are organic compounds which incorporate a carbonyl functional group, C=O. The carbon atom of this group has two remaining bonds that may be occupied by hydrogen or alkyl or aryl substituents. If at least one of these substituents is hydrogen, the compound is an aldehyde. If neither is hydrogen, the compound is a ketone. (2)

Figure 1. Structure of Aldehyde and Ketone

Aldehydes and ketones have higher boiling point than alkenes but lower boiling point than alcohol because it is more polar but cannot form intermolecular hydrogen bonds to other carbonyl groups. These two compounds are also known to be soluble in water but falls when chain lengthens. (4) In aldehydes, the carbonyl group has a hydrogen atom attached to it together with either a second hydrogen atom or, more commonly, a hydrocarbon group which might be an alkyl group or one containing a benzene ring. (14)

Figure 2. Examples of Aldehydes In ketones, the carbonyl group has two hydrocarbon groups attached. Again, these can be either alkyl groups or ones containing benzene rings. Again, we'll concentrated on those containing alkyl groups just to keep things simple. Notice that ketones never have a hydrogen atom attached to the carbonyl group. (14)

Figure 3. Examples of Ketones MATERIALS / REAGENTS & EXPERIMENTAL PROCEDURE The experimenters placed 3-5 drops of each sample (acetone, acetaldehyde, benzaldehyde and cyclohexanone) in dry test tubes and prepared on various tests. There will be two category of tests for this experiment, the solubility test which comprises of solubility in water and sodium hydroxide ad chemical test which includes Tollens’ test, 2,4-DNP, Iodofrom test and Jones’ test. After each test, their reactions were observed and classified as insoluble/non-reactive, slightly soluble/slow reaction and soluble/fast reaction. The solubility test involved: solubility in water, add 5 drops of distilled water in each sample and observe, while in NaOH, add 5 drops of 10% NaOH in each sample and observe. For chemical reactions - 2,4- DNP test: Add 1 mL (10 drops) of 95% ethanol and 1 mL 2,4dinitrophenylhydrazine reagent per sample. Shake and observe reaction. Tollen’s test: Add 2 mL (20 drops) of Tollen’s reagent per sample. Stand for 10 minutes and observe reaction. Iodoform test: Add 2mL of distilled water per sample. Place test tube in hot water bath (60˚C) for 3-4 minutes. Add lodoform reagent while shaking until brown color appeared for 2 minutes. Keep in water bath again for 5 minutes. Observe reaction. Jones’ Test: Add 4 drops of Jones’ Reagent (Chromic Acid) while shaking in each sample. Stand for 10 minutes and observe reaction. DATA & RESULTS The reaction of the different samples is classified into two different tests: the solubility test and the chemical test. The solubility test is comprises with solubility in water and in sodium hydroxide, while for the chemical tests are 2,4-DNP test, Tollen’s test, Iodoform test and lastly the Jone’s test. Below is the table for the reaction of each sample into different tests.

Table 1. Reaction Profile SOLUBILITY TEST Solubility in 2,4 – Solubility in sodium DNP water hydroxide Test

CHEMICAL TEST Tollen’s Test

Iodoform Test

Jone’s Test

Visible + Result SAMPLES acetone

++

-

++

-

++

-

acetaldehyde

++

-

++

+

-

-

benzaldehyde

+

++

++

++

-

++

cyclohexanone

-

++

++

-

-

+

SOLUBILITY IN WATER: The carbonyl residue in acetone (CH3COCH3) which is polar in nature due to the difference in electronegativity between C and O, forms an overall molecular dipole in acetone. This molecular dipole is nearly identical to water--in fact, acetone has a dielectric constant of about 77 while water's dielectric constant is about 80 at room temperature and thus would be soluble in water. Hydrogen bonding does occur between acetone and water as the oxygen of acetone's cabonyl can hydrogen bond with the O-H bonds of water. However, the presence of such hydrogen bonding would in fact only lend to the ability of the two types of molecules to be miscible with each other.

Acetaldehyde is soluble in all proportions. The reason for the solubility is that acetaldehyde can't hydrogen bond with themselves; they can hydrogen bond with water molecules. One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one of the lone pairs on the oxygen atom of an aldehyde or ketone for a hydrogen bond to be formed.

Figure 4. Bonding of Water and Carbonyl Group Benzaldehyde is soluble in water 0.6 g/100 ml (20 °C). It has fairly low solubility because it is a non-polar hydrocarbon with a low percentage of oxygen in the molecule. Cyclohexanone is slightly soluble in water (5-10 g/100 mL), but miscible with common organic solvents. But in the experiment the cyclohexanone is not soluble in water. It may be caused by the amount of cyclohexanone and the solvent that makes it insoluble in water.

SOLUBILITY IN NaOH: Acetone and Acetaldehyde is not soluble in sodium hydroxide, while benzaldehyde and cyclohexanone is indeed fast soluble in NaOH. The presence of cyclic structure may cause why this two samples are soluble in water. 2,4-DINITROPHENYLHYDRAZINE TEST: This test is also called Brady’s test. 2,4- DNP test is used to qualitatively detect carbonyl functionality of a ketone or aldehyde functional group. A positive test is signaled by a yellow or red precipitate (known as a dinitrophenylhydrazone.) If the carbonyl compound is aromatic, then the precipitate will be red; if aliphatic, then the precipitate will have a yellow color. (2) The reaction between 2,4-Dinitrophenylhydrazine and a ketone is shown below: RR'C=O + C6H3(NO2)2NHNH2 → C6H3(NO2)2NHNCRR' + H2O

This reaction can be described as a condensation reaction, with two molecules joining together with loss of water. It is also considered an addition-elimination reaction: nucleophilic addition of the NH2 group to the C=O carbonyl group, followed by the removal of a H2O molecule. The mechanism for the reaction between 2,4-dinitrophenylhydrazine and an aldehyde or ketone is shown below: (3)

Figure 5. Reaction and complete mechanism of Ketone with 2,4 – DNP. All of the samples have a fast and positive test in 2,4 – DNP, because all of them belong to then ketone or aldehyde functional group. (9) TOLLEN’S TEST: This chemical test is commonly used to determine whether a known carbonyl containing compound is an aldehyde of ketone. A positive test for Tollens’ reagent results in an elemental silver precipitating out of solution, occasionally onto the inner surface of the reaction vessel, producing a characteristic and memorable “silver mirror” on the inner vessel surface. (4,10)

Figure 6. Reaction of an Aldehyde with Tollens’ test. (5)

Figure 7. Reaction of Acetaldehyde with Tollens’ Test. (12)

Figure 8. Reaction of Benzaldehyde with Tollens’ Test.

Benzaldehyde and Acetaldehyde having the presence of aldehyde will indeed positively react with the Tollens’ test and will produce a silver mirror precipitate. The Acetone and Cyclohexanone will not react with Tollens’ test because they belong to the functional group of ketone, and Tollens’ test is used only to determine the presences of functional group, aldehyde. (11) IODOFORM TEST: Iodoform test or so called haloform test is used to determine the presence of ketone. This is chemical reaction which produces a haloform (CHX3, where X is halogen) through the process of halogenation. (6,7)

Figure 9. Reaction of Methyl Ketone with Iodine in basic medium/

Figure 10. Reaction of Acetone with Iodoform test. JONES’ TEST: The Jones Oxidation allows a relatively inexpensive conversion of secondary alcohols to ketones and of most primary alcohols to carboxylic acids. The oxidation of primary allylic and benzylic alcohols gives aldehydes. Jones described for the first time a conveniently and safe procedure for a chromium (VI)-based oxidation, that paved the way for some further developments such as Collins Reaction and pyridinium dichromate, which also enabled the oxidation of primary alcohols to aldehydes. (6)

Figure 11. Reaction of Aldehydes with Jones’ test. A positive test for aldehydes and primary or secondary alcohols consists in the production of an opaque suspension with a green to blue color. Tertiary alcohols give no visible reaction within 2 seconds, the solution remaining orange in color. Disregard any changes after 15 seconds. Aldehydes are better characterized in other ways. The color usually develops in 5-15 seconds.

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