The FGI Mindset
A functional group, in the NIOS definition, is "an atom or group of atoms which is responsible for the characteristic chemical properties" of a molecule. Functional group interconversion (FGI) is the deliberate replacement of one such group by another, leaving the carbon framework intact wherever possible. Almost every conversion problem in NEET is solved by answering two questions in order: does the carbon count change, and does the oxidation level change.
If neither changes, the conversion is a lateral swap, such as an alcohol becoming a halide. If the oxidation level rises or falls, it is a climb up or down the oxidation ladder. If the carbon count changes, only a small set of reactions can do that, and recognising them instantly is the single most valuable skill in this chapter. The schematic below shows the central hubs and the bridges between them.
Six hubs, a handful of bridges. The alcohol sits at the centre because it connects laterally to halides and alkenes, and vertically into the oxidation ladder.
The Alcohol–Halide Axis
Alcohol and alkyl halide are at the same oxidation level and the same carbon count, so swapping between them is the cleanest lateral FGI. Going from alcohol to halide is done with a hydrogen halide, with thionyl chloride, or with a phosphorus halide. Thionyl chloride is the cleanest preparatively because both by-products, $\ce{SO2}$ and $\ce{HCl}$, escape as gases.
$$\ce{R-OH + HX -> R-X + H2O}$$ $$\ce{R-OH + SOCl2 -> R-Cl + SO2 ^ + HCl ^}$$ $$\ce{3R-OH + PCl3 -> 3R-Cl + H3PO3}$$
The reverse direction, halide to alcohol, is a textbook nucleophilic substitution. NIOS notes that warming a haloalkane with aqueous KOH replaces the halogen by a hydroxyl group, with primary halides proceeding by $S_N2$ and tertiary halides by $S_N1$.
$$\ce{R-X + KOH(aq) ->[\Delta] R-OH + KX}$$
Aqueous versus alcoholic KOH
The single word "aqueous" or "alcoholic" silently changes the product. Aqueous KOH supplies the hydroxide as a nucleophile and gives the alcohol by substitution. Alcoholic KOH supplies it as a base and removes a β-hydrogen, giving the alkene by elimination (dehydrohalogenation).
aq. KOH → alcohol (substitution); alc. KOH → alkene (elimination, Saytzeff major product).
The Alcohol–Alkene Axis
Alcohols and alkenes differ by a molecule of water. NIOS describes the dehydration of an alcohol to an alkene; the reverse, hydration of an alkene, restores the alcohol. Both directions follow Markovnikov regiochemistry, and the dehydration of higher alcohols gives the more substituted (Saytzeff) alkene as the major product.
$$\ce{CH3CH2OH ->[conc.~H2SO4][443~K] CH2=CH2 + H2O}$$ $$\ce{CH3-CH=CH2 + H2O ->[H+] CH3-CH(OH)-CH3}$$
The acid-catalysed hydration places the hydroxyl on the more substituted carbon, giving the secondary alcohol from propene. When the anti-Markovnikov primary alcohol is required, hydroboration–oxidation ($\ce{BH3}$ followed by $\ce{H2O2}/\ce{OH-}$) is the standard tool, a route that featured directly in a recent NEET reagent-identification question.
Ascending & Descending the Chain
Most FGIs preserve the carbon count. A small toolkit changes it, and recognising these reactions is decisive. To ascend by one carbon, the workhorse is the reaction of an alkyl halide with alcoholic potassium cyanide, which installs a nitrile carbon. That nitrile is then hydrolysed to a carboxylic acid or reduced to a primary amine, both of which retain the extra carbon.
$$\ce{R-X ->[KCN] R-C#N ->[H3O+] R-COOH}$$
To descend by one carbon, the Hofmann bromamide degradation converts a primary amide to a primary amine with one fewer carbon, the carbonyl carbon leaving as carbonate. Decarboxylation of the sodium salt of a carboxylic acid with soda lime is the other classic one-carbon-down step, giving an alkane.
$$\ce{R-CONH2 ->[Br2,~KOH] R-NH2}$$ $$\ce{R-COONa + NaOH ->[CaO,~\Delta] R-H + Na2CO3}$$
Counting carbons before choosing the reagent
When a stem asks to convert a compound into one with a different carbon count, the answer is almost forced. One carbon up usually means KCN (then hydrolyse or reduce). One carbon down usually means Hofmann bromamide (amide to amine) or decarboxylation. If the carbon count is unchanged, eliminate every cyanide and Hofmann option immediately.
+1 C → KCN route · −1 C → Hofmann bromamide or decarboxylation.
The Cannizzaro and crossed-Cannizzaro reactions show how a single aldehyde can be pushed up and down the oxidation ladder at once — see Cannizzaro & Crossed-Cannizzaro.
The Oxidation Ladder
Climbing the oxidation ladder is the most frequently tested FGI family. A primary alcohol oxidises first to an aldehyde and then to a carboxylic acid; the difficulty is stopping at the aldehyde, which requires a mild, controlled oxidant such as PCC (pyridinium chlorochromate) that does not over-oxidise. A strong oxidant such as acidified $\ce{KMnO4}$ or $\ce{CrO3}$ carries the alcohol all the way to the acid. A secondary alcohol oxidises only to a ketone, which resists further oxidation.
$$\ce{R-CH2OH ->[PCC] R-CHO ->[KMnO4/H+] R-COOH}$$ $$\ce{R2CH-OH ->[oxidn.] R2C=O}$$
The ladder makes the reagent obvious: to move up, pick an oxidant; to move down, pick a hydride or catalytic hydrogen. PCC stops one rung short; strong oxidants go to the top.
The Reduction Ladder
Descending the oxidation ladder is reduction, and the reagent choice is governed by strength and selectivity. Sodium borohydride ($\ce{NaBH4}$) is mild and reduces only aldehydes and ketones to alcohols, leaving esters and acids untouched. Lithium aluminium hydride ($\ce{LiAlH4}$) is far stronger and reduces carboxylic acids, esters, amides and nitriles in addition to carbonyls; an acid is taken all the way down to a primary alcohol.
$$\ce{R-CHO ->[NaBH4] R-CH2OH}$$ $$\ce{R-COOH ->[LiAlH4] R-CH2OH}$$ $$\ce{R-C#N ->[LiAlH4] R-CH2-NH2}$$
A distinct, deeper reduction removes the oxygen entirely, converting a carbonyl group straight to a methylene ($\ce{C=O -> CH2}$). Two complementary reactions achieve this. Clemmensen reduction uses zinc amalgam with concentrated $\ce{HCl}$ and is the acidic-medium choice; Wolff–Kishner reduction uses hydrazine followed by a strong base with heat and is the basic-medium choice.
$$\ce{R-CO-R' ->[Zn(Hg),~conc.~HCl] R-CH2-R'}$$ $$\ce{R-CO-R' ->[NH2NH2][KOH,~\Delta] R-CH2-R'}$$
Choosing the right reducing agent
If a stem reduces a ketone in the presence of an ester or acid and the ester survives, the reagent must be $\ce{NaBH4}$, not $\ce{LiAlH4}$. If the target is a full $\ce{C=O \to CH2}$ deoxygenation, no hydride works — only Clemmensen or Wolff–Kishner. DIBAL-H is the specialist that stops an ester or nitrile at the aldehyde stage.
NaBH₄ = selective (C=O only); LiAlH₄ = powerful; Clemmensen/Wolff–Kishner = C=O to CH₂; DIBAL-H = stop at aldehyde.
Routes to Amines
Amines sit at the crossroads of several FGIs, and NEET rewards knowing which route gives a pure primary amine and which changes the carbon count. Direct alkylation of ammonia is avoided because it over-alkylates to a mixture of primary, secondary and tertiary amines. The reliable routes are tabulated below.
| Method | Reagents | Gives | Carbon change |
|---|---|---|---|
| Reduction of nitrile | LiAlH4 or H₂/Ni | 1° amine | same as nitrile (+1 vs halide) |
| Reduction of nitro | Sn/HCl or H₂/Ni | 1° amine | unchanged |
| Gabriel phthalimide | phthalimide, KOH, R–X, then hydrolysis | pure 1° amine | unchanged |
| Hofmann bromamide | Br2 + KOH on R–CONH₂ | pure 1° amine | one carbon less |
The Gabriel synthesis is prized because it gives an exclusively primary amine without contamination by higher amines, but it works only for alkyl halides — aryl halides do not undergo the required substitution. The Hofmann route is the one to reach for whenever the question demands an amine with one fewer carbon than the starting material.
The Diazonium Aryl Hub
For aromatic conversions, the benzenediazonium salt is the universal hub. A primary aromatic amine treated with nitrous acid (generated in situ from $\ce{NaNO2}$ and $\ce{HCl}$) at low temperature, 273–278 K, gives the diazonium salt, whose $\ce{-N2+}$ group is then displaced by a wide range of nucleophiles.
$$\ce{C6H5NH2 ->[NaNO2,~HCl][273-278~K] C6H5N2^+Cl^-}$$
From this single intermediate, the diazonium group can be replaced by $\ce{-Cl}$, $\ce{-Br}$ or $\ce{-CN}$ (Sandmeyer reaction, with cuprous salts), by $\ce{-OH}$ (warm with water), by $\ce{-I}$ (with KI), or by $\ce{-H}$ (with hypophosphorous acid). This makes aniline a gateway to almost any substituted benzene, which is why aromatic conversion problems so often begin by drawing the diazonium salt.
The Master Reagent Table
The table below condenses the standard, examination-safe toolkit into a single reference. Every entry is a reliable textbook conversion; the trick in the exam is to read the desired transformation and locate the matching row.
| Conversion | Reagent(s) | Note |
|---|---|---|
| Alcohol → halide | HX, SOCl2, PCl3/PCl5/PBr3 | SOCl₂ cleanest (gaseous by-products) |
| Halide → alcohol | aq. KOH | substitution (SN1/SN2) |
| Halide → alkene | alc. KOH | elimination, Saytzeff major |
| Alcohol → alkene | conc. H2SO4, Δ | dehydration, Saytzeff major |
| Alkene → alcohol | H2O/H⁺ (Markovnikov) | BH₃ then H₂O₂/OH⁻ for anti-Markovnikov |
| Halide → nitrile (+1 C) | alc. KCN | chain ascends by one carbon |
| Nitrile → acid | H3O+ | hydrolysis |
| Nitrile → 1° amine | LiAlH4 / H₂-Ni | reduction |
| 1° alcohol → aldehyde | PCC | mild; stops before acid |
| 1° alcohol → acid | KMnO4/H⁺, CrO3 | strong oxidant |
| 2° alcohol → ketone | CrO3, KMnO4 | no further oxidation |
| Carbonyl → alcohol | NaBH4, LiAlH4, H₂/cat | NaBH₄ selective for C=O |
| Acid → 1° alcohol | LiAlH4 | NaBH₄ will not reduce acids |
| C=O → CH₂ | Clemmensen [Zn(Hg)/HCl] · Wolff–Kishner [NH₂NH₂/KOH] | acidic vs basic medium |
| Amide → 1° amine (−1 C) | Br2 + KOH (Hofmann) | chain descends by one carbon |
| Aryl amine → diazonium | NaNO2/HCl, 273–278 K | aryl conversion hub |
Worked Multi-Step Roadmaps
The real test of FGI is stringing single steps into a route. Two examples show the method: identify the carbon-count change first, then walk the oxidation ladder.
Convert ethanol into propan-1-amine (one carbon longer, ends as a primary amine).
The product has one more carbon, so the KCN step must appear. Route: dehydrate or substitute ethanol to the halide, ascend with cyanide, then reduce.
$$\ce{CH3CH2OH ->[SOCl2] CH3CH2Cl ->[KCN] CH3CH2CN ->[LiAlH4] CH3CH2CH2NH2}$$
The nitrile carbon becomes the new terminal carbon; LiAlH₄ reduces the nitrile cleanly to the primary amine.
Convert propan-2-ol into propanone, then into propane (climb, then strip the oxygen).
The secondary alcohol oxidises only as far as the ketone; the ketone is then fully deoxygenated to the alkane by Clemmensen or Wolff–Kishner.
$$\ce{CH3CH(OH)CH3 ->[CrO3] CH3COCH3 ->[Zn(Hg),~HCl] CH3CH2CH3}$$
No hydride could perform the second step — only a C=O to CH₂ reduction strips the oxygen entirely.
FGI in one screen
- Solve every conversion by asking first: does the carbon count change, then does the oxidation level change.
- Lateral swaps: alcohol ↔ halide (HX/SOCl₂/PX₃ one way, aq. KOH the other); alcohol ↔ alkene (dehydration vs Markovnikov hydration).
- +1 carbon = KCN route (then hydrolyse to acid or reduce to amine); −1 carbon = Hofmann bromamide or decarboxylation.
- Oxidation up: 1° alcohol → aldehyde (PCC) → acid (KMnO₄/H⁺); 2° alcohol → ketone only.
- Reduction down: NaBH₄ (selective C=O), LiAlH₄ (powerful, acids → 1° alcohol, nitriles → amine); Clemmensen/Wolff–Kishner for C=O → CH₂.
- Aromatic conversions funnel through the benzenediazonium salt from aniline.