Question

The correct trend of acidic nature of the following alkynes is (a) \(\mathrm{CH} \equiv \mathrm{CH}>\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}>\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}\) (b) \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}>\mathrm{CH} \equiv \mathrm{CH}>\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}>\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{CH}>\mathrm{CH} \equiv \mathrm{CH}\) (d) \(\mathrm{CH} \equiv \mathrm{CH}>\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CCH}_{3}>\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}\)

Short answer

The correct trend of acidic nature is \text{(a) } \text{CH} \text{\textequiv} \text{CH}>\text{CH}_{3}-\text{C} \text{\textequiv} \text{CH}>\text{CH}_{3} \text{C} \text{\textequiv} \text{CCH}_{3}.

Step-by-step solution

Understand Acidity of Alkynes

The acidity of alkynes depends on the ability of the carbon atom bonded to the hydrogen atom to stabilize the negative charge after deprotonation. Alkynes with more s-character in the hybrid orbitals of carbon are more acidic since s-orbital electrons are closer to the nucleus.

Compare Alkyne Acidic Nature based on Substituents

Substituents can donate or withdraw electron density from the carbon atom. Alkyl groups, like methyl (CH3), are electron-donating groups that decrease acidity, while hydrogen (H) does not have this effect. Thus, alkynes with fewer alkyl substituents are more acidic.

Determine the Trend of Acidity

Based on the substitution, the order of acidity is as follows: Ethyne (CH≡CH) is the most acidic since it has two hydrogen atoms and no alkyl groups. Propyne (CH3-C≡CH) is less acidic than ethyne, but more than 2-butyne (CH3C≡CCH3) because it has one alkyl group. 2-butyne with two alkyl groups is the least acidic.

Select the Correct Option

Comparing all given options with the trend we've determined, we see that option (a) reflects the correct trend of acidity.

Key concepts

Chemical Properties of Alkynes

Alkynes are a unique class of hydrocarbons characterized by a carbon-carbon triple bond. This triple bond is composed of one sigma bond and two pi bonds, forming a linear structure. The chemical properties of alkynes are heavily influenced by the high electron density within the triple bond, making them relatively reactive. One of the noteworthy reactions is their ability to undergo addition reactions where the pi bonds act as nucleophiles to attract electrophiles.

As for their acidity, it's one feature that sets alkynes apart from other hydrocarbons. The sp-hybridized carbon of an alkyne has a 50% s-character, which means that the electrons shared in the bond with hydrogen are held closer to the atom's nucleus than those in sp2 (33% s-character) or sp3 (25% s-character) hybridized carbons. This makes the hydrogen atom of an alkyne more acidic, which is observable in terminal alkynes where the acidic hydrogen can be deprotonated to form an acetylide anion.

The acetylide anion is a strong base and nucleophile, making alkynes pivotal in many organic syntheses. It's stable when compared to other carbanions due to the s-character previously explained. Moreover, alkynes undergo reactions such as hydrohalogenation, hydration, and halogenation, and can be transformed into alkenes, alcohols, and vicinal dihalides, respectively, which exhibits their chemical versatility.

Acid-Base Behavior in Organic Chemistry

In organic chemistry, the acid-base behavior of compounds is a fundamental concept, often explained by the Bronsted-Lowry theory and sometimes the Lewis theory. Acidity refers to a molecule's ability to donate a proton (H+), while basicity refers to its ability to accept a proton.

Alkynes, particularly terminal alkynes with a hydrogen atom at one end, demonstrate a unique acid-base behavior. When a terminal alkyne is deprotonated, a stabilized anion known as an acetylide ion is formed. This ion is a useful synthetic intermediate in organic chemistry, thanks to its high nucleophilicity. The stability of this anion is critical in understanding acidity; the more stable the anion, the stronger the acid.

Terminal alkynes are more acidic than their aliphatic alkane and alkene counterparts because the negative charge on the acetylide anion is held in an orbital with more s-character, which is closer to the nucleus and thus more stable. The ability to generate such stable anions from terminal alkynes underlines the synthetic utility of these acids in organic reactions, particularly nucleophilic substitutions and carbon-carbon bond formations.

Alkyne Substituent Effects

Substituent effects play a significant role in the chemistry of alkynes, and they are crucial for understanding the acidity trends observed in different alkynes. Substituents are atoms or groups of atoms attached to the carbon atoms of the alkyne. Depending on their nature, substituents can either increase or decrease the acidity of the alkyne.

Electronic factors like induction and resonance have considerable impacts on alkyne acidity. Electron-donating groups, such as alkyl groups (e.g., methyl or ethyl), reduce acidity by pushing electron density towards the triple bond's sp-hybridized carbon, which destabilizes the negative charge on the acetylide anion formed upon deprotonation.

Conversely, electron-withdrawing substituents like halogens or groups containing electronegative atoms enhance acidity by drawing electron density away from the acidic hydrogen's carbon, stabilizing the negative charge of the acetylide anion. Thus, the more electron-withdrawing substituents an alkyne has, or the fewer electron-donating substituents, the higher its acidity. This explains why a terminal alkyne with no alkyl substituents, such as ethyne (HC≡CH), is more acidic than propyne (CH3C≡CH) or 2-butyne (CH3C≡CCH3) with their electron-donating alkyl substituents.