Organic Chemistry for the Chemistry Olympiad — NSEC and INChO

Weightage: Organic chemistry is tested through mechanisms and structure determination rather than memorised reactions. The syllabus builds on Classes XI and XII with extension, so confirm it with HBCSE. A reaction you can derive from electron flow is worth more than ten you have memorised.

1. Electronic effects and acidity

Electrons are pushed by inductive effects (through sigma bonds, weakening with distance), resonance (delocalisation through pi systems) and hyperconjugation. A stabilised charge means a stabler species.

• Acid strength rises when the conjugate base is stabilised: carboxylic acids are stronger than alcohols because the carboxylate delocalises its charge over two oxygens, and electron-withdrawing groups (Cl, NO${}_2$) strengthen an acid.
• Base strength falls when the lone pair is delocalised. Aniline is a weaker base than cyclohexylamine because its lone pair joins the ring.
• Carbocation stability runs tertiary $>$ secondary $>$ primary, and benzylic and allylic cations are stabilised by resonance.

2. Stereochemistry

A carbon with four different groups is a stereocentre. Assign R or S by ranking substituents with the CIP rules, orienting the lowest priority away from you, and reading the direction of the other three.

Counting rules:

• A molecule with $n$ stereocentres has up to $2^n$ stereoisomers.
• A molecule with a plane of symmetry among its stereocentres is meso, which reduces the count.
• Enantiomers are mirror images with opposite optical rotation. Diastereomers are stereoisomers that are not mirror images.

Worked example. 2,3-Dibromobutane has two stereocentres, so the formula gives four, but the $(R,S)$ form is meso, leaving three stereoisomers: one pair of enantiomers and one meso compound.

E/Z describes alkene geometry by the same priority rules. A racemic mixture is optically inactive because the two enantiomers cancel. Degrees of unsaturation for $C_c{H}_h{N}_n{X}_x$ are $\frac{2c+2+n-h-x}{2}$, so benzene, $C_6{H}_6$, has $4$.

3. Substitution and elimination

Elimination competes. E2 is a one-step process favoured by strong, bulky bases and heat, and it needs an anti-periplanar arrangement. E1 shares the carbocation of SN1. By Zaitsev's rule, the more substituted alkene usually dominates, unless a bulky base favours the Hofmann product.

4. Additions and aromatic substitution

Alkenes undergo electrophilic addition. For HX, Markovnikov's rule places the hydrogen on the carbon with more hydrogens, because the more stable carbocation forms. Peroxides reverse the regiochemistry for HBr by a radical path (anti-Markovnikov). Hydroboration-oxidation gives anti-Markovnikov alcohols with syn addition.

Benzene reacts by electrophilic aromatic substitution: the electrophile attacks, a resonance-stabilised intermediate forms, and a proton is lost to restore aromaticity. Substituents direct the next group:

• Activating, ortho/para directing: $-OH$, $-OR$, $-N{H}_2$, $-R$.
• Deactivating, meta directing: $-N{O}_2$, $-CN$, $-CHO$, $-COOH$.
• Halogens deactivate yet direct ortho and para.

Aromaticity needs a planar, cyclic, fully conjugated ring with $4n+2$ pi electrons (Huckel's rule). Cyclopentadienyl anion, with 6 electrons, is aromatic.

5. Carbonyl chemistry

The carbonyl carbon is electrophilic. Nucleophilic addition gives alcohols, cyanohydrins and imines, and aldehydes are more reactive than ketones (less hindered and more electron-poor).

• Aldol reaction: an enolate adds to another carbonyl, and dehydration gives an $\alpha ,\beta$-unsaturated carbonyl. It needs $\alpha$-hydrogens.
• Cannizzaro reaction: an aldehyde without $\alpha$-hydrogens disproportionates in strong base to an alcohol and a carboxylate.
• Carboxylic acid derivatives follow the reactivity order: acid chloride $>$ anhydride $>$ ester $>$ amide, by nucleophilic acyl substitution.
• Grignard reagents add to carbonyls to form alcohols, and are destroyed by protic solvents, so reactions are done in dry ether.

6. Spectroscopy

Infrared identifies functional groups: a strong $C=O$ stretch near $1700$ cm${}^{-1}$, a broad $O-H$ stretch around $3300$ cm${}^{-1}$, and an $N-H$ stretch near $3300$ to $3500$ cm${}^{-1}$.

Proton NMR gives, for each set of equivalent hydrogens, a chemical shift (the environment), an integration (the number of hydrogens) and a splitting by the $n+1$ rule for $n$ neighbouring hydrogens.

Worked example. Ethanol shows a triplet for $C{H}_3$ (two neighbours, $n+1=3$) integrating to $3$, a quartet for $C{H}_2$ (three neighbours) integrating to $2$, and a singlet for $OH$ integrating to $1$.

Mass spectrometry gives the molecular mass, and fragments reveal the structure. To solve a structure, start with the formula and its degrees of unsaturation, then add IR groups, then NMR connectivity, and verify every signal.

7. Planning a synthesis

Work backwards (retrosynthesis): identify the bond to be made, and the reagent pair that makes it. Protect a sensitive group if a reagent would attack it, and check each step for competing reactions.

Common traps

• Using Markovnikov's rule for a radical reaction.
• Counting $2^n$ stereoisomers without checking for meso forms.
• Calling a tertiary halide an SN2 substrate.
• Assuming halogens are meta directors.
• Forgetting that a Grignard reagent reacts with any acidic proton.

Memory aids

• "Stable cation, stable product": guide to regiochemistry.
• "n plus 1": NMR splitting.
• "Acid chloride, anhydride, ester, amide": acyl reactivity.

Summary

Organic chemistry is read through electron flow: inductive and resonance effects set acidity, basicity and cation stability, and mechanisms explain substitution, elimination, addition and substitution on aromatic rings.

Stereochemistry follows CIP rules and counting with meso forms, carbonyl chemistry rests on nucleophilic addition, and spectra convert formulas into structures.

Exam protocol

• Draw the mechanism with curly arrows before naming a product.
• Count stereocentres and check for symmetry.
• Match IR, NMR and formula before committing to a structure.
• Confirm the syllabus and format with HBCSE.