4.2.2 Haloalkanes Flashcards
The chemistry of the haloalkanes + The organohalogen compound in the environment
Explain how haloalkanes are named
A prefix is added to the name of the longest chain to indicate the halogen.
When 2 or more halogens are present in a structure they are listed in alphabetical order
How can haloalkanes be classified
Like alcohols, haloalkanes can be primary, secondary or teritary.
Dependant on the number of carbons attached to the C⁺ (follow markownikoff’s rule)
Describe the reactive properties of haloalkanes
Haloalkanes are:
- volatile liquids (low b.p.)
- immiscible (don’t mix) with water
Explain haloalkanes volatility and compare to alkanes
Haloalkanes have higher boiling points than alkanes because thay have permanent dipoles (𝛿⁺C - X𝛿⁻) and permanent dipole-dipole interactions are stronger than London forces. However permanent dipole-dipole interactions are still weak intermolecular forces so haloalkanes have relatively low boiling points
Explain why haloalkanes are not miscible with water
Haloalkanes are immiscible with water because they do not form hydrogen bonds
define nucleophile
an electron pair donor
list some nucleophiles
- hydroxide ions :OH⁻
- water molecules H₂O:
- ammonia molecule :NH₃
another common nucleophile is CN⁻ (cyonide)
define nucleophilic subsitution
A reaction in which a nucleophile is attracted to an electron-deficient carbon atom (C⁺), and replaces an atom or group of atoms on the carbon atom
short notes on nucleophilic substitution of haloalkanes
The halogen on a haloalkane can be replaced by another atom or group of atoms e.g. ⁻OH.
There is heterolytic fission of the C-X (carbon-halogen) bond
Define hydrolysis
Hydrolysis is a reaction with water that breaks a chemical compound into 2 compounds, the H and the OH in a water molecule becomes incorporated into the 2 compounds
It is an example of nucleophilic substitution
Explain the hydrolysis of a haloalkane
the halogen atom is replaced by an ⁻OH group (such as from NaOH → an aqueous alkali)
The C-X bond is broken by heterolytic fission
Outline the reaction mechanism for nucleophilic substitution
using ⁻OH
- The nucleophile, ⁻OH, approaches the carbon atom attached to the halogen on the opposite side of the molecule from the halogen atom.
- This 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 is also formed.
Outline how haloalkanes can be converted into alcohols
By the use of aqueous sodium hydroxide NaOH. Using hydrolysis.
The reaction is slow at room temperature so the mixture is heated under reflux to obtain a good yeild of product.
What is the rate of hydrolysis dependant on and what impact does this have
The rate of hydrolysis depends on the strength of the carbon-halogen bond in the haloalkane.
C-F bond is the strongest and the C-I bond is the weakest carbon-halogen bond.
This means that less energy is required to break the C-I bonds than other C-X bonds.
From the bond enthalpies we can predict that:
- iodoalkanes react faster than bromoalkanes
- bromoalkanes react faster than chloroalkanes
- fluoroalkanes are unreactive as a large quantity of energy is required to break the C-F bond.
what is the general equation for the hydrolysis of primary haloalkanes with water
using CH₃(CH₂)₃X
CH₃(CH₂)₃X + H₂O → CH₃(CH₂)₃OH + H⁺ + X⁻
Outline how the rate of hydrolysis of haloalkanes can be measured
The rate of each reaction can be followed by carrying out the reaction in the presence of aqueous silver nitrate (AgNO₃). As the reaction takes place halide ions, X⁻(aq), are produced which react with the Ag⁺(aq) ions to form a precipitate of the silver halide.
Ag⁺(aq) + X⁻(aq) → AgX(s)
Explain how the rate of hydrolysis of haloalkanes is be measured using AgNO₃ (aq)
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 the water and the haloalkane to mix and produce a single solution rather than 2 layers.
Describe the experiment for the hydrolysis of haloalkanes
no results
- Set up 3 test tubes:
Test tube 1: Add 1cm³ of ethanol and 2 drops of 1-chlorobutane
Test tube 2: Add 1cm³ of ethanol and 2 drops of 1-bromobutane
Test tube 3: Add 1cm³ of ethanol and 2 drops of 1-iodobutane - stand the test tubes in a water bath at 60℃.
- 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 silver nitrate to each of the test tubes. Immediately start a stop watch.
- Observe the test tubes for 5 minutes and record the time taken for the precipitate to form.
Give the results for the hydrolysis of haloalkanes experiment
From mixing ethanol, aqueous silver nitrate and an aqueous haloalkane.
The results:
- 1-chlorobutane produces a white precipitate very slowly.
- 1-bromobutane produces a cream precipitate that forms slower than with 1-iodobutane but faster than with 1-chorobutane.
- 1-iodobutane produces a yellow precipitate rapidly
Explain the results from the experiment of the hydrolysis of haloalkanes
The results from the hydrolysis of haloalkanes experiment where chloro reacts slowest and iodo reacts fastest can be explained by bond enthalpies of the C-X bond.
- 1-Chlorobutane reacts the slowest because the C-Cl bond is the strongest
- 1-iodobutane reacts the fastest as the C-I bond is the weakest
Therefore the rate of hydrolysis increases as the strength of the carbon-halogen bond decreases.
Why does rate of reaction differ within structural isomers of a haloalkane
rate can differ because of the location of the halogen-carbon bond.
tertiary haloalkanes are hydrolysed fastest whilst primary slowest.
Why is the rate of hydrolysis faster for tertiary haloalkanes
because primary haloalkanes react in a one step mechanism whereas tertiary react in a two step one. The increase rate can also be attributed to the increased stability of the tertiary carbocation compared to that of the primary.
outline the two step mechanism for tertiary haloalkanes hydrolysing
- carbon-halogen bond of the tertiary haloalkane breaks by heterolytic fission, forming a tertiary carbocation and a halide ion.
- a hydroxide ion attacks the carbocation to form the organic product.
