What is the overall rxn mechanism for this experiment?
C6H5Br + H2SO4/hno3
What is the source of the nitronium
ion for this rxn? What other reagents could be used?
Reacting sulfuric and nitric acid together produces the nitronium ion. Alone, nitric acid is not a strong enough electrophile to react with bromobenzene.
Acetic anhydride and nitric acid or NO2BH4
What order are the two acids mixed?
Sulfuric acid is added to the nitric acid. The reverse rxn can overheat and boil easily
What is special about the apparatus set up for the nitration of bromobenzene?
Includes a Claisen adaptor and thermometer. Necessary bc the temperature is very important and also don’t want any of the solvent/product to evaporate
Through which instrument is the bromobenzene added?
Pasteru Pipette is much longer than a regular pipette and prevents loss of material
Why is the bromobenzene added dropwise to the acidic solution?
To prevent excess heating, which would lead to polysubstitution
What other measure is taken to ensure that polysubstitution does not occur?
The reaction vessel cannot exceed 55 deg C
Temperature control is achieved by…
Cooling the rxn vessel, slowing the addition of bromobenzene, dropwise addition of bromobenzene
Why did we continuously have to shake the flask?
To thoroughly mix the contents. The nitration occurs at the surface of the bromobenzene and the acids. Also, bromobenzene has a low solubility in the acid solution so it’s very important that they are mixed well. If it was not mixed, the reactants would have acccumulated at the top, possibily leading to polysubstitution
Why is the flask warmed to 60 deg C after bromobenzene has been added?
The bromo group is an EWG and deactivates the benzene. To ensure the rxn occured, the additional heat helps to overcome this deactivation
Workup of products added to water?
Separates any remaining bromobenzene, nitric and sulfuric acid into the water
Workup of vacuum filtration?
Nitrobromobenzene is the only solid. para isomer
is the main
Workup of dissolve in ethanol?
More polar isomers (ortho, meta) have increased solubility in ethanol due to polarity. Para is not as soluble in ethanol.
Workup of evaporating 1/2 of ethanol, cool solution, second filtration?
Ortho isomer is the main
Why do you evaporate 1/2 of ethanol before 2nd crop?
Decreases the solubility of the nitrobromobenzene in solution
How do the melting points/TLC show the different isomers?
Para: symmetrical as the bromo and nitro groups creates dipoles in complete opposite directions. Symmetrical molecules have a higher MP due to the structure of the crystal lattice. Para has the highest MP. TLC uses concept of partitioning btwn a polar solid phase and less polar mobile phase. Since the para isomer is nonpolar, it won’t be very attracted to the solid phase and therefore dissolve more in the mobile phase, creating a higher Rf value.
Ortho/meta: nonsymmetrical: bromo and nitro groups create dipoles that go in the same relative direction. MP are significantly lower than para isomer bc they don’t have symmetry. Their Rf values are smaller bc they are more attracted to the stationary, polar solid phase and dissolve less in the mobile phase and do not travel as far
What are the major sources of experimental concern?
1. Source of nitronium ion- nitric acid is not a sufficient electrophile alone. But with sulfuric acid, it is strong enough.
2. Polysubstitution- once the bromobenzene reacts with one nitronium ion, it cant react with another. To prevent this polysubstitution, the temp is kept below a certain mark. The activation energy of the polysubstitution rxn is much higher bc nitrobromobenzene is more deactivated than bromobenzene. So, if the temp is kept low, there will simply not be enough energy to reach this high activation energy and the polysubstituted prodcut will not form
3. Solvent- bromobenzene is not very soluble in the acidic solution so must be vigorously mixed
4. Isomer separation- use diff polarities to separate the isomers. Ortho/meta are more polar and therefore have a high solubility in ethanol. In contrast, para is nonpolar and therefore symmetrical and does not as easily dissolve in ethanol.
Para isomer. Bc the bulky substituents are in 1,4 positions (symmetrical), the molecule is not polar, and thus is soluble in the nonpolar mobile phase on the TLC plate
Bromobenzene + HNO3/H2SO4 -> ortho, meta, para-bromobenzene + H2O
Bromobenzene and nitric acid are insoluble as solvent in each other and the formation of bromonitrobenzene occurs at the interface of the two substances. Without more nitric acid, the mixing might not be as successful and thus more unwanted byproducts.