Experiment 4 CHM143L

CHM143L / GROUP 2 Cruz, Patricia Anne D. De Guzman, Anne K.

Dela Dela Cruz, Cruz, Maria Maria Victor Victoria ia Duron, Marc John

Professor Oliver Villaflores

 Introduction to Carbonyl compounds  A

compound containing the carbonyl group is known as a carbonyl compound



Carbonyl group - a functional group composed of a carbon atom double-bonded to an oxygen atom

The carbon and oxygen atoms of the carbonyl group are sp2 hybridized and planar in geometry  This generally undergoes nucleophilic substitution, triggering an attack at 107 º

 The

carbonyl group is treated as an electrophile due to resonance and inductive effects:

 The

resonance structure show a positive charge on the carbonyl carbon (lack of electron density)



Due to the electronegativity of the oxygen atom, induction renders the carbon atom partially positive

 ALDEHYDE  A simple type of

carbonyl compound with at least one hydrogen atom as a substituent

KETONE  A simple type

of carbonyl compound with two substituent other than the hydrogen atom

The difference between aldehydes and ketones •

 Aldehydes are more reactive than ketones toward nucleophilic attack:  –

 –

Steric effects: ketone is more sterically  hindered than aldehyde Electronic effects: The partial positive charge of an aldehyde is less stabilized than that of a ketone:

Classifying carbonyl compounds:

CHEMICAL TESTS The ease of oxidation helps chemists distinguish aldehydes from ketones.

 Aldehydes are easily oxidized by all sorts of different oxidizing agents, ketones aren't.

1. Benedict’s test Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution. Benedict’s reagent has Cu2+ ions which gives it a deep blue color. During the oxidation, Cu2+ is reduced to Cu2O (a precipitate). If the blue color does not change no oxidation occurs. If the  blue color stays and you have a precipitate, oxidation occurs.

 Benedict’s test continued 

POSITIVE RESULT: ketone No change in the blue solution. aldehyde The blue solution produces a dark red precipitate

Classifying carbonyl compounds: CHEMICAL TESTS 2. Tollen’s test

Tollens’ reagent, is an alkaline solution of silver (Ag+) ion complexed with ammonia (NH3), which keeps the Ag+ ion in solution.  When Tollens’ reagent oxidizes an aldehyde, the Ag+ ion is reduced to free silver (Ag). Deposited on a clean glass surface, the silver produces a mirror.

Tollen’s

test continued  RESULTS:

ketone No change in the colorless solution. aldehyde The colorless solution produces a silver mirror on the test tube. Less spectacular, but  just as valid is the formation of a grey or  black precipitate.

CHEMICAL TESTS 3. Carbonyl group reactions  –

Condensation Reaction

REAGENT/S and USES: 2,4-dinitrophenylhydrazine - utilized as a test for the carbon-oxygen double bond in ketones and aldehydes MECHANISM: Nucleophilic addition-elimination rxn. The 2,4-dinitrophenylhydrazine first adds

across the carbon-oxygen double bond (the addition stage) to give an intermediate compound which then loses a molecule of water (the elimination stage).

POSITIVE RESULT:  A bright orange or  yellow precipitate shows the presence of the carbon-oxygen double bond in an aldehyde or ketone.

CHEMICAL TESTS - Addition Reaction REAGENT/S and USES: Sodium bisulfite (NaHSO3) - utilized as a test for the carbon-oxygen double bond in conjunction with the condensation rxn. MECHANISM: Addition reaction  This reaction only works well for aldehydes. In

POSITIVE RESULT: Formation of white crystals (precipitate) indicating presence of C=O group

the case of ketones, one of the hydrocarbon groups attached to the carbonyl group needs to be a methyl group. Bulky groups attached to the carbonyl group get in the way of the reaction happening. The reason that cyclohexanone react with sodium bisulfite and not in 3-pentanone is that cyclohexanone is an unhindered ketone while 3-pentanone only has ethyl groups and this is already bulky enough to interfere with the

3. Carbonyl group reactions

2,4-dinitrophenylhydrazine test

Sodium bisulfite test

CHEMICAL TESTS 5. Alkyl group reactions  – Haloform reaction The haloform reaction is the reaction of  

a methyl ketone with chlorine, bromine, or iodine in the presence of hydroxide ions to give a carboxylate ion and a haloform. There is one aldehyde that undergoes the haloform reaction, which is acetaldehyde.

Iodoform Test •

When the halogen used is iodine.

TEST COMPOUNDS

Acetaldehyde

Acetone

1.

OBSERVATIONS

Acetaldehyde+H2O+8M NaOH-Pale Yellow + I2 in KI- Golden Yellow +heat- pale yellow precipitate Acetone+H2O+8M NaOH-Clear + I2 in KI- Pale Yellow +heat- pale yellow precipitate

References Klein, David M. Organic Chemistry 2nd Ed. 2012 John Wiley and Sons, Inc.