Haloalkanes Flashcards
What are the three mechanisms in which haloalkanes can be synthesised?
- Free radical substitution
- Electrophilic addition reactions with alkenes
- Reacting alcohols with halide ions
What is the general equation for when an alcohol reacts with a halide ion to form a haloalkane?
ROH + HX —> RX + H2O
What is the catalyst used for the synthesis of a bromoalkane from reacting an alcohol with a bromide ion? Also state the reactants
Concentrated sulfuric acid
Reactant: NaBr
What is the catalyst used for the synthesis of an iodoalkane from reacting an alcohol with an iodide ion? Also state the reactants
Phosphoric acid
Reactant: NaI
What is a mechanism?
Pictorial representation of the movement of a pair of electrons during a chemical reaction
What does a curly arrow represent?
The movement of a pair of electrons
Describe the movement of electrons in heterolytic fission and homolytic fission
Heterolytic: Pair of electrons in bond moves to one atom (double-headed curly arrow)
Homolytic: Each electron in bond moves to a different atom - radicals formed (two half-headed curly arrows - do not need to draw this in A-Level)
Describe nucleophilic substitution
- Halogen in haloalkanes more electronegative than C so is slightly negative in the polar bond
- The electron-deficient C is readily attacked by a lone pair donor (nucleophile)
- Nucleophile donates its lone pair to C, forming a covalent bond and C-X bond breaks by heterolytic fission - halide ion released
What is a nucleophile?
Lone pair donor
Describe nucleophilic substitution when the nucleophile is a hydroxide ion
- Reagent: Aqueous NaOH or KOH. Conditions: Warm, aqueous
- Hydroxide donates its lone pair to to electron-deficient C atom. Breaks C-X bond and forms an alcohol
Describe the experiment that is used to show the rate of hydrolysis of the haloalkanes and strength of C-X bond
- Gently warm separate samples of 1-chloropropane, 1-bromopropane and 1-iodopropane in a water bath with a little added ethanol
- Add equal amounts aqueous silver nitrate
- 1-chloropropane: White ppt. 1-bromopropane: Cream ppt. 1-iodopropane: Yellow ppt
Explain the results of the ppt formed when silver nitrate is added to chloro, bromo and iodopropanes
C-X bond gets weaker as we go down the group - the shared electrons in the C-X bond get further away from the halogen nucleus, so bond becomes weaker, increasing reactivity, leading to a darker colour and a reduced reaction rate
Describe nucleophilic substitution when the nucleophile is a cyanide ion
- Reagent: KCN dissolved in ethanol (ethanolic KCN)
- Haloalkane warmed witth ethanolic KCN to form a nitrile, adds an extra carbon to chain. Nucleophile is -:CN
Describe nucleophilic substitution when the nucleophile is ammonia
- Reagent: NH3 dissolved in ethanol (ethanolic ammonia). Conditions: Excess NH3
- Haloalkane warmed in excess ethanolic ammonia in a sealed container, primary amines are formed (two hydrogen atoms attached to the nitrogen)
- Nucleophile is ammonia, in which the N atom has the lone pair of electrons
- Equation example: CH3CH2Br + :NH3 —> CH3CH2NH2 + HBr
Describe the further nucleophilic substitution of primary amines
- Primary amine is also a nucleophile - can donate their lone pair to another C atom on another haloalkane. Leading to additional carbon groups bonded to central nitrogen
- Produces a secondary amine. This can react with another haloalkane to produce a tertiary amine
- Tertiary amine produced can also react with another haloalkane to produce a quaternary ammonium salt. No lone pair on this so there can be no further substitution
What are chlorofluorocarbons (CFCs) and what are they used for?
- Haloalkanes containing both chlorine and fluorine atoms but no hydrogen e.g. CCl3F
- Short chain ones are gases and were used in refrigerators. Longer chain ones are used for dry cleaning and de-greasing solvents
- CFC gases are very unreactive under normal conditions but they eventually end up in the atmosphere, where they decompose to give chlorine free radicals
What is ozone and why is it important?
- Molecule made from three oxygen atoms: C3
- Absorbs harmful UV radiation from the sun that causes skin cancer by damaging DNA in skin cells
How do chlorine radicals decompose ozone molecules in the stratosphere?
- R-Cl –> R* + Cl*
- Cl* + O3 –> ClO* + O2
- ClO + O3 –> 2O2 + Cl*
As Cl* is not destroyed but regenerated we can say that it acts as a catalyst in the breakdown of ozone to oxygen
What are other radicals that break down ozone?
In high temperatures of combustion engines, nitrogen and oxygen react together to form nitrogen oxides. Nitrogen Monoxide, NO, is a radical.
What is the alternative to CFCs which do not contribute to decomposition of ozone layer?
1,1,1,2-tetrafluoroethane, primarily used in refrigeration and air conditioning. Only containes C-H and C-F bonds, does not produce any chlorine radicals as the bonds are stronger
