haloalkanes Flashcards
What is a nucleophile (with examples)?
● A species that donates lone pair of electrons.
● HO-, H2O, and NH3.
What makes HO- a bejer nucleophile than H2O?
it’s full charge
Describe and draw the general nucleophilic subsJtuJon of a haloalkane
SubsBtuBon here is swapping the halogen for another atom.
1. The nucleophile agacks the delta posi:ve carbon in the carbon-halogen polar bond to form a covalent bond.
2. At the same :me, the carbon-halogen bond breaks by heteroly:c fission forming a halide ion.
3. The geometry of the product switches.
● The nucleophile approaches from behind as its repelled by the delta nega:ve halogen.
For a terBary haloalkane, this is a 2-step process because the carbocaBon intermediate is stable on its own.
What is hydrolysis?
A type of reacJon involving the spli_ng of a molecule by water.
What are the condiJons and products for the hydrolysis of halogenoalkanes?
● Hea:ng under reflux with NaOH (aq) or KOH (aq).
● The product is an alcohol and a compound of the halogen
(e.g., NaBr or KBr for the hydrolysis of bromoalkanes).
The NaOH and KOH is needed so it dissociates into HO- nucleophiles. You can’t just have a pure soluBon of OH ions
What 2 things does the rate of the hydrolysis of haloalkanes depend on and how
● The bond enthalpy (i.e., strength) of the carbon-halogen bond; the weaker, the faster.
● The degree of the haloalkane; ter:ary is faster than secondary which is faster than primary.
Why does the degree of haloalkane affect the rate of hydrolysis?
The higher the degree, the more stable the carboca:on intermediate formed
How does the strength of the carbon-halogen bond vary between different haloalkanes and why?
● Bond enthalpy (i.e., strength) decreases for halogens further down the group ∴ broken more easily.
● Due to decreased agrac:on between the bonding pair of electrons and the nuclei of bonding pairs due to increased shielding and atomic radius.
This factor overrides the polarity of the bonds.
How can the rate of the hydrolysis of haloalkanes be measured and why?
● By adding AgNO3 (aq) and :ming how long it takes for a precipitate for form. The quicker it does, the faster the rate of reac:on.
● As soon as the halide ion is formed from nucleophilic subs:tu:on, it can react with the silver ion: Ag+ (aq) + X- (aq) → AgX (s).
What are CFCs, what are they used as and why?
● Chlorofluorocarbons (thus containing only carbon, chlorine, and fluorine).
● Used as propellants, aerosols, and refrigerants because of their stability/low-reac:vity, non-toxicity, and vola:lity.
What is in equilibrium in the upper atmosphere and how?
● The forma:on and destruc:on of ozone.
● O2 → 2O (broken by UV light to form diradicals).
● O3⇌O2+O.
Describe how CFCs lead to the destrucJon of ozone and explain why this is so effective
● The weaker C-Cl bond is broken by homoly:c fission under UV light forming Cl• which catalyses the breakdown of ozone:
○ Cl•+O3 →ClO•+O2
○ ClO•+O•→O2 +Cl•
● The overall equa:on is O3 + O• → 2O2
● The chlorine radical is reproduced so they have a long residence :me.
Nitrogen oxides work in a similar way.
What does the destrucJon of the ozone mean?
Exposed to UV light which can cause skin cancer.
