The hydrogen atoms present in acetylene molecule are :

1. Classification of Hydrocarbons: Alkanes, Alkenes, and Alkynes (basic)

At its simplest, organic chemistry is the study of hydrocarbons—compounds made up entirely of carbon and hydrogen atoms Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65. Carbon is unique because it can form four bonds, allowing it to create chains and rings of various lengths. We classify these molecules based on the nature of the bonds between the carbon atoms. The most fundamental division is between saturated hydrocarbons (which contain only single bonds) and unsaturated hydrocarbons (which contain double or triple bonds).

Saturated hydrocarbons are known as Alkanes. In these molecules, every carbon atom is bonded to the maximum possible number of hydrogen atoms through single covalent bonds. They follow the general formula CₙH₂ₙ₊₂. On the other hand, Unsaturated hydrocarbons include Alkenes (containing at least one double bond, CₙH₂ₙ) and Alkynes (containing at least one triple bond, CₙH₂ₙ₋₂) Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65-68. This distinction is vital in daily life; for instance, vegetable oils are unsaturated chains, while animal fats are typically saturated Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

Feature	Alkanes	Alkenes	Alkynes
Bond Type	Single (C–C)	Double (C=C)	Triple (C≡C)
Saturation	Saturated	Unsaturated	Unsaturated
General Formula	CₙH₂ₙ₊₂	CₙH₂ₙ	CₙH₂ₙ₋₂
Suffix	-ane (e.g., Ethane)	-ene (e.g., Ethene)	-yne (e.g., Ethyne)

One fascinating deeper detail involves hybridization. In Alkanes, carbon atoms are sp³ hybridized. In Alkenes, they are sp², and in Alkynes, they are sp hybridized. As we move from single to triple bonds, the "s-character" of the carbon atom increases (reaching 50% in alkynes). This makes the carbon atom in an alkyne more electronegative. This high electronegativity pulls the electron density away from the hydrogen atom in a C-H bond, making terminal alkynes like acetylene (ethyne) slightly acidic. This allows them to react with strong bases to form metal salts, a property not found in alkanes or alkenes.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.68; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

2. Hybridization in Carbon Compounds (sp, sp², sp³) (intermediate)

To understand carbon compounds, we must first look at tetravalency—the fact that carbon has four valence electrons and typically forms four covalent bonds Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.63. However, these bonds aren't always identical. To achieve its stable structures, carbon undergo hybridization, a process where its atomic orbitals (one s and three p orbitals) mix to create new, equivalent "hybrid" orbitals. Think of it like mixing different colors of paint to get a specific shade that works best for a particular structure.

Depending on whether carbon forms single, double, or triple bonds, it adopts one of three hybridization states. In sp³ hybridization, the s-orbital mixes with all three p-orbitals to form four identical bonds arranged in a tetrahedral shape (109.5°), typical of saturated compounds like ethane Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.62. In sp² hybridization, one s mixes with two p-orbitals, leaving one "pure" p-orbital to form a double bond in a trigonal planar layout (120°). Finally, in sp hybridization, the s-orbital mixes with only one p-orbital, resulting in a linear structure (180°) found in triple-bonded molecules like ethyne.

Hybridization	Bond Type	Geometry	s-character
sp³	Single (Saturated)	Tetrahedral	25%
sp²	Double (Unsaturated)	Trigonal Planar	33.3%
sp	Triple (Unsaturated)	Linear	50%

A critical nuance for your UPSC preparation is the concept of s-character. Since the s-orbital is closer to the nucleus than the p-orbital, a hybrid orbital with more "s-character" (like sp with 50%) holds its electrons more tightly. This makes the carbon atom more electronegative. In a triple bond (sp), this high electronegativity pulls electron density away from the attached hydrogen, making the C-H bond polarized and the hydrogen atom slightly acidic—a unique property of alkynes compared to other hydrocarbons.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.63

3. Electronegativity and Bond Polarity (basic)

In our previous steps, we explored how atoms share electrons to reach a stable, noble gas configuration (Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60). However, in the world of chemistry, sharing is rarely 50-50. This brings us to the vital concept of Electronegativity — the internal "tug-of-war" ability of an atom to attract a shared pair of electrons toward itself within a chemical bond.

When two atoms with different electronegativities form a covalent bond, the electron cloud isn't centered; it shifts toward the more "greedy" or electronegative atom. This creates Bond Polarity. We represent this shift using partial charges: the atom pulling the electrons closer becomes slightly negative (δ-), and the atom losing its grip becomes slightly positive (δ+). While carbon compounds generally do not form ions like salts do (Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59), these internal partial charges are the "engine" behind chemical reactivity.

Understanding polarity is crucial for the UPSC aspirant because it explains the behavior of molecules. For instance, the more polarized a bond is, the easier it might be for a molecule to react with a base or for a bond to break during a reaction. Atoms like Oxygen or Nitrogen are very electronegative, whereas Hydrogen has a much weaker pull. As we will see later, even the hybridization state of a Carbon atom can change its electronegativity, making it act more like a "strong" atom that pulls electrons away from its neighbors.

Type of Bond	Electron Sharing	Result
Non-Polar Covalent	Equal (between identical atoms)	No partial charges (e.g., H-H)
Polar Covalent	Unequal (between different atoms)	Partial charges δ+ and δ- (e.g., C-O)

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.60; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.59

4. Theories of Acids and Bases (intermediate)

To master organic chemistry, we must look beyond the simple definition of acids as sour substances. At its core, acidity is about the stability of a molecule after it loses a proton (H⁺). While classical theories like the Arrhenius model focus on substances producing H⁺ or OH⁻ ions in water Science, Acids, Bases and Salts, p.25, organic chemistry often relies on the Brønsted-Lowry theory (acids are proton donors) and the Lewis theory (acids are electron-pair acceptors).

A fascinating application of these theories is the acidity of hydrocarbons. You might expect all C-H bonds to be neutral, but they aren't. In organic molecules, the acidity of a hydrogen atom is heavily influenced by the hybridization of the carbon it is attached to. For instance, in acetylene (ethyne, C₂H₂), the carbon atoms are sp hybridized. This means the hybrid orbital has 50% s-character. Since s-orbitals are closer to the nucleus than p-orbitals, a higher s-character makes the carbon atom more electronegative. This electronegativity pulls the shared electrons away from the hydrogen, polarizing the C-H bond and making it easier for the hydrogen to leave as a proton (H⁺).

Hydrocarbon Type	Hybridization	s-character	Relative Acidity
Alkanes (e.g., Ethane)	sp³	25%	Very Weak
Alkenes (e.g., Ethene)	sp²	33.3%	Weak
Alkynes (e.g., Ethyne)	sp	50%	Strongest (among these)

Because of this unique property, terminal alkynes like acetylene can react with very strong bases such as sodamide (NaNH₂) or sodium metal to form metal acetylides, a reaction that does not occur with alkanes or alkenes. While still much weaker than mineral acids like HCl or organic acids like acetic acid Science, Acids, Bases and Salts, p.26, this trend is crucial for synthesizing complex organic structures.

Sources: Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.25; Science, class X (NCERT 2025 ed.), Acids, Bases and Salts, p.26

5. Applications of Acetylene and Ethylene in Industry (intermediate)

In the world of organic chemistry, Ethylene (Ethene, C₂H₄) and Acetylene (Ethyne, C₂H₂) are the workhorses of industry. Their utility stems from their unsaturation—the presence of double and triple bonds which makes them far more reactive than saturated alkanes. While saturated hydrocarbons generally burn with a clean blue flame, these unsaturated compounds typically produce a yellow, sooty flame due to incomplete combustion, unless they are supplied with pure oxygen Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.69. This high carbon content per molecule is precisely what makes them so energy-dense and useful.

Acetylene is uniquely characterized by its acidity. In an acetylene molecule, the carbon atoms are sp hybridized, meaning they possess 50% 's-character'. This high s-character makes the carbon atom more electronegative, pulling electron density away from the hydrogen atom. Consequently, the C-H bond becomes highly polarized, and the hydrogen can be released as a proton (H⁺). This allows acetylene to react with strong bases to form metal acetylides. Industrially, its most famous application is the oxy-acetylene flame. When mixed with pure oxygen, acetylene burns at temperatures exceeding 3000°C, which is sufficient to melt and weld metals—a much more intense application than the low-melting-point solder used for simple electronics Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54.

Ethylene, on the other hand, is the world's most produced organic compound, primarily serving as a building block for plastics like Polyethylene (Polythene). Beyond heavy industry, it plays a critical role in the agricultural sector. Ethylene acts as a natural plant hormone that regulates the ripening of fruits. Given that India is a global leader in the production of fruits like mangoes and bananas Geography of India, Majid Husain, Agriculture, p.99, the controlled use of ethylene gas is vital for ensuring that produce reached markets in optimal condition after being harvested unripe for transport.

Feature	Ethylene (C₂H₄)	Acetylene (C₂H₂)
Bonding	Double Bond (sp² hybridization)	Triple Bond (sp hybridization)
Key Property	Polymerization & Biogenic Hormone	Weak Acidity & High Combustion Heat
Primary Use	Plastics (Polythene), Fruit Ripening	Oxy-acetylene Welding, Chemical Synthesis

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.69; Science, class X (NCERT 2025 ed.), Metals and Non-metals, p.54; Geography of India, Majid Husain, Agriculture, p.99

6. Chemical Reactivity and Displacement Reactions (intermediate)

In chemistry, reactivity is essentially a measure of how easily an atom or molecule can lose or gain electrons to reach a stable state. A fundamental way to observe this is through displacement reactions. In basic inorganic chemistry, we see this when a more reactive metal displaces hydrogen from an acid to form a salt and hydrogen gas: Acid + Metal → Salt + H₂ Science, Class X, Acids, Bases and Salts, p.20. This principle isn't just for laboratory acids like HCl; it extends into organic chemistry, where certain hydrocarbons can act as very weak acids depending on their structure.

While most hydrocarbons (like methane or ethane) are chemically inert toward metals, alkynes—specifically terminal alkynes like acetylene (ethyne)—behave differently. The hydrogen atoms attached to the triple-bonded carbons in ethyne are acidic. This means they can be displaced by strong bases or active metals like Sodium (Na) or Sodamide (NaNH₂). This is a classic displacement reaction in organic chemistry where the metal replaces the hydrogen to form a metal acetylide and liberates hydrogen gas.

Why does this happen? The secret lies in the hybridization of the carbon atoms. In ethyne, the carbons are sp-hybridized, meaning they have 50% s-character. Since s-orbitals are closer to the nucleus than p-orbitals, an sp-hybridized carbon is more electronegative than the sp² or sp³ carbons found in alkenes and alkanes. This high electronegativity allows the carbon to pull the shared electron pair of the C-H bond closer to itself, making the hydrogen atom slightly positive and easier to remove as a proton (H⁺).

Hydrocarbon Type	Hybridization	s-character	Relative Acidity
Alkanes (Ethane)	sp³	25%	Lowest (Inert)
Alkenes (Ethene)	sp²	33%	Low
Alkynes (Ethyne)	sp	50%	Highest (Acidic)

Understanding this reactivity series of hydrocarbons allows us to predict how they will behave in industrial extractions or synthetic reactions Science, Class X, Metals and Non-metals, p.49. Just as we use a reactivity series to determine which metal will displace another from a salt solution Science, Class X, Metals and Non-metals, p.45, we use hybridization to determine which organic molecules will participate in displacement reactions.

Sources: Science, Class X, Acids, Bases and Salts, p.20; Science, Class X, Metals and Non-metals, p.45; Science, Class X, Metals and Non-metals, p.49

7. The Relationship between s-character and Acidity (exam-level)

To understand why some hydrocarbons behave like acids while others do not, we must look at the hybridization of the carbon atom. In organic chemistry, carbon atoms can hybridize their orbitals in three primary ways: sp³ (found in alkanes like ethane), sp² (found in alkenes like ethene), and sp (found in alkynes like ethyne). As we move from single to triple bonds, the s-character of the hybrid orbital increases. While carbon usually forms four bonds to satisfy its valency Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.62, the nature of those bonds changes significantly based on this hybridization.

The s-orbital is spherical and resides closer to the nucleus than the elongated p-orbital. Therefore, the more "s-character" a hybrid orbital has, the closer its electrons are held to the carbon nucleus. This makes the carbon atom effectively more electronegative. When carbon is sp hybridized (as seen in ethyne, C₂H₂), it possesses 50% s-character. This high electronegativity pulls the shared electron pair in the C-H bond strongly toward the carbon, leaving the hydrogen atom with a partial positive charge. This polarization allows the hydrogen to be released as a proton (H⁺) when treated with a strong base, which is the very definition of acidity.

Hydrocarbon Type	Hybridization	% s-character	Acidity Trend
Alkane (e.g., Ethane)	sp³	25%	Least Acidic
Alkene (e.g., Ethene)	sp²	33.3%	Intermediate
Alkyne (e.g., Ethyne)	sp	50%	Most Acidic

Because ethyne is an unsaturated carbon compound with a triple bond Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64, its C-H bond is significantly more acidic than the bonds in saturated compounds. In fact, ethyne can react with strong bases like sodium amide (NaNH₂) or sodium metal to form metal acetylides, a reaction that ethane or ethene simply cannot perform. This property is a direct consequence of the 50% s-character stabilizing the negative charge on the carbon after the proton is lost.

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.64

8. Acidic Nature of Ethyne (Acetylene) (exam-level)

In the world of organic chemistry, we usually think of hydrocarbons (like methane or ethane) as being neutral and non-reactive with bases. However, ethyne (acetylene) breaks this rule. It possesses a unique acidic nature that allows it to donate a proton (H⁺) when treated with very strong bases. To understand why, we have to look at the hybridization of the carbon atom.

In an ethyne molecule (HC≡CH), the carbon atoms are sp hybridized. This means the hybrid orbital is composed of 50% s-orbital character and 50% p-orbital character. In contrast, the carbons in ethene (sp²) have 33% s-character, and ethane (sp³) has only 25%. Because s-orbitals are closer to the atomic nucleus than p-orbitals, an increase in s-character makes the carbon atom more electronegative. This electronegative carbon pulls the shared pair of electrons in the C-H bond toward itself, significantly polarizing the bond and making the hydrogen atom "loose" enough to be released as a proton.

While ethyne is a very weak acid compared to mineral acids or even ethanoic acid (Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74), its acidity is high enough to react with active metals. For instance, when ethyne reacts with sodium metal (Na) or sodamide (NaNH₂), it displaces hydrogen gas and forms metal acetylides. This reaction is reminiscent of how metals like zinc react with acids or bases to liberate hydrogen gas (Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20). This property is exclusive to "terminal" alkynes (where the triple bond is at the end of the chain) and serves as a classic test to distinguish them from alkanes and alkenes.

Hydrocarbon	Hybridization	s-character	Acidity Order
Ethane (CH₃-CH₃)	sp³	25%	Least Acidic
Ethene (CH₂=CH₂)	sp²	33%	Intermediate
Ethyne (HC≡CH)	sp	50%	Most Acidic

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.74; Science, Class X (NCERT 2025 ed.), Acids, Bases and Salts, p.20

9. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of atomic structure and hybridization, you can see how the sp hybridization in acetylene (ethyne) serves as the chemical "smoking gun" for this question. In an sp-hybridized carbon atom, there is 50% s-character, which means the electrons are held much closer to the carbon nucleus compared to sp2 or sp3 hybridized carbons. This increased electronegativity of the carbon atom makes it "pull" the shared electron pair of the C-H bond toward itself, leaving the hydrogen atom with a partial positive charge. This polarization allows the hydrogen to be released as a proton (H+), which is why the hydrogen atoms are (A) acidic.

To arrive at this answer, a coach’s logic follows a clear path: High s-character → High electronegativity → Polarized C-H bond → Proton release. This is why acetylene can react with strong bases like sodamide (NaNH2) or sodium metal to form metal acetylides—a reaction that doesn't happen with ethane or ethylene. A common UPSC trap is to assume all hydrocarbons are neutral (Option D) because they don't turn litmus paper red; however, in the realm of organic chemistry, the relative acidity of terminal alkynes is a critical property. Option (B) basic is incorrect because acetylene acts as a proton donor, not an electron donor or proton acceptor, in these reactions. As noted in Iwanami Organic Chemistry, this specific reactivity is what differentiates alkynes from other simple hydrocarbons.