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Here's another chemistry lesson in which my brain grew legs and walked out the door. Dont give up. Ever. Love, E.
Haloalkanes and Their Angelic Reactions: Part One
Haloalkanes are more commonly referred to as halogenoalkanes. Obviously you’ve already read my post on halogenoalkanes and their properties so there’s no surprise that you’re itching to read what I’ve got to say about these beauties and their reactions! Should we delve in?
There are a few different kinds of reactions you must learn for the A Level exam that involve halogenoalkanes.
The first is the synthesis of chloroalkanes via the photochemical chlorination of the alkanes. I know it looks scary, but don’t worry, it is simpler than it sounds. It essentially means “forming chloroalkanes through chlorinating an alkane in the presence of sunlight”.
Chlorine will react with methane when UV light is present and will form several kinds of chloroalkanes and fumes of hydrogen chloride gas. Chloromethane was once commonly used as a refridgerant. Depending on how many chlorine molecules there are, there will be different compounds formed:
methane + chlorine -> chloromethane + hydrogen chloride
CH4 + Cl2 -> CH3Cl + HCl
or
methane + chlorine -> trichloromethane + hydrogen chloride
CH4 + 3Cl2 -> CHCl3 + 3HCl
When undergone in real life, mixtures of halogenoalkanes are produced with some long chain alkanes which can be separated out with fractional distillation.
To understand what happens in an overall chemical reaction, chemists use mechanisms. These basically show the step-by-step process that is usually shown by a simple symbol equation that summarises everything.
The chlorination of methane is something you must learn the mechanism for. It’s pretty easy but involves a lot of steps and must be revised periodically to remember them.
The actual reaction is a substitution reaction because one atom or group is replaced by another. Since the chlorine involved is a free radical, it can also be called a free-radical substitution reaction.
1. Initiation
UV light is essential for the first step in the mechanism. This breaks the Cl-Cl covalent bond so that each chlorine leaves with one electron from the shared pair. Chlorine free radicals, with one unpaired electron in the outer shell, are formed. Free radicals are only formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is homolytic fission.
2. Propagation
This has two sub-steps
(a) Chlorine free radicals (highly reactive) react with methane to form hydrogen chloride and leave a methyl free radical.
Cl• + CH4 -> HCl + •CH3
(b) This free radical then reacts with another chlorine to form chloromethane and another chlorine free radical. Producing free radicals is a chain reaction which is why it is such a problem in ozone depletion - a little amount can cause a lot of destruction.
•CH3 + Cl2 -> CH3Cl + •Cl
3. Termination
This step stops the chain reaction. It only happens when two free radicals collide to form a molecule in several ways:
Cl• + Cl• -> Cl2
UV light would just break down the chlorine molecule again, so although this is technically a termination reaction it is not the most efficient.
Cl• + •CH3 -> CH3Cl
Forming one molecule of methane uses one chlorine and one methyl free radical.
•CH3 + •CH3 -> C2H6
Ethane can be formed from two methyl free radicals - this is why there are longer chain alkanes in the mixture.
This whole process is how organic halogenoalkanes are the product of photochemical reactions of halogens with alkanes in UV light - made via free radical substitution mechanisms in chain reaction.
Another reaction you need to know is a nucleophilic substitution reactions. A nucleophile is an electron pair donor or proton acceptor - the name comes from Greek origins (”loves nucleus”) - such as hydroxide ions, cyanide ions or ammonia molecules. Hydroxide and cyanide ions are negative but ammonia is neutral.
Halogenoalkanes have a polar bond because of the difference between the highly electronegative halogen and the carbon atom. The 𝛿+ carbon can go under nucleophilic attack. The mechanism for negatively charged nucleophiles these in general is:
Nu represents the nucleophile. This example is with a bromoalkane. Make sure to include curly arrows that begin at a lone pair or the centre of a bond and end at an atom or centre of bond, and delta (slight) charges.
Lets look at a more specific example:
One nucleophile that can be used is a hydroxide ion, found in either water or sodium hydroxide. In this case, you need to know about aqueous sodium hydroxide or potassium hydroxide and a halogenoalkane. This takes place at room temperature but is slow so is often refluxed (continuously boiled and condensed back into the reaction flask). Reflux apparatus is shown below:
The halogenoalkane is dissolved into ethanol since it is insoluable in water and this solution along with the aqueous hydroxide can mix. The product produced is an alcohol, which is organic.
The general reaction is:
R-CH2X + NaOH -> CH3CH2OH + NaX
Where X represents a halogen.
You must learn the mechanism for this reaction. The lone pair on the hydroxide attacks the carbon atom attached to the halogen and this causes both carbon electrons to move to the halogen which becomes a halide ion.
The reaction of a hydroxide ion can also be classed as a hydrolysis reaction as it breaks down chemical bonds with water or hydroxide ions. The speed of reaction depends on the strength of the bond - a stronger carbon-halogen bond, a slower reaction.
C-I is the most reactive (reactivity increases down group 7) and C-F is therefore the least reactive and strongest.
Part two of this post will cover nucleophilic substitution of cyanide ions and ammonia molecules, as well as elimination reactions.
SUMMARY
You need to know about the synthesis of chloroalkanes via the photochemical chlorination of the alkanes. - “forming chloroalkanes through chlorinating an alkane in the presence of sunlight”.
Chlorine will react with methane when UV light is present and will form several kinds of chloroalkanes and fumes of hydrogen chloride gas. Depending on how many chlorine molecules there are, there will be different compounds formed.
When undergone in real life, mixtures of halogenoalkanes are produced with some long chain alkanes which can be separated out with fractional distillation.
To understand what happens in an overall chemical reaction, chemists use mechanisms. These basically show the step-by-step process.
The chlorination of methane is something you must learn the mechanism for. The actual reaction is a substitution reaction because one atom or group is replaced by another.
The first step is initiation - UV light is essential for the first step in the mechanism. This breaks the Cl-Cl covalent bond so that each chlorine leaves with one electron from the shared pair. Chlorine free radicals, with one unpaired electron in the outer shell, are formed. Free radicals are only formed if a bond splits evenly - each atom getting one of the two electrons.
Step two is propagation: (a) Chlorine free radicals (highly reactive) react with methane to form hydrogen chloride and leave a methyl free radical (b) this free radical then reacts with another chlorine to form chloromethane and another chlorine free radical. Producing free radicals is a chain reaction which is why it is such a problem in ozone depletion - a little amount can cause a lot of destruction.
To stop the chain reaction, the final step is termination. It only happens when two free radicals collide to form a molecule in several ways: two chlorine free radicals forming a chlorine molecule, two methyl FRs forming ethane or a chlorine FR and a methyl FR forming chloromethane.
Ethane contributes to the longer chain alkanes in the mixture.
Another reaction you need to know is a nucleophilic substitution reactions. A nucleophile is an electron pair donor or proton acceptor, such as hydroxide ions, cyanide ions or ammonia molecules. Hydroxide and cyanide ions are negative but ammonia is neutral.
Halogenoalkanes have a polar bond because of the difference between the highly electronegative halogen and the carbon atom. The 𝛿+ carbon can go under nucleophilic attack.
Nu represents the nucleophile. Make sure to include curly arrows that begin at a lone pair or the centre of a bond and end at an atom or centre of bond, and delta (slight) charges.
One nucleophile that can be used is a hydroxide ion, found in either water or sodium hydroxide. In this case, you need to know about aqueous sodium hydroxide or potassium hydroxide and a halogenoalkane. This takes place at room temperature but is slow so is often refluxed (continuously boiled and condensed back into the reaction flask). The halogenoalkane is dissolved into ethanol since it is insoluable in water and this solution along with the aqueous hydroxide can mix. The product produced is an alcohol, which is organic.
The general reaction is :R-CH2X + NaOH -> CH3CH2OH + NaX where X represents a halogen
The lone pair on the hydroxide attacks the carbon atom attached to the halogen and this causes both carbon electrons to move to the halogen which becomes a halide ion.
The reaction of a hydroxide ion can also be classed as a hydrolysis reaction as it breaks down chemical bonds with water or hydroxide ions.
The speed of reaction depends on the strength of the bond - a stronger carbon-halogen bond, a slower reaction. C-I is the most reactive (reactivity increases down group 7) and C-F is therefore the least reactive and strongest.
reacting halogenoalkanes with ethanol
Halogenoalkanes
Halogenoalkanes are a homologous series of saturated carbon compounds that contain one or more halogen atoms. They are used as refrigerants, solvents, flame retardants, anaesthetics and pharmaceuticals but their use has been restricted in recent years due to their link to pollution and the destruction of the ozone layer.
They contain the functional group C-X where X represents a halogen atom, F,Cl, Br or I. The general formula of the series is CnH2n+1X.
The C-X bond is polar because the halogen atom is more electronegative than the C atom. The electronegativity decreases as you go down group 7 therefore the bond becomes less polar. Flourine has a 4.0 EN whereas iodine has a 2.5 EN meaning it is almost non-polar.
The two types of intermolecular forces between halogenoalkane molecules are Van Der Waals and permanent dipole-dipole interactions. As the carbon chain length increases, the intermolecular forces (due to VDWs) increase as the relative atomic mass increases due to more electrons creating induced dipoles. Therefore the boiling point of the halogenoalkanes increases since more forces must be broken.
Branched chains have lower boiling points than chains of the same length and halogen because the VDWs are working across a greater distance and are therefore weaker.
When the carbon chain length is kept the same, but the halogen atom is changed, despite the effect of the changing polar bond on the permanent dipole-dipole interactions, the changing VDWs have a greater effect on the boiling point. Therefore as RAM increases, the boiling point increases meaning an iodoalkane has a greater boiling point than a bromoalkane if they have the same carbon chain length.
Halogenoalkanes are insoluble or only slightly soluable in water despite their polar nature. They are soluble in organic solvents such as ethanol and can be used as dry cleaning agents because they can mix with other hydrocarbons.
Summary
Halogenoalkanes are saturated carbon compounds with one or more halogen atoms. Their general formula is CnH2n+1X, where X is a halogen. Their functional group is therefore C-X.
They are used as refrigerants, solvents, pharmaceuticals and anaesthetics but have been restricted due to their link to the depletion of the ozone layer.
C-X bonds are polar due to the halogen being more electronegative than the carbon. The polarity of the bond decreases down group 7.
Van der Waals and permanent dipole-dipole interactions are the intermolecular forces in halogenoalkanes.
When carbon chain length increases, boiling points increase due to RAM increasing and the number of Van Der Waals increasing too.
In branched halogenoalkanes, Van Der Waals are working across a greater distance therefore attraction is weaker and boiling points are lower than an identical unbranched chain.
When the halogen is changed, the boiling point increases down the group due to the effect of a greater RAM - more VDWs mean more intermolecular forces to break.
Halogenoalkanes are insoluble in water but soluble in organic solvents like ethanol.
Bonus: free radical substitution reactions in the ozone layer
Ozone, O3, is an allotrope of oxygen that is usually found in the stratosphere above the surface of the Earth. The ozone layer prevents harmful rays of ultraviolet light from reaching the Earth by enhancing the absorption of UV light by nitrogen and oxygen. UV light causes sunburn, cataracts and skin cancer but is also essential in vitamin D production. Scientists have observed a depletion in the ozone layer protecting us and have linked it to photochemical chain reactions by halogen free radicals, sourced from halogenoalkanes which were used a solvents, propellants and refrigerants at the time.
CFCs cause the greatest destruction due to their chlorine free radicals. CFCs – chloroflouroalkanes – were once valued for their lack of toxicity and their non-flammability. This stability means that they do not degrade and instead diffuse into the stratosphere where UV light breaks down the C-Cl bond and produces chlorine free radicals.
RCF2Cl UV light ---> RCF2● + Cl●
Chlorine free radicals then react with ozone, decomposing it to form oxygen.
Cl● + O3 ---> ClO● + O2
Chlorine radical is then reformed by reacting with more ozone molecules.
ClO● + O3 ----> 2O2 + Cl●
It is estimated that one chlorine free radical can decompose 100 000 molecules of ozone. The overall equation is:
2O3 ----> 3O2
200 countries pledged to phase of the production of ozone depleting agents in Montreal, leading to a search for alternatives. Chemists have developed and synthesised alternative chlorine-free compounds that do not deplete the ozone layer such as hydroflurocarbons (HFCs) like trifluromethane, CHF3.
SUMMARY
Ozone, found in the stratosphere, protects us from harmful UV light which can cause cataracts, skin cancer and sunburn.
Ozone depletion has been linked to the use of halogenoalkanes due to their halogen free radicals.
CFCs were good chemicals to use because they have low toxicity and were non-flammable. The fact they don’t degrade means they diffuse into the stratosphere.
Chlorine free radicals are made when CFCs are broken down by UV light.
These go on to react with ozone to produce oxygen.
Chlorine free radicals are then reformed by reacting with more ozone.
It is a chain reaction that can deplete over 100 000 molecules of ozone.
There is a 200 country ban on their use and scientists have developed alternatives like hydrofluorocarbons to replace them
Happy studying!
Currently working on another halogenoalkanes post - can you tell I’ve got an ochem mock soon? It’ll be out soon for those interested :)
AS Flash Cards: HalogenoalkAnes
Halogenoalkanes are formed when a hydrocarbon has one or more hydrogen atoms substituted for halogen atoms.
Halogenoalkanes may be hydrolysed to form alcohols, using hot sodium hydroxide solution.
The hydroxide ion acts as a nucleophile when reacting with a halogenoalkane. A nucleophile is a lone pair electron donor.
The C–I bond is weaker than the C–Br and C–Cl bond, and so it breaks faster when attacked by nucleophiles.
USES
Chloroethene and tetrafluoroethene are used to make the polymers PVC and PTFE (Teflon) respectively.
CFCs (chlorofluorocarbons) were manufactured for use in aerosols and refrigerants because of their low reactivity and high volatility.
CFCs have caused the ozone layer to break down, leading to an increased intensity in damaging ultraviolet radiation on Earth.
Biodegradable CFCs and HCFCs are now used as alternatives to CFCs.
Physical Properties of Halogenoalkanes
Miscibility
Immiscible in water, but miscible in ethanol
To dissolve in water, the forces between the halogenoalkanes (VDW) and the forces between the water (hydrogen bonds). Both require energy
Energy is released when intermolecular forces are formed between the halogenoalkanes and water. However, the forces formed are weaker than the original hydrogen bond.
Not 'profitable' to dissolve in water
However hydrogen bonds in ethanol are weaker, so the hydrogenoalkane is miscible in ethanol
Viscosity
less viscous than water
weaker intermolecular forces in halogenoalkanes, therefore easier to slide over each other compared to water molecules
Density
Denser than water
Packages molecules better
b.p./m.p.
higher than alkanes/alkenes, but lower than water
Intermolecular forces (permanent and temporary dipole-dipole interactions) stronger than alkanes/alkenes (temporary dipole-dipole interactions), but weaker than water (hydrogen bonds)
Electroconductivity
Does not conduct electricity
No free electrons
Halogenoalkanes
These are created through radical substitution (see the post on that for more). The general formula for halogenoalkanes with one halogen is : "C(n)H(2n+1)X" where X shows a halogen such as Br or Cl There are Primary, Secondary and Tertiary halogenoalkanes, depending on the number of R groups or hydrogen atoms the adjoining carbon is attached to (like alcohols) The C-X bond has a dipole. The X is d- and the carbon is d+. This leaves it open to "attack" by negative nucleophiles, such as aqueous OH- ions. X- is a stable ion on its own, so X is substituted easily. This is known as nucleophilic subtitution. Because the C-F bond is the strongest (highest bond enthalpy), and the C-I bond is the weakest, fluoroalkanes are the least reactive and iodoalkanes are the most reactive.
image from http://www.chemhume.co.uk
Halogenoalkanes also react with water, in a hydrolysis reaction, to make alcohol. C2H5Br + H20 ---> C2H5OH + HBr.
