4C - Haloalkanes

Question 1

Mechanism for the hydrolysis of bromoethane?

Answer 1

Question 2

What are CFCs and HCFCs used?

Answer 2

Industry e.g. for aerosoles and refridgerators

Question 3

Apart from CFCs and HCFCs what else contributes to the depletion of the ozone layer

Answer 3

Nitrogen monoxide (NO) which naturally forms during lightning.

Question 4

What type of bond fission is the hydrolysis of a haloalkane?

Answer 4

Heterolytic fission

Question 5

Equations for how the ozone layer is formed?

Answer 5

O₂ → O• + O•

O₂ + O• → O₃

Question 6

Define electronegativity

Answer 6

A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond

Question 7

Regarding the depletion of the ozone layer due to CFCs. What role does the Cl radical take in this reaction?

Answer 7

It’s a reactive intermediate

Question 8

Why do iodoalkanes have the fastest rate of hydrolysis?

Answer 8

They contain the most number of sheilding electrons, decreases the nuclear attraction and the energy required to break the C-I bond is the least

Question 9

Give 3 examples of nucleophiles

Answer 9

H₂O

OH^-

NH₃

Question 10

Write an equation to show the photodissociation of CFCs in the stratosphere.

Conditions required?

What type of bond fission is this?

Answer 10

CF₂Cl₂ → CFCl• + Cl•

Condition = UV light

Homolytic fission

Question 11

Equation to show the depletion of the ozone layer

Answer 11

O₃ → O₂ + O•

Question 12

What does a curly arrow show in a mechanism?

Answer 12

The movement of an electron pair

Question 13

Out of radical substitution, electrophilic addition and nucleophilic substitution:

Which has the greated atom economy?

Answer 13

Nucleophilic substitution = 100% atom economy (produces only one product.

• Radical substitution - Too many possible isomers and byproducts.
• Electrophilic addition - Produce isomers.

Question 14

What does it stand for?

CFCs and HCFCs?

Answer 14

CFCs = Chlorofluorocarbons

HCFCs = Hydrochlorofluorocarbons

Question 15

3 Ways of synthesising a haloalkane?

Answer 15

1. Free radical substitution - Alkane + Halogen
2. Electrophilic addition - Alkene + Halogen or Alkene + Hydrogenhalide
3. Nucleophilic substitution - Alcohol + hydrogenhalide

Question 16

Write equations showing how NO contributes to the depletion of the ozone layer.

Overall equation?

Answer 16

Propagation:

NO• + O₃ → NO₂• + O₂

NO₂• + O → NO• + O₂

Overall equation:

O₃ + O → 2O₂

Question 17

The rates of hydrolysis of chloroethane, bromoethane and iodoethane are different.

• Describe how you would monitor the reaction rates.
• Explain why the different haloalkanes react at different rates.

(8 marks)

Answer 17

• Add 3 drops of each haloalkane into a test tube. Lable them accordingly.
• Add 1cm³ of ethanol to each of these 3 test tubes. The ethanol acts as a co-solvent.
• Get 3 new test tubes. Add 1cm³ of aqueous AgNO₃ to each of them. Place these 3 test tubes into a hot water bath. Hot water bath ensures that a constant temperature is maintained throughout experiment.
• Add one test tube of the aqueous AgNO₃ to each of the test tubes containing the haloalkanes and ethanol.
• Place the 3 test tubes into the hot water bath. Heat from hot water bath will provide heat to speed up the rate of the reaction.
• Use stop watch to time how long it takes for a coloured precipitate to form (this is the silver halide precipitate).

-Equation for hydrolysis (X = halogen):

CH₃CH₂X + H₂O → CH₃CH₂OH + X^-

-Equation for precipitation reaction:

Ag⁺ + X^- → AgX

Iodoethane reacted the fasted, chloroethane reacted the slowest. The strength of the C-X bond affects the reactivity of the haloalkane. C-I bond is the weakest of the 3 C-X bonds and so iodoethane had the fastest rate of reaction.

Question 18

What does the ozone layer protect us from?

Answer 18

UV radiation - over exposure increases chance of developing skin cancer

Question 19

Define nucleophile

Answer 19

An atom (or group of atoms) which is attracted to an electron deficient center or atom, where it donates a pair of electrons to form a new covalent bond.

Question 20

Where is the ozone layer found?

Answer 20

Stratosphere

Question 21

Preperation of an alchohol by hydrolysis of a haloalkane:

• Name reagent used
• Conditions?

Answer 21

Reagent = Aqueous NaOH

Conditions = Heat under reflux

Question 22

Radical mechanism for the depletion of the ozone layer due to CFCs

Conditions?

Overall equation?

Answer 22

Conditions = UV light

Initiation:

CF2Cl2 → CFCl• + Cl•

Propagation:

Cl• + O₃ → ClO• + O₂

ClO• + O → Cl• + O₂

Overall equation:

O₃ + O → 2O₂

Question 23

Why do organic reactions often require heating under reflux?

Answer 23

Organic compounds have a simple molecular structure so they ahve low boiling points. In order to keep the contents of the flask from boiling dry you need to heat the mixture under reflux.

Question 24

Order of rate of hydrolysis of chloro-, bromo- and iodoalkanes?

Answer 24

Iodoalkanes = Fastest

Bromoalkanes

Chloroalkanes = Slowest

Question 25

Why do you think haloalkanes are generally more reactive than alkanes?

Why is this not the case for fluoroalkanes?

Answer 25

Carbon-Halogen bond is weaker than the C-H bond which means that they require less energy to break so they can react more easily.

C-F bond is stronger than the C-H bond so the C-F bond requires more energy to break, therefore fluoroalkanes are less reactive than alkanes.

Question 26

In CFCs why does the C-Cl bond break instead of the C-F bond?

Answer 26

The C-F bond is stronger than the C-Cl bond