Why must Grignard reactions be anhydrous?

RMgX is a strong base; water immediately protonates it (RMgX + H2O -> RH + Mg(OH)X), destroying the reagent.

Why does C-I bond break more easily than C-F despite F being smaller?

Bond strength scales with orbital overlap — the diffuse 5p of I overlaps poorly with C's 2sp3, making C-I weakest. Also, I- is the most polarisable, stabilising the transition state.

When does Saytzeff's rule fail?

With bulky bases (potassium tert-butoxide, LDA), Hofmann's rule takes over: the less substituted alkene forms because the base cannot reach the more hindered beta-H.

Why do polar aprotic solvents accelerate SN2?

They solvate cations strongly but leave anionic nucleophiles 'naked' and highly reactive. Polar protic solvents H-bond with nucleophiles, deactivating them.

What stereochemical outcome distinguishes SN1 from SN2?

SN2 inverts the configuration at the stereocentre (Walden inversion). SN1 racemises because the planar carbocation can be attacked from either face, often giving slight excess of inversion if the leaving group blocks one face.

Which substrate undergoes SN2 fastest?

CH3Br. SN2 favours the least hindered carbon; methyl > 1° > 2° >> 3° (effectively zero).

Grignard reagent CH3MgBr reacted with HCHO and worked up gives:

Ethanol. CH3- + HCHO -> CH3CH2O- -> CH3CH2OH after H3O+.

The mechanism of (CH3)3CBr hydrolysis with dilute aqueous NaOH is:

SN1. Tertiary substrate, dilute base, polar protic solvent — classic SN1.

Williamson ether synthesis works best with:

Primary alkyl halide and alkoxide. Primary alkyl halides undergo SN2 with the alkoxide; tertiary substrates give E2 with alkoxide as base.

Wurtz reaction of CH3Br and CH3CH2Br with Na in ether gives:

A mixture of ethane, propane, and butane. Two methyls couple to ethane, methyl + ethyl to propane, two ethyls to butane — Wurtz with two different halides gives all three.

Alkyl Halides MDCAT 2026 — Notes, Key Concepts & MCQs (Chemistry)

Alkyl Halides

Alkyl Halides averages 2 MCQs per MDCAT paper, focused on SN1 vs SN2, E1 vs E2, Grignard chemistry, and Wurtz/Williamson syntheses.

Alkyl Halides is a Chemistry chapter on the official PMDC MDCAT 2026 syllabus, contributing roughly 2 MCQs to the 45-MCQ Chemistry section. Mastering the core concepts below typically secures the full chapter weightage.

Classification and bond polarity

Alkyl halides R-X (X = F, Cl, Br, I) are classified by the carbon to which X is attached: 1° (RCH₂X), 2° (R₂CHX), or 3° (R₃CX). The C-X bond is polarised δ+C-Xδ−; bond strength decreases C-F > C-Cl > C-Br > C-I, but reactivity in nucleophilic substitution increases I > Br > Cl > F because the weaker bond is more easily broken and the larger halide is a better leaving group. Methyl iodide is the canonical SN2 substrate; tert-butyl bromide the canonical SN1. The FSc Punjab Textbook Chemistry XII Chapter 11 and Clayden Chapters 15-17 give the rigorous treatment.

SN2 — concerted backside attack

Bimolecular: rate = k[RX][Nu⁻]. The nucleophile attacks the carbon from the side opposite the leaving group; the transition state has five groups around C in a trigonal bipyramidal geometry. Result: inversion of configuration (Walden inversion). Steric hindrance kills SN2: methyl > 1° > 2° >> 3° (essentially zero). Polar aprotic solvents (DMSO, DMF, acetone) accelerate SN2 by leaving the nucleophile "naked." Strong nucleophiles (OH⁻, CN⁻, RO⁻, RS⁻, I⁻, N₃⁻) drive SN2.

SN1 — two-step via carbocation

Unimolecular: rate = k[RX], independent of nucleophile concentration. Step 1 is rate-limiting: ionisation to a carbocation. Step 2 is fast capture by the nucleophile. Carbocation stability dictates: 3° > 2° >> 1° (no SN1 for primary except resonance-stabilised allylic/benzylic). Polar protic solvents (H₂O, ROH) stabilise the developing charge. Result: racemisation (planar carbocation attacked from either face) and possible carbocation rearrangement.

E1 vs E2 elimination

E2: bimolecular, concerted; strong base (OH⁻, RO⁻, NH₂⁻) abstracts a β-H while the leaving group departs; anti-periplanar geometry required. Saytzeff's rule: more substituted (more stable) alkene predominates; bulky bases (t-BuO⁻) flip to Hofmann (less substituted alkene). E1: unimolecular, two-step via the same carbocation as SN1; loss of β-H from the cation gives the alkene. Under heating with concentrated base, elimination dominates over substitution; in cold dilute base, substitution wins. Tertiary substrates strongly prefer SN1/E1; primary prefer SN2.

Named reactions: Grignard, Wurtz, Williamson

Grignard reagents RMgX form when R-X meets Mg in dry ether (anhydrous — water destroys them). They are nucleophiles and strong bases (R^δ−). Reactions: with HCHO → 1° alcohol; with RCHO → 2° alcohol; with R₂C=O → 3° alcohol; with CO₂ then H₃O⁺ → carboxylic acid (one carbon longer); with epoxide → alcohol two carbons longer; with nitrile then hydrolysis → ketone. Wurtz: 2R-X + 2Na → R-R + 2NaX in dry ether — symmetric coupling. Williamson ether synthesis: R-O⁻ + R'X → R-O-R' + X⁻; works best with primary R'X to avoid E2. Morrison & Boyd Chapters 6 and 17 are the canonical sources.

Key Concepts

Worked MCQs

Frequently Asked Questions

How Alkyl Halides Is Tested

MDCAT questions on Alkyl Halides are a mix of recall (definitions, classifications), application (predict outcomes, interpret diagrams), and basic numerical/analytical reasoning. PMDC papers from 2020–2025 emphasized the concepts above; older UHS papers (2008–2019) tested them too, with slight variations in question framing.