3.3 Halogenoalkanes Flashcards
What is a primary Haloalkane?
The carbon with halogen is attached to one alkyl group
I
C-C-I
I
Only one link from the CH2 to the alkyl
What is a secondary haloalkane?
Carbon with halogen is attached to two other groups
-C-C-C
I
Br
What is a tertiary haloalkane?
Carbon holding the halogen is attached to three other alkyl group
C
I
C-C-C
I
Br
What is the trend in haloalkane bond enthalpies?
Get weaker going down the group due to greater distance and shielding
What is hydrolysis?
Using water to break up a compound
What is the formula for the addition of silver nitrate?
Ag+ + X —> AgX
Why is iodine the fastest to form a precipitate?
It’s the biggest atom so weakest attraction (more distance and shielding) so precipitation is formed quicker
What colour precipitates does Iodine,Bromine and Chlorine form with silver nitrate?
Iodine- Yellow
Bromine- Cream
Chlorine- White
What is the definition of a nucleophile?
A negatively charge ion or an atom with a delta negative charge
Has a lone pair
Electron donator
What are the common nucleophiles?
Hydroxide ions- :OH-
Ammonia- :NH3
Cyanide- : CN
In a haloalkane nucleophillic substitution reaction what is the leaving group ?
Halide ion
What is the order of reactivity with Haloalkanes?
C - Hal bond determines reactivity
C-F stronger (fewer shells)
What do nucleophilic substitution of OH ions form?
- Alcohols
What happens in nucleophilic substitution of OH?
- Lone pair on the O
- Attacks positive C atom
- Halogen is replaced by OH
- HYDROLYSIS
What happens if OH ions undergo nucleophilic substitution in water?
- Reaction works same as normal
- Slower than other reactions
What do CN- ions form with Haloalkanes?
- Forms nitriles
What happens in a nucleophilic substitution reaction with CN- ions?
- Weakly nucleophilic
- Partially positive carbon atom attacked by lone pair on CN-
What happens to the length of the carbon chain in Nucleophilic Substitution with CN ions?
It increases as the CN adds another carbon to the halogen
What conditions must CN- nucleophilic substitution reactions take place under?
- Reflux conditions
- Ethanol
Why must CN- nucleophilic substitution not occur in aqueous conditions?
- The water would act as a nucleophilic and take over
What happens in the nucleophilic substitution of NH3
- Lone pair on NH3 attacks partially charged Carbon
- C-N bond replace C-Hal and has a positive charge
- Annother Ammonia molecule then picks up a H+ from the NH3 leaving an amine group
What conditions are required for NH3 to undergo nucleophilic substitution?
- Ethanol
- Excess Ammonia
Why must there be excess Ammonia for NH3 nucleophilic substitution?
- Because the amine group created is also nucleophilic
- Would react again with haloalkane
- To prevent further substitution
What are formed as a by product of nucleophilic substitution with NH3?
- Quartenery ionic salts
- Extra H+ is picked up by NH3
- Forms NH4 +
What conditions are required for elimination?
- Ethanol
- Reflux reactions
- OH-
-Hot
How does elimination work?
- OH- uses its lone pair to bond with one of the hydrogens on the adjacent carbon to the halogen
- electron pair from c - h forms a c-c double bond
- Bromine takes pair of electrons and leaves as a Br-
How do you name amines?
- Add amino prefix to hydrocarbon
Why does silver nitrate not form an immediate precipitate?
Lots of strong covalent bonds
No free ions
Why is ethanol needed in elimination?
To act as a solvent
Where does the water come from in Oh- elimination?
- It is an aqueous solution
What is the ozone formula?
O3
What is the ozone layer?
A layer of ozone making up a protective layer protecting earth from harmful UV radiation
Why is UV radiation harmful?
- Can cause DNA damage
- Cause of skin cancers
How is ozone being formed and broken at the same rate without intervention of CFC’s?
O+O2 = O3 (reversible reaction)
What did CFC’s used to be used in?
- Coolants in fridges
- Propellants in aerosols
- Dry cleaning
What are the features of CFC’s?
- Contain cholorine,fluorine and carbon
- Very stable (high bond enthalpy)
- Non toxic
What was decided at the Montreal Protocol?
- 24 countries banned use of CFC’s
- However some old fridges still contain them
How do CFC’s destroy ozone?
CFCl3 (example) dissociates into CFCl2 and Cl free radicals
Cl’ + O3 —> ClO’ + O2
ClO’ + O —>Cl’ + O2
Overall 2O3 turns into 3O2
What is the effect of one CFC molecule?
- Can destroy thousands of O3
