Question

The most acidic hydrogen atoms are present in (a) ethane (b) ethene (c) ethyne (d) benzene.

Short answer

The most acidic hydrogen atoms are present in (c) ethyne.

Step-by-step solution

Understand Acidity of Hydrocarbons

The acidity of hydrogen atoms in hydrocarbons is related to the stability of the resulting anion after deprotonation. Hydrocarbons with sp hybridized carbons, such as alkynes, tend to have more acidic hydrogen atoms because the negative charge on the sp carbon anion is held closer to the nucleus, making it more stable.

Evaluate the Hybridization of Carbons

Ethane consists of sp3 hybridized carbons, ethene (ethylene) has sp2 hybridized carbons, and ethyne (acetylene) contains sp hybridized carbons. Benzene also contains sp2 hybridized carbons, similar to ethene. Acidity increases with greater s-character; hence, sp hybridized carbons are more acidic than sp2 which are more acidic than sp3.

Determine the Most Acidic Hydrogens

Since ethyne has sp hybridized carbons and an sp hybridized carbon anion is more stable due to increased s-character, the hydrogen atoms in ethyne are the most acidic compared to those in ethane, ethene, and benzene.

Key concepts

SP Hybridization

Understanding the concept of sp hybridization is crucial when evaluating the acidity of hydrocarbons, as it directly influences the electron distribution around a carbon atom. An sp hybridized carbon is formed when a carbon atom mixes one s orbital with one p orbital, leading to two degenerate sp orbitals. This unique hybridization confers a linear geometry and a 180-degree bond angle.

Due to the higher s-character (50%) of the sp orbitals compared to sp² (33%) and sp³ (25%), the electrons are held closer to the nucleus. This closer proximity leads to a stronger attraction between the nucleus and the electron pair, creating a bond with greater electron density along the axis, and ultimately, more acidic hydrogen atoms when attached to such carbons.

Stability of Carbanions

The stability of carbanions plays a pivotal role in determining the acidity of hydrocarbons. A carbanion is formed when a hydrogen atom is removed from a carbon atom, thus leaving behind a negatively charged ion. The stability of this negatively charged species is influenced by several factors, including the hybridization of the carbon atom.

Carbanions with greater s-character in their hybrid orbitals are more stable because the negative charge is held more tightly by the nucleus. As a result, for hydrocarbons like alkynes with sp hybridized carbons, the carbanions formed are significantly more stable compared to those formed from sp² or sp³ carbons. This increased stability leads to enhanced acidity of the hydrogens attached to such carbons.

Hydrogen Acidity

Hydrogen acidity in the context of hydrocarbons is the tendency of a hydrogen to be released as a proton (H⁺). This property is vital in many chemical reactions, such as those occurring in organic synthesis and biochemical pathways. The acidity is not uniform across all hydrocarbons and varies with changes in structure and hybridization of the carbon atoms to which the hydrogens are attached.

The acidity of a hydrogen atom is assessed by the stability of the conjugate base, which, for hydrocarbons, is the carbanion. A stable carbanion equates to a more acidic hydrogen. Therefore, in the lineup of ethane, ethene, ethyne, and benzene, ethyne's hydrogen atoms are most acidic due to the resulting sp hybridized stable carbanion.

Alkyne Chemistry

Alkyne chemistry is a rich field due to the unique properties of the carbon-carbon triple bond featured in alkynes. Alkynes are hydrocarbons with at least one pair of carbon atoms connected by a triple bond, consisting of one σ bond and two π bonds. This bond formation involves sp hybridized carbons, which results in a straight-line molecular geometry.

With regard to acidity, the terminal hydrogen atoms (those attached to the carbon atoms at the end of the triple bond) in alkynes are significantly more acidic than those in alkenes or alkanes. The high acidity makes alkynes interesting candidates for deprotonation reactions, leading to the formation of alkynyl anions, which can be used as nucleophiles in various organic synthesis reactions, showcasing the importance of understanding hydrogen acidity in alkyne chemistry.