Chemistry · Organic Chemistry · Chapter notes
Organic Reagents and Their Roles · Conditions Decide the Product
Organic chemistry notes on what each reagent actually does: alcoholic against aqueous KOH, concentrated against dilute H2SO4, the peroxide effect, KMnO4 hot and cold, bromine's three faces, PCC against dichromate, NaBH4 against LiAlH4, and Lindlar against sodium in liquid ammonia.
In short
Students do not confuse reagents, they skip conditions. The same KOH gives an alkene in alcohol and an alcohol in water; the same sulphuric acid removes water when concentrated and adds it when dilute; the same HBr follows Markovnikov without peroxide and reverses with it. Read the word before the formula, because that word is usually the answer.
Contents
The letter match you will never forget
The first three letters match the product. This is not a mnemonic somebody invented, it is a coincidence in the spelling that happens to be true, which is exactly why it sticks.
The condition switches
| Reagent | Condition | What it does | Product |
|---|---|---|---|
| KMnO4 | cold, dilute, alkaline | adds two OH across C=C | syn diol (Baeyer's test) |
| KMnO4 | hot, concentrated | cleaves the C=C entirely | acids or ketones |
| Br2 | in CCl4, dry | adds across C=C, colour dies | test for unsaturation |
| Br2 | in water | with phenol | white 2,4,6-tribromophenol |
| Br2 | with Fe or FeBr3, dry | substitutes on the ring | bromobenzene |
The peroxide effect works for HBr only, never HCl or HI. H-Cl is too strong to break; H-I gives a radical too unreactive to carry the chain.
How far it goes: strength decides the product
| Reagent | Strength | Reduces or oxidises |
|---|---|---|
| NaBH4 | mild reducer | aldehyde, ketone, acid chloride only |
| LiAlH4 | strong reducer | all of the above, plus acid, ester, amide, nitrile |
| PCC | mild oxidiser | 1° alcohol to aldehyde, and stops there |
| K2Cr2O7 / H+ | strong oxidiser | 1° alcohol all the way to carboxylic acid |
| H2 / Lindlar | poisoned catalyst | alkyne to cis alkene. Lindlar lets them lie together. |
| Na / liquid NH3 | dissolving metal | alkyne to trans alkene. Sodium sends them opposite ways. |
- Ends in H4 (BH4−, AlH4−): it is carrying hydride, so it is a reducer.
- Rich in oxygen (MnO4−, Cr2O72−, O3): it is an oxidiser.
- A metal with an acid (Zn/HCl, Sn/HCl): nascent hydrogen, so again a reducer.
- Anhydrous AlCl3 or FeBr3: a Lewis acid catalyst. Water destroys it, which is why the word anhydrous is always written.
Say it once, keep it forever
In water, phenol drops white: two, four, six.
With iron and dry, the ring stays whole, one H just changed its role.
Dichromate kicks it clean off its hinge, and the acid walks out.
Fehling turns red, but the benzene ring is refused.
- Cover the right-hand column above. Read a reagent and its condition, and say the product out loud.
- Any you miss, say the matching rhyme once, out loud. Rhythm survives exam stress; a list does not.
- Then reverse it: name a product, and give the reagent and the condition.
Before the exam
What the paper actually asks from this chapter
Read next
More chemistry notes
Now play it.
Reading a chapter and understanding it are different things. In the app this chapter becomes a game you play, a short read, then practice that tests you at every step. A full chapter, understood, in 45 to 60 minutes. Free to start.
- PlayA game built for the concept
- ReadThe short NCERT explainer
- PracticeTimed, until it sticks
Still stuck at 11pm? TarQPro reads the answer you got wrong and teaches the idea behind it, not just the right option.