Bromination as a tool for structure determination

1. Bromination as a tool for structure determination Experimental: • Dissolve the sample in DCM (1 mL) • Add dropwise a 1% bromine solution of DCM • Positive test, if red color disappears Conclusion: The sample contains a multiple bond, which undergoes bromination

2. Hypothesis: The sample is an alkene if the red color disappears Br2 consumed red color disappears

3. Sample 2 also an alkene

4. No Reaction? Why? Let’s take a look at the mechanism of alkene bromination Electrophilic Additon

5. Electrophilic Additon Alkene is the Nucleophile The stronger the nucleophile, the faster is the reaction An electron-rich alkene makes a stronger nucleophile,

6. What makes an electron rich alkene? Is a maleate an electron rich alkene?

7. Let’s take a look at the substituents Electron-Withdrawing • Reduces the electron density of the double bond • Makes the double bond less nucleophilic

8. Maleate is not electron-rich enough (even though it is an alkene) Conclusion: “False” Negative

9. What if we “helped” maleate to go over the activation energy barrier? by heating it Drawbacks: We might evaporate the whole solvent expected product a White Solid

10. Sample 3 notan alkene an aromatic compound aromatic compounds do not undergo electrophilic addition reactions BUT red color disappears

11. Is anisole undergoing an electrophilic addition? Or could it be an aromatic electrophilic substitution (EAS)? (the first step of which is an electrophilic addition anyway) Can this happen in the absence of a Lewis Acid? Check the Substituent

12. Methoxy group, • a strong activator will enable anisole undergo bromination (EAS) in the absence of a Lewis Acid can HBr be detected?

13. red color disappears Conclusion: “False” Negative Further Conclusions (if HBr is detected): Sample is an aromatic compound with a strongly activating substituent