Question

Acidic hydrogen is present in: (a) Ethyne (b) Ethene (c) Benzene (d) Ethane

Short answer

(a) Ethyne

Step-by-step solution

Understanding Acidic Hydrogen

An acidic hydrogen is one that can be released as a proton (H⁺) in a reaction. This typically occurs when the hydrogen is bonded to a more electronegative element, such as a carbon atom attached to a triple bond or adjacent to an electron-withdrawing group.

Analyzing Ethyne

Ethyne ( ext{C}_2 ext{H}_2) has a hydrogen atom directly attached to a carbon-carbon triple bond. Due to the high s-character of the sp-hybridized orbitals, the hydrogen is more acidic compared to other hydrogen atoms in hydrocarbons.

Analyzing Ethene

In ethene ( ext{C}_2 ext{H}_4), the hydrogen atoms are bonded to sp²-hybridized carbons forming a double bond. This configuration makes the hydrogen atoms much less acidic than those in ethyne.

Analyzing Benzene

In benzene ( ext{C}_6 ext{H}_6), the hydrogen atoms are bonded to a sp²-hybridized carbon in an aromatic ring. These hydrogen atoms are not considered acidic due to the absence of significant s-character of the C-H bonds in the aromatic system.

Analyzing Ethane

Ethane ( ext{C}_2 ext{H}_6) consists of carbon-carbon single bonds with hydrogen atoms bonded to sp³-hybridized carbons. These hydrogens are the least acidic of the group due to the very low s-character of the sp³ orbitals.

Conclusion

Out of the given options, ethyne has the hydrogen bonded to the sp-hybridized carbon, making it the most acidic among the choices.

Key concepts

Ethyne

Ethyne, also known as acetylene, is a simple alkyne with the chemical formula \(\text{C}_2\text{H}_2\). It is characterized by a carbon-carbon triple bond, which is the defining feature of alkynes. In ethyne, each carbon atom is connected to one hydrogen atom and is triple-bonded to the other carbon atom. This bond structure gives ethyne its linear geometry.

One key property of ethyne is the presence of acidic hydrogen. The hydrogen atoms attached to the carbon in the triple bond are more acidic than those in other hydrocarbons like ethene or ethane. This is because the carbon atoms in ethyne are sp-hybridized, which lends an s-character to its bond with hydrogen that increases acidity.

Ethyne is often used in welding processes due to its ability to burn at high temperatures. Its acidic hydrogen can also participate in chemical reactions involving proton donations.

sp Hybridization

sp Hybridization occurs in molecules where each carbon atom forms two bonds, including one triple bond or two double bonds. This type of hybridization involves the combination of one s and one p orbital from the carbon atom to form two sp-orbitals, resulting in linear geometry with a bond angle of 180°. This is typical in molecules like ethyne, where the carbon atoms are bonded together by a triple bond.

The presence of sp hybridization is crucial in determining the acidity of hydrogen atoms attached to the carbon. When carbon is sp-hybridized, the orbital containing the hydrogen has a higher s-character compared to sp² or sp³ hybridized carbons, providing more effective overlap of orbitals. This makes the C-H bond in ethyne more polar and stabilizes the conjugate base formed when the hydrogen is removed, leading to increased acidity.

Understanding hybridization helps in predicting the chemical behavior and properties of molecules, notably their geometry and reactivity.

Electronegativity

Electronegativity is a chemical property that describes an atom's ability to attract and bind with electrons. It's a critical concept in understanding molecular structure and reactivity. Among the elements, fluorine is the most electronegative, while cesium and francium are among the least.

In the context of hydrocarbons, carbon's electronegativity plays a role in determining bond characteristics and molecule reactivity. For instance, the spanned carbon atoms in ethyne are more electronegative due to the higher s-character in sp-hybridization, making the adjacent hydrogens slightly more acidic.

This effect arises because the carbon atom's ability to hold onto electrons in a bond is balanced by its hybridized state. More electronegative carbon can polarize the C-H bond, facilitating the removal of hydrogen as a proton. Thus, electronegativity is key in explaining why some hydrogens in hydrocarbons are more acidic than others.

Hydrocarbon Acidity

Hydrocarbon acidity refers to the tendency of certain hydrogen atoms within hydrocarbons to behave as acids, capable of being released as protons (H⁺). This characteristic varies depending on the hydrocarbon structure and the type of hybridization associated with the carbon atoms to which these hydrogens are attached.

In hydrocarbons like ethyne (acetylene), the hydrogen atoms are more acidic compared to those in ethane, ethene, or benzene. This is due to the sp-hybridization of the carbon atoms in ethyne, which increases the s-character of the orbital forming the C-H bond. Consequently, this leads to a more stable conjugate base upon hydrogen removal.

The concept of hydrocarbon acidity is crucial in organic chemistry, especially in reactions where the acidity of hydrogen can influence pathways and outcomes. It allows chemists to predict which hydrogens within complex molecules will most likely participate in proton transfer reactions.