LiroxtronIV (LiroxtronIV)-
explain why bp of RX increases down Group 17
increasing amount of energy required to overcome increasing strength of id-id attraction b/w RX molecules as no. of e- per molecule to be polarised increases down group 17
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why RX insoluble in water?
cannot form H bonds/ion-dipole interactions w/ water >
insufficient energy released to overcome strong H bonds b/w water
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prepare RX from alkane
reaction: free radical substitution
r&c: LIMITED X2/EXCESS alkane, UV light
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prepare RX from C=C
reaction: electrophilic addition
r&c: dry HX(g), rtp
OR
r&c: X2 in inert organic solvent(CCl4), absence of light
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prepare RX from ROH
reaction: nucleophilic substitution
r&c: PCl5(s), rtp
obs:white fumes of HCl(g)
OR
r&c:PX3, heat
OR
SOCl2, heat
OR
conc. HCl, anhydrous ZnCl2 catalyst, heat
OR
conc. H2SO4, KX(s)/NaX(s), heat
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what affects reactivity of RX?
1.strength of C-X bond
- weaker C-X bond > C-X bond breaks > readily > >reactive RX to NS
2. stability of leaving group
- > stabilized -ve charge on anion(leaving group) > weaker base strength of leaving group > less reactive to NS
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describe unimolecular nucleophilic substitution(SN1)
GN
2 step mechanism
Generation of carbocation
C-X bond breaks, e- pair donated to X forming carbocation + anion
Nucleophilic attack
nucleophile attacks carbocation from either side of plane w/ equal probability; 2 pdt of = proportion > forms racemic mixture, optically inactive
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describe bimolecular nucleophilic substitution(SN2)
1 step mechanism
Generation of pentavalent transition state
nucleophile does backside attack on RX, as C-O bond forms, C-X bond breaks simultaneously > inversion of structure occurs
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RX react with OH-
RX + OH- -> ROH + X-(aq)
reaction: NS
r&c: NaOH(aq), heat
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RX react with CN-
RX + CN- -> R-CN + X-
reaction: NS
r&c: NaCN, ethanol, heat
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RX react with :NH3
RX + NH3 -> R-NH2 + HX
reaction: NS
r&c: excess NH3, ethanol, heat in sealed tube
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RX form C=C
reaction: elimination
r&c: NaOH, ethanol, heat
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prepare aryl halides
benzene + X2 -> aryl halide + HX
reaction: ES
r&c: X2, anhydrous FeX3/AlX3/Fe, heat
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why aryl halides resistant to NS?
p-p orbital overlap results in delocalisation of lp of e- on X into π e- system of benzene ring > formation of partial db character in C-X bond, strengthens C-X bond > partially +ve C < e- deficient, less susceptible to NS
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why aryl halides require harsher conditions than benzene?
X atom electronegative, EWG > benzene ring < e--rich, deactivates benzene ring towards electrophile attack > X is EWG but 2,4-directing
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why rate of NS increases for RX going down the group?
bond strength of C-X decreases down the group >
bond gets longer & weaker as < effective overlap of orbitals >
weaker bond, > reactive RX is to NS, faster appearance of AgX ppt >
rate of reaction increases
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identify RX
1. heat R-X w/ NaOH (NS)
2. cool mixture (prevent decomposition of AgNO3)
3. acidify w/ dilute HNO3 (neutralize excess NaOH)
4. add AgNO3(aq) (ppt)
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what are the uses of CFCs?
1. refrigerant; low-toxicity, low BP, inert
2. aerosol propellants; volatile
3. fire-extinguishers; dense, non-flammable
4. as solvents; clean & degrease machinery
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what are the effects of CFCs on ozone layer?
C-F, C-Cl bonds strong > CFCs chemically unreactive & stable >
unreactivity allows CFCs to diffuse into upper stratosphere >
strong UV radiation cleaves relatively weak C-Cl bond, generate Cl radicals > radicals catalyse decomposition of O3 >
O2, ozone layer depletes >
allows more harmful UV radiation to reach earth
