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Classification Tests for Hydroxyl- & Carbonyl- Containing Co
Transcript of Classification Tests for Hydroxyl- & Carbonyl- Containing Co
Monday, March 17, 2014
PT 1y2-2, Grp. III
PROCEDURE AND RESULTS
1. Distinguish whether a compound is hydroxyl- or carbonyl-containing.
2. Differentiate the three types of alcohols
3. Differentiate aldehydes from ketones.
4. Explain the mechanism involved in the differentiating tests.
Classification Tests for Hydroxyl- & Carbonyl- Containing Compounds
SOLUBILITY OF ALCOHOLS IN WATER
*Most organic compounds are not soluble in water, except for low molecular-weight amines and oxygen-containing compounds.
*Low molecular-weight compounds are generally limited to those with fewer than 5 carbon atoms.
*The Lucas reagent is an aqueoussolution of strong acid (HCl) and zinc chloride(ZnCl).
*The alcohol starting material must be sufficiently soluble in aqueous environments for the reaction to take place.
CHROMIC ACID TEST
*Distinguishes primary and secondary alcohols from tertiary.
*Chromic acid will oxidize a primary alcohol first to an aldehyde and then to a carboxylic acid and it will oxidize a secondary alcohol to a ketone. Tertiary alcohols do not react.
TEST OR (2,4-DNP)
*Serves as a derivative formation. Both aldehydes and ketones react with 2,4-dinitrophenylhydrazine to form a solid 2,4-dinitrophenylhydrazone (DNP) derivative.
*If the solid is yellow, this most often means that the carbonyl group in the unknown is unconjugated.
* A reddish-orangecolor most likely means that the carbonyl group is conjugated.
*Compounds in which the carbonyl group is not conjugated produce orange precipitates.
*Presence of aldehydes and not ketones is detected by reduction of the deep blue solution of copper (II) to muddy green solution, and then form a brick-red precipitate of insoluble cuprous oxide.
*This test is commonly used for reducing sugars but is known to be not specific for aldehydes.
TOLLENS' SILVER MIRROR TEST
*Used to distinguish between aldehyde and a ketone. It exploits the fact that aldehydes are readily oxidized, whereas ketones are not.
*The reaction is accompanied by the reduction of silver ions in Tollens’ reagent into metallic silver.
*Ketones are not oxidized by the Tollens’ reagent, so the treatment of a ketone with Tollens’ reagent in a glass test tube does not result in a silver mirror.
*The Iodoform test indicates the presence of an aldehyde or ketone in which one of the groups directly attached to the carbonyl carbon is a methyl group.
*The sample is allowed to react with a mixture of iodine and base.
*Formation of a yellow precipitate therefore indicates the presence of a methyl group directly attached to the carbonyl.
Chromic acid reagent
Fehling's A and B
5% NaOCl solution
Iodoform test reagent
1. Label 5 test tubes.
2. Place 10 drops of each of the alcohols. (ethanol, n-butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, benzyl alcohol)
3. Add 1mL of water and shake the mixture after every addition.
Cloudiness of the solution indicates insolubility of that specific alcohol to water.
If cloudiness results, continue adding water .25mL at a time until a homogenous dispersion results.
Note the total of volume added.
If no cloudiness results after the addition of 2mL water, the alcohol is soluble in water.
SOLUBILITY OF ALCOHOLS
1. Add about 2-3 drops of sample to 1 mL of the reagent.
2. Shake vigorously for a few seconds.
3. Allow to stand at room temperature.
Perform this test on:
Observe the rate of formation of the cloudy suspension of formation of 2 layers.
*The alcohols that were soluble in water are polar compounds since water is polar.
*The lower the number of carbon atoms present, the more soluble or more miscible a substance is.
*Branching of carbon chains also affect solubility. The more branching present, the more soluble a compound is. However, this is only true for organic compounds that have the same number of carbon atoms present.
n-butyl alcohol was soluble in Lucas reagent while sec-butyl alcohol and tert-butyl alcohol were observed to have a formation of cloudy layer.
Tert-butyl alcohol took the shortest time to form the layer while sec-butyl alcohol took the longest time.
3˚ alcohols formed the second layer in less than a minute. 2˚ alcohols required 5-10 minutes while 1˚
alcohols were usually unreactive.
CHROMIC ACID TEST
1. Dissolve 1 drop of liquid sample in 1mL of acetone.
2. Add 2 drops of10% aqueous Potassium dichromate solution and 5 drops of 6M Sulfuric acid.
Perform this test on:
1˚ and 2˚ alcohols and aldehydes reduced the orange-red chromic acid/sulphuric acid reagent to an opaque green or blue suspension of Cr (III) salts in 2-5 seconds.
1˚ alcohols reacted with chromic acid to yield aldehydes, which arefurther oxidized to carboxylic acids.
2˚ alcohols reacted with chromic acid to yield ketones, which do not oxidize further.
3˚ alcohols were usually unreactive and aldehydes were oxidized to carboxylic acids
(OR 2,4-DNP) TEST
1. Place a drop of liquid sample into a small test tube.
2. Add 5 drops of 95% ethanol. Shake well.
3. Add 3 drops of 2,4-Dinitrophenylhydrazone. If no yellow or orange-red precipitate forms, allow the solution to stand for at least 15 mins.
test detects the presence of carbonyl groups and tests positive for aldehydes and ketones
Perform this test on:
A result of red-orange precipitate indicates the presence of conjugated carbonyl compounds while a result of yellow precipitate indicates the presence of unconjugated carbonyl compounds.
Most aromatic aldehydes and ketones produce red dinitrophenylhydrazone while many nonaromatic aldehydes and ketones produce yellow products.
1. Place 1mL of Fehling's reagent (made by mixing equal amounts of Fehling's A and Fehling's B) into each test tube.
2. Add 3 drops of the sample to be tested.
3. Place the tubes in a beaker of boiling water and observe changes within 10-15 mins.
aldehydes gave a positive result of brick-red precipitate while ketones did not produce any reaction
Perform the test on:
1. Prepare 4 test tubes with 1mL of Tollens' reagent.
2. Add 2 drops each of the samples separately: acetaldehyde, benzaldehyde, acetone, n-butryaldehyde, and acetophenone.
3. Shake the mixture. Allow to stand for 10 mins. If no reaction has occured, place the tube in a beaker of warm water for 5 mins.
The preparation of Tollens’ reagent was based on the formation of a silver diamine complex that is water soluble in basic solution.
Aldehydes were oxidized to carboxylic acids while ketones did not undergo oxidation except alpha-hydroxyketone.
aldehydes were expected to be oxidized while ketones did not undergo any oxidation
1. Place 2 drops of each sample.
2. Add 20 drops of 10% KI solution.
3. Add 20 drops of chlorine bleach to each tube and mix.
Iodoform test was used to detect the presence of methyl carbinol (2˚ alcohol with adjacent methyl group) and methyl carbonylgroups.
Yellow crystals or precipitate gave a positive result. An alkalinesolution of sodium hypoiodite, formed fromsodium hydroxide and iodine, convertedacetaldehyde and aliphatic methyl ketones intoiodoform
Distinguish whether a compound is hydroxyl- or carbonyl- containing
Differentiate the 3 types of alcohols
are alcohols in which the carbon carrying the -OH group is only attached to one alkyl group (generally represented by an R)
are alcohols in which the carbon carrying the -OH group is attached to two alkyl group (generally represented by an R) which may be the same or different
are alcohols in which the carbon carrying the -OH group is attached to three alkyl group (generally represented by an R) which may be the combination of same or different.
Differentiate aldehydes from ketones
the (C=O) is found at the carbon chain’s end. This means that the (C) carbon atom will be bounded to a hydrogen atom plus another carbon atom.
the (C=O) group is usually found at the center of the chain.