10C. Thiols + Disulfides

Question 1

Thiols

Answer 1

• compounds containing an -SH group (sulfhydryl) bonded to a tetrahedral carbon atom

C - S - H

Question 2

Naming

Answer 2

• similar to alkenes + alkynes, except that the parent chain contains the -SH group + is numbered from the end closest to the -SH group
• the location of the -SH group is shown + ‘thiol’ is suffixed to the name of the parent alkane- common names for thiols are derived by naming the alkylgroup bonded to -SH, followed by ‘mercaptan’
• mercaptan is the common name for any molecule containing an -SH group. It is Latin, meaning ‘mercury capturing’
• mercaptans react with mercuric ions + ions of other heavy metals to form precipitates

Question 3

Physical Properties

Answer 3

• compounds of sulfur in general, + low weight molecular thiols in particular, are noted for their disagreeable odour
• thiols are non-polar covalent compounds as the electronegativity difference between sulfur + hydrogen is small, therefore there isn’t hydrogen bonding, so thiols have lower boiling points + are less soluble in water + other polar solvents than alcohols of similar molecular weight

Question 4

Reactions

Answer 4

• thiols are weak acids, compared in strength to phenols
• they react with strong bases to form thiolate salts
• thiols react with Hg²⁺ in aqueous solution to give sulfide salts as insoluble precipitates - hence the name mercaptan
• the most common reaction of thiols in a biological system is their oxidation to sulfides, most readily by molecular oxygen
• disulfides are easily reduced to thiols by several reducing agents
• the easy interconversion between thiols + disulfides is very important in protein chemistry

Question 5

Disulfides

Answer 5

• compounds containing an - S - S - group
• when deriving the common name of a disulfide, the groups bonded to each sulfur atom are named, followed by ‘disulfide’