Experiment: # 5

Nitration of Chlorobenzene

Name: Ali Marzuk, 4855366

CHM 2123, Section B24

TA: Kirk Mulligan

Date: October 20th, 2008

Department of Chemistry

University of Ottawa

1. Theory and mechanism:

The reaction that occurred in this experiment was the electrophilic addition of NO2

+ to chlorobenzene. To have this occur, we must (1), protonate nitric acid by mixing it with sulfuric acid. This produces NO2

+ which can be (2), added to chlorobenzene to form chloronitrobenzene at the ortho or para position. NO2 attacks the ortho and para positions due to the effect of the chlorine substituent. It has a weak deactivating effect, which means (3) it has some resonance stability at the ortho and para positions.

Balanced Chemical Equation:

Mechanisms: Activation of the nitric acid using sulfuric acid:

O

N+

O-

OH

+

Nitric Acid Sulfuric Acid

H

O

S

O

OHO

O

N+

O-

OH2+

+

O

S

O

OHO-

N+

OO

Nitronium Ion

Reaction of electrophillic nitronium ion with nucleophilic benzene:

H

+ N+

OO

N+

O

O-CH

+H

NO2Benzene

Nitrobenzene

+ H+

2. Table of reactants and solvents:

Compound Molecular weight ( g/mol)

Mass (g)

Density(g/ml) Volume (mL)

Mmol (millimoles)

Eq (equivalents

)

Concentrated nitric acid

63.0128 - 1.42 6.0 0.113 6.3

Concentrated Sulfuric acid

98.0734 - 1.84 6.0 0.094 5.2

Chlorobenzene 112.5585 2.107 1.11 2.0 0.018 1

Ethanol 46.1 - - Enough to

dissolve crude

product

- -

3. Experimental procedure:

For the complete procedure, please refer to lab manual “Organic Chemistry Laboratory: CHM 2123”, fall 2008, pg: 40-41.

4. Observation:

Nitration of chlorobenzene

Procedure Observation

Nitric acid, sulfuric acid and chlorobenzene mixed

Cloudy, translucent, white

After cooling in ice water bath Pale yellow chunks in cloudy/white solution

Crude after filtration Product was extremely yellow and chunky and hard in texture

Separation of p-chloronitrobenzene from o-chloronitrobenzene

Procedure Observation

Dissolving the crude product in ethanol on a hot plate

Light , clear yellow solution

After letting the crude product stand at room temperature undisturbed for 7-10 minutes

Large needles of about length 1-1.5 cm grew at the bottom of the flask

After addition of distilled water White needle-like crystals formed

5. Results :

Data Results

Mass of crude product 3.54 g

Mass of refined product 0.66 g

Ratio of para to ortho isomer 7:3

Percent yield 33.67 %

Melting point obtained 90-91.5 oC

Theoretical melting point 83-84 oC

6. TLC Plates:

7. Sample Calculations:

Amount of chlorobenzene = 2.0 g (0.018 mol)Amount of HNO3 = 6.0 mL x 1.42 g/mL = 8.52 g

No. of moles of chlorobenzene = no. of moles of chloronitrobenzene Mass of chlorobenzene = (0.018 mol)(157.45 g/mol) = 2.8 g

The p-isomer is 70% of the product So the theoretical yield of the p-chloronitrobenzene = 0.70 x 2.8 g = 1.96g

% yield = 0.66 g/ 1.96 g x 100% = 33.67 %

Rf (A) = A/x = 3.4 cm/ 4.6 cm = 0.739

8. Discussion:

The percent yield is 33.7 % for this experiment. The product isolated was needle-like crystals that were white in color and appeared to have a flaky texture. In this reaction, the chlorobenzene reacts with the nitronium ion in a preferential fashion. This is because there is a directing effect produced by the chloro group on the benzene. The literature melting point for 1-chloro-4-nitrobenzene is 83ºC-84ºC, whereas the experimental melting point was 90.0 ºC – 91.5ºC. This slight change and broadening of the melting point range shows that the product isolated was not in its purest form, however it is still relatively pure. The slight impurity is also seen in the TLC slide that shows a small amount of by-product that has a smaller RF than the desired compound of 1-chloro-4-nitrobenzene.

It is believed that some of the ortho product remained as a part of the re-crystallized product. The para isomer is less polar and thus had a larger RF value in the TLC analysis than did the ortho isomer, which is more polar and was adsorbed more strongly to the surface of the silica plate. This polarity difference is due to the fact that the para product (1-4) is more of a symmetrical molecule with the electron withdrawing groups of chlorine and nitronium having a canceling-out effect on one another, thus reducing the overall polarity of the molecule. On the other hand, in the ortho (1,2) isomer, the two electron withdrawing groups are close to one another and there is no symmetry to the molecule, thus the overall polarity of the molecule is increased, as proven in the TLC analysis.

The general mechanism for the electrophilic addition starts when the electrons form a double bond from the resonance benzene ring attacks a great electrophile attached to a great leaving group. The electrophile attaches to one side of the double bond, which leaves the other side with a positive carbocation. This ion of this intermediate step is known as the arenium ion. Next a base, most likely the leaving group deprotonated or attacks the hydrogen that is bonded to the same carbon as the electrophile. The electrons from the hydrogen go back into the ring reforming the benzene ring.

9. Questions: