Haloalkanes Flashcards
Why are fluoroalkane un reactive?
due to the high strength of the Carbon - Fluorine bond.
This is why the surface of some non- stick pans is covered in a fluoroalkane.
What are the iodoalkanes the most reactive of the haloalkanes?
because the bond is weak and easily broken.
What can haloalkanes be classified as?
primary, secondary or tertiary.
What are primary haloalkanes?
The -halo group is attached to a carbon atom with at least two H atoms attached.
What are secondary haloalkanes?
The -halo group is attached to a carbon
atom with only one H atom attached.
What are tertiary haloalkanes?
The -halo group is attached to a carbon atom with no H atoms attached.
What are the physical properties of haloalkanes?
halogen has a higher electronegativity than the carbon to which it is attached. The presence of the polar bond and the slight charges on both the carbon and the halogen results in increased intermolecular forces between molecules. It is these permanent dipole-dipole forces which must be overcome in order to melt or boil a sample. There is therefore an increase in boiling points,
melting points and density in comparison to the corresponding alkanes. Because the haloalkanes do not possess hydrogen bonding, they are often insoluble in water, although haloalkanes are more soluble in water than the corresponding alkanes and the alkenes.
The intermolecular forces between haloalkanes are-
PDD
The intermolecular forces between a haloalkanes and water would be-
Hydrogen bonding
d) We would expect 2-chlorobutane to have a boiling point than butane.
higher
Define the term electronegativity
the ability of an atom to attract electrons to a covalent bond
What is the carbon-halogen bond of haloalkanes and what does this do?
haloalkanes is polar and this gives rise to its reactivity.
what does this mean?
-carbon atom attracts electrons less strongly than the chlorine atom. This
makes one end of the molecule δ+ and the other end of the molecule δ-
. In this way the bond is polarised and the positive carbon atom can be attacked by negatively charged species known as nucleophiles
What do nucleophiles have?
lone pair
When does nucleophilic substitution occur?
a molecule is attacked by an electron pair donor and an atom or group of atoms in the molecule is replaced by the
nucleophile
What are nucleophiles attracted to?
positive/ partial positive charge
what are reactions that haloalkanes can undergo (nucleophilic substitutions)?
alkaline hydrolysis of a haloalkane
haloalkanes with cyanide ions
ammonia with haloalkanes
what are the reagents and conditions for the hydrolysis of a haloalkane? and equations
-Sodium hydroxide aqueous/ KOH
-Warm (or reflux)
water
-CH3Cl + NaOH—>NaCl + CH3OH
reflux
What should never be included in the mechanism?
The metal ion as it is a spectator ion
what happens when haloalkanes are reacted with an ethanolic solution of sodium or potassium cyanide?
Nucleophilic substitution of the cyanide ion in ethanol will take place to form nitriles
What is the source of CN- ions?
KCN
Why is nucleophilic substitution with cyanide an important reaction?
IMPORTANT
As it extends the length of the carbon chain
What happens when haloalkanes are treated with excess ethanolic ammonia?
reacts to produce the amine, aminoethane
what are the general equations for the reaction with ammonia and haloalkanes?
CH3CH2Br + 2NH3—ethanol—> CH3CH2NH2 + NH4+ Br-
CH3CH2Br + NH3 —> CH3CH2NH2 + HBr
Hbr+ NH3—> NH4Br
the plus on NH4+ is the charge
What does the reactivity of the haloalkanes depend on?
on the strength of the carbon halogen
bond.
What happens as we go from C-F to C-I
the bond enthalpies decrease (the bonds get weaker);
• it becomes easier to break the carbon to halogen bond and therefore
easier to lose the halide ion;
• the haloalkanes become more reactive.
what is the practical to demonstrate the rates of hydrolysis of haloalkanes (due to strength of bond)
haloalkane is mixed with water and aqueous ethanolic silver nitrate and
placed in a water bath at 60oC.
Ethanol is used as it dissolves the
haloalkanes better.
• The water acts as a nucleophile as it has a lone pair of electrons on its
oxygen atom but it is a slow reaction. As the hydrolysis reaction occurs, the
halide ion, X- is displaced.
• The halide ions, X- produced react with the Ag+ ion in the silver nitrate to form
a precipitate of the silver halide.
Ag+ (aq) + X- (aq) → AgX (s)
• By timing the first appearance of a silver halide precipitate it is possible to
deduce the rate of hydrolysis.
What are the results of the experiment?
1-chlorobutane Slow production of a pale white precipitate
1-bromobutane Cream precipitate produced more rapidly
1-iodobutane Immediate production of a yellow precipitate
Why is ethanol used in the practical?
better solvent for the haloalkane
What is the role of water and the practical?
acts as a nucleophile
what does heating in a water bath do?
Increases rate of reaction
What does the silver nitrate do?
produces a precipitate of the silver halide
what are some direct applications of Haloalkanes?
-polymers such as polychloroethene PVC used in plastic carrier bags
-Polymers such as polytetrafluoroethene, Teflon (PTFE) used for non stick pans
-Chlorofluorocarbons such as CF2Cl2 and CF3Cl, used as a refrigeran
what do CFCs do? (chlorofluorocarbons.)
responsible for the thinning of the ozone layer
what does CFCs do when they reach the stratosphere?
CFCs absorb ultraviolet radiation, which causes photodissociation of the carbon-chlorine bonds. CF2Cl2 → CF2Cl● + ●Cl
what is formed when the stratosphere absorbs ultraviolet radiation? and equations
very reactive chlorine radicals which catalyse the decomposition of ozone to oxygen
Cl• + O3 —> O2 + ClO•
ClO• + O —> O2 + Cl•
overall: O3 + O—> 2O2
how do nitrous oxide affect the ozone there? and equations
Radicals from NOx gas from thunderstorms and aircraft may also catalyse the breakdown of ozone.
•NO+ O3 —>NO2 + O2
•NO2+O —> •NO + O2
overall: O3 + O—> 2O2
What does the term radical mean?
a Species that has an unpaired electron
Where is the ozone layer found?
In the stratosphere
