Halogenalkanes Flashcards
What are the two types of reactions that can occur with haloalkanes?
Only elimination or substitution reactions can occur
Describe and explain nucleophilic substitution
The C atom of the haloalkane is slightly positive, as the halogen it is attached to is more electronegative (and weaker).
This means the negative nucleophile is attracted to the C
The nucleophile bonds to the C, causing the halogen bond to break (weakest bond, can only have 4 bonds)
State what is meant by the term nucleophile
An electron pair donater.
What is hydrolysis?
Splitting molecules using water
How can we test for halogenoalkanes? (brief)
Do a substitution reaction which turns the halogen into a halide ion
Then test for that ion with silver nitrate soln.
Describe how you would test for haloalkanes
Warm the haloalkane with NaOH soln. in a mix of ethanol and H2O
Halogen displaced to become an ion
Acidify with dilute nitric acid and then add silver nitrate
Various precipitates may be formed
You can confirm the precipitate by adding ammonia soln.
Why do you acidify the halide ion mixture with nitric acid?
To prevent any unreacted OH- ions reacting with the silver and giving a confusing precipitate
Adding silver nitrate soln. to a halide ion produces a white precipitate. What ion is it?
Cl-
Adding silver nitrate soln. to a halide ion produces a very pale yellow precipitate. What ion is it?
I-
Adding silver nitrate soln. to a halide ion produces a very pale cream precipitate. What ion is it?
Br-
What are the conditions for haloalkane substitution by OH- ions?
Aqueous and warm
What are the conditions for haloalkane substitutions by CN- ions?
Aqueous/alcohol
Warm
What is produced by haloalkane substitution with CN- ions?
A nitrile
What are the conditions for haloalkane substitution by NH3?
Excess ammonia (NH3) Sealed tube
Give a general reaction for the substitution of a haloalkane by NH3
R-X + 2NH3 –> R-NH2 = NH4+X-
In substitution by NH3, NH3 acts in two ways. What are they?
As a nucleophile and as a base
What are the reagents and conditions for elimination reactions?
Reagents - NaOH/KOH
Conditions - dissolved in ethanol
Describe and explain elimination reaction
The OH- takes one of the Hs - its electrons go to the C it was previously attached to
These two electrons move to form double bond b/wn the 2 C atoms
The approach of those electrons repels the electrons in the C-Br bond right out onto the bromine
The Br is thrown off as a negative ion
What does CFC stand for?
Chlorofluorocarbons
What are the common uses of CFCs and why?
Low toxicity, low reactivity and low flammability led to common use as refrigerants and aerosol propellants
What is the ozone layer?
A thin layer of the atmosphere consisting of O3, that filters out some UV
What does the ozone layer do?
Filter out shorter wavelengths of UV light that would be harmful to most life in larger doses
Why do HCFCs not affect the ozone layer?
They don’t have the C-Cl bond
What is an ozone molecule?
O3
CFCs affect the ozone layer. Name the reaction that occurs
Free radical substitution
Give an overall balanced formulaic equation for the free radical substitution of ozone
2O3 –> 3O2
Why are CFCs such a problem?
Attack ozone
Cl* regenerated so can affect others
CFCs have long lifetime (20-100yrs)
Explain why an excess of ammonia is needed in the substitution of NH3 to produce a high yield of amine
NH3 has two roles - firstly as a nucleophile and secondly as a base
What is the role of Cl in the decomposition of ozone?
Acts as a catalyst
What is the suffix of a nitrile?
-anenitrile
CFCs affect the ozone layer. Describe the reaction that occurs
UV light breaks C-Cl bond --> C*F3 + Cl* Cl radicals attack ozone --> regenerate more Cl*s Cl* + O3 --> ClO* + O2 ClO* + O3 --> Cl* + 2O2 Cl* + Cl* --> Cl2
