O- Halogenoalkanes Flashcards
What is the reactivity of halogenoalkanes compared to that of alkanes?
halogenoalkanes- ↑ reactive
What are the 3 main uses of halogenoalkanes?
- refigerants
- solvents
- pharmaceuticals
What are halogenoalkanes?
compound made of C, H & halogen
Give ine way of making halogenoalkanes + state conditions required
- free radical substitution
- UV light
What is a nucleophile?
- e- (lone) pair donor
- attracted to areas of ↓ e- density
- :Nu
What is an electrophile?
- e-/ lone pair acceptor
- attracted to areas of ↑ e- density
- E+
What are the 3 types of halogenoalkanes?
- primary 1°- 1 R group (end of chain)
- secondary 2°- 2 R groups
- tertiary 3°- 3 R groups (x H on central carbon)
Explain the trends in boiling points of halogenoalkanes
- ↑ as chain length ↑
- ↑ as size of halogen substituent ↑
What are the 3 factors influencing the boiling points of halogenoalknaes?
- chain length
- type of halogen substituent
- position of substituent (1°/ 2°/ 3°)
Explain whether iodopentane or iodopropane has a higher boiling point. (4)
- iodopentane
- longer C chain
- ↑ vdW forces between molecules
- require ↑ energy to break
What is the trend in boiling points as the halogen substituents go down the group?
- ↑
- 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
Explain the trend in boiling points as isomers go from primary to tertiary.
- primary = ↑ bp
- molecules pack ↑ closely tgt
- ↑ s.a.
- ↑ vdW
- ↑ energy to break
Are halogenoalkanes soluble in water?
- No =(
- dissolve = attractions betw. halogenoalkanes (vdW & dipole-dipole) + H bonds betw, H2O molecules must be broken
- too energetically unfavourable
What does reactivity depend on?
- bond length (↑ down the group)
- bond strength (↓ down the group)
- molecular size (↑)
Explain why C-I bond is the weakest.
smallest electronegativity diff.
What happens in nucleophilic substitution (in general)?
- nucleophile attacks atom w/ +ve charge
- replaces one of the groups/ atoms
What are the 3 nucleophiles?
- hydroxide ion :OH-
- cyanide ion :CN-
- ammonia :NH3
Conditions for reaction of OH- with halogenoalkanes
-
boiled under reflux (prevent loss of reactants upon heating)
– condenser placed vertically (used in distillation) used to keep cool - aqueous NaOH/ KOH
Equation of reaction with hydroxide ions
R-X + OH- –> R-OH + X-
- alcohol
- halide ion
Conditions of reaction with cyanide ions
- boiled under reflux
- KCN
- aq. ethanol
Equation for reaction with cyanide ions
R-X + CN- –> R-CN + X-
- forms nitrile (C三N)- extending C chain (when at end)!
- eg. propanenitrile
+1×C– eg. Propane–>butane
Conditions for reaction with ammonia
- heated w/ ethanolic ammonia
- in a sealed tube
Equation for reaction with ammonia
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
Factors affecting rate of substitution reaction
- temp
- solvent
- halogen being substituted
- type of halogenoalkane- 1° vs 2° vs 3°
Would a 1° or 2° halogenoalkane react faster?
- 1°
- C ↑ accessible
- ↓ sterically hindered
Conditions for elimination reaction
- heat
-
NaOH in ethanol (solvent) x water
– … + NaOH (ethanol)
– OH- = base
How do we control selectivity of substitution reactions?
- control temp
- control solvent
elimination vs nucleophilic substitution:
- temp: heat vs cold
- solvent: ethanol vs water (aq)
How does the type of halogenoalkane affect the rate of reaction?
- primary prefers nucleophilic substitution
- secondary- both
- tertiary- elimination
primary usually longer (straight chains) ∴ the long one
Reactivity of CFC’s (chlorofluorocarbons)
very unreactive
Uses of CFC’s
- 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
Why are CFC’s harmful for the environment?
- decompose in the atmosphere
- release Cl atoms–> react w/ ozone
- holes in the ozone layer (ozone depletion)
Equations for reaction of chlorine with ozone
- heterolytic fission (2 diff. atoms)- F2Cl2C –> F3C* + *Cl
- *Cl + O3 –> *ClO + O2
- ClO + O3 –> **Cl + 2O2 (Cl radical regenerated)
- 2 moles of ozone reacted in total
Who introduced CFC’s?
Thomas Midgley
