HALOGENOALKANES
general formula: C_nH_2n+1X [n=1,2,.. X = F,Cl,Br,I]
functional group: halogen group; F: fluoro-, Cl: chloro-, Br: bromo-, I: iodo-
halogenoalkanes: primary(1°), secondary(2°), tertiary(3°)
primary: C atom has 2/3 H atoms attached to it
secondary: C atom - 1 H atoms attached
tertiary: C atom has no H atoms on it

Physical properties
-lower members w/ low RMM > gases at rtp(small VDW forces) [CH₃Cl, CH₃Br, CH₃CH₂Cl]
higher members > liquid / solid [CH₃I]
-characteristic smell
-insoluble in polar solvents(water), soluble in non-polar solvents

Chemical properties
-Nucleophilic substitution: hydrolysis, conversion to alkanonitrile, conversion to amine
-Elimination: dehydrohalogenation to form alkene

Nucleophilic substitution
C-X bond: polar, X more electronegative than C atom > X: nucleophilic, C: electrophilic


Alkaline hydrolysis to form alcohol
reagent: NaOH(aq)
condition: heat under reflux
equation: -C-X + NaOH(aq) >(reflux)> -C-OH + NaX

if products neutralise w/ acid (NaOH reacts w/ Ag⁺), then add AgNO₃: silver halide ppt, AgX, X= Cl-white ppt, Br-pale yellow ppt, I-yellow ppt

Mechanism of nucleophilic substitution in alkaline hydrolysis of bromoethane
-nulceuphile attacks +ve C atom of C-Br bond (due to polarisation) > transition state complex formed


-carbanion decomposed to give ethanol


Energy diagram of alkaline hydrolysis of bromoethane


slow step mechanism involves 2 subs: OH^- & CH₃CH₂Br > a 2^nd -order reaction, rate eqn: rate = k[CH₃CH₂Br][OH^-], k = rate const. [ ] = molar conc of reactant
(hydrolysis of 3° halogenoalkane undergoes a diff nuclephilic subs mechanism: carbocation intermediate)

Mechanism of 3° halogenoalkane


Energy diagram of reaction



1^st-oder reaction involving only (CH₃)₃Br, rate eqn: rate = k[(CH₃)₃Br]

Conversion to alkanonitile
reagents: aqueous ethanolic potassium cyanide, KCN (KCN dissolved in mixed solvent of ethanol + water, ethanol: solvent for halogenoalkane, water: solvent for KCN)
conditions: heat under reflux
equation: -C-X + KCN (ethanolic) >(reflux)> -C-CN + KX
[X replaced by cyano group, -CN > C atom increased by 1]
alkanonitrile can be converted to amine (by reduction) or acid (by hydrolysis) then to alcohol
RCN + 4[H] >(reduction: LiAlH₄/ dry ether, reflux)> RCH₂NH₂ (amine)> alcohol
RCN + 2H₂O >(hydrolysis: dil HCl, heat)> RC(=O)OH (acid) + NH₄Cl > alcohol

Conversion to amine
reagent: ethanolic ammonia (ammonia dissolved in ethanol)
condition: heat in sealed vessel (to prevent escape of gas)
equation: RX + NH₃ (ethanolic) >(heat in seal vessel)> RNH₂ + HX [HX(acidic) + NH₃ > NH₄X]
primary(1°) amine: 2 H atoms attached to N atom
secondary(2°) amine: 1 H atoms attached to N atom
tertiary(3°) amine: no H atom attached to N atom
(excess halogenoalkane: mixture of 1°, 2°, 3°, quarternary alkylammonium salt)
CH₃CH₂Br (excess) + 2NH₃ > CH₃CH₂-NH₂ + NH₄Br
CH₃CH₂Br + CH₃CH₂-NH₂ (nulceophilic) > NH(CH₂CH₃)₂ + HBr
CH₃CH₂Br + NH(CH₂CH₃)₂ (nulceophilic) > N(CH₂CH₃)₃ + HBr
CH₃CH₂Br + N(CH₂CH₃)₃ (nulceophilic) > N(CH₂CH₃)₄⁺Br^-
Elimination
Dehydrohalogenation to form alkene
hydrogen halide molecule, HX removed from halogenoalkane to form alkene
reagent: ethanolic NaOH
condition: heat under reflux
equation:

HALOGENOARENES (aryl halides)
cpds w/ halogen atom attached to C in benzene ring
molecular formula: C₆H₅X [X = halogen]

formation of halogenoarenes
C₆H₆ + X₂ >(Fe)>C₆H₅X + HX

Chemical properties
halogenoarenes: unreactive compared to halogenoalkanes
due to CX bond in halogenobenzene less polar than CX bond in halogenoalkane
p π-electrons of halogen interact w/ 6 π-electrons of benzene ring > delocalised > CX bond stronger + less polar
halogenoalkane doesn't react w/ dil NaOH(aq) even under reflux > need drastic conditions (conc NaOH(aq), heat under pressure + temp-350°C, 150atm)

phenol, C₆H₅OH: weak acid, not alcohol, reacts w/ NaOH > ionic sodium phenoxide salt, C₆H₅O^-Na⁺

CFC = ChloroFluoroCarbons (chlorofluoroalkanes)
C-F bond strongest halogen-C bond > stable (C-Cl bond 2^nd strongest) > chlorofluoroalkanes > stable & unreactive org cpd

Properties of CFC
-odourless
-low toxicity
-chemically unreactive
-non-flammable
-low bp
-good organic solvent

Uses of CFC
Refrigerant: dichlorodifluoromethane (CCl₂F₂) (bp= -30°C) in air-conditioners or refrigerators. has high RMM w/ large intermolecular VDW forces > liquefied easily by compression & cooling (Joule-Thompson's effect), it is also non-corrosive & non-toxic
Aerosal propellant: dichlorodifluoromethane (CCl₂F₂) produces spray of 'mist' containing active subs eg perfume, insecticide, paint,..

Solvent: 1,2-dichloro-1,1,2,2-tetrafluoroethane (CF₂ClCF₂Cl) (bp =48°C, volatile (l) at rtp)- degreasing solvent for machinery parts & electronic circuit components

Ozone depletion
formation of ozone(O₃):
O₂(g) >(UV from sun)> 2O(g) (oxygen atoms)
O(g) + O₂(g) > O₃(g)
photochemical reaction bet CFC and ozone:
CF₂Cl₂ > CF₂C• + Cl• (chlorine atom-FR)
Cl• + O₃ > •OCl + O₂(g)
•OCl + O > Cl• + O₂
(Cl• reproduced > continue reaction w/ other ozone molecules)

Substitutes for CFC
-CO₂,N₂, propene gas, methylpropane as aerosal propellants
disadvantages: CO₂, N₂: don't produce fine 'mist', propane: flammable
-HCFC-hydrochlorofluorocarbon (CHF₂Cl): halogenoalkanes w/ H atoms, H atoms make C-Cl bond more reactive > HCFC destroyed before reaches stratosphere to deplete ozone

Uses of other halogenated alkanes
-in fire extinguishers: BCF (halon 1211), bromochlorofluoromethane (CF₂ClBr) in fire extinguishers for oil, gasoline, electric and chemical fires
-anaesthetic: fluothane or halothane before surgery, 2-bromo-2-chloro-1,1,1-trifluoroethane (CF₃CHBrCl)
non-halogenated anaesthetic: N₂O- laughing gas-dinitrogen oxide, novocaine, xylocaine




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