15.1 Flashcards
Haloalkanes define
Compounds, contain carbon, hydrogen, and at least 1 halogen
How indicate identity of halogen in haloalkane name?
Prefix before longest chain (eg bromomethane)
When 2+ halogens present in haloalkane, how named?
Alphabetically (eg 2-bromo-1-iodo-propane)
Difference between primary, secondary, tertiary haloalkanes?
Primary - carbon that halogen is connected to is bonded to 0-1 other carbons
Secondary - carbon that halogen is connected to is bonded to 2 other carbons
Tertiary - carbon that halogen is connected to is bonded to 3 other carbons
Why does the carbon atom in carbon-halogen bond have slightly positive charge?
Carbon-halogen bond in polar as halogen more electronegative, so electron pair closer to halogen so carbon slightly positive
Because the carbon in the carbon-halogen bond is slightly positive, what can it do?
Attract species containing a lone pair of electrons
Nucleophiles define + some examples
An atom (or group of atoms)which is attracted to an electron-deficient centre or atom, where it donates a pair of electrons to form a new covalent bond
OH-
H2O
NH3
What happens when haloalkane reacts with nucleophile?
Nucleophile replaces halogen in substitution reaction
New compound produced containing different functional group
Reaction mechanism called nucleophilic substitution
Hydrolysis define
A reaction with water that breaks a chemical compound into 2 compounds, the H and the OH in a water molecule becomes incorporated into the 2 compounds
Steps in nucelophilic substitution reaction of haloalkane with OH-
1) nucleophile OH- approaches carbon atom attached to halogen from opposite side of molecule to halogen. This direction of attack by OH- minimises repulsionbetween nucleophile and partially negative charged halogen atom
2) lone pair electrons on OH- attracted to and donated to partially positive carbon atom
3) new bond formed between O in OH- and partially positive carbon atom
4) carbon-halogen bond breaks through heterolytic fission
5) new organic product is alcohol, halide ion also formed
Haloalkanes can be converted ro alcohols using…
Conditions for reaction?
Aqueous NaOH
Heat reacting mixture under reflux
Rate of hydrolysis depends on…
Strength of carbon-halogen bond in haloalkane
Trend in bond enthalpies going down group 7…
Less energy needed to break carbon-halogen bond down the group (bond enthalpy decreases down the group)
How to measure rate of hydrolysis in primary haloalkanes
1) set up 3 test tubes with the following:
- 1cm3 ethanol, 2 drops 1-chlorobutane
- 1cm3 ethanol, 2 drops 1-bromlbutane
- 1cm3 ethanol, 2 drops 1-iodobutane
(Ethanol used as the haloalkane is insoluble in water. The ethanol allows the haloalkane and water to mix in one solution instead of forming 2 separate layers
2) stand test-tubes in water bath at 60C
3) place test tube with 0.1mol/dm3 AgNO3 in water bath and allow to reach constant temperature
4) add 4cm3 AGNO3 to each test tube quickly, and immediately start stopwatch
5) observe test tubes for 5 mins, record time taken for precipitate to form
As C-Cl has highest bond enthalpy, should have least precipitate formed, and as C-I has lowest, should have most precipitate formed
Which carbocation is the most stable (primary, secondary, tertiary)
Tertiary, then secondary, then primary
Hence tertiary haloalkanes can lose the halogen much more easily than a primary, as the tertiary is already very stable. Therefore the rate of hydrolysis is highest in tertiary haloalkanes
