Why use thioglycollic acid? (2 of 2)
Typical results for the reagent ammonium thioglycollate are shown in Table 1. The concentrations of free amino acids produced from the hair protein by the reaction were measured and it was found that 63% of S-S bonds had been broken after 30 minutes of treatment with 1 mol dm-3 ammonium thioglycollate at pH 7.5, for example.
|pH||Relative reaction rate|
Table 1: Types of dirt and stains
Looking at experimental results enabled cosmetic chemists to choose the best reagent and conditions – thioglycollic acid in relatively alkaline conditions has proved suitable.
Explaining the results
These results have been rationalised as follows
At a relatively low pH (i.e. fairly acidic conditions), thioglycollic acid loses a proton from its carboxylic acid group:
H-SCH2COO-H +OH- H-SCH2COO- + H2O
This reacts with hydroxide ions (contained in the product as potassium hydroxide) to form a dianion
H-SCH2COO- + OH- -SCH2COO-+ H2O
It is the S- end of the dianion that is active in breaking the disulfide bridges. As we are only interested in this part of the molecule, we can simplify it to R’S- to save us having to write out the full structure each time. The reaction occurs in 4 stages;
Figure 14: Mechanism of breaking the disulfide bridge in cystine
a) Nucleophilic attack by S in RS- on one of the S atoms in cystine. This breaks the disulfide bridge in cystine and generates a new one between a cysteine molecule and the thioglycollate ion.
b) The anion generated in step (a) strips a proton off water to generate a cysteine molecule.
c) A second thioglycollate molecule attacks the first to break the just formed disulfide bridge and remake it between the thioglycollate molecules.
d) The anion generated in step (c) strips a proton off water to generate the second cysteine molecule.
The S- on -SCH2COO- acts as an even better nucleophile than the O- on H-SCH2COO- for two reasons:
• The ion has a double negative charge
• The lone pair electrons on sulfur are more available for donation than those on oxygen.
Explain why the lone pair electrons on sulfur are more available for donation than those on oxygen.
They are further from the nucleus than those on oxygen yet they feel approximately the same nuclear charge.