Bromination of Toluene

1. Bromination of Toluene Chemistry 318 Fall 2018

3. Schedule of day • PPE and Pre-lab check – at the door • Quiz • Recitation • Safety • Put bags away • Goggles • Gloves • Lab Coat • Lab part 1 • IR Review • Lab part 2

4. Due Dates • Today • Notebook copies of Bromination of Toluene before leaving the lab • Next Week • Spectroscopy Problem Set (part I & part II #1) • At beginning of lab – Bromination of Toluene Report

5. In Lab Today • Review of Electrophilic Aromatic Substitution • Bromination of toluene experiment • Review of IR Spectroscopy • Demonstration of FT-IR; salt plates; %Transmittance vs. Absorption mode

6. Notebook Keeping • Notebooks should be prepared according to the instructions in the Manual pp. 1 – 20. • For this experiment, a Data Table that includes chemical safety notes, MW, density, mass and calculated moles (as measured during lab) is appropriate. • Information such as bpt., mpt., density, MW, etc. should always be entered in your notebook as part of your pre-lab and obtained from approved sources. See the Lab Manual “Citation” section. No Wikipedia – No Chembook (the Wikipedia of the chemistry world) – no web-based information.

7. Notebook Keeping (cont.) • Record all observations – everything you see, smell, feel (temperature changes), etc. Be clear and specific in your descriptions. • All calculations (in your notebook and on your report) must be clearly shown and labeled – show your work and pay attention to units and significant digits.

8. Experiment Notes • Bromine (Br2) is a hazardous and extremely corrosive substance • Lab coat, gloves, and goggles at all times! Change gloves as necessary. • Keep vials in hood and prepare salt plates there • Your instructor will handle the toluene and bromine. • Preparation of staple: sand it vigorously to remove plastic coating. Bend to fit in bottom of vial. • Moisten the pH paper before inserting it in the air space above the liquid in the vial.

9. Experiment Notes • During the (forty-five minute) wait time we will review spectroscopy. • Do not change any setting on the IR. • NaClplates are very fragile and expensive. Handle with great care and only by the edges.

10. IR of Product • This is what the IR spectrum of your product mixture might look like (in B&W)  • You will need to draw in the baseline and complete approximate peaks before calculating the area. See Mohrig p. 306 and Lab Manual pp.42-43.

11. Peak Area by Triangulation Peak Area = h * w½ * 1.064 Where h = Peak Height w½ = width of peak at ½ the peak height Note: The curve is assumed Gaussian and the h * w1/2 value is about 6.4% less than the true value, thus the correction factor. Total Peak Area = sum of component peak areas Mole Fraction of component = component peak area/total peak area Mole Percent = mole fraction x 100

12. Lab Report & Forms • See the Report Form for Bromination of Toluenein the Lab Manual (page 163). • Follow the instructions for writing reports found at the end of this experiment in the Lab Manual and on BlackBoard under “Notebook & Report Formats” and “Lab Reports-in General”. • Assemble and staple your report in this order: • Report Form • Calculations • IR spectrum • Written report

13. Electrophilic Aromatic Substitution See the Solomons lecture text for an in-depth discussion of Electrophilic Aromatic Substitution.

14. Electrophilic Aromatic Substitution(Overall Reaction) • Although they resemble alkenes, aromatic compounds do not undergo electrophilic addition reactions as alkenes do. • Rather, they undergo two-step substitution reactions in order to preserve the stable aromatic ring.

15. Electrophilic Aromatic Substitution(mechanism) • The electrophile is attacked by the electrons of the aromatic ring. • A new sigma bond is formed in the resonance stabilized arenium ion intermediate. • A proton is lost and the aromaticity is restored. -H+

16. Electrophilic Aromatic Substitution(mechanism) • Electrophiles (electron deficient species; Lewis acids), are attacked by the aromatic sextet of pi electrons in the aromatic ring. • An arenium ion intermediate is formed in which the ring has lost its aromaticity. • The arenium ion breaks the sigma bond to H+, and the electron pair becomes a pair of pi electrons in the aromatic ring. • There are several substitution reagents used in organic chemistry synthesis: • Halogenation – X2 (X = Cl, Br) • Nitration – HONO2/H2SO4 • Sulfonation – SO3/H2SO4 • Friedel-Crafts Alkylation – RCl/AlCl3 • Friedel-Crafts Acylation – R(C=O)Cl), AlCl3

17. Electrophilic Aromatic Substitution Bromination • In bromination of arenes, the electrophilic bromine is generated by reacting Br-Br with a Lewis acid metal catalyst. • A common catalyst is FeBr3. However, this substance can be produced by the very fast reaction between Fe0 and Br2 (as in this experiment). • The electrophile is likely to be the very electrophilic Brd+ in the catalytic complex [Br─BrFeBr3], although frequently Br+ itself is shown in textbooks.

18. Electrophilic Aromatic Substitution Reactivity & Orientation • The reactivity of BENZENE itself (no substituent) is used as a reference compound. • When there is a substituent group (G) onthe benzene ring, the substituent determines both the positionand the rate of substitution of the INCOMING electrophile. • Substituents affect which position (orientation ) on the ring is attacked bythe electrophile in the reaction (o-, m-, or p-). • Substituents also affect how fast the electrophile attacks the ring (reactivity).

19. Electrophilic Aromatic Substitution Reactivity & Orientation • Bromination of a benzene ring containing an electron donating group (methyl group) is faster than bromination of benzene itself. • Substitution of bromine at the ortho/para positions is favored .