|Lecture Notes: 22 June||
Acidity: Thiols are significantly more acidic than alcohols, just as Hydrogen sulfide (pKa= 7.0) is a stronger acid than water (pKa= 15.7). Thus Ethane thiol has a pKa= 8.5 compared to Ethanol with a pKa= 15.9.
Oxidation: Thiols are readily oxidized. In biological systems redox reactions using thiols are widely used. One example is the oxidation of thiol side chains in the amino acid cysteine to give disulfide cross bridges to stabilize extra-cellular proteins:
This reaction occurs readily in the presence of oxygen, which maintains these bonds in the extracellular environment. On the other hand it is readily reversible under reducing conditions, so these bonds are not useful in the reducing ienvironment of the cell interior!
Synthesis: Thiols may be prepared from alkyl halides by attack by sulfur nucleophiles such as hydrogen sulfide via an SN2 mechanism:
Sulfides are prepared by reacting a primary or secondary alkyl halide with a thiolate ion, RS-, via an SN2 mechanism:
The reaction goes well since thiolate anions are among the best known nucleophiles. (This synthesis is essentially the same as the Williamson ether synthesis.)
Benzene Structure: Benzene is a six carbon, highly unsaturated ring. The formula is consistant with its being a conjugated triene - C6H6, thus it was originally written as:
As we have seen for alkenes, both conjugated and unconjugated, would expect an addition product with Br2, C6H4Br2, but in fact this is not formed, rather we get a substitution product, C6H5Br. Benzene turns out to be remarkably stable - it does not react with bromine, HBr or other reagents which add to double bonds. it is also resistant to oxidation when compared to alkenes.
So why does this happen? What's different about benzene? Of course from our discussion of conjugated molecules we would expect resonance structures as below:
That is all of the bonds should be the same, intermediate between single and double bonds. This is found to be the case: C-C bond length = 0.154 nm, C=C bond length = 0.134 nm, whereas the bond length for benzene is 0.139 nm. Note that benzene will be planar, with t torus of electron density symmetrically above and below the ring. Now we can understand why benzene does not undergo electrophilic addition: the addition of the electron pair would disrupt benzene's conjugated sytem, leading to a product with higher energy, since the electrons would not be able to delocalize themselves as readily, as seen below:
Aromatic compounds are referred to as arenes. Many aromatic names must simply be memorized. Memorize the names and structures of: Benzene, Toluene, phenol, aniline, benzaldehyde, benzoic acid, anisole, styrene, xylene.
For systematic naming we use -benzene as the parent name, and add substituents as we have for other hydrocarbons [examples: isopropylbenzene, 1,2,4-Trichlorobenzene].
There are two standard benzene based substituents: phenyl for the -C6H5 substituent, and benzyl for the C6H4CH2- group.
Disubstituted benzene rings are named using the "omp" system. Ortho- for 1,2; meta- for 1,3; and para- for 1,4. [examples o, m, and p nitrobenzaldehyde].
IUPAC: 2,4,6-Trinitrotoluene (TNT)
Polynuclear Aromatic Hydrocarbons (PAHs): Note naphthalene, Anthracene and Benzo[a]pyrene.
Phenols: Phenols are named as substituted aromatic compounds with the parent being - phenol instead of -benzene. Thus we can have p -Chlorophenol, 2,4,5-Tribromophenol, etc.
Acidity of Alcohols and Phenols: Alcohols and phenols are weakly acidic, as we might expect due to their similarity to water. They can also function as weak Lewis bases, becoming protonated to give oxonium ions:
Note that phnols are much stronger acids than alkyl alcohols. Some pKa values: CH3CH2OH = 16.00; H2O = 15.74; CH3OH = 15.54; Phenol = 10.00; p -Nitrophenol = 7.15; HCl = -7.00.
So why are the pKa's of phenol and its derivatives so low - we see a difference of 7 orders of magnitude! Can explain on the basis of resonance structures (you should come up with any additional contributing resonance structure(s)) of the ion:
Remember what these resonance structures mean: the charge is now spread over a greater area, rather than concentrated on the oxygen alone. This means the oxygen will be less attractive to protons, so the phenolate ion will reman ionized at higher H+ concentrations.
Phenols are synthesized from aromatics by first sulfonating to give an arenesulfonic acid, which is then converted to a phenol by treatment with sodium hydroxide at high temperature:
Phenols cannot be converted to halides by treatment with HX (the aryl ring is not subject to nucleophilic substitution - it is itself electron rich), nor can they be dehydrated by treatment with acid (they are already unsaturated).
Ether synthesis: On the other hand phenols readily undergo Williamson ether synthesis since they are more acidic than alcohols, and are thus easily converted to good anionic nucleophiles:
Note that sodium or potassium are not needed to make a phenoxide ion for this reaction.
Oxidation: Phenol can be oxidized by strong oxidizing agents such as sodium dichromate to give quinone (cyclohexadienedione):
Quinones are of particular interest because they are readily reduced to hydroquinones which are in turn readily reoxidized to quinones:
The quinones/hydroquinones are widely used in biological systems as the base of redox cofactors and redox agents (including defensive secretions etc. - Bombardier beetle stores hydroquinone and hydrogen peroxide in separate compartments, then mixes them upon disturbance. The reaction heat results in an explosive ejection of vapor and oxidizing quinones, with popping noise).
Last modified 22 June 2004