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La Chimie H2 > Les Dérivés d'halogène > Flashcards

Les Dérivés d'halogène Flashcards

1
Q

What is the general formula of halogenoalkanes?

A

CnH2n+1X

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2
Q

Explain physical properties of halogenoalkanes

A
  1. (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

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3
Q

Name type of rxn, reagent and condition of preparation FROM alcohols to RX

A

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
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4
Q

Define Nucleophilic substitution

A

Rxn where halogen atom in organic molecule is substituted by a nucleophile

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5
Q

What two ways does nucleophilic sub n occur through?

A

a. SN2 mechanism for primary halogenoalkanes

b. SN1 mechanism for tertiary halogenoalkanes

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6
Q

Describe mechanism for hydrolysis of bromoethane

A

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:-)

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7
Q

What does energy profile diagram of SN2 hydrolysis reaction have?

A
  • Vertical axis: Potential energy
  • Horizontal axis: Rxn pathway
  • Ea
  • dH
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8
Q

Explain why the optical isomerism of product is reversed for SN2?

A

Nucleophile attack C atom fr side opp to halogen atom. If C is chiral, pdt wld hv inversion of config as compared to rxt

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9
Q

Explain steric effect/hindrance

A
  • 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
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10
Q

Describe mechanism for hydrolysis of 2-bromo-2-methylpropane (HINT: tertiary RX)

A

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

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11
Q

Explain why a racemic mixture forms when aa halogenoalkane undergoing SN1 is chiral?

A
  • 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
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12
Q

Explain why a tertiary RX undergoes SN1 but primary RX does not

A
  • 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)
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13
Q

What is one exception to steric consideration?

A

(CH3)3CCH2Cl
- despite being primary halide, SN1 mechanism favoured instead of SN2 due to steric hindrance of bulky -C(CH3)3 grp

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14
Q

Name one exception due to electronic config

A

(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

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15
Q

What is the order of reactivity of halogenoalkanes towards nucleophilic sub n?

A

R-I > R-Br > R-Cl > R-F

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16
Q

Name reagent, condition and type of reaction of hydrolysis to form alcohol (nucleophile: OH-)

A

R&C: NaOH (aq) or KOH (aq), heat
*aq! bcos another rxn w ethanol as solvent

Type of rxn: nucleophilic sub n

17
Q

Name reagent, condition and type of reaction of formation of ether (nucleophile: OR-)

A

R&C: Na in excess alcohol (form R’O- Na+), heat

Type of rxn: nucleophilic sub n

18
Q

Name reagent, condition and type of reaction of formation of nitriles (nucleophile: CN-). What to note about this reaction?

A

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)

19
Q

What are two applications of nucleophilic sub n rxn w CN- as Nu?

A
  • 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
20
Q

Name general equations for reactions of nitriles, reagents and conditions

A

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

21
Q

Name reagent, condition, type of reaction of formation of amine from RX (Nu: NH3)

A

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)

22
Q

Name reagent condition, type of reaction of elimination of RX

A

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)

23
Q

Describe a chemical test for halogenoalkanes

A
  1. Heat compound w aq NaOH followed by add n dilute HNO3
  2. 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)

24
Q

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?

A
  1. to provide OH- (aq) as nucleophile to sub RX form X-
  2. provide enough energy to break C-X bond in R-X
  3. 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
25
Q

Name reagents, conditions and type of reaction for preparation of halogenoarenes

A

R&C: Cl2(g), ANHYDROUS AlCl3
type of rxn: electrophilic sub

26
Q

Why is there difference in reactivity between halogenoarenes and halogenoalkanes?

A
  • 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
27
Q

Why do halogenoalkenes not undergo nucleophilic substitution?

A

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

28
Q

Name reagents, conditions and observations for hydrolysis of CH3CH2Cl
Explain reactivity of CH3CH2Cl

A

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

29
Q

Name reagents, conditions and observations for hydrolysis of chlorobenzene
Explain reactivity of chlorobenzene

A

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

30
Q

Name reagents, conditions and observations for hydrolysis of acyl chloride [ CH3C=O(Cl) ]
Explain reactivity of acyl chloride

A

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

31
Q

State and explain properties of fluoroalkanes and chlorofluoroalkanes

A
  • 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
32
Q

Describe uses of fluoroalkanes & chlorofluoroalkanes (CFCs)

A
  • 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
33
Q

Explain the environmental concern of chlorofluoroalkanes

A
  • 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

34
Q

Explain difference in rate of hydrolysis of halogenoalkanes

A
  • due to bond energy, not bond strength
    => the lower the bond strength, the easier RX hydrolyses

La Chimie H2 (25 decks)

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