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chemistry > Halogen Derivatives > Flashcards

Halogen Derivatives Flashcards

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1
Q

Physical properties of halogen compounds

A

Structure: Simple molecular structure, polar molecule
1. Higher MP and BP than alkanes with similar Mr (pdpd>idid)
2. BP increases down the group
(electron cloud size increases)
3. insoluble in water (pdpd between H2O and C-X molecule < pdpd between C-X molecules + hydrogen bonds between H2O molecules)
4. Soluble in organic solvents (idid between organic solvent and C-X molecule > idid between organic solvent molecules and pdpd between C-X molecules)

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2
Q

Explain why iodoalkanes have the highest boiling point compared to
bromoalkanes, chloroalkanes and fluoroalkanes with the same alkyl chain.

A
  • Iodoalkanes have the largest electron cloud size.
  • The electron cloud of the iodoalkanes is the most easily polarised.
  • The iodoalkane contains stronger idid interactions between their molecules.
  • Hence, more energy is needed to overcome the interactions in iodoalkane
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3
Q

Synthesis of halogenoalkanes

A
  1. FRS of alkanes
  2. Electrophilic addition of alkenes
  3. Nucleophilic substitution of alcohols
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4
Q

Electrophilic addition of alkenes

A

Dry HCl/ HBr/ HI(g), room temperature
OR
Cl2 in CCl4 / Br2 in CCl4, dark room, room temperature

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5
Q

Nucleophilic addition of alcohols

A
  1. PCl5, room temperature
  2. PCL3, heat under reflux
  3. SOCl2, heat under reflux
  4. Concentrated HCl, Zn catalyst, heat under reflux
  5. PBr3, heat under reflux
  6. NaBr3(s)/KBr(s), concentrated H2SO4, heat under reflux
  7. PI3, heat under reflux
  8. NaI(s)/KI(s), concentrated H2SO4, heat under reflux
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6
Q

Nucleophilic substitution of halogenoalkanes ( formation of alcohol)

A

NaOH/KOH(aq), heat under reflux

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7
Q

Nucleophilic substitution of halogenoalkanes (formation of nitriles)

A

Ethanolic NaCN/KCN, heat under reflux
(one of the two step-up reactions)

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8
Q

Nucleophilic substitution of halogenoalkanes ( formation of amines)

A

Mono-sub: Limited RX groups, excess concentrated NH3 in ethanol, heat in sealed tube
Multi-sub: Excess RX group, limited concentrated NH3 in ethanol, heat in sealed tube

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9
Q

Elimination to form alkenes

A

Ethanolic NaOH/KOH, heat under reflux (synthesis of alkenes)
* must have at least one H beside the C-X bond

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10
Q

Reaction of nitriles

A
  1. Hydrolysis
  2. Reduction
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11
Q

Hydrolysis of nitriles and its products

A

Acidic hydrolysis:
H2SO4/HCl(aq), heat under reflux
Product: CN–> COOH + NH4+

Basic hydrolysis:
NaOH(aq), heat under reflux
Product: CN–> COO- +NH3

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12
Q

Reduction of nitriles

A

LiAlH4 in dry ether or H2 gas, Ni catalyst heat
Product: amine

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13
Q

SN2 mechanism

A
  • primary RX/ secondary RX
  • one-step reaction
  • Factor: steric hindrance
  • Rate equation: k[RX][Nu]
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14
Q

Why primary RX undergoes SN2?
(Steric hindrance)

A
  • primary RX has less bulky alkyl groups bonded which will less likely hinder the nucleophile’s approach, thus facing less steric hindrance
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15
Q

SN1 mechanism

A
  • tertiary RX/ secondary RX
  • two-step reaction
  • Factor: Stability of carbocation
  • Rate equation: K[RX]
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16
Q

Why tertiary RX undergoes SN1?

A
  • tertiary RX has more e-donating alkyl groups
  • positive charge on carbocation dispersed to a larger extent
  • 3 degree carbocation more stable
17
Q

Why halogenoarenes do not undergo nucleophilic substitution?

A
  1. Partial double bond character of the C-X bond
    - The p-orbital of the halogen atom can overlap with the pi electron cloud of the benzene ring.
    - The lone pair of electrons on Cl can be delocalised into the benzene ring.
    - This results in the C–X bond having a partial double bond character which makes the C–X bond stronger and harder to break.
  2. Interelectronic repulsion of nucleophile
    - Nucleophiles are electron-rich species which will face interelectronic repulsion with the pi electron cloud of the benzene ring.
    - Benzene provides high steric hindrance.
18
Q

Distinguishing test for for RX

A
  1. Add NaOH(aq) and heat –> nucleophilic sub to release halide ion
  2. Add excess dilute HNO3 after cooling –> neutralise any excess base (OH-) to prevent AgOH ppt from forming
  3. Add AgNO3(aq) –> form ppt

White ppt AgCl –> chloroalkane
Cream ppt AgBr –> bromoalkane
Yellow ppt AgI –> Iodoalkane

chemistry (10 decks)

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