Easy Guide: Physical & Chemical Properties of Thiols
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
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.
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.
Bonding
Moreover, the S-H bonds present in thiolalcohol are weaker than that of O-H bonds of alcohol.
Chemical Properties of Thiol
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
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.
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.
FAQs on Thiol: Structure, Properties, and Applications
1. What is a thiol and what is its general formula?
A thiol is an organic compound that features a sulfhydryl group (-SH). It is essentially the sulfur analogue of an alcohol (R-OH), where the oxygen atom has been replaced by a sulfur atom. Due to this relationship, thiols are also widely known as mercaptans. Their general chemical formula is written as R-SH, where 'R' represents an alkyl or other organic substituent.
2. What is the most characteristic physical property of thiols?
The most distinguishing physical property of low-molecular-weight thiols is their extremely strong and pungent odour, often described as resembling garlic or rotten eggs. This powerful smell is so potent that the human nose can detect it at very low concentrations. For example, the defensive spray of a skunk is primarily composed of various thiol compounds.
3. How are thiols named according to the IUPAC system?
In the IUPAC nomenclature system, thiols are named by taking the name of the parent alkane and adding the suffix -thiol. The main carbon chain is numbered to assign the lowest possible number to the carbon atom bonded to the -SH group. For instance, a two-carbon chain with a thiol group (CH₃CH₂SH) is named ethanethiol, and a three-carbon chain with the thiol group on the central carbon is named propane-2-thiol.
4. What are the key differences in properties between thiols and alcohols?
While structurally similar, thiols and alcohols exhibit several key differences in their properties:
- Acidity: Thiols are significantly more acidic than their corresponding alcohols. The S-H bond is weaker than the O-H bond, making it easier to donate a proton.
- Boiling Point: Thiols have lower boiling points than alcohols of similar molecular mass. This is because the hydrogen bonding in thiols is much weaker than in alcohols due to the lower electronegativity of sulfur compared to oxygen.
- Odour: Thiols have a strong, repulsive smell, whereas simple alcohols have a milder, often sweet smell.
5. What are some important real-world applications of thiols?
Thiols have several vital applications in both nature and industry. Key examples include:
- Gas Odorants: Small quantities of thiols like ethanethiol are added to odourless natural gas and LPG to serve as a powerful warning agent, allowing dangerous leaks to be detected easily by smell.
- Biochemistry: The amino acid cysteine contains a thiol group. This group is fundamental in protein chemistry for forming disulfide bridges (S-S bonds), which help stabilise the three-dimensional structure of proteins.
- Antioxidants: The thiol group in compounds like glutathione plays a crucial role as an antioxidant in living organisms, protecting cells from damage by reactive oxygen species.
6. Why are thiols more acidic than their alcohol counterparts?
Thiols are stronger acids than alcohols for two main reasons related to the properties of sulfur versus oxygen:
- Weaker Bond Strength: The bond between sulfur and hydrogen (S-H) is weaker and longer than the oxygen-hydrogen (O-H) bond. This is because sulfur's valence electrons are in the 3p orbital, further from the nucleus, resulting in less effective overlap. A weaker bond requires less energy to break, making the proton (H⁺) easier to release.
- Stability of the Conjugate Base: When a thiol loses a proton, it forms a thiolate ion (RS⁻). The negative charge is spread over a larger sulfur atom, making it more stable than the corresponding alkoxide ion (RO⁻), where the charge is concentrated on a smaller oxygen atom. A more stable conjugate base corresponds to a stronger acid.
7. How does the oxidation of a thiol differ from the oxidation of an alcohol?
The oxidation of thiols and alcohols leads to different types of products. When a primary alcohol is oxidised, it typically forms an aldehyde and then a carboxylic acid. In contrast, the mild oxidation of a thiol does not add oxygen but instead causes two thiol molecules to couple together, forming a disulfide (R-S-S-R'). This reaction involves the removal of hydrogen atoms from the two -SH groups and the formation of a sulfur-sulfur bond, a process crucial for forming disulfide bridges in proteins.
8. How is a thiol structurally different from a thioether and a thiophene?
It is important to distinguish between these related sulfur-containing organic compounds:
- A Thiol (R-SH) has a sulfhydryl group where a sulfur atom is bonded to a hydrogen atom and a carbon atom.
- A Thioether (R-S-R'), also called a sulfide, has a sulfur atom bonded to two carbon-containing groups. It is the sulfur analogue of an ether and lacks an S-H bond.
- A Thiophene (C₄H₄S) is a specific aromatic heterocyclic compound with a five-membered ring containing one sulfur atom and four carbon atoms. It is a distinct cyclic structure and not a functional group like a thiol.
