15.1 The chemistry of the haloalkanes
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What are haloalkanes
Compounds containing the elements carbon, hydrogen and at least one halogen
When naming haloalkanes…
…a prefix is added to the name of the longest chain to indicate the identity Eg. Fluoro- Chloro- Bromo- Iodo-
Haloalkanes have a carbon-halogen bond in their structure. Halogen atoms are more _______ than carbon atoms. The electron pair in the carbon-halogen bond is therefore _______ to the halogen atom than the carbon atom. The carbon-halogen bond is ______.
Electronegative Closer Polar
Carbon atoms in haloalkanes
Have a slightly positive charge and can attract species containing a lone pair of electrons
What are nucleophiles
Species that donate a lone pair of electrons An atom or group of atoms that is attracted to an electron deficient carbon atom
Examples of nucleophiles
Hydroxide ions, :OH- Water molecules, H2O: Ammonia molecules, :NH3
What happens when a haloalkane reacts with a nucleophile
The nucleophile replaces the halogen in a substitution reaction New compound is produced containing different functional group Reaction mechanism is nucleophilic substitution
What is substitution
A reaction in which one atom or group of atoms is replaced by another atom or group of atoms
What is hydrolysis
A chemical reaction involving water or an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
Steps in nucleophilic substitution Hydrolysis of haloalkane
• Nucleophile, OH- approaches carbon attached to halogen on opposite side of molecule • Direction of attack by OH- minimises repulsion between nucleophile and δ- halogen • Lone pair of electrons on OH- ion attracted and donated to δ+ carbon atom • New bond formed between oxygen atom of OH - ion and carbon atom • Carbon-halogen bond breaks by heterolytic fission • New organic product is alcohol, halide ion formed
How can haloalkanes be converted to alcohols
By using aqueous sodium hydroxide Very slow reaction at room temp. so mixture heated under reflux
What do curly arrows show
The movement of electron pairs Start from either lone pair or bonding pair of electrons and point to atom to which it moves
What happens to the carbon-halogen bond in hydrolysis
It is broken and the -OH group replaces the halogen in the haloalkane
What does the rate of hydrolysis depend on
The strength of the carbon-halogen bond in the haloalkane
Which bond is the strongest Which is the weakest C-F, C-Cl, C-Br, C-I
C-F is the strongest C-I is the weakest Less energy is needed to break C-I bond
Which reacts faster, iodoalkanes or bromoalkanes
Iodoalkanes
Which reacts faster, bromoalkanes or chloroalkanes
Bromoalkanes
Fluoroalkanes are unreactive because…
…a large quantity of energy is required to break the C-F bond
How can we measure the rate of hydrolysis of primary haloalkanes
Carry out rewaction in presence of aqueous silver nitrate Halide ions produced which react with Ag ions to form precipitate of silver halide Nucleophile in reaction is water Haloalkanes are insoluble in water and reaction carried out in presence of ethanol solvent
Why do we use ethanol when measuring the rate of hydrolysis of primary haloalkanes
It allows water and the haloalkane to mix and produce a single solution rather than two layers
Required practical detailed method Hydrolysis of haloalkanes
• Set up 3 test tubes of haloalkanes with 1cm^3 ethanol • Stand tubes in water bath at 60 degrees • Place test tube containing 0.1moldm^-3 silver nitrate in water bath, allow all to reach constant temp. • Add 1cm^3 silver nitrate quickly to each test tube • Start stop clock • Observe for 5 mins and record time taken for precipitate to form
Hydrolysis of haloalkanes - The compound with the slowest rate of reaction is the one that has the ______ carbon-halogen bond
Strongest
The rate of hydrolysis increases as the strength of the carbon-halogen bond…
…Decreases.
Which is hydrolysed the fastest, a primary haloalkane, secondary or tertiary
Primary is slowest, tertiary is fastest Primary reacts by one-step mechanism while tertiary is two-step