C15- Haloalkanes

Question 1

What are haloalkanes?

Answer 1

Compounds containing carbon, hydrogen and at least one halogen.
Formed when one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine).

Question 2

What is the reactivity of haloalkanes?

Answer 2

Haloalkanes have a carbon-halogen bond.
- As halogen atoms are more electronegative, the electron pair in the bond is closer to the halogen atom.
The bond is therefore polar.
So haloalkanes are reactive, except fluorine.
- the carbon atom is delta positive and can attract species containing a lone pair of electrons. (nucleophiles)

Question 3

How do you name haloalkanes?

Answer 3

• prefix is added to the name of the longest chain to indicate the identity of the halogen.
• when 2+ halogens are present, they are listed in alphabetical order.
• carbons are numbered based on which one is most substituted.

Question 4

What is the rate of hydrolysis of haloalkanes? What does it depend on and why?

Answer 4

Depends on the strength of the carbon-halogen bond.

• for a reaction to happen the C-X bond must be broken. The weaker the bond, the less energy required and so the rate is faster.
• as you go down group 7 the bond enthalpies (strength) of the halogens decreases = reactivity increases.
• however fluoroalkanes are unreactive due to the large amount of energy needed to overcome the bond.

Question 5

What are nucleophiles? With examples.

Answer 5

Species that donate a lone pair of electrons.

• It is an atom/group of atoms that is attracted to an electron deficient C atom, where it donates a pair to form a new covalent bond.
• hydroxide ions :OH-
• water molecules H2O:
• ammonia molecules :NH3

Question 6

What is nucleophilic substitution?

Answer 6

A reaction in which the nucleophile replaces the halogen.

A new compound containing a different functional group is produced.

Question 7

What is hydrolysis?

Answer 7

A chemical reaction involving water/ an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule, resulting in the formation of two products.

Question 8

What is the difference between primary, secondary, and tertiary haloalkanes?

Answer 8

1. Primary:
   - carbon that carries halogen atom is only attached to one other alkyl group.
   Methyl halides = primary even though there is no other alkyl group attached to that C.
2. Secondary:
   - carbon attached directly to 2 other alkyl groups. Can be same or different.
3. Tertiary:
   - carbon atom attached directly to 3 other alkyl groups.

Question 9

Explain the general mechanism for the hydrolysis of a haloalkane.

Answer 9

1. Nucleophile approaches the C atom attached to the halogen from the opposite side of the halogen.
2. This direction of attack minimises repulsion nucleophile and the delta - halogen atom.
3. Lone pair of electron on nucleophile is attracted and donated to delta + C atom.
4. A bond is formed between the C and the atom of the nucleophile which contained the lone pair.
5. C-X bond broken by heterolytic fission.
6. Alcohol and halide ion formed.

Question 10

How can haloalkanes be converted to alcohols?

Answer 10

Using aqueous sodium hydroxide.

- reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good yeild of product.

Question 11

What is the general equation for the hydrolysis of haloalkanes with water?

Answer 11

CH3CH2CH2CH2X + H2O = CH3CH2CH2CH2OH + (H+) + (X-)

X= halogen.

Question 12

Explain how you can measure the rate of hydrolysis of haloalkanes?

Answer 12

By carrying out the reaction in presence of aqueous silver nitrate.
- as the reaction takes place halide ions (X-), are produced which react with Ag+ ions to form a precipitate of the silver halide.
Ag+ + X- = AgX
- nucleophile= water (present in aq silver nitrate).
- haloalkanes = insoluble in water, reaction carried out in presence of ethanol solvent.
- ethanol allows water and haloalkane to mix and produce single solution instead of two layer.
1-chlorobutane=white precipitate formed slowly.
1-bromobutane=cream precipitate formed faster than ^.
1- iodobutane=yellow precipitate formed rapidly.

Question 13

Out of tertiary and primary haloalkanes, which hydrolyses faster and why?

Answer 13

Tertiary haloalkanes.
Primary react by a one step mechanism. Forms primary carbocation (unstable)
Tertiary have a 2 step mechanism:
1. C-X bond of tertiary haloalkane breaks by heterolytic fission, forming tertiary carbocation(very stable) and halide ion.
2. Hydroxide ion attacks carbocation to form the organic product.

Question 14

What are organohalogen compounds and their uses?

Answer 14

Molecules that contain at least one halogen atom joined to a carbon chain. Rarely found in nature as they aren’t broken down naturally in the environment.

• general solvents.
• dry cleaning solvents.
• making polymers.
• flame retardants.
• refrigerants.
• pesticides.

Question 15

What is the ozone layer?

Answer 15

• found at the edge of the stratosphere.
• only a small fraction of its gases is ozone, but this is enough to absorb most biologically damaging uv radiation (UV-B) from the sun’s rays.

Question 16

Why must UV-B be removed and what is the role of ozone in this?

Answer 16

Ozone absorbes the uv radiation.
-continued depletion of the ozone layer will allow more uv-b to reach the earth’s surface. Leading to increased genetic damage and cancer.
In the stratosphere, ozone is continuously being formed/broken down by UV radiation:
1. Very high energy UV breaks oxygen molecules into O radicals.
2. (Steady state) involves O2 and O radicals. Ozone forms and then breaks down. Rate of formation = rate of breaking down.

Question 17

What are the properties of chlorofluorocarbons?

Answer 17

-very stable due to strength of the C-X bond.
- highly volatile.
- non-toxic.
- unreactive.
Were used as refrigerants, in air-conditioning, as aerosol propellants.

Question 18

What happens when CFCs reach the stratosphere?

Answer 18

They are no longer as stable.

• they begin to break down, forming chlorine radicals.
• these radicals catalyse the breakdown of the ozone layer

Question 19

Explain how CFCs deplete the ozone layer.

Answer 19

• due to the strength of the c-x bond they have long residence time in troposphere.
• once in the stratosphere, uv radiation provides sufficient energy to break the c-x bond by homolytic fission to form radicals.
• C-Cl bond breaks as it has lowest bond enthalpy.
• as radiation initiates the breakdown, process is called photodissociation.

Question 20

What is the mechanism for the depletion of the ozone layer by CFCs?

Answer 20

Initiation: CF2CL2 = CF2Cl▪️+ Cl▪️
Propagation: Cl▪️ + O3 = ClO▪️ +O2
ClO▪️ + O = Cl▪️+O2
-chlorine radical formed in initiation can react with an ozone molecule, breaking it down into oxygen.
- propagation step two regenerates a Cl radical, which can attack and remove another molecule of ozone in step one. These two steps repeat in a cycle and one CFC molecule can promote the breakdown of 100000 ozone molecules.

Overall equation: O3 + O = 2O2

Question 21

What other radicals catalyse the breakdown of ozone?

Answer 21

Nitrogen oxide (NO), which are formed naturally during lightning, and also as a result of aircraft travel in the stratosphere.
They cause the break down by a similar mechanism as CFCs 
(equation in textbook)