lab 7 - unknown aldehyde or ketone

which test confirms the presence of either an aldehyde or ketone?

The reaction of an aldehyde or ketone with 2,4-dinitrophenylhydrazine (2,4-DNP), yielding insoluble 2,4-dinitrophenylhydrazone

what are the two different types of positive results with 2,4-DNP?

a yellow ppt formed indicates a saturates aldehyde or ketone. A orange-red ppt indicates conjugation with a carbon-carbon double bond or with an aromatic ring.

which three tests can be used to distinguish between aldehydes and ketones?

Benedict’s. Tollens’, and Chromic Acid tests

the tests to distinguish between aldehydes and ketones ask the question…

can the unknown be oxidized?

Benedict’s Reagent

a citrate complex of the cupric ion (blue colored) which is reduced to a yellow-to-red ppt (cuprous oxide) by an aliphatic aldehyde

Tollens’ reagent

an ammonical silver nitrate solution which is reduced to metallic silver (a silver mirror) by most aldehydes

Chromic acid test

chromic acid in acetone rapidly oxidizes aldehydes to carboxylic acids (forming a green ppt) whereas ketones are attacked slowly or not at all by the reagent. Aliphatic aldehydes are oxidized somewhat faster than aromatic aldehydes. (orange → green ppt due to change in oxidation state of chromium)

carbonyl band in IR

intense band in the 1780-1660 cm^-1 region

C-H stretch of aldehyde group in IR spectra

weak bands near 2820 cm^-1 or 2700 cm^-1

only a ____ ketone will produce a positive result in the iodoform test

methyl

what is the formula for iodoform? What does it look like?

CHI₃, appears as a yellow ppt

what is the only aldehyde that will form iodoform?

acetaldehyde

what is the only primary alcohol that will form iodoform?

ethyl alcohol

4 general steps of the reaction of a carbonyl compound with a derivative of ammonia?

1. protonation of carbonyl → creates a more electrophilic carbon atom

2. nucleophilic nitrogen adds to now electrophilic position

3. forms a carbinol amine like intermediate

4. formation of water + regeneration of acid catalyst

what are the effects of low pH on the reaction of a carbonyl compound with a derivative of ammonia?

• activates the carbonyl group toward nucleophilic attack (must have enough of the protonated carbonyl)

• decreases the nucleophilicity of the ammonia derivative (must have enough of the free nitrogen compound)