PREPWISER.
Practise online

Hydrocarbons: Alkanes revision notes

Every idea in the Hydrocarbons: Alkanes chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 7 and download the PDF.

Sign in to read all 7Download PDF

Read the first part free. Sign in free with Google to read the rest and download the PDF: 6 A4 pages.6 A4 pages, ready to print.

How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Preparation of alkanes

Methods of preparation
1× asked, last 2025
2 RX+2 Na→dry etherR−R+2 NaX2 RCOONa+2 HX2O→electrolysisR−R+2 COX2+HX2+2 NaOHRCOONa+NaOH→ΔCaORH+NaX2COX3,RX→Zn, dil. HClRHRX→Mg, etherRMgX→HX2ORH,RC≡CH→HX2/Pt, Pd or NiRCHX2CHX3\begin{gathered}\ce{2RX + 2Na ->[\text{dry ether}] R-R + 2NaX}\\ \ce{2RCOONa + 2H2O ->[\text{electrolysis}] R-R + 2CO2 + H2 + 2NaOH}\\ \ce{RCOONa + NaOH ->[\text{CaO}][\Delta] RH + Na2CO3},\qquad \ce{RX ->[\text{Zn, dil. HCl}] RH}\\ \ce{RX ->[\text{Mg, ether}] RMgX ->[H2O] RH},\qquad \ce{RC#CH ->[\text{H}_2/\text{Pt, Pd or Ni}] RCH2CH3}\end{gathered}
Wurtz, Kolbe: R–R, never CH₄; one RX (or RCOONa) gives even-C; two different R give 3 alkanes, R–R′ may be odd-C.
Br
CH₃Br
+
Br
C₂H₅Br
Na, dry ether
 
ethane
+
propane
(odd C)
+
butane
MethodC inC outProduct
Wurtz: 2 C₂H₅Br + 2 Na24
Kolbe: 2 CH₃COONa22
soda lime: C₂H₅COONa32
Zn/HCl: C₂H₅Br22
Wurtz and Kolbe join two R (C doubles); soda lime removes the COO carbon; reduction keeps the count.
  • Soda lime: one C fewer. Reduction keeps the C count; Zn/dil. HCl reduces RCl, RBr, RI but not RF.
  • Quick method. Halides that give one alkane by reduction (Zn/HCl, or RMgX then water) = its monohalo structural isomers: 2-methylbutane has four distinct C–H sets, so 4 RBr.

Practise Hydrocarbons: Alkanes questions as you read. Free, and you can start without signing up.

Practise Hydrocarbons: Alkanes

6 more concepts in 2 topics

Free-radical halogenation and Conformations and properties. Read the first part free. Sign in free with Google to read the rest and download the PDF.

Sign in to read the restYou come straight back to this spot.Download the PDF

Free-radical halogenation

  • Mechanism: initiation to termination2×
  • Selectivity: 3° > 2° > 1° H3×
  • Counting halogenation products4×

Conformations and properties

  • Staggered vs eclipsed ethane
  • Boiling and melting points1×
  • Combustion and controlled oxidation2×

Practise Hydrocarbons: Alkanes next

About 20 past JEE Main questions from this chapter, with instant answer checking and step-by-step solutions where available. Free, and you can start without signing up.

Practise Hydrocarbons: Alkanes

Also free on PrepWiser

Spotted a mistake? Write to support@prepwiser.in and we will fix it in the next version.

Version 1, October 2026. Free to share for study, as it is. Please don't sell it or remove the PrepWiser name.

PractiseSign in to read all 7Download PDF
Free with Google, takes a few seconds6 pages, free