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Chemistry Y12 > O- Halogenoalkanes > Flashcards

O- Halogenoalkanes Flashcards

1
Q

What is the reactivity of halogenoalkanes compared to that of alkanes?

A

halogenoalkanes- ↑ reactive

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2
Q

What are the 3 main uses of halogenoalkanes?

A
  1. refigerants
  2. solvents
  3. pharmaceuticals
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3
Q

What are halogenoalkanes?

A

compound made of C, H & halogen

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4
Q

Give ine way of making halogenoalkanes + state conditions required

A
  • free radical substitution
  • UV light
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5
Q

What is a nucleophile?

A
  • e- (lone) pair donor
  • attracted to areas of ↓ e- density
  • :Nu
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6
Q

What is an electrophile?

A
  • e-/ lone pair acceptor
  • attracted to areas of ↑ e- density
  • E+
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7
Q

What are the 3 types of halogenoalkanes?

A
  1. primary 1°- 1 R group (end of chain)
  2. secondary 2°- 2 R groups
  3. tertiary 3°- 3 R groups (x H on central carbon)
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8
Q

Explain the trends in boiling points of halogenoalkanes

A
  1. ↑ as chain length ↑
  2. ↑ as size of halogen substituent ↑
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9
Q

What are the 3 factors influencing the boiling points of halogenoalknaes?

A
  1. chain length
  2. type of halogen substituent
  3. position of substituent (1°/ 2°/ 3°)
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10
Q

Explain whether iodopentane or iodopropane has a higher boiling point. (4)

A
  • iodopentane
  • longer C chain
  • ↑ vdW forces between molecules
  • require ↑ energy to break
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11
Q

What is the trend in boiling points as the halogen substituents go down the group?

A
  • ↑
  • although Cl has largest permanent dipole
  • large ↑ in no. of e- completely outweighs the loss of permanent dipoles in the molecules
  • effect of vdW > dipoles
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12
Q

Explain the trend in boiling points as isomers go from primary to tertiary.

A
  • primary = ↑ bp
  • molecules pack ↑ closely tgt
  • ↑ s.a.
  • ↑ vdW
  • ↑ energy to break
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13
Q

Are halogenoalkanes soluble in water?

A
  • No =(
  • dissolve = attractions betw. halogenoalkanes (vdW & dipole-dipole) + H bonds betw, H2O molecules must be broken
  • too energetically unfavourable
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14
Q

What does reactivity depend on?

A
  1. bond length (↑ down the group)
  2. bond strength (↓ down the group)
  3. molecular size (↑)
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15
Q

Explain why C-I bond is the weakest.

A

smallest electronegativity diff.

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16
Q

What happens in nucleophilic substitution (in general)?

A
  • nucleophile attacks atom w/ +ve charge
  • replaces one of the groups/ atoms
17
Q

What are the 3 nucleophiles?

A
  1. hydroxide ion :OH-
  2. cyanide ion :CN-
  3. ammonia :NH3
18
Q

Conditions for reaction of OH- with halogenoalkanes

A
  1. boiled under reflux (prevent loss of reactants upon heating)
    – condenser placed vertically (used in distillation) used to keep cool
  2. aqueous NaOH/ KOH
19
Q

Equation of reaction with hydroxide ions

A

R-X + OH- –> R-OH + X-
- alcohol
- halide ion

20
Q

Conditions of reaction with cyanide ions

A
  1. boiled under reflux
  2. KCN
  3. aq. ethanol
21
Q

Equation for reaction with cyanide ions

A

R-X + CN- –> R-CN + X-
- forms nitrile (C三N)- extending C chain (when at end)!
- eg. propanenitrile
+1×C– eg. Propane–>butane

22
Q

Conditions for reaction with ammonia

A
  1. heated w/ ethanolic ammonia
  2. in a sealed tube
23
Q

Equation for reaction with ammonia

A

R-X + 2NH3 –> R-NH2 + NH4X
- primary amine formed
2 steps:
1. normal- :NH3 attacks C
2. :NH3 acts as a base–> attacks H on NH3+ (that has lost e-) connected to C

24
Q

Factors affecting rate of substitution reaction

A
  1. temp
  2. solvent
  3. halogen being substituted
  4. type of halogenoalkane- 1° vs 2° vs 3°
25
Q

Would a 1° or 2° halogenoalkane react faster?

A
  • 1°
  • C ↑ accessible
  • ↓ sterically hindered
26
Q

Conditions for elimination reaction

A
  1. heat
  2. NaOH in ethanol (solvent) x water
    – … + NaOH (ethanol)
    – OH- = base
27
Q

How do we control selectivity of substitution reactions?

A
  1. control temp
  2. control solvent
    elimination vs nucleophilic substitution:
    - temp: heat vs cold
    - solvent: ethanol vs water (aq)
28
Q

How does the type of halogenoalkane affect the rate of reaction?

A
  • primary prefers nucleophilic substitution
  • secondary- both
  • tertiary- elimination

primary usually longer (straight chains) ∴ the long one

29
Q

Reactivity of CFC’s (chlorofluorocarbons)

A

very unreactive

30
Q

Uses of CFC’s

A
  • short chain- gases –> 1. aerosol propellants 2. refrigerants 3. blowing agents for foams (eg. expanded polystyrene)
  • long chain- 1. dry cleaning 2. de-greasing solvents
31
Q

Why are CFC’s harmful for the environment?

A
  • decompose in the atmosphere
  • release Cl atoms–> react w/ ozone
  • holes in the ozone layer (ozone depletion)
32
Q

Equations for reaction of chlorine with ozone

A
  1. heterolytic fission (2 diff. atoms)- F2Cl2C –> F3C* + *Cl
  2. *Cl + O3 –> *ClO + O2
  3. ClO + O3 –> **Cl + 2O2 (Cl radical regenerated)
  • 2 moles of ozone reacted in total
33
Q

Who introduced CFC’s?

A

Thomas Midgley

Chemistry Y12 (33 decks)

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