Alkanes — Structure, Naming and Reactions

• Apply IUPAC rules to name alkanes with multiple branches and longest-chain selection
• Draw and interpret Newman projections for staggered and eclipsed conformations
• Write the full free-radical halogenation mechanism (initiation, propagation, termination)
• Predict monochlorination products using selectivity ratios for Cl₂ and Br₂
• Balance combustion equations and relate enthalpy to chain length
• Distinguish thermal and catalytic cracking and their industrial significance

Level 1 — Foundations

General formula: $C_nH_{2n+2}$

IUPAC naming rules:

1. Find the longest carbon chain — this is the parent alkane.
2. Number the chain from the end that gives substituents the lowest locants.
3. Name and number substituents: methyl (−CH₃), ethyl (−C₂H₅), propyl (−C₃H₇).
4. List substituents alphabetically (ignore di/tri prefixes for alphabetical order).
5. Use commas between numbers, hyphens between number and letter.

Types of carbon:

• Primary (1°): bonded to 1 other carbon
• Secondary (2°): bonded to 2 carbons
• Tertiary (3°): bonded to 3 carbons
• Quaternary (4°): bonded to 4 carbons (no H)

Conformations of ethane:

Newman projection: look along C–C bond. Front carbon = dot; back carbon = circle. Staggered has H atoms of back carbon bisecting the H–H angle of front carbon.

Butane conformations (C2–C3 bond):

Level 2 — JEE Depth

Free-radical halogenation mechanism (using Cl₂/hν on RH):

Initiation: $\text{Cl}_2 \xrightarrow{h\nu} 2\,\text{Cl}^\bullet$

Propagation (chain-carrying steps): $\text{Cl}^\bullet + \text{R-H} \rightarrow \text{R}^\bullet + \text{HCl}$ $\text{R}^\bullet + \text{Cl}_2 \rightarrow \text{R-Cl} + \text{Cl}^\bullet$

Termination: $\text{Cl}^\bullet + \text{Cl}^\bullet \rightarrow \text{Cl}_2$ $\text{R}^\bullet + \text{R}^\bullet \rightarrow \text{R-R}$ $\text{R}^\bullet + \text{Cl}^\bullet \rightarrow \text{R-Cl}$

Selectivity (relative reactivity per H atom):

Br₂ is more selective because the transition state is product-like (Hammond postulate); the more stable radical forms faster. Cl₂ has an exothermic, early transition state — less selective.

Combustion: $C_nH_{2n+2} + \frac{3n+1}{2}\,O_2 \rightarrow n\,CO_2 + (n+1)\,H_2O$

Standard enthalpy of combustion: more negative as chain length increases (more C–H and C–C bonds broken/formed).

Cracking:

Isomerisation: straight-chain alkane → branched (higher octane) using AlCl₃/HCl at 60–70°C. Reforming: naphtha → aromatics using Pt catalyst at 450°C.

Example 1: Name the compound with structure: CH₃CH₂CH(CH₂CH₃)C(CH₃)₂CH₂CH₃. Count primary, secondary, and tertiary carbons.

Step 1 — Identify longest chain.
The molecule has: C-C-C-C-C as one possible chain. Let us build it:

Main chain attempt (7 carbons):
C1: CH₃–
C2: CH₂–
C3: CH– (has ethyl branch)
C4: C– (has two methyl branches)
C5: CH₂–
C6: CH₃

Wait — that is only 6 carbons. Let us retrace with the ethyl group included:

Structure re-read: The C4 also has a CH₂CH₃. So longest chain is:
CH₃–CH₂–CH–C(CH₃)₂–CH₂–CH₃
              |
             CH₂CH₃

Count: 6 carbons in main chain → hexane parent.
Branch at C3: –CH₂CH₃ (ethyl)
Branch at C4: two –CH₃ (two methyls)

Numbering from left:
Position 3: ethyl; Position 4: two methyls → 4,4-dimethyl
Full name: 3-ethyl-4,4-dimethylhexane

Verify lowest locant set:
From left: {3, 4, 4} → sum 11
From right: {3, 3, 4} → sum 10 ← lower

Re-number from right:
C1: CH₃–
C2: CH₂– (was C6)
C3: C(CH₃)₂– (was C4, two methyls)
C4: CH– (was C3, ethyl)
C5: CH₂–
C6: CH₃

Name: 3-ethyl-4,4-dimethylhexane (same from either end after applying lowest-locant rule properly)

Actually, re-checking: from the right, methyls are at C3,C3 and ethyl at C4 → {3,3,4} sum 10.
From the left: methyls at C4,C4 and ethyl at C3 → {3,4,4} sum 11.
Choose from right: 4-ethyl-3,3-dimethylhexane.

Carbon types:
- C1, C2, C5, C6 of main chain + two methyl branches = primary (6 total)
- C4 (main chain, between two methyls and ethyl) = no — C4 is quaternary (bonded to 4 C)
- C3 of ethyl group = primary
- C3 (main chain) = tertiary? No: bonded to C2, C4, and two methyl Cs at C3 — it's quaternary.

Let us be explicit for 4-ethyl-3,3-dimethylhexane:
Quaternary C: C3 of the main chain (bonded to C2, C4, CH₃, CH₃)
Tertiary C: C4 (bonded to C3, C5, CH₂ of ethyl)
Primary C: both CH₃ at C3; terminal CH₃ of ethyl; C1; C6 → 5 primary carbons
Secondary C: C2, C5, CH₂ of ethyl → 3 secondary carbons
Tertiary C: C4 → 1 tertiary
Quaternary C: C3 → 1 quaternary

Example 2: In free-radical chlorination of butane (CH₃CH₂CH₂CH₃), find all possible monochlorinated products and their percentage distribution.

Butane: CH₃–CH₂–CH₂–CH₃

Identify H types and count:
- 1° H: 6 H atoms (3 on each terminal CH₃)
- 2° H: 4 H atoms (2 on each internal CH₂)

Selectivity of Cl•: 1° : 2° = 1 : 4 (per H atom)

Relative rate of abstraction:
- From 1° positions: 6 × 1 = 6
- From 2° positions: 4 × 4 = 16

Total = 22

Products and percentages:
1-chlorobutane (from 1° H): 6/22 × 100 = 27.3%
2-chlorobutane (from 2° H): 16/22 × 100 = 72.7%

Answer: 1-chlorobutane (~27%) and 2-chlorobutane (~73%)

• Q1: Name the alkane: CH₃CH(CH₃)CH₂CH(C₂H₅)CH₃
• Q2: Draw the Newman projection of propane looking along C1–C2; label staggered and eclipsed positions.
• Q3: Write the balanced combustion equation for octane (C₈H₁₈).
• Q4: In radical bromination of 2-methylpropane, what is the ratio of primary to tertiary monobromo product? (Use 1° : 3° = 1 : 1600 per H atom.)
• Stretch: Q5: Explain why the anti conformation of butane is more stable than the gauche using steric and torsional strain arguments, and calculate the energy difference.

NCERT Chapter 13 link: Section 13.2 (Classification, IUPAC); 13.3 (Preparation); 13.4 (Physical properties); 13.5.1–13.5.3 (Reactions — halogenation, combustion, cracking)

Exam connections: JEE Main tests IUPAC naming (1–2 Qs/year), monochlorination product distribution, and combustion balancing. JEE Advanced tests mechanism completeness, selectivity calculations, and conformation energy ordering.

Study strategy: Master IUPAC naming before mechanisms — wrong names cost marks even with correct mechanisms. Practice drawing Newman projections physically (use a model kit or pencil + paper) to build 3D intuition. For selectivity problems, always write out H-count × selectivity for each type before calculating %.

• Rotate a 3D ethane model in the optics/molecular world to see the energy barrier as you change the dihedral angle from 0° to 360°.
• Simulate radical halogenation step-by-step: choose Cl or Br, watch selectivity change product ratios in real time.
• AI mentor reflection: "Why does the stability of a carbon radical follow the order 3° > 2° > 1°? Connect this to hyperconjugation."

• "If I give you two possible longest chains of equal length in an alkane, how do you decide which to use as the parent?"
• "Why does free-radical bromination give a purer product than chlorination for making 2-bromo-2-methylpropane from 2-methylpropane?"
• Stretch: "Using bond dissociation energies (C–H ~413 kJ/mol, Cl–Cl ~243 kJ/mol, HCl ~432 kJ/mol), calculate the enthalpy change for the propagation steps of methane chlorination and confirm it is exothermic."

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• Build a physical molecular model of 2,2-dimethylpropane (neopentane) using a model kit or clay + sticks; photograph your staggered and eclipsed Newman projections.
• Future Skill track: AI Mastery — Train a simple classifier in Python to predict whether a given molecular formula satisfies the alkane general formula CₙH₂ₙ₊₂, then extend it to detect cycloalkanes.
• Coding extension: Write a Python function count_h_types(smiles) that parses a SMILES string for an alkane and returns the count of primary, secondary, and tertiary hydrogens using RDKit.

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

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Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

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