Organic Chemistry Reactions of Aldehydes and Ketones

ALDEHYDES AND KETONES

OBJECTIVE(S):

* Observe the reactions of aldehydes and ketones

EXPERIMENTAL RESULTS (QUALITATIVE ANALYSIS)

Table 1. Reaction with 2,4-dinitrophenlhydrazine

Tests for: aldehydes and ketones Fails for: alcohols carboxylic acids and esters

Compound Observation

C2H5OH + CH3COCH3 + 2,4-DNPH Orange ppt. formed

C2H5OH + C6H5CHO + 2,4-DNPH Orange ppt. formed

C2H5OH + 2,4-DNPH No ppt. formed

Positive Test: gives yellow/orange/red ppt Negative Test: stays orange but clear

The 2,4-DNP Test:

2,4-DNP (2,4-dinitrophenylhydrazine)

This chemical will add to only carbonyls of aldehydes and ketones and not carboxylic acids or esters. When the product is formed it will be a yellow or orange or red solid (ppt). Other chemicals will not react and therefore will leave a clear (but orange colored) solution with no solids. The active reagent for this test is the carbonyl group.

Table 2. Tollen's Test( Silver ion in ammonia)

Tests for: aldehydes Fails for: ketones and tertiary alcohols

Compound Observation

Tollen's reagent + CH3COCH3 No cool silver mirror neither formation of ppt

Tollen's reagent + HCHO Form silver mirror

Tollen's reagent + C6H5CHO Form silver mirror and black ppt. formed

Positive Test: gives black ppt or cool silver mirror Negative Test: stays colorless

This will test only for aldehydes. The Tollens‟ reagent, composed of a silver-ammonium ion, will form solid silver as it oxidizes that aldehyde. This silver will plate a glass surface creating a mirror.

Table 3. Schiff's Test

Tests for: aldehydes Fails for: ketones, alcohols, acids and esters

Compound Observation

Schiff's reagent + HCHO Dark-violet color

Schiff's reagent + CH3COCH3 Bloody red color

Schiff's reagent + C6H5CHO Red-violet and black ppt. formed

Positive Test: gives purple color after exposure to air. Negative Test: stays colorless

Like the Tollens reaction, this reaction will only react with aldehydes and its product is easily biodegradable. The reaction is rather fast for the first step, but it takes time for O2 from the air to remove the last hydrogens and let the product form a second conjugated ring that makes it the purple color.

Both Tollen's test and Schiff's test are tests for aldehydes because they had the same structural formula but differs in general formula.

Table 4. Iodoform Test

Tests for: alpha-methyl ketones Fails for: aldehydes , alcohols

Compound Observation

10% NaOH + KI + CH3COCH3 Milky yellow fine ppt

10% NaOH + KI + C2H5OH Milky white

Positive Test: milky yellow ppt Negative Test: stays clear or milky white

This will test specifically for methyl ketones by giving a yellow precipitate. It should be noted that aldehydes can also add the reagent but will form a white precipitate (ppt), and it does not test for ketones in general.

Table 5. Effect of Acid Concentration on Rate of Addition of 2,4-DNPH

Compound Observation

2,4-DPNH + CH3COCH3 Least ppt

2,4-DPNH + CH3COCH3 + HOAC More ppt

2,4-DPNH