4.2.2 Haloalkanes Flashcards
What are haloalkanes
Compounds containing carbon, hydrogen, and at least one halogen
What is the rule to remember when naming haloalkanes
If more than one halogen present, name alphabetically
(e.g. 2-chloro-1-iodopropane)
What are haloalkanes structured as
Primary, secondary, tertiary
- dependent on position of halogen, and how many carbons are attached to it
What are the 3 ways to make haloalkanes
Free radical substitution, electrophilic addition, nucleophilic substitution
Free radical substitution to make haloalkanes
Alkane + halogen ===> haloalkanes
(Need UV light)
Electrophilic addition to make haloalkanes
Alkene + hydrogen halide ===> haloalkanes
(room temperature)
Nucleophilic substitution of haloalkanes
Alcohol + hydrogen hapkido ===> haloalkane
(Hear under reflux)
(Or can use sodium halide and H2SO4)
Why is the halogen-carbon bond polar
Halogens are more EN than Carbon
What is the nature of the C atom attached to the halogen in haloalkanes
- X-C bond is polar
- so the electron pair is closer to the halogen
- so the carbon atom attached to it is slightly positive
- (so can attract species containing lone pair of e-)
What is a nucleophile
An atom or group of atoms that is attracted to an electron deficient carbon atom, where it donated a pair of electrons to form a new (dative) covalent bond
What are examples of nucleophiles
:OH-
H2O:
:NH3
- must have lone pair of electrons to donate
- can be charged or neutral
What is nucleophilic substitution
When a haloalkane reacts with a nucleophile, the nucleophile replaces the halogen in a substitution reaction
What is a substitution reaction
Reaction in which one atom or a group of atoms is replaces by another atom or group of atoms
What undergoes nucleophilic substitution
Primary haloalkanes to form a primary product, variety of
What is hydrolysis
Chemical reaction involving water/an aqueous solution of a hydroxide that causes the breaking of a bond in a molecule
- means molecule is split into 2 products
- e.g. in haloalkanes, where the -OH group replaces the halogen atom
What is the mechanism of hydrolysis nucleophilic substitution
1) the nucleophile, OH-, approaches the a C atom attached to the halogen on the opposite side of the molecule containing the C atom
2) this direction AS this minimises the repulsion between the OH- nucleophile attacking and the the slightly negative H atom
3) a lone pair of electrons on the OH- ion is attracted and is donated to the slightly positive C atom
4) a new bond forms between the O of the OH- and the C atom
5) the C-X bond breaks via heterocyclic fission
6) new product formed is an alcohol and a halide ion
Where can you get the OH- ion from in hydrolysis
Aqueous NaOH
- slow at room temp, so heated under reflux to maintain a good yield
- the Na will later react with a halide
explain how you would draw the reaction mechanism for a haloalkane and OH- ion
1) draw the haloalkane with the δ+ and δ- charges on the C and X
2) draw the :OH- below
3) draw a line coming from the : lone pair on the ion to the C attached to the halogen, making sure the arrow is on the opposite side of the halogen
4) draw an arrow coming from the C-X bond to the halogen
5) draw your products, being the alcohol and halide ion
how would you draw the nucleophilic substitution of water and a haloalkane
1) draw a line coming from the :O-H-H bond pair to the C attached to the C-X bond, making sure arrow is on opposite side of the halogen (also add on δ+ and δ- charged too)
2) draw the alcohol formed but with a + on the one side of the O attached to the OH, and on the other side, draw the H bonded
3) draw a line coming from the O-H bond to the O attached to the OH
4) add + halide ion on here
5) then, draw the resulting alcohol, + halide ion, + H+ ion
what do you need to remember when reacting water with a haloalkane
- need a solvent, such as ethanol, to allow the haloalkane and water to mix
- it would form a hydrogen halide instead, instead of sodium halide as before
what is the rate of hydrolysis of haloalkanes dependent on
dependent on the strength of the C-X bond, as this is what gets broken
how does the strength of bonds affect the rate of reaction, therefore the rate of enthalpy
- the stronger the bond (C-F is strongest)
- the higher its enthalpy
- so the harder it is to break
- so it is the least reactive
why does bond strength decrease as you go down group 7
- the size of the halogen atom increases
- so the C-X bond length increases
- so the C-X bond enthalpy decreases
- so weaker, and requires less energy to break (hence the C-I bond is the most reactive, as is the weakest)
how can you measure the hydrolysis of haloalkanes
- haloalkanes react with water
- haloalkane + H2O ===> alcohol + H+ + X-
- the X- ion (halide ion) reacts with aqueous silver nitrate to produce a precipitate of silver halide
- Ag+ + X- ===> AgX(s)
- you can measure the rate that the precipitate is produced
why do you add ethanol when measuring rate of hydrolysis of haloalkanes
- haloalkanes are insoluble in water
- but you need to get the nucleophile, H2O:, from the aqueous AgNO3
- so you add ethanol as solvent
- means that you will end up with single solution, and not 2 layers
explain the steps in measuring the rate of hydrolysis of haloalkanes
1) set up 3 test tubes, each with 1cm^3 of ethanol and drops of haloalkanes (making sure they’re all the same structure, primary, and carbon length, butane), from Cl to I
2) stand all in water bath at 60 degrees
3) add test tubes of 0.1 moldm^-3 of AgNO3 to water bath too
4) add 1cm^3 into each tube, and start the stop clock
5) observe for 5 minutes, and see time for precipitate to form
what should be your results for testing the rate of hydrolysis of haloalkanes
CHLORO= white, SLOW
BROMO= cream, MIDDLE
IODO= yellow, FAST
what are organohalogen compounds
molecules that contain at least 1 halogen atom joined to a carbon chain
what are the uses of organohalogen compounds
pesticides, solvents, making polymers, flame retardants, refrigerants, aerosol cans, air conditioning
explain organohalogen compounds in nature
- rarely found in nature
- not broken down naturally in the environment
where is the ozone layer found
the outer edge of the stratosphere
what is the role of the ozone layer
contains ozone - O3
- which absorbs most of the biologically damaging UV radiation, UVB , from sunrays, only allowing a small amount to reach the surface
what will a depletion of O3 result in
increase in sunburns and skin cancer (caused by the genetic damage of UV)
explain how ozone is continually broken down and made in the stratosphere
- due to the actions of UV radiation
1) high energy UV breaks O2 molecules into O radicals
- O2 ===> 2O (UV on arrow)
2) a STEADY STATE is set up involving O2 and the O radicals, in which O3 forms and breaks
- O2 + O === O3
what is the equilibrium of ozone production, and what has human activity done to this
rate of formation of O3 is equal to the rate at which O3 is broken down
- a STEADY STATE
- however, human activity, such as CFCs, has upset this equilibrium
what does CFC stand for
chlorofluorocarbons
what are CFCs used for
- refrigerants
- air conditioning units
- aerosol propellants
why are CFCs very stable
have strong C-X bonds in their molecules:
- so have a long residence time in the troposphere (don’t react very well)
- takes years to reach the stratosphere
what happens to CFCs once they reach the stratosphere
- the UV radiation provides enough energy to break the C-X bonds in CFCs, via homolytic fission
- and forms radicals
- the C-Cl bond will always be the one to break, as C-Cl bond has lower enthalpy than C-F
what is the first step in the free radical mechanism of CFCs
PHOTODISSOCIATION - is the initiation step, as the radiation initiated the breakdown
- the C-Cl bond breaks via homolytic fission, and forms a Cl radical and a CFC radical
how would you draw the first step in CFC mechanism
1) draw the CFC
2) you CAN show the fish hook arrows in the C-Cl bond, going to each atom, but not needed
3) draw arrow, going to the 2 radicals formed
what do the Cl. radicals act as in the propagation step of CFC mechanism
act as an intermediate, as are very reactive
what are the equations for the propagation of the CFC mechanism
1) Cl. + O3 ===> ClO. + O2
2) ClO. + O ===> Cl. + O2
OVERALL: O + O3 ===> 2O2
- repeats in cycle in a chain reaction
why can the Cl. radical be seen as a catalyst in the CFC reaction mechanism
- it is not used up in the reaction (same used up as produced)
- but it lowers the AE and increases the ROR
what is the other radical responsible for catalysing the breakdown of O3
nitrogen oxide radicals
where are NO radicals found
- naturally in lightning strikes and thunderstorms
- as a result of aircraft travel in the stratosphere
what is the equation for making NO radicals
N2 + O2 ===> 2NO.
(need high temp on arrow, as both N≡N and O=O have high bond enthalpies, so require a lot of energy to break)
explain the steps of NO. reacting with O3
1) NO. + O3 ===> NO2. + O2
2) NO2. + O ===> NO. + O2
OVERALL: O3 + O ===> 2O2
what is the montreal protocol
an international agreement phasing out CFCs (and bromine containing compounds), and finding alternatives where possible
even though the montreal protocol exists, why will CFCs still take time to phase out
- still until 2060s and 2070s:
- CFCs are very stable in lower atmosphere, so have a long life (C-F and C-Cl have strong, high bond enthalpies, need lots of energy to break)
- CFCs take a long time to reach the stratosphere (years)
- CFCs are still being released
- Cl radicals can catalyse O3 depletion many times before being terminated
reasons why CFCs are good for aerosols specifically
- volatile
- non-flammable
- non-toxic
- low reactivity
what is a biodegradable alternative to using CFCs in aerosol cans
hydrochlorofluorocarbon
- (contains H too in structure)
