Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons ?

1. The Versatile Nature of Carbon (basic)

Welcome to the fascinating world of Organic Chemistry! To understand why we have an entire branch of chemistry dedicated mostly to one element, we must first look at the versatile nature of carbon. Carbon is the structural backbone of everything from the DNA in your cells to the fuels that power our world. In fact, the number of known carbon compounds is estimated to be in the millions, far outnumbering the compounds of all other elements combined Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 62.

This "superpower" of carbon arises from two unique chemical properties working in tandem:

• Catenation: This is carbon’s unique ability to form strong covalent bonds with other carbon atoms, creating long chains, branched structures, or even rings. While other elements like Silicon can form chains (up to 7 or 8 atoms), they are highly reactive and unstable. Carbon-carbon bonds, however, are exceptionally strong and stable, allowing for the existence of massive, complex molecules Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 62.
• Tetravalency: Carbon has a valency of four. This means a single carbon atom can bond with four other atoms—whether they are other carbon atoms or atoms of elements like Hydrogen (H), Oxygen (O), Nitrogen (N), or Chlorine (Cl). This creates an almost infinite variety of geometric arrangements and functional combinations Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 63.

Historically, scientists believed these complex compounds could only be produced by a "vital force" within living organisms. However, this theory was famously overturned in 1828 when Friedrich Wöhler synthesized urea (an organic compound) from ammonium cyanate (an inorganic material) Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p. 63. This proved that the chemistry of life follows the same physical laws as the rest of the universe, rooted in the versatile bonding of the carbon atom.

Property	Description	Result
Catenation	Self-linking ability via strong C-C bonds.	Formation of long chains and rings.
Tetravalency	Ability to form four covalent bonds.	High variety in molecular structure and complexity.

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

2. Classification of Hydrocarbons (basic)

At the very heart of organic chemistry lies a vast family of compounds known as hydrocarbons. As the name suggests, these are molecules composed exclusively of carbon and hydrogen atoms. We classify them primarily based on the types of bonds holding the carbon atoms together. Think of this as the "skeleton" of organic chemistry—once you understand how these bonds are structured, the rest of the subject becomes much more intuitive. According to Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65, we divide them into two major categories: saturated and unsaturated.

Saturated hydrocarbons, commonly known as alkanes, contain only single bonds between carbon atoms. Because every carbon atom is bonded to the maximum possible number of hydrogen atoms, we say it is "saturated." These compounds form a homologous series, where each successive member differs from the previous one by a –CH₂– unit. For instance, methane (CH₄) is followed by ethane (C₂H₆), then propane (C₃H₈), and butane (C₄H₁₀). As you add more carbon atoms, the molecular weight increases predictably—specifically by 14 units for every CH₂ unit added (since Carbon is 12u and Hydrogen is 1u) as noted in Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.66.

On the other hand, unsaturated hydrocarbons contain one or more double or triple bonds between carbon atoms. Those with at least one double bond are called alkenes (e.g., ethene, C₂H₄), while those with at least one triple bond are called alkynes (e.g., ethyne, C₂H₂). These bonds make the molecules more reactive than alkanes. Interestingly, this classification has direct implications for our health. Vegetable oils generally consist of long unsaturated carbon chains and are considered healthier for cooking, whereas animal fats often contain saturated fatty acids, which can be harmful if consumed in excess Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71.

Type	Bonding	General Name	Example
Saturated	Single (C–C)	Alkanes	Methane (CH₄)
Unsaturated	Double (C=C)	Alkenes	Ethene (C₂H₄)
Unsaturated	Triple (C≡C)	Alkynes	Ethyne (C₂H₂)

Sources: Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.65; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science, class X (NCERT 2025 ed.), Carbon and its Compounds, p.71

3. IUPAC Nomenclature: The Logic of Naming (intermediate)

In organic chemistry, naming isn't arbitrary; it follows a systematic logic established by the IUPAC (International Union of Pure and Applied Chemistry). Think of it as a grammar for molecules where the name tells you exactly how the carbon "skeleton" is built. The foundation of any name is the parent chain, which is determined by the number of carbon atoms. For instance, a single carbon atom starts with the prefix meth-, while a four-carbon chain starts with but-. As the number of carbon atoms increases, the molecular weight of the compound also rises predictably with each added CH₂ unit Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64.

Number of Carbons	Prefix	Alkane Example (Saturated)
1	Meth-	Methane (CH₄)
2	Eth-	Ethane (C₂H₆)
3	Prop-	Propane (C₃H₈)
4	But-	Butane (C₄H₁₀)

Beyond the number of carbons, the name changes based on saturation. If all carbon atoms are linked by single bonds, the suffix is -ane (saturated). However, if there is a double bond, the suffix becomes -ene, and for a triple bond, it is -yne. For example, a three-carbon chain with a double bond is called propene Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. If the carbons form a closed loop rather than a straight line, we add the prefix cyclo-, such as in cyclohexane (C₆H₁₂), which consists of six carbons in a ring Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65.

Finally, we account for functional groups — specific atoms or groups like alcohols (-OH) or ketones (>C=O) that give the molecule its chemical personality. When a functional group is present, the terminal 'e' of the parent alkane name is usually replaced by a specific suffix. For example, a three-carbon ketone is named by taking propane, dropping the 'e', and adding '-one' to get propanone Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. This logical layering ensures that a chemist anywhere in the world can draw the exact structure of a molecule just by reading its name.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.64; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.65; Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.67

4. Hydrocarbons in Energy: CNG, LPG, and LNG (intermediate)

In the world of energy, the simplest members of the alkane family play a starring role. As we move from Methane (CH₄) to Butane (C₄H₁₀), the molecular weight increases (CH₄ ≈ 16 g/mol to C₄H₁₀ ≈ 58 g/mol), which significantly alters their physical properties and how we store them for use. Natural Gas is primarily composed of Methane (often 80-90%), with smaller amounts of Ethane, Propane, and Butane Majid Hussain, Environment and Ecology, p.15. Because methane is a very light molecule, it remains a gas at standard temperatures and requires high pressure or extreme cooling to be transported efficiently. To make these hydrocarbons useful for society, we categorize them into three main commercial forms:

• CNG (Compressed Natural Gas): This is mostly methane stored at very high pressure. It is increasingly used as a cleaner transport fuel because it produces fewer pollutants than petrol or diesel NCERT, Contemporary India II, p.115.
• LNG (Liquefied Natural Gas): This is also methane, but it has been cooled to roughly -162°C to turn it into a liquid. This reduces its volume by about 600 times, making it viable for shipment across oceans in specialized tankers.
• LPG (Liquefied Petroleum Gas): Unlike the others, LPG consists mainly of Propane (C₃H₈) and Butane (C₄H₁₀). These heavier molecules are much easier to liquefy under moderate pressure at room temperature, which is why they are perfect for the portable steel cylinders used in our kitchens.

Beyond fossil reserves, we can produce these hydrocarbons through Bio-energy. In rural areas, the anaerobic decomposition of organic waste in biogas plants (often called 'Gobar gas plants') yields a methane-rich gas NCERT, Contemporary India II, p.117. This process not only provides a clean cooking fuel with high thermal efficiency but also produces nutrient-rich slurry that serves as excellent manure for farmers Majid Hussain, Geography of India, p.30.

Fuel Type	Primary Hydrocarbon	Storage Method	Primary Use
CNG	Methane (CH₄)	High Pressure (Gas)	Transport (Buses, Autos)
LPG	Propane & Butane	Moderate Pressure (Liquid)	Domestic Cooking
LNG	Methane (CH₄)	Cryogenic Cooling (Liquid)	Long-distance Bulk Trade

Sources: Environment and Ecology, Majid Hussain, Distribution of World Natural Resources, p.15; NCERT, Contemporary India II, Energy Resources, p.115; NCERT, Contemporary India II, Energy Resources, p.117; Geography of India, Majid Husain, Energy Resources, p.30

5. Environmental Impact: Hydrocarbons as GHGs (intermediate)

In our study of organic chemistry, we have seen how alkanes like methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) increase in complexity and molecular weight. However, these molecules are not just industrial fuels; they play a critical role in the Earth's energy balance. Hydrocarbons, particularly methane, are potent Greenhouse Gases (GHGs). While Carbon Dioxide (CO₂) is the most abundant GHG, methane is significantly more efficient at trapping heat on a per-molecule basis. It is primarily generated through anaerobic processes—organic decomposition occurring in the absence of oxygen—such as in the intestinal tracts of livestock and the waterlogged soils of rice fields Environment and Ecology, Majid Hussain, Climate Change, p.11. To understand the environmental impact of different hydrocarbons, we use a metric called Global Warming Potential (GWP). GWP measures two key factors: how effectively a gas absorbs energy (heat) and how long it remains in the atmosphere before breaking down Environment, Shankar IAS Academy, Climate Change, p.260. Although methane has a relatively short atmospheric lifetime of about 12 years compared to the centuries CO₂ can persist, its ability to trap heat is vastly superior in the short term.

Gas	Atmospheric Lifetime (Years)	GWP (100-year horizon)
Carbon Dioxide (CO₂)	Variable (approx. 100)	1
Methane (CH₄)	12	21
Nitrous Oxide (N₂O)	120	310

Beyond simple alkanes, there are specialized organic compounds like Hydrofluorocarbons (HFCs)—hydrocarbons where hydrogen atoms are replaced by fluorine. These are termed "super greenhouse gases" because their GWP can be thousands of times higher than CO₂ Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.426. Understanding this chemistry is vital for policy-making, such as India's push for Compressed Natural Gas (CNG) in transport to reduce particulate pollution, while simultaneously needing to manage the risk of methane leaks which would exacerbate warming Environment, Shankar IAS Academy, India and Climate Change, p.315.

Sources: Environment and Ecology, Majid Hussain, Climate Change, p.11; Environment, Shankar IAS Academy, Climate Change, p.260; Environment, Shankar IAS Academy, Environment Issues and Health Effects, p.426; Environment, Shankar IAS Academy, India and Climate Change, p.315

6. Homologous Series and Molecular Mass Trends (exam-level)

In organic chemistry, carbon atoms have the unique ability to link together in long chains, a property known as catenation. This allows for the formation of a homologous series: a family of compounds that share the same functional group and a general formula, but differ from one another by a specific repeating unit, typically a CH₂ unit Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.62. Because they share the same functional group, the chemical properties of all members in the series remain very similar. For example, all alcohols (like Methanol or Ethanol) will react in a similar fashion because their behavior is dictated by the –OH group, regardless of the chain length Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.66.

As we move from one member to the next in a series, there is a steady increase in molecular mass. In the alkane series, each successive member adds one carbon and two hydrogen atoms, meaning the molar mass increases by approximately 14 units (12 for Carbon + 2 for Hydrogen) at each step. This creates a clear ascending sequence of weights, as seen in the first four alkanes:

Alkane	Formula	Approx. Molar Mass (g/mol)
Methane	CH₄	16.05
Ethane	C₂H₆	30.08
Propane	C₃H₈	44.11
Butane	C₄H₁₀	58.14

This increase in molecular mass leads to a noticeable gradation in physical properties. As the molecules get larger and heavier, the attractive forces between them strengthen. Consequently, melting and boiling points increase with increasing molecular mass Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.67. This is why the simplest alkanes are gases at room temperature, while those with much longer chains (higher mass) eventually become liquids and then solids like paraffin wax.

Sources: Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.66; Science class X (NCERT 2025 ed.), Carbon and its Compounds, p.67

7. Solving the Original PYQ (exam-level)

You’ve just mastered the homologous series of alkanes, where each successive member differs by a CH₂ unit. This question tests your ability to apply that structural understanding to a physical property: molecular weight. Since carbon and hydrogen atoms have fixed atomic masses, adding more of them naturally increases the total mass of the molecule. By recognizing the prefixes meth- (1 carbon), eth- (2 carbons), prop- (3 carbons), and but- (4 carbons), you can visualize the chain lengthening and the weight rising accordingly.

To solve this, recall the general formula for alkanes: C_nH_2n+2. As 'n' increases from 1 to 4, we move from Methane (CH₄) to Ethane (C₂H₆), then Propane (C₃H₈), and finally Butane (C₄H₁₀). Each step adds approximately 14 atomic mass units (12 for Carbon and 2 for Hydrogen). Therefore, the sequence that follows the ascending order of carbon atoms will also be the correct increasing order of molecular weights, which leads us directly to Option (A). As noted in Science, class X (NCERT), these foundational structures are the building blocks of organic chemistry.

UPSC frequently uses directional traps to catch candidates off guard. Options (B), (C), and (D) are incorrect because they either present a decreasing order or a completely jumbled sequence. A common mistake is to confuse the numerical prefixes or misread the requirement for 'increasing' versus 'decreasing' order. In competitive exams, always double-check the prefix sequence—Meth, Eth, Prop, But—to ensure you are tracking the growth of the carbon skeleton accurately before selecting your answer.