Haloalkanes & Haloarenes — NEET Glossary — Notes

Term	Definition	NEET Context
Haloalkane	Organic compound where halogen replaces H in an alkane (R-X)	General category; subclassified as primary, secondary, tertiary
Haloarene	Organic compound where halogen is directly attached to benzene ring (Ar-X)	Less reactive than haloalkane due to resonance
Nucleophile	Electron-pair donor that attacks electrophilic centers; Lewis base	Strong (OH-, CN-, I-) favor SN2; weak ( $H_{2}O$ , ROH) favor SN1
Electrophile	Electron-pair acceptor; Lewis acid	C in C-X is electrophilic (C^δ+) due to halogen's electronegativity
Leaving group	Group that departs with bonding electrons (as stable anion/neutral)	Best leaving groups = weakest bases: I- > Br- > Cl- >> F-
SN1	Substitution Nucleophilic Unimolecular; two-step via carbocation; Rate = k[RX]	Favored by 3° substrate, polar protic solvent, weak nucleophile
SN2	Substitution Nucleophilic Bimolecular; one-step concerted; Rate = k[RX][Nu]	Favored by 1° substrate, polar aprotic solvent, strong nucleophile
Carbocation	Carbon with only 3 bonds and a positive charge; sp2, planar	Key intermediate in SN1 and E1; stability: 3° > 2° > 1° > methyl
Walden inversion	Complete inversion of configuration at stereocenter in SN2	SN2 always gives inversion (not racemization)
Racemization	Production of 50:50 R:S mixture from a chiral substrate	SN1 always gives racemization (planar carbocation attacked from both sides)
Backside attack	Nucleophile approaches C-X from directly opposite the leaving group (180°)	Defines SN2 geometry; causes Walden inversion
Polar protic solvent	Polar solvent with O-H or N-H; can H-bond; examples: $H_{2}O$ , ROH	Favors SN1 (solvates ions); weakens nucleophile (solvation cage)
Polar aprotic solvent	Polar solvent without O-H/N-H; cannot H-bond; examples: DMSO, DMF, acetone	Favors SN2 (leaves nucleophile unsolvated/reactive)
E1	Elimination Unimolecular; two steps via carbocation; Rate = k[RX]; competes with SN1	High temperature favors E1 over SN1
E2	Elimination Bimolecular; one-step concerted; Rate = k[RX][Base]	Favored by strong bulky base; gives Saytzeff product
Saytzeff's rule	More substituted alkene is the major elimination product (thermodynamic control)	Default rule for E1 and E2 with non-bulky base
Hofmann's rule	Less substituted alkene is major product (steric control with bulky base)	Only with very bulky base (t-BuO-)
Hyperconjugation	Delocalization of σ C-H electrons into adjacent π or p orbital	Stabilizes alkenes (more substituted) and carbocations (more alkyl groups)
Finkelstein reaction	RCl + NaI (acetone) → RI + NaCl↓; halogen exchange to iodide	Driven by NaCl precipitation; Le Chatelier's principle
Swarts reaction	RBr + AgF → RF + AgBr↓; halogen exchange to fluoride	Driven by AgBr precipitation; only practical route to fluoroalkane
Dow process	$C_{6}H_{5}Cl$ + NaOH → $C_{6}H_{5}OH$ + NaCl (623 K, 300 atm); NAS	Industrial phenol production; harsh conditions due to Ar-Cl resonance
Grignard reagent	R-MgX; prepared from R-X + Mg in dry ether; carbon is nucleophilic (C^δ-)	Used for C-C bond formation; destroyed by water
p-π conjugation	Overlap of halogen p-orbital with benzene π system; gives partial C=X in Ar-X	Explains why haloarenes are less reactive (stronger Ar-C-X bond)
Meisenheimer complex	Anionic intermediate in nucleophilic aromatic substitution (NAS)	Stabilized by EWG at ortho/para; explains why EWG activates NAS
Phosgene ( $COCl_{2}$ )	Toxic gas formed by oxidation of $CHCl_{3}$ ; carbonyl chloride	$CHCl_{3}$ stored in dark/amber to prevent formation
DDT	Dichlorodiphenyltrichloroethane; insecticide; non-biodegradable, lipophilic	Biomagnification through food chain; environmental persistent
CFCs (freons)	Chlorofluorocarbons; C-Cl bond cleaved by UV → Cl•; catalytic $O_{3}$ destruction	Montreal Protocol banned CFCs; replaced by HFCs (no Cl)
Biomagnification	Increasing concentration of persistent pollutant at higher trophic levels in food chain	DDT: concentrates in fat, increases from plant → herbivore → carnivore → apex predator
Bond energy	Energy required to break one mole of a covalent bond homolytically	C-F (485) > C-Cl (339) > C-Br (285) > C-I (213) kJ/mol