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"Convert ethanamine into methanamine." "Convert methanol to ethanoic acid." These questions appear in NCERT exercises and CBSE papers every year, and students often stare at them with no idea where to start. The fix is a method, not more memorising. This post gives you a roadmap of functional group links, a four-step planning method, and three fully worked conversions.
Why work backwards from the product?
Professional chemists plan syntheses backwards. The Nobel Prize in Chemistry 1990 went to Elias James Corey "for his development of the theory and methodology of organic synthesis". The Nobel press release explains that his approach, retrosynthetic analysis, starts from the target molecule and asks which bonds must be broken, simplifying the structure step by step until known starting materials appear.
Your conversions are the school-sized version. Students who work forwards from the starting material often try reactions at random. A study of graduate students by Bhattacharyya and Bodner (2005), titled "It gets me to the product", probed how they wrote mechanisms for reactions that lacked the usual cues, and discussed limitations in their strategies. Even advanced students need a plan.
What does the roadmap look like?
Learn these links in both directions where they exist. Each one is in the NCERT Class 12 chemistry units on haloalkanes, alcohols, carbonyl compounds and acids and amines.
Alkyl halide ↔ alcohol: aqueous KOH replaces –X with –OH; HX, PCl₅ or SOCl₂ go back.
Primary alcohol → aldehyde → carboxylic acid: mild oxidation stops at the aldehyde (for example PCC); strong oxidation such as KMnO₄ goes on to the acid.
Carboxylic acid → amide: with ammonia and heat.
Amide → amine: LiAlH₄ keeps every carbon; Br₂ with NaOH (Hoffmann bromamide) removes one.
Alkyl halide → nitrile: KCN adds one carbon. A nitrile then gives an acid on hydrolysis, or an amine on reduction.
Primary aliphatic amine → alcohol: nitrous acid releases N₂ and gives the alcohol.
Aromatic amine → diazonium salt → many groups: NaNO₂ and HCl at low temperature, then Cu₂X₂ for –Cl or –Br, KI for –I, HBF₄ or NaBF₄ and heat for –F, warming in water for –OH.
Nitrobenzene → aniline: Sn and HCl.
The four-step method
Count carbons in the start and the product.
Choose the carbon move. Same count: change groups only. One more: cyanide or Grignard. One fewer: Hoffmann bromamide, decarboxylation, or the haloform reaction for a methyl ketone.
Work backwards from the product to a group you can reach from the start.
Check each arrow for a real reagent and a real product, with no impossible steps.
Worked example 1: one carbon more
Methanol to ethanoic acid (an NCERT Unit 9 exercise). C1 to C2, so you need a carbon-adding step.
Work backwards: ethanoic acid comes from hydrolysis of ethanenitrile, CH₃CN.
CH₃CN comes from CH₃Br and KCN.
CH₃Br comes from methanol with HBr or PBr₃.
Forwards: CH₃OH → CH₃Br → CH₃CN → CH₃COOH. Three steps, one carbon added at the cyanide step.
Worked example 2: one carbon fewer
Ethanamine to methanamine (also an NCERT exercise). C2 to C1.
Work backwards: methanamine comes from ethanamide by Hoffmann bromamide degradation, which loses one carbon.
Ethanamide comes from ethanoic acid with ammonia and heat.
Ethanoic acid comes from oxidising ethanol, and ethanol comes from ethanamine with nitrous acid.
Forwards: CH₃CH₂NH₂ → CH₃CH₂OH → CH₃COOH → CH₃CONH₂ → CH₃NH₂.
Worked example 3: same carbons, different groups
The CBSE Class XII Chemistry sample paper for 2026–27, Question 26, asks for benzoic acid to benzylamine. Both have seven carbons, so no carbon move is needed. The marking scheme route is C₆H₅COOH with NH₃ and heat to give benzamide, then LiAlH₄ to give C₆H₅CH₂NH₂.
Compare it with the NCERT exercise "benzoic acid to aniline". Aniline has six carbons, so this time you use Br₂ and NaOH on benzamide to remove one. Same first step, different second step, decided entirely by the carbon count.
The same paper's other part, nitrobenzene to fluorobenzene, follows the aromatic branch of the map: Sn/HCl to aniline, NaNO₂/HCl to the diazonium salt, then NaBF₄ and heat.
Which mistakes cost marks?
Using Hoffmann bromamide when the carbons stay the same. Check the count again.
Gabriel synthesis for aniline. NCERT notes that aromatic primary amines cannot be made this way, because aryl halides do not undergo the needed substitution.
Leaving out reagents or conditions. Each arrow needs its reagent; marking schemes often award marks per step.
Oxidising a primary alcohol "to an aldehyde" with KMnO₄. It goes on to the acid.
Named reactions are the arrows on this map, so a map of named reactions is a useful companion. For aromatic routes, see our guide to amines and diazonium salts. When you cannot see the route, the Fermi Problem Library has a step-by-step solver that explains the reasoning, so you can compare where your plan went off course.
A practice routine for this fortnight
Week 1, three days: Copy the roadmap from memory, then correct it. Add the reagent on every arrow.
Week 1, two days: Ten conversions a day from NCERT exercises, writing the carbon count first.
Week 2: Mixed sets of eight from board papers and entrance questions. Time each one, and mark any you planned forwards instead of backwards.
Take one conversion you could not do last week, write the carbon count at the top, and plan it backwards today.
