O- Halogenoalkanes

Question 1

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

Answer 1

halogenoalkanes- ↑ reactive

Question 2

What are the 3 main uses of halogenoalkanes?

Answer 2

1. refigerants
2. solvents
3. pharmaceuticals

Question 3

What are halogenoalkanes?

Answer 3

compound made of C, H & halogen

Question 4

Give ine way of making halogenoalkanes + state conditions required

Answer 4

• free radical substitution
• UV light

Question 5

What is a nucleophile?

Answer 5

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

Question 6

What is an electrophile?

Answer 6

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

Question 7

What are the 3 types of halogenoalkanes?

Answer 7

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)

Question 8

Explain the trends in boiling points of halogenoalkanes

Answer 8

1. ↑ as chain length ↑
2. ↑ as size of halogen substituent ↑

Question 9

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

Answer 9

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

Question 10

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

Answer 10

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

Question 11

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

Answer 11

• ↑
• 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

Question 12

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

Answer 12

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

Question 13

Are halogenoalkanes soluble in water?

Answer 13

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

Question 14

What does reactivity depend on?

Answer 14

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

Question 15

Explain why C-I bond is the weakest.

Answer 15

smallest electronegativity diff.

Question 16

What happens in nucleophilic substitution (in general)?

Answer 16

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

Question 17

What are the 3 nucleophiles?

Answer 17

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

Question 18

Conditions for reaction of OH- with halogenoalkanes

Answer 18

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

Question 19

Equation of reaction with hydroxide ions

Answer 19

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

Question 20

Conditions of reaction with cyanide ions

Answer 20

1. boiled under reflux
2. KCN
3. aq. ethanol

Question 21

Equation for reaction with cyanide ions

Answer 21

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

Question 22

Conditions for reaction with ammonia

Answer 22

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

Question 23

Equation for reaction with ammonia

Answer 23

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

Question 24

Factors affecting rate of substitution reaction

Answer 24

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

Question 25

Would a 1° or 2° halogenoalkane react faster?

Answer 25

- 1° - C ↑ accessible - ↓ sterically hindered

Question 26

Conditions for elimination reaction

Answer 26

1. heat 2. **NaOH in ethano**l (solvent) *x water* -- ... + NaOH (ethanol) -- OH- = base

Question 27

How do we control selectivity of substitution reactions?

Answer 27

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

Question 28

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

Answer 28

- **primary** prefers **nucleophilic substitution** - secondary- both - tertiary- elimination | primary usually longer (straight chains) ∴ the long one

Question 29

Reactivity of CFC's (chlorofluorocarbons)

Answer 29

very unreactive

Question 30

Uses of CFC's

Answer 30

- **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

Question 31

Why are CFC's harmful for the environment?

Answer 31

- **decompose** in the atmosphere - release **Cl atoms**--> **react w/ ozone** - holes in the ozone layer (**ozone depletion**)

Question 32

Equations for reaction of chlorine with ozone

Answer 32

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

Question 33

Who introduced CFC's?

Answer 33

Thomas Midgley