last updated Wednesday, February 10, 2016

Meeting 11 (Condensed Key)

1. a. The bromophenol and iodophenol are both suitable reagents for the Suzuki reaction. While the iodophenol is more reactive than the bromophenol, which allows for shorter reaction times and less energy use. The atom economy for the iodo reaction is lower because of the larger leaving group. Iodophenol is also much more expensive than bromophenol (Aldrich: bromo: ~$80/mol, iodo: ~$600/mol).

b. Potassium carbonate deprotonates the phenol to yield a phenolate which is more water-soluble and easier to undergo an oxidative addition due to its negative charge. In addition, the carbonate ion also serves as ligand in the ligand exchange part of the catalytic cycle (X=Br, I).

XC₆H₄OH   +   CO₃^2-     ----- >     XC₆H₄O^-   +    HCO₃^-

c. Elemental palladium metal on carbon (Pd/C) is used as catalyst. It goes through a sequence of oxidation addition and reductive elimination during the reaction. It can easily be recovered after the reaction and be recycled. The disadvantage is that the catalyst is fairly expensive (~$10/g) which makes the reaction fairly costly if performed in larger scale.

d. The addition of hydrochloric acid causes a protonation of the phenolate resulting in the precipitation of the neutral phenol. In addition, the carbonate is protonated leading to the formation of carbon dioxide.

C₆H₅C₆H₄O^-  +       H⁺   ------>    C₆H₅C₆H₄-OH
          CO₃^2-  +  2 H⁺  ------>     H₂O  +  CO₂ ↑

e. A mixture of methanol and water (1:1) is ultimately used for recrystallization. The crude consisting of the catalyst and the phenol is first dissolved in methanol. After the catalyst is removed by decanting the solution, water is added.  The addition of water increases the polarity of the solution and causes the weakly polar product to precipitate. The mixture is brought to a gentle boil to dissolve the product and allow for controlled crystallization upon cooling.

f. The compound displays eight signals in the ¹³C{¹H}-NMR spectrum in the range from δ=116 ppm to δ=155 ppm. The signal at δ=155 ppm is due to the phenol carbon and the one at δ=116 ppm due to the ortho carbon to the phenol group.