Chapter 15.1 - The chemistry of haloalkanes Flashcards
What is a haloalkane?
Compounds containing carbon, hydrogen, and at least one halogens
Describe the reactivity of haloalkanes
- Haloalkanes are much more reactive than alkanes
- Halogens are more electronegative than carbon atoms and so the C-Halogen bond is polar
- Causing the carbon to carbon to carry a partial positive and the halogen a partial negative charge
- So undergo two key types of reactions; nucleophilic substitution and elimination reactions
Define the term nucleophile
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
Define the term electrophile
An atom or group of atoms that is attracted to an electron-rich centre, where it accepts a pair of electrons
What is a nucleophilic substitution reaction?
A reaction in which a nucleophile is attracted to an electron-deficient carbon atom and replaces an atom or group of atoms on it
Give some examples of nucleophiles
- hydroxide ions :OH⁻
- water molecules H₂O:
- ammonia molecules :NH₃
Describe what happens in the nucleophilic substitution of haloalkanes
- The nucleophile replaces the halogen in a substitution reaction
- A new compound is produced containing a different functional group
What is hydrolysis?
A reaction with water or an aqueous solution of hydroxide that breaks a chemical compound into two compounds
Describe what happens during the hydrolysis of a haloalkane ( e.g. hydroxide and chlorine)
- The nucleophile (hydroxide) approaches the carbon atom attached to the halogen (chlorine) on the opposite side of the molecule from the halogen molecule (to minimise repulsion between the nucleophile and partially negative halogen atom)
- A lone pair on the nucleophile is attracted and donated to the partially positive positive carbon atom
- A new bond is formed between the oxygen atom of the hydroxide ion and the carbon atom
- The C-Halogen bond breaks by heterolytic fission
- An alcohol and halide is formed
Describe the conditions of the nucleophilic substitution of a haloalkane into an alcohol
- Aqueous NaOH
- Reflux
Describe the conditions of the ?? of an alcohol into a haloalkane
- NaX
- H₂SO₄
- Reflux
What factors affects the rate of hydrolysis?
- Polarity
- Bond enthalpy
Why does the bond enthalpies decrease down group 7?
- As you go down the group, the atomic radii of the atoms increase
- So the nuclear attraction between the positive nucleus and electrons decreases
- And a smaller energy supply is required to break the bonds
- Therefore, the bond enthalpies decreases
Predict the bond enthalpies of iodine, bromine, chlorine, and fluorine
I < Br < Cl < Fl
Why are fluoroalkanes unreactive?
As a large supply of energy is required to break the C-F bond
How do you measure the rate of hydrolysis?
- Set up three test tubes, each with 1cm³ of ethanol and two drops of a haloalkane (chloro, bromo, and iodo)
- Place the test tubes in a water bath at 60°C
- Place a tube containing 0.1 mol dm⁻³ aqueous silver nitrate(AgNO3) in the water bath and allow all tubes to reach a constant temperature
- Add 1cm³ of the silver nitrate to each tube and start a stop-clock
- Record the time taken for the precipitate to form
How to ensure that the test is reliable?
- Use equal amounts, in moles, of each halogenoalkane
- Use halogenoalkanes of the same chain length
- Use a water bath to ensure a constant temperature
What are the observations?
- Chlorobutane - a white precipitate forms very slowly
- Bromobutane - a cream precipitate forms slower that iodo but faster than chloro
- Iodobutane - a yellow precipitate forms rapidly
Why is ethanol used?
Haloalkanes are insoluble in water and so ethanol allows water and the haloalkane to mix and produce a single solution rather than two layers
Why is the silver nitrate aqueous?
Contains the nucleophile, water(H20)
Write out the general ionic formula for the formation of precipitates, include state symbols
Ag⁺(aq) + X⁻(aq) -> AgX(s)
Describe the rate of hydrolysis in primary, secondary, and tertiary alcohols
tertiary > secondary > primary
Why do tertiary haloalkanes hydrolyse the fastest?
This is because a primary haloalkane will react by a one step mechanism whereas a tertiary haloalkane reacts by a two step mechanism
- formation of carbocation intermediate
- formation of the organic produce
Also tertiary carbocations are more reactive than primary and secondary ones
