M4 B4 Haloalkanes
No description yet. This curated deck is ready for daily study.
Card preview (24)
What is a haloalkane?
An alkane that contains a halogen (Cl,Br,I,F) One or more hydrogen atoms have been replaced by a halogen atom.
What is the general formula for a haloalkane?
Cn H 2n+1 X X= halogen
What is nucleophilic substitution?
The replacement of a halogen by a nucleophile
What is a nucleophile?
An electron pair donor (usually negative or with a negative dipole) e.g- ..OH-, ..NH3, ..CN-, H2O.. (..= where lone pair is)
Which bond in a haloalkane is polar?
The C-X bond because the halogen has a greater electronegativity than carbon.
What happens to the carbon attached to the halogen during nucleophilic substitution?
The carbon atom is attacked by a species with a lone pair of electrons, this species is a nucleophile, and it replaces the halogen atom.
Define: hydrolysis
A reaction with water that breaks a chemical compound into 2 chemical compounds.
Hydrolysis of haloalkanes: What is the.. Reagent? Conditions? Equation?
Reagent: NaOH (aq) Conditions: Reflux, its used in a water solvent, so (aq) is essential Equation: C2H5Br + NaOH = C2H5OH + NaBr
Describe nucleophilic substitution with an OH- ion (hydrolysis)
• The nucleophile is the OH- ion. • It attacks the carbon (positive dipole), causing the C-X halogen bond to break. • The halogen atom becomes a halide. • A new covalent bond forms between the C atom and the nucleophile. • The halogen has been substituted by the OH.
What is used to slow the reaction of nucleophilic substitution (looking at rates of hydrolysis)
Water (HOH) is used Haloalkane + water = alcohol + hydrogen halide
The X- ion lost from the haloalkane can be identified using what?
Using aqueous silver nitrate- to form a precipitate.
What is the method/ procedure for measuring the rate of hydrolysis (nucleophilic substitution) for haloalkanes.
• Place a measured sample of each haloalkane (equal molar amounts) into separate test tubes and place into a water bath at 50 degrees (c). • Into another test tube place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath. • Once at equal temperature, add equal volumes of the ethanol mixture and haloalkane solution together. • Time how long it takes for each precipitate to form.
What is the reason for the different rates of hydrolysis for different haloalkanes?
• As the bond enthalpy decreases, it is easier to break the C-X bond. • The stronger the bond the harder it is to break, and the slower the rate of reaction will be. • The weaker the bond the faster the rate of reaction will be. C-I is the weakest bond so fastest rate of reaction C-Cl is the strongest bond so the slowest rate of reaction (more energy required to break this bond)
What does CFC stand for?
Chlorofluorocarbon
Useful properties of CFC’s (chlorofluorocarbons)? (3)
• Non-toxic • Chemically inert (non-flammable) • Highly volatile
Give 3 names of CFC’s (examples)
• Trichlorofluoromethane • Dichlorodifluoromethane • 1,1,1trichloro 2,2,2trifluoroethane
What are the uses for CFC: Trichlorofluoromethane (2)
• Refrigeration • Air conditioning
What are uses for CFC: Dichlorodifluoromethane (2)
• Aerosol propellants • Blowing agent for polystyrene
What are uses for CFC: 1,1,1trichloro 2,2,2trifluoroethane (2)
• Dry cleaning solvent • Degreasing agents for circuit boards
What does the Ozone layer do?
• The Ozone layer (O3- toxic) absorbs large amounts of ultra-violet radiation, protecting the Earth’s surface from high intensity UV- which could lead to skin cancers and cateracts
The ozone layer is maintained in a balance with oxygen, what is this balanced equation?
O2 + O ⇌ O3
How do CFC’s affect the ozone layer?
• CFC’s are very stable and cause depletion of the Ozone layer as they are unreactive. • However, when they reach the stratosphere they are broken down by UV radiation • The formation of CFC radicals (e.g- Cl.) turns O3 and O into O2, causing an imbalance in the amount of O3 and O, shifting the equilibrium of the balance of oxygen in the Ozone layer.
What is the mechanism for Ozone depletion? (radical substitution)
Initiation: CFCl3 = Cl. + .CFCl2 Propagation: CL. + O3 = ClO. + O2 ClO. + O = Cl. + O2 Termination: Cl. + Cl. = Cl2 Overall reaction= O3 + O = 2O2
What are alternatives to CFC’s?
HFC’s (hydrofluorocarbons) HClFC’s (hydrochlorofluorocarbon) And alkanes (HC’s)