Halogenalkanes

Question 1

Forming Alkenes

Answer 1

Elimination

Conditions:
-Reflux
- NaOH dissolved in ethanol

:OH- acts as a base

Question 2

Forming alcohols

Answer 2

Nucleophilic substitution

Conditions:
-Reflux
- NaOH (aq)
- Propanone is a mutual solvent

:OH- reacts with alkene and the Halide ion is then released.

OR

Conditions:
- Hot water

:OH2 -> Alcohol + Hydrogen Halide

Question 3

Forming an Amine

Answer 3

Nucleophilic substitution

Conditions:
- Concentrated alcoholic ammonia solution
- Heat in sealed tube

:NH3 –> NH2 joins to carbon and HCl is formed as a by-product

Question 4

Forming a Nitrile and what is the benefit

Answer 4

Nucleophilic substitution
- Lengthens a carbon chain

Conditions:
- Reflux
- KCN dissolved in alcohol

:CN- forms Nitrile with C triple bonded to N and KCl side product

Question 5

Describe the test for a Halogenalkane

Answer 5

1. Nucleophilic substitution to release Halide ion
   - Dissolve in propanone (mutual solvent)
   - NaOH (aq)
   - Alcohol formed
2. Nitric acid added to remove excess OH- that may obscure precipitate results with Ag+.
3. Add Silver Nitrate
   Cl-: White ppt
   Br-: Cream ppt
   I-: Yellow ppt
4. Dilute ammonia solution and then concentrated ammonia solution to distinguish between 3
   - Dilute dissolved AgCl ppt
   - Concentrated dissolves AgBr ppt
   - Dilute/Concentrated both cannot dissolve AgI ppt

Question 6

How do you measure the rate of hydrolysis

Answer 6

1. Measure equal volumes of R-X into 3 seperate test tubes
2. Add propanone to each test tube as a mutual solvent
3. Add 5cm^3 of silver nitrate into 3 other test tubes.
4. Place all 6 tubes into a hot water bath for several minutes.
5. Add the AgNO3 (aq) to each R-X test tube and time how long it takes for the silver halide ppt to form.

The water acting as the solvent of the AgNO3 is a nucleophile. Water will form an alcohol whilst the halide ion being substituted from R-X will form AgX ppt. This allows us to measure the rate at which R-X bonds are hydrolysed.

Question 7

How is the rate of Nucleophilic substitution of Halogenoalkanes affect by Bond enthalpy and bond polarity

Answer 7

Bond enthalpy has a more significant impact than bond polarity does.

Rate:
Fastest Slowest
1-Iodopropane > 1-Bromopropane > 1-Chloropropane

Bond Strength
1-Chloropropane > 1-Bromopropane > 1-Iodopropane

Less energy is require to hydrolyse R-I therefore faster rate.

Chlorine is more electronegative, so it should cause the Carbon to be more delta+ and, therefore, more electron deficient and react faster with the Nucleophile. However, this is not the case because the more electronegative halide also results in a stronger bond. R-Cl, thus, has a very slow rate of hydrolysis due to the strength of R-Cl bonds.

Question 8

Describe the use of halogenoalkanes as solvents and the dangers

Answer 8

Often used as a degreasing agent

Exposure to vapours can lead to harm to the nervous system and organs such as the kidney and liver.

Question 9

Describe the use of halogenoalkanes as anaesthetics and the dangers

Answer 9

Chloroform was commonly used.

Affects heart and is also oxidised slowly by atmospehric oxygen ot form highly toxic carbonyl dichloride.
Extremely flammable

Question 10

Describe the use of halogenoalkanes as refrigerants and aerosols and the dangers

Answer 10

Chlorofluorocarbons CFCs were used as refrigerants and aerosol propellants.

Leads to ozone depletion as the CF2CL2 encounters UV radiation in the stratosphere and forms Chlorine radicals and CF2Cl radicals from the C-Cl bond homolytic fission.

Radical reacts with ozone molecule forming Oxygen molecules.

2 O3 –> 3 O2

Question 11

Alternatives to CFCs

Answer 11

Hydrochlorofluorocarbons HCFCs
- The hydrogen atom makes it much less stable in the lower atmosphere so fewer reach the ozone layer :)
- HCFCs however are a potent greenhouse gas :(

Hydrofluorocarbons HFCs
- No chlorine therefore less likely to form radicals as C-F bond is very strong :)