9.4.4. Properties

Physical properties of alkynes follow the same trend of alkenes and alkanes. First three members are gases, the next eight are liquids and the higher ones are solids. All alkynes are colourless. Ethyene has characteristic odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter than water and immiscible with water but soluble in organic solvents like ethers, carbon tetrachloride and benzene. Their melting point, boiling point and density increase with increase in molar mass.

Alkynes, similar to alkenes and alkanes, show a consistent pattern in their physical properties based on molecular weight. The first three alkynes (like ethyne) exist as gases at room temperature, while those with larger molecular weights transition into liquids and eventually solids. For example, while ethyne (C2H2) is a gas, larger alkynes like octyne (C8H14) are liquid at room temperature. Despite their generally low polarity, alkynes tend to dissolve well in organic solvents, which means they can mix with substances like ether while being insoluble in water. This is because the polar nature of water does not interact well with the mostly non-polar alkyne molecules. As the size of the alkyne increases, the melting point, boiling point, and density also rise due to greater intermolecular forces.

Think of alkynes like a group of friends at a party. The lighter alkynes (like ethyne) are very sociable and can freely move about like gas, but as you introduce heavier friends (with larger carbon chains), they tend to cluster together, becoming liquids, and ultimately as they're even heavier, they might be more like solid, unmovable friends at the party.

Alkynes show acidic nature, addition reactions, and polymerisation reactions as follows: A. Acidic character of alkyne: Sodium metal and sodamide (NaNH2) are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas. These reactions have not been observed in case of ethene and ethane thus indicating that ethyne is acidic in nature in comparison to ethene and ethane. Why is it so?

Alkynes demonstrate unique chemical behaviors, especially their acidic property. When sodium metal (Na) or sodamide (NaNH2) interacts with ethyne, ethyne acts as an acid, thereby releasing hydrogen gas and forming sodium acetylide. This reaction showcases that the hydrogen atoms attached to the carbons in alkynes are more acidic than those in alkenes or alkanes. The reason for this acidic nature lies in the hybridization of the carbon atoms. In ethyne, the carbons are sp hybridized, giving a higher s-character (50%), which makes the C-H bonds stronger and more polar. This polarity helps in the easier release of the hydrogen ion (H+). In contrast, in ethene (sp²) and ethane (sp³), the hybridized carbons have lower s-character, which results in their hydrogen atoms being less acidic.

Imagine a game of tug-of-war. In ethyne, the sp hybridized carbons are like players that are really strong and can pull hard, easily letting go of the rope (the hydrogen), while in ethene and ethane, the players are not as strong and struggle to let go of the rope, making it harder to release the hydrogen.

Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen, hydrogen halides etc. Formation of the addition product takes place according to the following steps.

Alkynes are quite reactive due to the presence of their triple bonds, and they readily undergo addition reactions. For example, when alkynes react with dihydrogen gas (H2) in the presence of a catalyst (like nickel, palladium, or platinum), they convert into alkenes, and with more hydrogen, they can become alkanes. In the case of halogens, the addition is swift, turning an alkyne into a saturated compound containing halogens. This addition can be explained in several steps: First, the triple bond breaks, allowing the hydrogen or halogen atoms to add and form a new compound. This process is generally called electrophilic addition because the multiple bond serves as a target for electrophiles (positively charged species). The reactivity of alkynes allows for a variety of complex organic compounds to be synthesized, ranging from simple to intricate.

Think of the addition reactions of alkynes as a game of 'musical chairs.' At first, you have three chairs representing the triple bond. When players (hydrogen or halogen atoms) come in, they take seats (the addends) — the chairs representing the triple bond get resolved into more chairs (forming a double bond with alkenes and then single with alkanes) as they play along, leading to more variations and mixtures in the game.

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