Module 4: Haloalkanes Flashcards
What are the haloalkanes?
They are the homologous series of compounds with the general formula CnH2n+1X (where X is the halogen)
What are primary/secondary/Tertiary haloalkanes?
Primary - The halogen is bonded to a C that is bonded to 1 other C
Secondary - The halogen is bonded to 2 other C’s.
Tertiary - The halogen is bonded to a C that is bonded to 3 other C’s.
What are the 3 ways you can synthesise haloalkanes?
- Free radical substitution
- Electrophillic addition
- Reacting alcohols with halide ions
What is the General equation for reacting an alcohol with a hydrogen halide?
ROH + HX —–> RX + H2O
What hydrogen halide ion do we use for a chloroalkane?
NaCl
What hydrogen halide ion do we use for a bromoalkane?
NaBr in the presence of concentrated sulfuric acid (acts as a catalyst)
What hydrogen halide ion do we use for an iodoalkane?
NaI in the presence of phosphoric acid
What is a mechanism?
The movement of a pair of electrons
What do curly arrows show?
The movement of electrons
What is nucleophilic subsitution?
- The electron deficient carbon atom is readily attacked by a species that contains a lone pair of electrons ( the nucleophile)
- The nucleophile donates its lone pair of to carbon - as it does this it forms a covalent bond with the C atom
- The 2 electrons in the C-X bond moves onto the halogen atom, breaking the C- X bond ( heterolytic fission)
- By gaining the electrons from the bond, the halogen atom forms a halide ion.
What’s heterolytic fission?
Pair of electrons in bond move to one atom so that negative ions are formed.
What’s homolytic fission?
Each electron in the bond moves to a different atom so that a radical is formed.
What’s a nucleophile?
A lone pair donor
Name the 3 types of nucleophilic substitution?
- Hydrolysis reaction with Hyrdroxide ions forming alcohols
- Cyanide Ions, forming nitriles
- Excess ammonia - forming primary amines.
What is hydrolysis with :OH- ions? State the conditions, overall equation and reaction equation
Conditions: warm, aqueous, NaOH reagent
Overall equation = RX + NaOH —–> ROH + NaBr
Reaction equation = RX + :OH- —— ROH + :Br-
How do we test for the rate of hydrolysis?
- Get warm samples of 1-Chloropropane etc in a water bath with some added ethanol
- Add equal amounts of aqueous silver nitrate to each test tube
- Time how long it takes for ppt to form (how long it takes for bond to break)
What is the pattern with the rate of hydrolysis reactions for group 7?
The rate of hydrolysis increases as we go down the group, the bond enthalpy ( energy required to break 1mol of a bond) decreases due to the electronegativity decreasing.
What is nucleouphilic substitution in terms of Cyanide ions? State the conditions, overall equation and reaction equation
Conditions: Warmed with an ethanolic solution of potassium cyanide.
Reaction equation: RBr + :CN- ——> RCN + :Br-
What is nucleophilic substitution in terms of excess ammonia (NH3)? State the conditions, overall equation and reaction equation
Conditions: Excess NH3 (Reagent = NH3 dissolved in ethanol)
RBr + :NH3 ——-> RNH2 + HBr
What are Chlorofluorocarbons (CFCs)?
Haloalkanes containing both chlorine and fluorine, with low hydrogen content
Example: Trichlorofluoromethane (CCl3F)
What are the uses of short-chain CFCs?
Used as gases in refrigerators
Short-chain CFCs are typically gaseous at room temperature.
What are the uses of longer-chain CFCs?
Used for dry cleaning and as de-greasing solvents
How do CFCs behave under normal conditions?
Very unreactive but eventually decompose in the atmosphere
What do CFCs decompose to produce in the atmosphere?
Chlorine free radicals
What is ozone?
A molecule made from three oxygen atoms, O3
Why is the ozone layer important?
It absorbs harmful UV radiation from the Sun that causes skin cancer
What is the equilibrium reaction for ozone formation?
O2 + O = O3
What effect do chlorine radicals have on ozone?
They decompose ozone in the stratosphere
What happens to the C-Cl bond in the presence of UV radiation?
It breaks to produce Cl•
What is the first step in the reaction of chlorine radicals with ozone?
Cl• + O3 → ClO• + O2
What is the role of Cl• in the breakdown of ozone?
Acts as a catalyst; it is regenerated and attacks more ozone molecules
What is the overall equation showing the breakdown of ozone?
2 O3 → 3 O2
