Question

Which of the following shows acidic character? (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) (b) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}\) (c) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) (d) \(\mathrm{CH}_{3} \equiv \mathrm{C}-\mathrm{CH}_{3}\)

Short answer

Compound (b) \\(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}\\) shows acidic character.

Step-by-step solution

Understanding Acidic Character

Acidic character in organic compounds can often be determined by the presence of hydrogen atoms that can be easily removed as protons (H⁺). Hydrocarbons with sp-hybridized carbons are more acidic than those with sp² or sp³ hybridization because they hold the electrons closer to the nucleus, stabilizing the resulting negative charge of the conjugate base.

Analyze Compound (a)

Compound (a) \(\mathrm{CH}_{3}-\mathrm{CH}_{3}\) is ethane, a saturated hydrocarbon with sp³ hybridized carbons. The C-H bonds are strong, making it unlikely to donate a proton, hence not showing acidic character.

Analyze Compound (b)

Compound (b) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}\) is propyne, an alkyne with a terminal hydrogen attached to a carbon-carbon triple bond. The hydrogen is attached to an sp-hybridized carbon, making it relatively acidic as it can easily lose a proton.

Analyze Compound (c)

Compound (c) \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) is ethene, an alkene with sp² hybridized carbons. The hydrogen atoms attached to these carbons are less acidic than those attached to sp-hybridized carbons, hence ethene does not show significant acidic character.

Analyze Compound (d)

Compound (d) \(\mathrm{CH}_{3} \equiv \mathrm{C}-\mathrm{CH}_{3}\) is 2-butyne, an internal alkyne with hydrogen atoms attached to sp³ hybridized carbons. This structure lacks the terminal hydrogen typical of acidic alkynes, so it does not show acidic character.

Key concepts

Hybridization in Organic Chemistry

Understanding hybridization in organic chemistry is key to predicting the properties and reactivity of organic compounds. Hybridization involves the mixing of atomic orbitals to create new, hybrid orbitals that form covalent bonds. In simple terms, it's like mixing ingredients to create a new dish. Here's how it works in hydrocarbon compounds:

• sp³ Hybridization: This occurs in structures like alkanes where each carbon forms four sigma bonds. These bonds are formed by overlapping sp³ hybrid orbitals. Such carbons are typically not very acidic because they contain single (sigma) bonds, making the bonds strong and less likely to donate protons.
  


• sp² Hybridization: Happens in alkenes where a double bond exists between carbons. The carbons form three sigma bonds and one pi bond, using sp² hybrid orbitals. The pi bond is less strong than sigma bonds, slightly increasing the likelihood of releasing a proton, but still not very acidic.
  


• sp Hybridization: Found in alkynes, especially those with a terminal triple bond. The carbons form two sigma bonds and two pi bonds using sp hybrid orbitals. These carbons hold electrons closer to the nucleus, making the attached hydrogen atoms more acidic as they can easily release a proton.
  

Comparative Acidity of Hydrocarbons

The acidity of hydrocarbons is crucial for understanding organic chemistry reactions, such as substitutions and eliminations. To determine acidity, we look at the hybridization of the carbon atoms holding the hydrogen that may be released:

• Alkanes (\( ext{sp}^3\)): These hydrocarbons are the least acidic because their sigma bonds between carbon and hydrogen are strong. Additionally, the carbon's lower electronegativity doesn't encourage proton donation.
  


• Alkenes (\( ext{sp}^2\)): These have slightly higher acidity than alkanes due to the presence of a pi bond, which doesn't hold protons as strongly. However, they are still not very reactive or acidic compared to alkynes.
  


• Alkynes (\( ext{sp}\)): These are the most acidic of the hydrocarbons, particularly those terminal alkynes. The higher s-character of the sp hybrid orbital means its electrons are held closer to the nucleus, stabilizing the conjugate base when a proton is lost.
  

Understanding these differences helps predict how molecules will behave under various conditions and which would act as an acid in chemical reactions.

Properties of Alkanes, Alkenes, and Alkynes

Alkanes, alkenes, and alkynes each have unique properties that determine their behavior and application in chemistry. Knowing these properties is important when analyzing problems like identifying acidic characters.

• Alkanes: Saturated hydrocarbons with only single bonds. They are non-polar, chemically inert, and generally exhibit low reactivity. Their lack of pi bonds makes them poor in showing any acidic character.
  


• Alkenes: These unsaturated hydrocarbons contain at least one double bond, contributing to their reactivity. The presence of a pi bond makes them slightly more prone to react than alkanes, but their acidity remains low. They are important in polymerization processes and addition reactions.
  


• Alkynes: Unsaturated hydrocarbons with at least one triple bond. This gives them unique properties such as higher acidity, especially terminal alkynes, due to their sp-hybridization. They are more reactive in addition reactions, often serving as starting materials in organic synthesis.
  

Recognizing these properties helps in understanding why compounds like propyne have more acidic characters compared to others like ethene or ethane.