M4 Haloalkanes Flashcards
What is a primary haloalkane?
When the halogen is joined to a carbon that is joined to one other carbon atom only
What is a secondary haloalkane?
When the halogen is joined to a carbon atom that is joined to two other carbon atoms
What is a tertiary haloalkane?
When the halogen is joined to a carbon atom that is attached to three other carbon atoms
Describe the carbon-halogen bond and the reactivity of haloalkanes
- Haloalkanes have a carbon-halogen bond in their structure.
- Halogen atoms are more electronegative than carbon atoms.
- The electron pair in the carbon-halogen bond is therefore closer to the halogen atom than the carbon atom.
- The carbon-halogen bond is polar.
- Therefore the carbon atom has a slightly positive charge and can attract species containing a lone pair of electrons (nucleophile).
- When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction. A new compound is produced with a different functional group. This is known as nucleophilic substitution.
What is a nucleophile?
A nucleophile is an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond.
(donates an electron pair)
What are examples of nucleophiles?
Hydroxide ions :OH-
Water molecules H2O:
Ammonia molecules :NH3
What is hydrolysis?
- Hydrolysis is a chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule.
- This results in the molecule being split into two products.
- In hydrolysis if a haloalkane, the halogen atom is replaced by an -OH group.
Describe a hydrolysis reaction with nucleophilic substitution mechanism
- The nucleophile (OH-), approaches the carbon atom attached to the halogen on the opposite side of the molecule from the hydrogen atom.
- The direction of attack by the OH- ion minimises repulsion between the nucleophile and the δ- halogen atom.
- A lone pair of electrons on the hydroxide ion is attracted and donated to the δ+ carbon atom.
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom.
- The carbon-halogen bond breaks by heterolytic fission.
- The new organic product is an alcohol. A halide ion is also formed.
How can haloalkanes be converted to alcohols?
- Haloalkanes can be converted to alcohols using aqueous sodium hydroxide.
- The reaction is very slow at room temperature so the mixture is heated under reflux to obtain a good yield of product.
eg. 1-bromobutane + NaOH —> butan-1-ol + NaBr
How does the rate of hydrolysis depend on the strength of the carbon-halogen bond?
- In hydrolysis the carbon-halogen bond is broken and the -OH group replaces the halogen in the haloalkane.
- The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane.
- C-F bond is the strongest carbon-halogen bond and C-I is the weakest halogen bond (bond strength decreases down the group). Less energy is required to break the C-I bond than the other carbon-halogen bonds.
- Therefore iodoalkanes react faster than bromoalkanes, bromoalkanes react faster than chloroalkanes and fluoroalkanes are unreactive as a large amount of energy is required to break the C-F bond.
How do you measure the rate of hydrolysis of primary haloalkanes?
Comparing the rate of hydrolysis of: 1-chlorobutane, 1-bromobutane and 1-iodobutane:
- The general equation for hydrolysis of any of these haloalkanes with water is:
haloalkane + water —> alcohol + H+ + X-
- The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate. As the reaction takes place halide ions (X- (ag)), are produced which react with Ag+ (aq) ions to form a precipitate of the silver halide.
Ag+ (aq) + X- (aq) —> AgX (s) - precipitate of silver halide
- The nucleophile in the reaction is water, which is present in the aqueous silver nitrate. Haloalkanes are insoluble in water, and the reaction is carried out in the presence of an ethanol solvent. Ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers.
Describe the experiment for the hydrolysis of haloalkanes
- Set up three test tubes adding 1cm3 of ethanol in each, and two drops of 1-chlorobutane/1-bromobutane/1-iodobutane in each.
- Stand the test tubes in a water bath at 60°C.
- Place a test tube containing 0.1mol dm-3 silver nitrate in the water bath and allow all tubes to reach a constant temperature.
- Add 1cm3 of the silver nitrate quickly to each of the test tubes and immediately start a stop-clock.
- Observe the test tubes for 5 minutes and record the time taken for the precipitate to form.
Observations:
• 1-chlorobutane: a white precipitate forms very slowly
• 1-bromobutane: a cream precipitate forms slower than 1-iodobutane but faster than with 1-chlorobutane
• 1-iodobutane: a yellow precipitate forms rapidly
- These observations are explained by considering the bond enthalpies of the carbon-halogen bonds. The compound with the slowest rate of reaction has the strongest carbon-halogen bond.
- Rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
How do primary, secondary and tertiary haloalkanes affect the rate of hydrolysis?
- Tertiary haloalkanes are hydrolysed most quickly, while hydrolysis of primary haloalkanes are slowest.
- This is because primary haloalkanes react by a one-step mechanism, whereas a tertiary haloalkanes reacts by a two-step mechanism.
- In the first step, the carbon-halogen bond of the tertiary haloalkane breaks by heterolytic fission, forming a tertiary carbocation and a halide ion.
- In the second step, a hydroxide ions attacks the carbocation to form the organic product.
- The increased rate and difference can be explained by the increased stability of the tertiary compound compared to that of the primary carbocation.
What are the uses of organohalogen compounds?
- Organohalogen compounds are molecules that contain at least one halogen atom joined to a carbon chain.
- They have many uses such as general solvents, dry cleaning solvents, making polymers, flame retardants, refrigerants and in many pesticides.
- Organohalogen compounds are rarely found in nature and, as they are not broken down naturally in the environment, have become the focus of some concern.
What is the ozone layer?
- The ozone layer is found at the outer edge of the stratosphere, at a height that varies from 10-40km above the Earth’s surface.
- Only a tiny fraction of gases making up the ozone layer is ozone, but that is enough to absorb most of the biologically damaging UV radiation from the Sun’s rays, allowing only a small amount to reach the Earth’s surface.
- UV radiation is commonly linked with sunburn and has other harmful effects.
- It is feared that continued depletion of the ozone layer will allow more UV radiation to reach Earth’s surface. This may lead to increased genetic damage and greater risk of skin cancer.
