Question

The greater acidity of thiols compared to alcohols can be explained by the fact that sulfur (a third period element) is larger than oxygen (a second- period element). The negative charge on an alkylsulfide ion ( \(\left.\mathrm{RS}^{-}\right)\)is delocalized over a larger area and is therefore more stable than the negative charge on an alkoxide ion \(\left(\mathrm{RO}^{-}\right)\). Thiols are sufficiently strong acids so that when dissolved in aqueous sodium hydiroxide, they are converted completely to alkylsulfide salts.

Short answer

Answer: Thiols are more acidic than alcohols because the larger sulfur atom can better accommodate and delocalize a negative charge in the conjugate base (alkylsulfide ion) compared to the oxygen atom in alcohols. When thiols react with aqueous sodium hydroxide, they form alkylsulfide salts and water due to their higher acidity and complete reaction with the strong base.

Step-by-step solution

Understand the acidity difference between thiols and alcohols

Acidity of a compound depends on the stability of its corresponding conjugate base. In this case, we have a thiol (\(\mathrm{RSH}\)) and an alcohol (\(\mathrm{ROH}\)). When a thiol loses its proton, it forms an alkylsulfide ion (\(\mathrm{RS}^{-}\)), and when an alcohol loses its proton, it turns into an alkoxide ion (\(\mathrm{RO}^{-}\)). The stability of these ions determines the acidity of the respective compounds.

Explain the effect of atom size on the stability of ions

The larger the atom, the better it can accommodate a negative charge as it can be delocalized over a larger area. Since sulfur is larger than oxygen, the charge on the sulfur atom in the alkylsulfide ion (\(\mathrm{RS}^{-}\)) is delocalized over a larger area, making it more stable compared to the alkoxide ion (\(\mathrm{RO}^{-}\)). Therefore, the thiol (\(\mathrm{RSH}\)) is more acidic than alcohol (\(\mathrm{ROH}\)).

Describe the interaction between thiols and sodium hydroxide

Sodium hydroxide (\(\mathrm{NaOH}\)) is a strong base, which can react with thiols. The reaction can be represented as: \(\mathrm{RSH + NaOH \to RS^{-}Na^{+} + H_{2}O}\) Since thiols are sufficiently strong acids, they react completely with sodium hydroxide, resulting in alkylsulfide salts (\(\mathrm{RS^{-}Na^{+}}\)).

Key concepts

Thiols vs Alcohols

In the world of organic chemistry, thiols and alcohols are two important classes of compounds that often come up. They both have a similar structure, consisting of a hydrocarbon chain attached to a functional group. Thiols contain a sulfur atom, while alcohols have an oxygen atom. This seemingly small difference plays a big role in their chemical behavior.

• Alcohols have a hydroxyl group \((\mathrm{OH})\), whereas thiols have a sulfhydryl group \((\mathrm{SH})\).
• The sulfur in thiols is larger than the oxygen in alcohols, which affects how they interact with other substances.

Thiols tend to have a stronger unpleasant odor compared to alcohols, which is due to the larger, more polarizable sulfur atom in thiols. Moreover, when it comes to acidity, thiols are more acidic than alcohols. This difference in acidity is fundamental and arises from the differences in the atoms that each functional group contains, influencing their chemical reactivity in aqueous solutions as observed in reactions with bases like sodium hydroxide (\(\mathrm{NaOH}\).

Conjugate Base Stability

The acidity of a compound is closely related to the stability of its conjugate base. In simpler terms, when a compound loses a proton (\(\mathrm{H}^{+}\)), it transforms into its conjugate base. The more stable the conjugate base is, the stronger the acid the compound becomes.For example, when a thiol loses a proton, it forms an alkylsulfide ion \((\mathrm{RS}^{-})\). Similarly, when an alcohol loses a proton, it forms an alkoxide ion \((\mathrm{RO}^{-})\).

• A stable conjugate base means that it can hold the negative charge comfortably.
• The stability of the conjugate base (alkylsulfide vs. alkoxide ion) greatly influences the corresponding acid's strength.

Since alkylsulfide ions are more stable than alkoxide ions, thiols are more acidic than alcohols. This outcome is due to the greater ability of the larger sulfide ion to spread out and stabilize the negative charge.

Alkylsulfide Ion

The alkylsulfide ion \((\mathrm{RS}^{-})\) is a crucial player in understanding why thiols are more acidic than alcohols. The process involves the thiol losing a hydrogen atom and forming an alkylsulfide ion, a transition that's particularly favorable due to the nature of sulfur.

• The sulfur atom in \((\mathrm{RS}^{-})\), being larger, can spread out the negative charge more effectively than the smaller oxygen atom in alkoxide ion \((\mathrm{RO}^{-})\).
• This delocalization of charge leads to greater stability for the \((\mathrm{RS}^{-})\) ion.

As a result, alkylsulfide ions do not readily get involved in chemical reactions, making the thiol a stronger acid that fully reacts with bases like sodium hydroxide \((\mathrm{NaOH})\) to form alkylsulfide salts.

Atom Size and Charge Delocalization

The concept of atom size and charge delocalization is fundamental in explaining the stability differences between alkylsulfide and alkoxide ions. Generally, larger atoms can hold a negative charge better because the charge can be delocalized over a bigger area.

• Sulfur, in the case of thiols, is larger than oxygen, allowing the \(\mathrm{RS}^{-}\) ion to spread its negative charge over a larger volume.
• Oxygen atoms in alcohols are smaller, leading to more localized and less stable charges on \(\mathrm{RO}^{-}\) ions.

This ability to distribute a negative charge over a larger space results in greater stability for the conjugate base of thiols, and hence, more acidity compared to alcohols. Understanding these properties helps explain why thiols can react completely with bases, forming stable salts in reactions.