Was to Investigate a Selection of Diagnostic Tests Used to Identify Carbonyl Compounds and to Use the Products to Identify the Unknown Carbonyls

Aim

Was to investigate a selection of diagnostic tests used to identify carbonyl compounds and to use the products to identify the unknown carbonyls.

Introduction

The carbonyl compounds aldehydes and ketones share the same carbonyl functional group which is carbon double bonded to the oxygen with no hydrogen’s. But in aldehydes at least one hydrogen is bonded to the carbonyl carbon. In all cases the carbon(s) that are attached to the carbonyl group may not be part of an aromatic ring or may be part of an aromatic ring. Since they share the carbonyl group, aldehydes and ketones share much of their chemistry, but they are different enough to be considered different classes of compounds.  This situation is similar to that of alcohols and phenols which both share the -OH group. Aldehydes and ketones undergo a reaction called nucleophilic addition where by in less acidic conditions a nucleophile donates a pair of electrons towards a carbonyl carbon forming a single bond, (Smith, 2011).

As the double bond between oxygen and carbon becomes a single bond and one pair of bonding electrons in the double bond moves and become an unshared pair on the oxygen. So the oxygen becomes single bonded to the carbon and has three pairs of unshared electrons and has a negative charge. So the oxygen picks up a proton from somewhere (possibly one that was attached to the nucleophilic atom that attacked the carbonyl carbon) and becomes an -OH group. Under more acidic conditions the results are pretty much the same, but the sequence in which they happen is more or less reversed.  In this case, a proton (from an acid) attaches itself to one of the unshared pairs of electrons on the oxygen.  The carbonyl group now has a +1 charge and is very inviting to even a weak nucleophile. [You can’t have a strong nucleophile in an appreciably acidic solution because strong nucleophiles are moderate to strong bases and moderate to strong bases don’t hang out in acidic solutions.]  So, a nucleophile attacks the carbonyl carbon forming a bond and the doubly bonded oxygen of the carbonyl becomes an -OH, as before, (Smith, 2011).

The properties of aldehydes and ketones is intermolecular forces. The only intermolecular forces are dipolar and LDF. According to (Tan 2013) the double bond makes the CO double bond larger dipole than the single CO bond which means CO double bond exert more attractive force than ethers. Another physical properties of aldehydes and ketones is solubility. Aldehydes and ketones cannot give hydrogen bonds but can receive two hydrogen bonds from water since carbonyl has two lone pairs. Since they have larger dipole than alcohols water will form stronger hydrogen bonds to them. Since aldehydes and ketones cannot give hydrogen bonds the only force that holds the molecules of pure substances together is dipole-dipole attractions and LDF attractions. Aldehydes and ketones are neutral which means they are neither acidic nor basic and one of the chemical properties is that these are flammable. The ones which are have lower molecular weight and have lower boiling points will be more volatile.

Apparatus

Solid DNP

Concentrated sulphuric acid

Distilled water

Test tubes

Beaker

Test tube rack

Buchner funnel

Filter paper

Dilute sulphuric acid

Potassium dichromate

Conical flask

Sodium carbonate

Stirring rod

Water bath

Ethanol

Method

A solution of 2.4-dinitrophenylhydrazine was prepared by dissolving 1.167g of solid DNP in 10.00ml of concentrated sulphuric acid, while stirring the solution was added to 15.00ml of distilled water and 50.00ml ethanol.

Five test tubes were inserted to the test tube rack and in each test tube 10.00ml 0f the DNP solution was added to each of them and the tubes were labeled A to E. and to the five test tubes 3.00ml of each of the five unknowns provided was added. After 15 minutes filtration was performed to isolate the products and the products were allowed to dry after filtration.

In test tube D nothing formed and the contents were discarded. For test tube D the original unknown added to it 2.00ml of it was added to the separate test tube and 4.00ml of dilute sulphuric acid was added too, then 7 drops of potassium dichromate were added then the test tube was heated on a water bath while shaking for 7 minutes. Few pinches of sodium dichromate powder was added to the test tube.

Results & discussion

1. Note your observations when each of the five solutions were added to the DNP solution.

• In test tube A when reagent A was added a yellow precipitate formed with two layers
• In test tube B when reagent B was added crystal like orange precipitate formed with two layers
• In test tube C when reagent C was added cloudy like orange precipitate formed at the middle of the liquid
• In test tube D when reagent D was added nothing was formed
• In test tube E when reagent E was added a strong yellow precipitate formed at the middle of the liquid

1. Based on your observations in (a) above, complete the following sentence:

Since there was a positive test with DNP for unknowns A, B, C, & E                       , it can be deduced that each of these contain a Carbonyl  group. Since there was a negative test for unknown         D     , it can be deduced that these organic species are not     aldehydes    nor       ketones      .

1. Note your observations when the solutions prepared in part 2 of the method were heated gently.

• No change occurred when the solution in test tube was heated gently

1. Based on your observation in (c), what can you deduce about the general identification of the organic species in (c) above?

• It is an alcohol

1. Note your observations when sodium carbonate powder was added to the solutions preparation in part 2 of the method after heating.

• When sodium carbonate powder was added the bubbles were formed, but after forming bubbles nothing changed to the test tube.

1. Based on your observation in (e), deduce the identity of the organic species that were present in these tubes from the original five organic reagents used in this experiment.

Tube       D           contain     an alcohol                                  

                                                             OR

The unknown with label       D        is  propan-1-ol.

1. Using molecular formulae, provide chemical reaction(s) to represent the observations made in (c). State which reaction pertains to which unknown/test-tube.

Tube containing   reagent D

Chemical reaction:

1. State with reasons, what observations would be noted if a sample of 2-methylpropan-2-ol was subjected to part 2 of the method above.

1. Complete the following table pertaining to part 2 of the method.

Table 1 Theoretical and experimental melting points of some DNP-derivatives

...