Les Dérivés d'halogène Flashcards
What is the general formula of halogenoalkanes?
CnH2n+1X
Explain physical properties of halogenoalkanes
- (i) bp of halogenoalkanes > bp of alkanes of similar Mr
bcos more energy needed to overcome stronger pd-pd attract n btw polar halogenoalkane molecules vs weaker id-id attract n btw non-polar alkane molecules
(ii) bp increase w larger halogen size
bcos,
- size e- cloud: CH3I > CH3Br > CH3Cl
- extent distort n e- cloud: CH3I > CH3Br > CH3Cl
- extent intermolecular id-id attract n: CH3I > CH3Br > CH3Cl
- amt energy needed to overcome id-id attract n: CH3I > CH3Br > CH3Cl
2.
Solubility
- halogenoalkanes soluble in organic solvents bcos energy released by form n id-id attract n btw halogenoalkane & organic solvent molecules enough to overcome id-id btw organic molecules & btw halogenoalkane molecules
- insoluble in water
bcos energy released by form n of weak id-id attract n btw halogenoalkane & water molecules inadequate to overcome strong H bond btw water molecules & id-id attract n btw halogenoalkane molecules
Name type of rxn, reagent and condition of preparation FROM alcohols to RX
Type: Nucleophilic sub n
(Reagent, condition -> products)
R-OH +,
- HCl (g), heat —> R-Cl +H2O
- anhydrous PCl5 —> R-Cl + POCl3 + HCl
- anhydrous SOCl2 —> R-Cl + SO2 + HCl
- anhydrous PCl3 —> R-Cl + H3PO3
- NaBr, conc H2SO4, heat OR HBr(g), heat —> R-Br + H2O
- anhydrous PBr3 —> R-Br +H3PO3
- red P + I2 (PI3 is unstable) —> R-I + H3PO3
Define Nucleophilic substitution
Rxn where halogen atom in organic molecule is substituted by a nucleophile
What two ways does nucleophilic sub n occur through?
a. SN2 mechanism for primary halogenoalkanes
b. SN1 mechanism for tertiary halogenoalkanes
Describe mechanism for hydrolysis of bromoethane
Name: SN2 mechanism
*one step process
i. OH- nucleophile attack e- def C atom fr opp side of Br
(Arrow from lp of OH:- to C, arrow fr C-Br bond to Br w dipoles)
ii. In pentavalent trans state, OH- start form bond w C atom while C-Br bond start break
(Big square bracket w double decker sign, dotted lines to show half form half broken bond)
iii. Inversion of config of pdt formed (if C of C-X bond is chiral, + Br:-)
What does energy profile diagram of SN2 hydrolysis reaction have?
- Vertical axis: Potential energy
- Horizontal axis: Rxn pathway
- Ea
- dH
Explain why the optical isomerism of product is reversed for SN2?
Nucleophile attack C atom fr side opp to halogen atom. If C is chiral, pdt wld hv inversion of config as compared to rxt
Explain steric effect/hindrance
- Relative rate of SN2 rxn of halogenoalkanes:
Methyl>1°>2°>3° - It is easier for nucleophile to attack C atom if there r less bulky R grp ard C atom
Describe mechanism for hydrolysis of 2-bromo-2-methylpropane (HINT: tertiary RX)
Step1: Arrow fr C-Br bond w dipoles to Br —(slow)—> carbocation intermediate + Br-
Step 2: arrow fr -:OH to C+ —(fast)—> C(CH3)3OH
*If C of C-X bond is chiral, pdt formed wld b racemic mixture
Explain why a racemic mixture forms when aa halogenoalkane undergoing SN1 is chiral?
- nucleophile attack sp2 hybridised, trig planar carbocation intermediate fr either top or bottom of plane w equal probability
=> forms equal amt of each (+) & (-) enantiomers, so a racemic mixture forms
Explain why a tertiary RX undergoes SN1 but primary RX does not
- relative rate of rxn of SN1 rxn:
3°>2°>1°>methyl - bcos of stability of carbocation formed (more R gro means stable and faster rxn)
What is one exception to steric consideration?
(CH3)3CCH2Cl
- despite being primary halide, SN1 mechanism favoured instead of SN2 due to steric hindrance of bulky -C(CH3)3 grp
Name one exception due to electronic config
(Chloromethyl)benzene, C6H5CH2Cl
- being primary favours SN1 mechanism due to resonance stabilisat n of benzyl carbocation by delocalisat n of +ve charge into benzene ring
What is the order of reactivity of halogenoalkanes towards nucleophilic sub n?
R-I > R-Br > R-Cl > R-F
Name reagent, condition and type of reaction of hydrolysis to form alcohol (nucleophile: OH-)
R&C: NaOH (aq) or KOH (aq), heat
*aq! bcos another rxn w ethanol as solvent
Type of rxn: nucleophilic sub n
Name reagent, condition and type of reaction of formation of ether (nucleophile: OR-)
R&C: Na in excess alcohol (form R’O- Na+), heat
Type of rxn: nucleophilic sub n
Name reagent, condition and type of reaction of formation of nitriles (nucleophile: CN-). What to note about this reaction?
R&C: KCN in ethanol/ alcoholic KCN, heat UNDER reflux
Type of rxn: nucleophilic sub n
NOTE: impt rxn, since no of C atoms increases (compared to another rxn)
What are two applications of nucleophilic sub n rxn w CN- as Nu?
- increase no of C atoms in chain (1 C for each sub n of X w CN) (step-up rxn)
- resulting nitrile pdt, RCN can be hydrolysed form carboxylic acid or reduced to form amines
Name general equations for reactions of nitriles, reagents and conditions
Hydrolysis
i. Acidic
RCN + 2H2O + H+ —> RCOOH + NH4+
R&C: HCl(aq) or H2SO4(aq), heat under reflux
ii. Basic
RCN + H2O + OH- —> RCOO- + NH3
R&C: NaOH(aq) or KOH(aq), heat under reflux
Reduction
iii. Reduct n
RCN + 4[H] —> RCH2NH2
R&C: LiAlH4
RCN + 2H2 —> RCH2NH2
R&C: H2 w Ni catalyst, heat
OR
H2 w Pd or Pt catalyst
Name reagent, condition, type of reaction of formation of amine from RX (Nu: NH3)
R&C: Excess conc NH3 in ethanol, heat in sealed tube
Type of rxn: Nucleophilic sub
*In excess RX, mixture of amines forms (RNH2, R2NH, R3N)
Name reagent condition, type of reaction of elimination of RX
R&C: KOH/NaOH in ethanol, heat
Type of rxn: Elimination
*major pdt is alkene w greatest no of R grp attached to C=C (Saytzeff’s rule)
Describe a chemical test for halogenoalkanes
- Heat compound w aq NaOH followed by add n dilute HNO3
- Add AgNO3(aq)
Observat n:
i. white ppt AgCl formed, RCl is present
ii. cream ppt AgBr formed, RBr is present
iii. yellow ppt AgI formed, RI is present
Eqn:
RX + NaOH –> ROH + NaX (nucleophilic sub)
Ag+ (aq) + X-(aq) –> AgX (s) (precipitat n)
In the chemical test for halogenoalkanes why:
1. is NaOH(aq) added in first step?
2. should mix be heated?
3. is HNO3 added and not other acids?
- to provide OH- (aq) as nucleophile to sub RX form X-
- provide enough energy to break C-X bond in R-X
- b4 add AgNO3, HNO3 is added to neutralise excess OH- in 1st step, prevent form n AgOH which is white ppt.
HCl(aq) & H2SO4(aq) should not be added bcos AgNO3 form white ppt w AgCl & Ag2SO4
Name reagents, conditions and type of reaction for preparation of halogenoarenes
R&C: Cl2(g), ANHYDROUS AlCl3
type of rxn: electrophilic sub
Why is there difference in reactivity between halogenoarenes and halogenoalkanes?
- halogenoarenes hv shorter & stronger C-X bond
- lp of e- on halogen atom is delocalised into benzene ring
=> strengthens C-X bond in halogenoarenes due to presence of partial double bond character - π electron cloud of benzene ring repel lp of e- of incoming nucleophile, rendering nucleophile’s attack difficult
Why do halogenoalkenes not undergo nucleophilic substitution?
lp on halogen atom delocalised w adjacent C=C bond.
=> strengthens C-X bond in halogenoalkene due to presence of partial double bond character, so nucleophilic sub no occur under normal condit n
Name reagents, conditions and observations for hydrolysis of CH3CH2Cl
Explain reactivity of CH3CH2Cl
R&C: NaOH(aq), heat, followed by AgNO3(aq)
Observat n:
- on heating w NaOH(aq), followed by dilute HNO3, then aq AgNO3, white ppt AgCl is seen
bcos,
- oni 1 electro-ve chlorine atom bonded to alkyl C atom
- so, alkyl C atom less e- deficient
- less susceptible to nucleophilic sub vs acyl chloride
Name reagents, conditions and observations for hydrolysis of chlorobenzene
Explain reactivity of chlorobenzene
R&C: NaOH (aq), prolonged heating, followed by AgNO3(aq)
observat n:
- no white ppt seen even on prolonged boiling w NaOH(aq) & add n of AgNO3 (aq)
bcos,
- lp of e- on Cl atom delocalised into benzene ring
- strengthen C-X bond bcos presence of partial double bond character
- nucleophilic sub no occur under normal condit n
Name reagents, conditions and observations for hydrolysis of acyl chloride [ CH3C=O(Cl) ]
Explain reactivity of acyl chloride
R&C: AgNO3(aq)
observat n:
white ppt AgCl seen immediately
bcos,
- carbonyl C atom bonded to 2 electro-ve atoms O & Cl
- makes carbonyl C atom highly e- deficient
- so, carbonyl C atom vv susceptible to nucleophilic sub which occurs readily
State and explain properties of fluoroalkanes and chlorofluoroalkanes
- generally stable, unreactive due to strong carbon-fluorine (C-F) bond
- combinat n of C-F & C-Cl bonds in molecule lead to great stability & non flammability
Describe uses of fluoroalkanes & chlorofluoroalkanes (CFCs)
- refrigerant
CFCs eg CCl2F2 commonly used since easily liquefied due to low bp - aerosol propellant
CFCs r inert, non-toxic & volatile, so ideal choice - Fire extinguisher
CBr2ClF being fully halogenated is non-flammable, volatile & dense, so ideal choice
Explain the environmental concern of chlorofluoroalkanes
- decomposed by UV light, produce Cl radicals in stratosphere (damage ozone layer)
bcos,
CFC undergo homolytic fission, form Cl radical continuously under UV light
*NOTE: C-F bond not broken! So, fluorocarbons don’t harm ozone layer
Cl radical produced attack ozone molecules
=> depletion of ozone layer results in UV radiation causing skin cancer, damages to vegetat n, accelerat n of ghg effect, affecting climate change
Explain difference in rate of hydrolysis of halogenoalkanes
- due to bond energy, not bond strength
=> the lower the bond strength, the easier RX hydrolyses
