MHT-CET Chemistry · Alkanes

The Alkane Series: Formula, Isomers, Carbon Types and Boiling Points

Alkanes are saturated hydrocarbons CₙH₂ₙ₊₂; each member differs from the next by CH₂ (14 g/mol), the number of structural isomers grows with n, and boiling point rises with chain length and falls with branching.

All Alkanes notes MHT-CET Chemistry strategy

Why this matters

14 PYQs, three MODERATE. Seven count isomers or carbon types, four are homologous-series arithmetic, and three are boiling points and uses. Three cards.

Concept 1 of 3: The Homologous Series and Its Molar Masses

Every step along the series adds one CH₂, so formula and molar mass are both linear in n. That turns any 'which member' or 'difference in molar mass' question into one line of arithmetic.

Definition

General formula: CₙH₂ₙ₊₂; molar mass 14n + 2.
Successive members differ by CH₂ = 14 g/mol, whatever the series.
Names: methane 16, ethane 30, propane 44 (the third member), butane 58 … decane C₁₀H₂₂, undecane C₁₁H₂₄.

Alkane molar mass

\mathrm{C_nH_{2n+2}}:\quad M = 14n + 2\ \text{g mol}^{-1}

Worked example

The first member of a homologous series has molar mass 46 g. What is the molar mass of the third member?

Practice this conceptself-check · 2 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 2nd May Shift 2 · Q71Easy

Example 1 · Alkanes · Nomenclature, Structural Isomers and Physical Properties

What is the molar mass of third member of homologous series if the molar mass of first member is 46 g?

Starting the count at ethane

The third alkane is propane, 44 — count from methane. 58 is butane, the fourth.

Drill 3 more on the homologous series and its molar masses

Concept 2 of 3: Structural Isomers and Primary, Secondary, Tertiary Carbons

Branching the chain gives structural isomers: two for butane, three for pentane, five for hexane. Questions run the table backwards — 'an alkane with five isomers' means six carbons, so n moles of it hold 6n moles of carbon. A carbon's type is simply how many other carbons it is bonded to.

Definition

Isomer counts: C₄H₁₀ 2, C₅H₁₂ 3, C₆H₁₄ 5, C₇H₁₆ 9. C₁–C₃ have one each.
Carbon types: primary (1°) bonded to one C, secondary (2°) to two, tertiary (3°) to three, quaternary (4°) to four.
Isobutane (CH₃)₃CH: one 3° carbon. Isopentane (CH₃)₂CHCH₂CH₃: one 2° carbon.
(CH₃)₄C is neopentane; IUPAC name 2,2-dimethylpropane.

Isomers	Alkane	Carbon moles in n mol
2	C₄H₁₀	4nQ
3	C₅H₁₂	5nQ
5	C₆H₁₄	6nQ

Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 11th May Shift 2 · Q89Moderate

Example 2 · Alkanes · Nomenclature, Structural Isomers and Physical Properties

What is the number of moles of secondary carbon atoms in n mole isopentane?

Neopentane as the IUPAC name

Neopentane is the common name, and it is offered. The IUPAC name of (CH₃)₄C is 2,2-dimethylpropane.

Drill 6 more on structural isomers and primary, secondary, tertiary carbons

Concept 3 of 3: Boiling Points and Uses

Alkanes hold together only by dispersion forces, which grow with surface area. A longer chain has more surface, so it boils higher; branching makes the molecule compact, so for the same formula the straight chain boils highest.

Definition

Boiling point rises with chain length: n-pentane > n-butane.
Isomers: branching lowers bp: n-hexane > 2-methylpentane > 3-methylpentane > 2,2-dimethylbutane.
Uses by chain length: C₁–C₄ fuel gases; petrol, kerosene, diesel in between; more than 35 C — road surfacing (bitumen).

Question	Answer
Highest bp: 2-methoxypropane, n-butane, 2-methylbutane, n-pentane	n-PentaneQ
Highest bp among the C₆H₁₄ isomers	n-HexaneQ
Alkanes used for road surfacing	More than 35 C atomsQ

Practice this conceptself-check

The same idea in a real exam question:

MHT-CET · 2025 · 21 April Shift I · Q83Easy

Example 3 · Alkanes · Nomenclature, Structural Isomers and Physical Properties

Which of the following compounds has highest boiling point?

Drill 2 more on boiling points and uses

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The Homologous Series and Its Molar Masses
Alkane molar mass
$\mathrm{C_nH_{2n+2}}:\quad M = 14n + 2\ \text{g mol}^{-1}$

Reference tables (2)

Structural Isomers and Primary, Secondary, Tertiary Carbons3 rows

Isomers	Alkane	Carbon moles in n mol
2	C₄H₁₀	4nQ
3	C₅H₁₂	5nQ
5	C₆H₁₄	6nQ

Boiling Points and Uses3 rows

Question	Answer
Highest bp: 2-methoxypropane, n-butane, 2-methylbutane, n-pentane	n-PentaneQ
Highest bp among the C₆H₁₄ isomers	n-HexaneQ
Alkanes used for road surfacing	More than 35 C atomsQ

Watch out for (2)

Starting the count at ethane
→ The Homologous Series and Its Molar Masses
Neopentane as the IUPAC name
→ Structural Isomers and Primary, Secondary, Tertiary Carbons

Test yourself on Alkanes

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Alkanes chapter test Drill all 14 questions