3.3.3 Halogenoalkanes Flashcards
Why do halogenoalkanes contain polar bonds?
Halogen is > electroneg. than C.
What does a halogenoalkane being polar cause the C to be and what happens to it as a result?
Carbon becomes slightly e- deficient and will regularly be attacked by nucleophiles and the halogen is lost.
What is a nucleophile?
A molecule/ion with a lone pair of e-s that are attracted to the e- deficient carbon atom.
What usually results when a nucleophile attacks a C?
Replacement of an atom> group of atoms usually attached to the C.
What are the three nucleophiles we need to know?
OH- (with dots), CN- (with dots), NH3 (with dots).
Under what conditions do halogenoalkanes undergo nucleophilic substitution with OH-? What is formed?
Halogenoalkanes are warmed with aqueous sodium hydroxide or potassium hydroxide to form an alcohol.
What do the arrows in the mechanism have to touch?
The letter and the lone pair.
In a nucleophilic substitution with OH- what happens to the C-X(halogen) bond?
C-X is polar so the C δ+ attracts a lone pair of e-s from the OH- ion.
What happens to the C-X bond after the C δ+ attracts a lone pair? What is formed?
Br takes both e-s and a new bond between the C and the OH- forms, making an alcohol.
What happens to the C-X bond after the C δ+ attracts a lone pair? What is formed?
Br takes both e-s and a new bond between the C and the OH- forms, making an alcohol.
Under what conditions do halogenoalkanes undergo nucleophilic substitution with CN-? What is formed?
Halogenoalkanes are warmed with an alcoholic solution of potassium cyanide to form a nitrile.
Do all nucleophilic substitutions follow the same procedure?
Yes but with different ions.
Under what conditions do halogenoalkanes undergo nucleophilic substitution with NH3? What is formed?
Halogenoalkanes are warmed with an excess of ammonia in a sealed container to form a primary amine.
If the hydroxide is aqueous what does it act as and what is formed and how is it formed?
A nucleophile, forms an alcohol by nucleophilic substitution.
What can the hydroxide also act as? What does it do and how by? What does it form?
A base, removing a H and a halogen by an elimination reaction. Forms an alkene.
How can we tell if it’s an elimination rection?
Higher temp. and ethanol.
How can we tell if it’s a substitution reaction?
Lower temperature and aqueous.
What does CFC stand for?
Chlorofluorocarbon.
What is a CFC?
A hydrocarbon where all the Hs have been subbed by both F and Cl.
Why are CFCs very unreactive?
The bonding between C-F and C-Cl is very strong, making them unreactive.
What is the molecular formula of ozone?
O3.
What is ozone to oxygen?
An allotrope.
Why is ozone destruction blamed on CFCs that make their way into the stratosphere?
Because they react with UV light (from the sun) to form very reactive Cl radicals, chemical reactions between the Cl radicals and ozone convert ozone gas to oxygen gas.
What is the reaction for O and UV light?
O2 -> O• + O•
How do O and O• react?
O2 + O• -> O3
How does ozone and UV light react?
O3 -> O2 + O•
What radicals destroy ozone?
Cl and Br.
What are the propagation steps for Cl• reacting with O3?
Cl• + O3 -> ClO• + O2
ClO• + O3 -> 2O2 + Cl•
What is the overall reaction for Cl•s reacting with O3?
2O3 -> 3O2, chlorine cancels out.
What is replacing CFCs?
HCFCs/
What does HCFC stand for?
Hydrochlorofluorocarbons.
What do HCFCs contain?
A C-H bond.
What makes HCFCs not destroy ozone?
Easily attacked by OH radicals in the lower atmosphere (troposphere), the products dissolve in the water and end up as rain so they don’t reach the stratosphere.
What are uses of halogenoalkanes and what do they produce?
Anaesthetics, aerosol, solvents, refrigerants, insecticides. CFCs.
How do you tell the difference between nucleophilic sub and electrophilic add?
Nucleophilic sub always involves a halogenoalkane, electrophilic add always involves an alkene.
Explain why a cyclic structure is a weaker base.
Lone pair on N, spread into a ring, less available for donation.
