- Classification primary 1 other C secondary 2 tertiary 3 methyl CH3X haloarene C6H5Cl direct ring benzyl C6H5CH2Cl side chain alkyl nomenclature CH3Cl chloromethane CH2Cl2 dichloromethane CHCl3 chloroform 1 parent 2 parents joint family.
- C-X polar C delta+ X delta- length F 139 to I 216 increases size strength F greater than Cl greater than Br greater than I reactivity I greater than Br greater than Cl greater than F tug of war larger rope weaker more reactive preparation alcohols PCl5 RCl POCl3 HCl SOCl2 Darzens best gases escape Lucas HCl ZnCl2 alkenes HX Markovnikov Br2 vicinal dibromide Finkelstein NaI acetone RI NaCl ppt drives Swarts AgF RF.
- Physical boiling higher than alkanes RI greater than RBr greater than RCl greater than RF size polar van der Waals heavier harder to lift density heavy halogen higher CH2Cl2 heavier than water sinks solubility water low non-polar dominates main reaction nucleophilic substitution C delta+ attacked by nucleophile OH- alcohol CN- cyanide NH3 amine replacement actor.
- SN2 one step backside attack inversion umbrella flip transition pentavalent Rate=k[RX][Nu-] second order favoured primary methyl less steric strong nucleophile polar aprotic acetone DMSO DMF good leaving I- back door entry inversion bus back door SN1 two steps carbocation formation slow RX to R+ +X- planar then Nu both sides racemic 50R50S Rate=k[RX] first order favoured tertiary secondary stable carbocation weak nucleophile H2O polar protic water alcohol stabilizes carbocation old leaves empty seat new can sit either side racemic primary SN2 tertiary SN1.
- Elimination dehydrohalogenation RX+alc KOH heat alkene+HX Saytzeff more substituted alkene major stable more neighbours stable bridge 2-bromobutane but-2-ene major but-1-ene minor Wurtz 2RX+2Na dry ether R-R symmetrical alkane ethane Grignard RX+Mg dry ether RMgX reactive alcohols isocyanide AgCN RNC foul smell carbylamine test.
- Polyhalogen CH2Cl2 methylene chloride paint remover solvent propellant low boiling CHCl3 chloroform ethanol acetone + bleaching powder sweet smell anesthetic earlier phosgene COCl2 poisonous air light dark brown ethanol stabilizer solvent CCl4 carbon tetrachloride non-polar solvent fire extinguisher banned ozone depletion DDT ClC6H4-CH(CCl3)-C6H4Cl two chlorobenzene rings CCl3 insecticide mosquitoes malaria non-biodegradable biomagnification birds eggshell thin banned powerful poison stays long.
- Haloarene low reactivity vs haloalkane resonance C-Cl partial double bond lone pair Cl to ring p-pi strengthens hard break sp2 holds tighter unstable phenyl carbocation double lock strong preparation benzene+Cl2 FeCl3 chlorobenzene electrophilic diazonium C6H5N2+Cl- CuCl HCl Sandmeyer C6H5Cl+N2 gas bubbles electrophilic substitution Cl o p directing but deactivating resonance donation directs o p inductive withdrawing deactivates slower than benzene further substitution o p positions chloronitrobenzene Wurtz Fittig C6H5Cl+CH3Cl+Na toluene Fittig 2C6H5Cl+Na biphenyl next guest beside or opposite party slower.
- Applications solvents CH2Cl2 paint remover CHCl3 lab solvent CCl4 fire extinguisher banned PVC polyvinyl chloride pipes bottles vinyl chloride polymerization Teflon PTFE non-stick tetrafluoroethylene CF2=CF2 non-stick pan pesticides DDT mosquitoes malaria banned biomagnification BHC 666 lindane gamma isomer environmental CFCs freon refrigeration ozone depletion UV Montreal protocol CCl4 ozone DDT non-biodegradable food chain accumulation birds eggshell thin harm biodegradable alternatives needed shield destroyer guest never leaves.
Chapter 6 - Haloalkanes and Haloarenes
Beginner
Topper
1. Introduction - Classification & Nomenclature
Haloalkanes are alkanes where one or more H replaced by halogen F Cl Br I. Think of methane CH4 replacing one H with Cl gives CH3Cl chloromethane refrigerant.
Classification by carbon attached to halogen: primary 1 degree halogen attached to carbon which attached to only one other carbon CH3CH2Cl chloroethane primary. Secondary 2 degree carbon attached to two other carbons CH3CHClCH3 2-chloropropane secondary. Tertiary 3 degree carbon attached to three other carbons (CH3)3CCl tert-butyl chloride tertiary. Also methyl halides CH3X only one carbon.
Haloarenes halogen directly attached to benzene ring chlorobenzene C6H5Cl. Side chain halogen benzyl chloride C6H5CH2Cl halogen on side chain not ring still alkyl halide.
Nomenclature common: methyl chloride CH3Cl, IUPAC chloromethane. CH2Cl2 dichloromethane, CHCl3 trichloromethane chloroform.
Visual Learning Method: Methane CH4 to CH3Cl, primary CH3CH2Cl, secondary CH3CHClCH3, tertiary (CH3)3CCl, haloarene C6H5Cl chlorobenzene, benzyl chloride side chain, IUPAC names.
Daily Analogy: Primary secondary tertiary like generations. Primary child has one parent connection single carbon, secondary two connections, tertiary three connections crowded like joint family more branching.
Classification & Nomenclature
Revision Points - Easy:
- Haloalkane alkane H replaced by halogen.
- Primary carbon attached to 1 other C, secondary 2, tertiary 3.
- Haloarene halogen directly on benzene ring chlorobenzene.
- Benzyl chloride side chain halogen alkyl halide.
- CH3Cl chloromethane CHCl3 chloroform.
2. Nature of C-X Bond & Preparation Methods
C-X bond polar because halogen more electronegative than carbon carbon delta plus halogen delta minus. Bond length increases F 139 pm to I 216 pm larger halogen longer bond weaker. Bond strength decreases F strongest I weakest. So reactivity I greater than Br greater than Cl greater than F.
Preparation: From alcohols ROH + PCl5 gives RCl + POCl3 + HCl best method no side products? Actually SOCl2 best gives RCl + SO2 + HCl gases escape Darzens procedure pure RCl thionyl chloride method best. ROH + HCl ZnCl2 Lucas gives RCl. From alkenes add HX Markovnikov RCH=CH2 + HBr -> RCHBrCH3, add Br2 gives vicinal dibromide. Halogen exchange Finkelstein RCl + NaI acetone -> RI + NaCl NaCl precipitates drives reaction. Swarts RCl + AgF -> RF + AgCl.
Visual Learning Method: C-X polarity C delta+ X delta-, bond length increasing F to I, strength decreasing, reactivity I greater than Br greater than Cl, preparation alcohols PCl5 SOCl2 Darzens best, alkenes +HX Markovnikov +Br2 dibromide, Finkelstein NaI acetone, Swarts AgF.
Daily Analogy: C-X bond like tug of war halogen stronger electronegative pulls electrons delta minus carbon delta plus. Larger halogen longer rope weaker easier to break more reactive I weakest most reactive.
C-X Bond & Preparation
Revision Points - Easy:
- C-X polar C delta+ X delta- halogen more electronegative.
- Bond length increases F to I larger atom longer weaker.
- Reactivity I greater than Br greater than Cl greater than F.
- From alcohols PCl5 SOCl2 Darzens best pure.
- From alkenes +HX Markovnikov +Br2 dibromide.
- Finkelstein RCl+NaI->RI NaCl ppt drives Swarts RF via AgF.
3. Physical Properties & Nucleophilic Substitution Overview
Physical: boiling point haloalkanes higher than alkanes due to larger size polar more van der Waals. Order RI greater than RBr greater than RCl greater than RF same carbon. Density also increases with heavy halogen CH2Cl2 heavier than water sinks. Solubility water low because cannot H-bond non-polar part dominates but slightly soluble polar C-X.
Chemical: main reaction nucleophilic substitution X replaced by nucleophile OH- CN- etc because C delta+ attacked by nucleophile negative loving. Nucleophile examples OH- gives alcohol, CN- gives cyanide nitrile, NH3 gives amine, SH- gives thiol.
Two mechanisms SN1 and SN2 decide based on structure, nucleophile, solvent, leaving group.
Visual Learning Method: Boiling point graph RI greater than RBr greater than RCl, density CH2Cl2 sinks water, nucleophilic substitution C delta+ nucleophile OH- CN- NH3 replaces X.
Daily Analogy: Boiling point higher with heavier halogen like heavier person harder to lift needs more heat to boil. Nucleophilic substitution like replacement actor X leaves stage nucleophile new actor enters stage C is stage.
Physical Properties & Substitution Overview
Revision Points - Easy:
- Boiling point higher than alkanes RI greater than RBr greater than RCl greater than RF.
- Density heavy halogen higher sinks water.
- Water solubility low non-polar dominates.
- Main reaction nucleophilic substitution X replaced by nucleophile OH- CN- NH3.
4. SN1 vs SN2 - Two Different Roads
SN2 bimolecular one step backside attack nucleophile attacks opposite to leaving group forms transition state pentavalent carbon both Nu and X half bonded then X leaves inversion Walden inversion like umbrella flipped inside out configuration inverted. Rate = k [RX][Nu-] depends both second order. Favoured by primary methyl less steric hindrance small carbon easy backside attack, strong nucleophile I- OH- CN-, polar aprotic solvent acetone DMSO DMF no H-bond to nucleophile keeps strong, good leaving group I- weak base.
SN1 unimolecular two steps first slow carbocation formation RX -> R+ + X- carbocation planar, second fast Nu- attacks either side gives racemic mixture 50% R 50% S if chiral. Rate = k[RX] only depends on substrate first order. Favoured by tertiary secondary stable carbocation tertiary more stable hyperconjugation, weak nucleophile H2O neutral, polar protic solvent water alcohol stabilizes carbocation via solvation, good leaving group.
Example tert-butyl bromide (CH3)3CBr with water SN1 gives racemic if chiral, CH3Br with OH- SN2 gives inversion.
Visual Learning Method: SN2 backside umbrella flip inversion one step transition state, Rate k[RX][Nu-], primary favoured. SN1 carbocation planar two steps racemic mixture both sides attack, Rate k[RX], tertiary favoured.
Daily Analogy: SN2 like entering bus from back door while old passenger leaves front simultaneously one step inversion. SN1 like old passenger leaves first creating empty seat carbocation then new passenger can sit from either side front or back racemic two possible sitting sides.
SN1 vs SN2 Mechanisms
Revision Points - Easy:
- SN2 one step backside attack inversion umbrella flip Rate=k[RX][Nu-] second order primary favoured strong nucleophile aprotic solvent.
- SN1 two steps carbocation slow then Nu fast racemic both sides Rate=k[RX] first order tertiary favoured weak nucleophile protic solvent stabilizes carbocation.
- Primary SN2 tertiary SN1.
5. Elimination & Other Reactions - Removing HX
Elimination dehydrohalogenation RX + alcoholic KOH heat -> alkene + HX. Removes H and X from adjacent carbons. Saytzeff rule more substituted alkene major more stable double bond more alkyl groups. Example 2-bromobutane CH3CHBrCH2CH3 with alc KOH gives but-2-ene major more substituted CH3CH=CHCH3 plus but-1-ene minor CH2=CHCH2CH3.
Wurtz reaction 2RX +2Na dry ether -> R-R +2NaX alkane higher symmetrical alkane formed. Example 2CH3Br+2Na->CH3CH3 ethane.
With metals: R-Mg-X Grignard reagent RX + Mg dry ether -> RMgX highly reactive for making alcohols. R-X + AgCN gives isocyanide RNC carbylamine foul smell test.
Visual Learning Method: Elimination H and X removed Saytzeff more substituted alkene major, Wurtz 2RX+2Na->R-R symmetrical, Grignard RMgX formation, isocyanide RNC foul smell.
Daily Analogy: Elimination like removing two neighbours H and X from house row creates double bond bridge alkene between houses. Saytzeff more neighbours stable bridge major.
Elimination & Other Reactions
Revision Points - Easy:
- Elimination RX+alc KOH heat alkene+HX Saytzeff more substituted alkene major stable.
- 2-bromobutane but-2-ene major but-1-ene minor.
- Wurtz 2RX+2Na dry ether R-R symmetrical alkane.
- Grignard RX+Mg dry ether RMgX reactive for making alcohols.
- AgCN gives isocyanide RNC foul smell carbylamine test.
6. Polyhalogen Compounds - Chloroform DDT CCl4
Methylene chloride CH2Cl2 used as paint remover solvent propellant low boiling.
Chloroform CHCl3 preparation ethanol or acetone + bleaching powder or CH4+Cl2. Sweet smell anesthetic earlier but produces phosgene COCl2 poisonous gas with air light so stored dark brown bottles with ethanol stabilizer. Reaction with acetone + KOH gives chlorobenzene? Actually chloroform + acetone + KOH? Main use solvent.
Carbon tetrachloride CCl4 non-polar solvent fire extinguisher earlier, now banned ozone depletion.
DDT dichlorodiphenyltrichloroethane ClC6H4-CH(CCl3)-C6H4Cl insecticide powerful kills mosquitoes malaria but non-biodegradable accumulates food chain biomagnification birds eggshell thin banned many countries environmental harm. Structure two chlorobenzene rings connected via CCl3 CH.
Visual Learning Method: CH2Cl2 paint remover, CHCl3 chloroform sweet anesthetic phosgene COCl2 poisonous dark bottle, CCl4 fire extinguisher banned ozone, DDT two chlorobenzene rings CCl3 insecticide mosquito malaria biomagnification banned.
Daily Analogy: DDT like powerful poison kills insects quickly but stays in environment long non-biodegradable like plastic accumulates food chain big fish eats small fish accumulates biomagnification birds affected.
Polyhalogen Compounds
Revision Points - Easy:
- CH2Cl2 paint remover solvent propellant.
- CHCl3 chloroform sweet smell anesthetic earlier phosgene COCl2 poisonous with air light dark bottle ethanol stabilizer.
- CCl4 solvent fire extinguisher banned ozone depletion.
- DDT insecticide mosquitoes malaria non-biodegradable biomagnification birds eggshell thin banned.
7. Haloarenes - Chlorobenzene Less Reactive
Haloarenes halogen directly on benzene ring low reactivity vs haloalkanes because resonance C-Cl partial double bond character lone pair Cl donates to ring p-pi conjugation strengthens bond hard to break, sp2 carbon more electronegative holds tighter, and unstable phenyl carbocation if SN1.
Preparation: benzene + Cl2 FeCl3 dark gives chlorobenzene electrophilic substitution, from benzene diazonium chloride C6H5N2+Cl- + CuCl HCl Sandmeyer gives C6H5Cl + N2 gas.
Reactions: electrophilic substitution Cl is ortho para directing but deactivating due to resonance donation directs ortho para but inductive electron withdrawing deactivates ring slower than benzene. So further substitution occurs ortho para positions to Cl. Example chlorobenzene nitration gives o and p chloronitrobenzene.
Wurtz Fittig and Fittig reactions: C6H5Cl +2Na + CH3Cl -> toluene +2NaCl Wurtz Fittig mixed alkyl aryl, 2C6H5Cl+2Na-> biphenyl C6H5C6H5 Fittig biphenyl formation.
Visual Learning Method: Resonance C-Cl partial double bond, sp2 carbon, preparation benzene+Cl2 FeCl3 Sandmeyer diazonium CuCl, o p directing deactivating electrophilic substitution o p chloronitrobenzene, Wurtz Fittig toluene biphenyl.
Daily Analogy: Haloarene C-Cl strong like double lock resonance makes bond stronger hard to break vs haloalkane single lock easy break. o p directing like Cl tells next guest to sit beside or opposite but deactivates party slower overall.
Haloarenes - Chlorobenzene
Revision Points - Easy:
- Haloarene low reactivity vs haloalkane resonance C-Cl partial double bond strong hard break sp2 holds tighter.
- Prep benzene+Cl2 FeCl3 chlorobenzene, diazonium Sandmeyer CuCl HCl C6H5Cl+N2.
- Cl o p directing but deactivating resonance donation o p inductive withdrawing deactivates slower.
- Further substitution o and p positions to Cl.
- Wurtz Fittig C6H5Cl+CH3Cl+Na toluene Fittig biphenyl C6H5C6H5.
8. Applications & Environmental Effects - Uses and Harms
Uses: solvents CH2Cl2 paint remover, CHCl3 lab solvent earlier anesthetic, CCl4 dry cleaning fire extinguisher now banned, PVC polyvinyl chloride plastic from vinyl chloride CH2=CHCl polymerization pipes bottles, Teflon PTFE non-stick from tetrafluoroethylene CF2=CF2 polymerization.
Pesticides DDT insecticide mosquitoes malaria but banned biomagnification, BHC benzene hexachloride 666 insecticide lindane gamma isomer used.
Environmental: chlorofluorocarbons CFCs freon refrigeration causes ozone depletion UV rays harm, carbon tet CCl4 ozone depletion banned Montreal protocol, DDT non-biodegradable accumulates food chain fish birds eggshell thin harms ecosystem.
So need biodegradable alternatives.
Visual Learning Method: Solvents CH2Cl2 paint remover, PVC pipes bottles, Teflon non-stick pan, pesticides DDT mosquito, BHC 666, environmental ozone depletion CFCs UV, DDT biomagnification fish birds eggshell thin, Montreal protocol banned.
Daily Analogy: CFCs like invisible shield destroyer ozone is shield from sun UV rays. DDT like guest who never leaves house accumulates food chain biomagnification stays long harms future generations.
Applications & Environmental Effects
Revision Points - Easy:
- Solvents CH2Cl2 paint remover CHCl3 lab solvent CCl4 fire extinguisher now banned.
- PVC pipes bottles from vinyl chloride Teflon non-stick from tetrafluoroethylene.
- DDT insecticide mosquitoes malaria banned non-biodegradable biomagnification BHC 666 lindane.
- CFCs freon refrigeration ozone depletion UV Montreal protocol banned.
- DDT accumulates food chain birds eggshell thin environmental harm need biodegradable alternatives.
Gold Reference Table - All Formulas + Daily Life Visual
| Concept | Formula / Easy Fact | Daily Life Visual / Uses |
|---|---|---|
| Classification | Primary 1 other C Secondary 2 Tertiary 3 Methyl CH3X Haloarene C6H5Cl direct ring Benzyl C6H5CH2Cl side chain alkyl | 1 parent 2 parents 3 joint family branching |
| Nomenclature | CH3Cl chloromethane CH2Cl2 dichloromethane CHCl3 chloroform CCl4 tetrachloromethane | Common names methyl chloride |
| C-X bond | Polar C delta+ X delta- Length F 139 to I 216 increases Strength F>Cl>Br>I Reactivity I>Br>Cl>F | Tug of war larger rope weaker more reactive |
| Preparation alcohols | ROH+PCl5 RCl+POCl3+HCl SOCl2 Darzens best RCl+SO2+HCl gases escape ROH+HCl ZnCl2 Lucas | Thionyl chloride best pure |
| Preparation alkenes | RCH=CH2+HX Markovnikov RCHBrCH3 +Br2 vicinal dibromide RCHBrCH2Br | Alkenes add HX Br2 |
| Exchange | Finkelstein RCl+NaI acetone RI+NaCl NaCl ppt drives Swarts RCl+AgF RF+AgCl | NaI acetone RI |
| Physical boiling | Higher than alkanes RI>RBr>RCl>RF same C size polar van der Waals | Heavier harder to lift higher boiling |
| Density solubility | Heavy halogen density higher CH2Cl2 heavier than water sinks Water solubility low non-polar dominates | Sinks water low solubility |
| Substitution main | Nucleophilic substitution X replaced by Nu- OH- alcohol CN- cyanide NH3 amine SH- thiol | Replacement actor stage C |
| SN2 | One step backside attack inversion umbrella flip Rate=k[RX][Nu-] second order favoured primary methyl less steric strong Nu- polar aprotic acetone DMSO DMF good leaving I- | Back door entry inversion bus back door |
| SN1 | Two steps carbocation formation slow RX->R+ +X- planar then Nu both sides racemic 50R50S Rate=k[RX] first order favoured tertiary secondary stable carbocation weak Nu H2O polar protic water alcohol stabilizes carbocation | Old leaves empty seat new can sit either side racemic |
| Elimination | RX+alc KOH heat alkene+HX Saytzeff more substituted alkene major stable 2-bromobutane but-2-ene major but-1-ene minor | Remove H X neighbours bridge alkene more neighbours stable major |
| Wurtz | 2RX+2Na dry ether R-R+2NaX symmetrical alkane 2CH3Br+2Na CH3CH3 ethane | Higher alkane |
| Grignard | RX+Mg dry ether RMgX reactive for alcohols | Making alcohols |
| Isocyanide | RX+AgCN RNC carbylamine foul smell test | Foul smell test |
| Polyhalogen CH2Cl2 | Methylene chloride paint remover solvent propellant low boiling | Paint remover |
| CHCl3 | Chloroform ethanol/acetone+bleaching powder sweet smell anesthetic earlier phosgene COCl2 poisonous with air light dark brown ethanol stabilizer solvent | Anesthetic earlier phosgene poisonous dark bottle |
| CCl4 | Carbon tetrachloride non-polar solvent fire extinguisher banned ozone depletion | Fire extinguisher banned |
| DDT | ClC6H4-CH(CCl3)-C6H4Cl insecticide mosquitoes malaria non-biodegradable biomagnification birds eggshell thin banned | Mosquito killer non-biodegradable food chain accumulation |
| Haloarene low reactivity | Resonance C-Cl partial double bond lone pair Cl to ring p-pi strengthens hard break sp2 holds tighter unstable phenyl carbocation | Double lock strong hard break |
| Prep haloarene | Benzene+Cl2 FeCl3 chlorobenzene electrophilic Diazonium C6H5N2+Cl- +CuCl HCl Sandmeyer C6H5Cl+N2 | Sandmeyer N2 gas bubbles |
| Electrophilic Cl | o p directing but deactivating resonance donation directs o p inductive withdrawing deactivates slower than benzene further substitution o p chloronitrobenzene | Next guest beside or opposite but party slower |
| Wurtz Fittig Fittig | C6H5Cl+CH3Cl+2Na toluene Wurtz Fittig 2C6H5Cl+2Na biphenyl C6H5C6H5 Fittig | Toluene biphenyl |
| Uses PVC Teflon | PVC pipes bottles vinyl chloride CH2=CHCl polymerization Teflon PTFE non-stick CF2=CF2 tetrafluoroethylene | Pipes non-stick pan |
| Pesticides | DDT BHC 666 lindane gamma isomer insecticide | Mosquitoes |
| Environmental | CFCs freon refrigeration ozone depletion UV Montreal protocol CCl4 ozone DDT non-biodegradable biomagnification food chain birds eggshell thin biodegradable alternatives needed | Shield destroyer guest never leaves accumulates |
Final Exam Ready Revision - Easy Day-to-Day Hooks: