Haloalkanes and Haloarenes — Part 2: Reactions and polyhalogen compounds: visual lesson, CBSE Class 12 Chemistry

Lesson

Reactions of halogen compounds

Part 2 — substitution, elimination and reactions with metals, and why aryl halides are so unreactive.

In this lesson you will

Write products of nucleophilic substitution and explain ambident nucleophiles
Compare SN1 and SN2 mechanisms, their kinetics, stereochemistry and reactivity orders
Predict elimination products by Saytzeff’s rule and write Grignard, Wurtz and Fittig reactions
Explain the low reactivity of haloarenes, their electrophilic substitution, and the uses and hazards of polyhalogen compounds

Concept

Nucleophilic substitution

A nucleophile replaces the halogen: $\ce{Nu- + R-X -> R-Nu + X-}$ . Ambident nucleophiles such as $\ce{CN-}$ and $\ce{NO2-}$ can attack through either of two atoms.

\ce{CH3CH2Br + KCN -> CH3CH2CN + KBr}

\ce{CH3CH2Br + AgCN -> CH3CH2NC + AgBr}

Table of nucleophilic substitution reagents, the attacking nucleophile and the product formed
Reagent	Nucleophile	Product
Aqueous KOH	$\ce{OH-}$	Alcohol
Sodium alkoxide	$\ce{RO-}$	Ether (Williamson)
KCN (ionic)	$\ce{CN-}$ via C	Nitrile (alkyl cyanide)
AgCN (covalent)	via N	Isocyanide
KNO₂	$\ce{NO2-}$ via O	Alkyl nitrite
AgNO₂	via N	Nitroalkane
NH₃ (excess)	$\ce{NH3}$	Amine
LiAlH₄	$\ce{H-}$	Alkane

Concept

The SN2 mechanism

Substitution, Nucleophilic, bimolecular: one step. The nucleophile attacks from the side opposite the halogen while the C–X bond breaks. Rate $= k[\ce{RX}][\ce{Nu-}]$ (second order). The configuration is inverted. Reactivity: $\ce{CH3X}$ > 1° > 2° > 3° (bulky groups block the backside).

Table of relative SN2 rates for methyl, primary, secondary and tertiary halides with the steric reason
Halide	Relative SN2 rate	Reason
Methyl	Fastest	No groups to block attack
Primary	Fast	Little crowding
Secondary	Slow	More crowding
Tertiary	Very slow	Backside blocked by three groups

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