Haloalkanes

Question 1

What is the general formula for haloalkanes?

Answer 1

CnH2n+1X

Question 2

Definition of nucleophilic substitution

Answer 2

Replacement (of a halogen) by a nucleophile.

Question 3

Definition of a nucleophile

Answer 3

An electron pair donor (lone pair).

Question 4

List 4 examples of nucleophiles.

Answer 4

:OH- :NH3 :CN- H2O:

Question 5

Explain the mechanism of nucleophilic substitution (3 points).

Answer 5

1. C-halogen bond is polar as the halogen has a greater electronegativity than the carbon atom.
2. This means the C atom will be attacked by a species with a lone pair of electrons to donate, these species are nucleophiles.
3. The nucleophile replcaes the halogen atom.

Question 6

Definition of hydrolysis

Answer 6

A reaction with water that breaks a chemical compound into two chemical compounds.

Question 7

What is the reagent used for thew hydrolysis of haloalkanes and why is it used?

Answer 7

NaOH(aq). Using a metal hydroxide speeds up the rate of hydrolysis.

Question 8

What is the condition for the hydrolysis of haloalkanes?

Answer 8

Reflux, in water solvent

Question 9

Draw the mechanism for the hydrolysis of of C2H5Br

Answer 9

*Refer to hand-written notes

Question 10

What do curly arrows represent in mechanisms?

Answer 10

The movement of a pair of electrons.

Question 11

Explain the mechanism of the hydrolysis of C2H5Br (3 points).

Answer 11

1. The nucleophile is the OH- ion and it attacks the delta-positive carbon.
2. The C-halogen bond breaks and the halogen atom becomes a halide ion.
3. A new covalent bond forms between the C atom and the nucleophile. The bromine is substituted by the OH.

Question 12

Write the genreral reaction equ for how the halide ion can be identified by using silver nitrate (AgNO3(aq)) in ethanol.

Answer 12

Ag+(aq) + X-(aq) …. AgX(s)

Question 13

Describe the method for finding the difference in the rate of hydrolysis (4 points).

Answer 13

1. Place a measured sample of each halogenalkane, (equal molar amounts- concentration), into separate test tubes and place in to a water bath at 50 degrees (speeds up reaction).
2. Into another test tube, place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath.
3. Once at equal temperatures, add equal volumes of the ethanol mixture and halolalkane solutions together.
4. Timehow long it takes for each precipitation to form.

Question 14

Results of hydrolysis rates reaction

Answer 14

Chloro- white, AgCl(s)
Bromo- cream, AgBr(s)
Iodo- yellow, AgI(s)

Question 15

Describe how the rate of the hydrolysis reaction can be calculated

Answer 15

1 / time taken for the precipiate to form

Question 16

Explain the reason for the different rates of the hydrolysis of haloalkanes (3 points).

Answer 16

1. As the bond enthalpy strength decreases down the group, it is easier to break the C-X bond.
2. The stronger the bond, the harder it is to break and the slower the rate of reaction will be.
3. The weaker the bond, the faster the rate will be.

Question 17

Rank the C-X bonds in terms of hydrolysis rates

Answer 17

C-I: Weakest bond, so fastest reaction as less energy to break bond.
C-Br: Reaction rate between C-I and C-Cl as bond enthalpy is higher than C-I but lower than C-Cl.
C-Cl: Strongest bond so slowest reaction.
C-X bond becomes weaker down group 7.

Question 18

List 3 useful properties of CFC’s (chlorofluorocarbons).

Answer 18

1. Non-toxic
2. Inert
3. Volatile

Question 19

What are the 2 uses of trichlorofluoromethane?

Answer 19

1. Refrigeration
2. Air conditioning

Question 20

State 2 dichlorodifluoromethane uses.

Answer 20

1. Aerosal propellants
2. Blowing agent for polystyrene

Question 21

What are the 3 uses of 1,1,1-trichloro-2,2,2-trifluoroethane?

Answer 21

1. Dry cleaning solvents
2. Degreasing agents
3. Circuits boards

Question 22

Write the reaction for how the ozone is maintained in a balance with oxygen and explain how this is established

Answer 22

O2 + O …. O3
Reversible reaction
From the breakdown of O2 by UV-radiation

Question 23

Draw the mechanism for the ozone depletion

Answer 23

*Refer to hand-written notes

Question 24

Advantages, disadvantages and uses of HFC’s (hydrofluorocarbons)

Answer 24

ADV- Non-flammable, non-toxic, ozone friendly.
DIS- Can cause greenhouse effect.
Use- Aerosal propellant, refrigeration

Question 25

Advantages, disadvantages and uses of HCFC’s (hydrochlorofluorocarbons)

Answer 25

ADV- Non-flammable, non-toxic.
DIS- Can damage ozone layer but at a slower rate than CFC’s.
Use- Refrigerant

Question 26

Advantages, disadvantages and uses of alkanes

Answer 26

ADV- Ozone friendly
DIS- Flammable
Use- Aerosal propellant