Thiol Chemistry Structure Properties and Reactions

What is Thiol?

Thiols are an organic chemical compound with similar characteristics of alcohol and phenols. However, it has a sulphur atom instead of an oxygen atom. Moreover, it has a –SH functional group. Besides, it is the sulphur analogue of hydroxyl or alcohol group. This functional group is referred to as either a thiol group or a sulfhydryl group.

Notably, thiols are known as mercaptans. They share similar molecular structure with alcohol.

Structure and Bonding of Thiol 

Thiol structure, i.e. R-SH, refers to Alkanethiols or Alkyl thiols. Here an alkyl group is joined to a sulfanyl group. Moreover, thiols and alcohol have similar molecular structure. Since sulphur is a bigger element compared to oxygen, the C-S bond's length (typically around 180 picometers) is around 40 picometers longer than a usual C-O bond. 

However, the C-S-H bond angle in thiol is nearly 90 degerees compared to that of C-O-H bond of alcohol. Moreover, in a solid or liquid state, the hydrogen bonding between individual thiol groups is weak. Additionally, the main cohesive force here is Van der Waals interactions among the highly polarised divalent sulphur solutions. 

Additionally, the S-H bond is weaker compared to the O-H bond, as witnessed in their individual Bond Dissociation Energy (BDE). Furthermore, for CH3S−H, BDE is 366 kJ/mol (87 kcal/mol), whereas, for CH3O−H, BDE is 440 kJ/mol (110 kcal/mol).

Nevertheless, owing to this minor difference in electronegativity of sulphur and hydrogen, S-H bonds becomes moderately polar. On the other hand, O-H bonds in hydroxyl groups are more polar, in contrast. Additionally, thiols have a lesser dipole moment compared to its corresponding alcohols.

Physical Properties of Thiol

1. Odour

Thiols are generally colourless but have a sharp smell which has resemblance with garlic. Typically, this smell is strong and repulsive. Moreover, this compound binds strongly to skin proteins. Hence, they are responsible for this persistent and intolerable smell produced by the spraying of skunks. Additionally, such smells are noticed by a human nose at a concentration of 10 parts per billion.

Furthermore, it is also accountable for a particular class of wine faults. An unintentional reaction between sulphur and yeast is responsible for this mishap. Besides, it produces the sharp odour of beer when it is exposed to the ultraviolet light.

However, not every thiol group has an unpleasant scent. For instance, furan-2-ylmethanethiol is responsible for the aroma of roasted coffee. On the other hand, grapefruit mercaptan, which is a monoterpenoid thiol, it contributes to the smell of grapefruit. Additionally, a monoterpenoid thiol is not present at a higher concentration. Nevertheless, pure mercaptan has a bad smell.

Furthermore, after the deadly explosion at New London School in Texas, USA natural gas distributors started adding thiols to natural gas. Prior to this incident in 1937, gas distributors were odorising such gasses with ethanethiol. However, these days, natural gas companies use a mixture of mercaptans and sulphide along with t-butyl mercaptan for this purpose. Moreover, ethanethiol is now used in liquefied petroleum gases. 

On the other hand, to destroy this odour, an oxidising catalyst is used. Additionally, a copper-based oxidation catalyst is employed to neutralise the unstable thiols and alter them into inert products.

2. Boiling Point And Solubility

Thiols have relatively lower boiling points, as they have a minor relationship, through hydrogen bonding with water and among themselves. Hence, they have a lower solubility in water and other polar solvents compared to alcohols of the same molecular weight. 

Furthermore, owing to these characteristics, thiols and its related sulphide functional group isomer have similar boiling points and solubility. However, it is not similar for alcohols and its corresponding isomers.

3. Bonding

Moreover, the S-H bonds present in thiolalcohol are weaker than that of O-H bonds of alcohol.

Chemical Properties of Thiol

1. Synthesis

The production method of thiols is similar to that of ethers and alcohols. Moreover, the reactions, in this case, are quicker and more yielding, as sulphur anions are superior nucleophiles compared to oxygen atoms. Additionally, thiols are produced when a halogenoalkane is heated in a solution of sodium hydrosulphide.   

• CH3CH2Br + NaSH heated in ethanol (aq) → CH3CH2SH + NaBr

Moreover, disulphides can be easily reduced by lowering agents like lithium aluminium hydride in dry ether to produce two thiols.  

• R-S-S-R' → R-SH + R'-SH

2. Reactions

The thiol group is a sulphur analogue of the hydroxyl group (-OH) of alcohols. Moreover, sulphur and oxygen are from the same group of the periodic table. Hence, they have some similarities in terms of chemical bonding. For example, generally, alcohol is a deprotonated form of RS –,, i.e. a thiolate. It has additional chemical reactivity than a protonated thiol from RSH.

Hence, the chemistry of thiols is closely related to that of alcohols. Different forms of thiols such as thioacetals, thioether, and thioesters are equivalent to ester, ethers, and acetals. Additionally, a group of thiols can react with alkenes to produce thioether. Moreover, groups of thiols can react with vinyl groups to make thioether linkages.

3. Acidity

Sulphur atoms of thiols are more nucleophilic than that of oxygen atoms in alcohol. These thiol groups are fairly acidic with a pKa of around 10 to 11. Moreover, in the existence of a foundation, a thiolate anion is created that is a fairly powerful nucleophile. Additionally, this group and its associated anions are readily oxidised. Reagents such as bromine produce an organic disulphide (R-S-S-R).

• 2R-SH + Br2 → R-S-S-R + 2HBr

Furthermore, oxidation by more powerful reagents like hydrogen peroxide or sodium hypochlorite makes sulfonic acids (RSO3H).

• R-SH + 3H2O2 → RSO3H + 3H2O

Nomenclature of Thiol

The nomenclature of thiol is relatively similar to that of alcohol. However, instead of joining –ol suffix as in case of alcohols, thiols uses the suffix –thiols for this naming purpose. Additionally, thiols produced from alkanes retain the –e in its name derived from an alkane. For instance, buthanethiol and ethnethiol. It stands in contrast to that of alcohols butanol and ethanol.

Furthermore, the –SH group of thiols are known as a mercapto group. Hence, the prefix mercapto- is included in the name of specific compounds. For example, 2-mercaptobenzothiazole and 2-mercaptoethanol. Additionally, when thiols are named according to their nearest oxygen compound, the prefix thiol- is used like thiophenol.

Biological Importance of Thiol

Being the functional group of amino acid cysteine, thiol groups play a vital role in biological systems. Moreover, when these groups of two cysteine residue in monomers or constituent units are brought close to each other during protein folding oxidation can produce cystine units with a disulphide bond (-S-S-).

Additionally, disulphide bonds contribute to the tertiary structure of proteins. However, it is only possible if cysteine is part of the same peptide chain, or subsidise to the quaternary structure of multi-unit proteins. It forms a strong covalent bond among different peptide chains. Furthermore, such chains of antibodies, be it heavy or light are kept together by disulphide bridges.

A practical example here is the curls of curly hair. They are a result of cysteine formation. Moreover, the chemical used for hair straightening is reductants. It reduces cysteine disulphide bridges and frees cysteine sulfhydryl groups. On the other hand, chemicals used for hair curling are oxidants. They oxidise cysteine sulfhydryl groups and creates cysteine disulphide bridges.

Thionyl Chloride

Thionyl chloride is an inorganic substance, and its chemical formula is SOCI2. This substance is colourless and moderately volatile, having an unpleasant, acrid smell. Moreover, this liquid is mainly used as a chlorinating reagent. Furthermore, it is toxic, and it reacts with water. Besides, it can be used to produce a chemical weapon, as it is listed under the Chemical Weapons Convention.

Notably, it is often confused with sulphuryl chloride (SO2Cl2). They are completely different compounds in term of their properties. Moreover, thionyl chloride can produce chloride ions, whereas sulphuryl chloride has chlorine.

Thiophene

Thiophene is a heterocyclic liquid, and its chemical formula is C4H4S. Moreover, this liquid has no colour and has a smell similar to benzene. Additionally, it consists of a planar five-membered ring and has an aroma, as suggested by various substitution reactions. Lastly, in most of its reactions, it shares a resemblance to benzene.

Thiol is undoubtedly a difficult topic, and students often struggle to comprehend its related concepts. However, this chapter is vital, and with guidance, they can easily prepare it. Consequently, our live classes conducted by subject experts from all over that country can help students to clear their queries, and prepare better for their exams. Now you can even download our Vedantu app for enhanced accessibility.