2.2.2 HALOGENOALKANES
Introduction
The haloalkanes occupy an important part in organic synthesis because they can undergo a multiplicity of different chemical reactions to form a multitude of new organic materials.
Naming
They are named using the prefix depending on the halogen present and the position of that halogen on the alkane (parent) chain.
F- Fluoro, Cl- Chloro, I- Iodo, Br- Bromo
Fluoroalkanes are unreactive 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. The iodoalkanes are the most reactive of the haloalkanes because the bond is weak and easily broken.
Student activity 1
Write the names of the following haloalkanes:
1.
a) CH3CH2CH2CH2Cl ......
b) ......
c) ......
d) ......
2. Draw skeletal formula for each of the following halogenoalkanes
a) 2-chlorobutane
b) 2-iodopentane
c) 3-chloro-5-methyloctane
d) 1,1,1-trifluoro-3-methylbutan-2-ol
e) 1-bromo-2-chloro-3-propylcyclohexane
f) Z-2,3-dibromo-4-methyl-4-iodohept-2-ene (challenging)
Classifying halogenoalkanes
Halogenoalkanes can be classified as primary, secondary or tertiary.
Primary halogenoalkanes:
The -halo group is attached to a carbon atom
with at least two H atoms attached.
Use the formula C3H7Cl to draw a primary
halogenoalkane.
Secondary halogenoalkane:
The -halo group is attached to a carbon
atom with only one H atom attached.
Use the formula C3H7F to draw a secondary
halogenoalkane
Tertiary Halogenoalkanes:
The -halo group is attached to a carbon atom
with no H atoms attached.
Use the formula C5H11I to draw the structure of
a tertiary halogenoalkane.
Physical Properties
There is an increase in boiling points, melting points and density in comparison to the alkanes. This arises from the increased intermolecular attractions occurring between haloalkane molecules. The presence of the polar bond and the slight charges on both the carbon and the halogen results in increased attractions. It is these dipolar forces which must be overcome in order to melt or boil a sample. Typically, halogenoalkanes are volatile liquids that do not mix with water.
What sort of intermolecular forces exist between haloalkanes?
…………………………………..
What sort of intermolecular forces exist between haloalkane molecules and water?
…………………………………..
Halogenoalkanes are more soluble in water than the alkanes and the alkenes, but they are still only slightly soluble.
Chemical Reactivity of the halogenoalkanes
The chemistry of the haloalkanes results from the polar nature of the carbon-halogen bond.
1. Define the term electronegativity
......
......
2. Explain why the bond between carbon and chlorine is polar.
......
......
3. The reactivity of the halogenoalkanes also depends on the strength of the carbon halogen bond. The standard bond enthalpies (in kJmol-1) are:
C-F = 467
C-Cl = 340
C-Br = 280
C-I = 240
Using the data in the table above predict which of the halogenoalkanes will be the most reactive. Explain your answer referring to the bond enthalpies. We will consider this now in practical 18 where we hydrolyse with hot aqueous alkali, neutralise and then add aqueous silver nitrate; the important bit is the hydroxide ion.
......
......
......
Summary of comparison of the ease of hydrolysis
Hydrolysis of the halogenoalkanes occurs most rapidly with iodoalkanes. The reactivity is shown below.
Iodoalkanes > bromoalkanes > chloroalkanes > fluoroalkanes
This can also be observed by adding aqueous ethanolic (this dissolves the haloalkanes better) silver nitrate to the reaction mixture and timing the first appearance of a silver halide precipitate. This is observed as soon as sufficient halide ions have been formed by the hydrolysis of the halogenoalkane. The important bit is water.
The X- ions liberated will then react with the silver ions in the solution to form the precipitate
Experimental Results
Reagents / Aqueous ethanolic silver nitrate1-chlorobutane / Slow production of a pale white precipitate
1-bromobutane / Cream precipitate produced more rapidly
1-iodobutane / Immediate production of a yellow precipitate
You must look at and learn a summary of the method for this practical. See experiment 18. It is also worth looking at the demo we did: D10. This is actually a nucleophilic substitution reaction from an alcohol to a haloalkane – the reverse of what we did above.
Student activity 2 – Name the functional groups present
Reactivity of the halogenoalkanes
The carbon-halogen bond is polar and this gives rise to its reactivity.
The carbon atom repels electrons and the chlorine atom pulls electrons towards itself. 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 is attacked by negatively charged species known as nucleophiles.
What is the definition of a nucleophile?
………………………………………………………………………………..
Reactions of the halogenoalkanes
The reaction with a nucleophile is a substitution reaction which results in hydrolysis – the reaction of a compound with water which results in the decomposition (splitting up) of the water (NB not the same as hydration) and a chemical change to produce two or more products
Nucleophilic Substitution Reactions
General Mechanism for nucleophilic substitution:
Nucleophiles are ions or molecules, which are attracted towards a positive charge. They are negatively charged species or have a lone pair of electrons. They are defined as electron pair donors.
Examples: - OH-, CN-, NH3, H2O
Nucleophilic Substitution reactions of the haloalkanes
Reagents: Sodium hydroxide aqueous
Conditions: Warm (or reflux)
Equation: RX + NaOH NaX + ROH
Ethanol is formed rapidly when the halogenoalkane is heated with the sodium hydroxide under reflux. This is the best method for preparing an alcohol, as the yield is considerably better than reacting the halogenoalkane with water which is a much poorer nucleophile (why?) – refer back to Experiment 18 and the notes.
The overall equation for the reaction does not show us the whole picture of how the reaction occurs. A reaction mechanism is a sequence of steps that shows how the reaction takes place.
Be careful about the difference between hydration and hydrolysis!
Hydration: adding water across a double bond (you get 1 product).
Hydrolysis: splitting water by reaction with another compound (you get 2 products).
Mechanism
The nucleophile approaches the positively charged carbon atom on the opposite side to the halogen. In this way the repulsion between the nucleophile and the halogen is at a minimum. As a bond forms between the nucleophile and the positively charged carbon atom, the carbon halogen bond is broken. One in and one out – simple. See over the page!
The reaction of chloromethane with OH-:
General equation:
Mechanism:
The reaction of chloromethane with H2O:
General equation:
Mechanism:
Ammonia reacting with bromoethane
…………………………………………………………………………………………………
…………………………………………………………………………………………………
Two moles of ammonia are needed.
The Uses of Halogen Compounds
Halogenoalkanes occupy an important part in organic synthesis. They are intermediates in the production of many other materials.
Halogenoalkanes also have direct applications including: -
· Polymers such as poly(chloroethene) 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 refrigerants, aerosol propellants, blowing agents for the production of foams and in air conditioning units (low reactivity, volatility and non-toxicity)
· Chlorofluorocarbons such as CCl2FCCl2F, used as a dry cleaning solvent or as a degreasing agent
Chemists and the environment
CFCs are regularly blamed for causing damage to the environment. Although they absorb much more infrared radiation than carbon dioxide, their contribution to the greenhouse effect is very low due to their low abundance in the atmosphere. CFCs are responsible for the thinning of the ozone layer in the stratosphere. Ozone absorbs significant proportions of ultraviolet radiation and thus protects us from skin cancer.
CFCs are still used in air conditioning units and were used in refrigerants until recently. They are unreactive, non-flammable and non-toxic, but the stability of the CFCs has been part of the problem in ozone depletion. This has led to the build up of CFCs in the atmosphere.
When they reach the stratosphere CFCs absorb ultraviolet radiation, which causes photodissociation of the carbon-chlorine bonds.
e.g. CF2Cl2 à CF2Cl + Cl
Very reactive chlorine radicals are formed. These radicals catalyse the decomposition of ozone to oxygen
Cl
2O3 à 3O2
Scientists are currently working to develop new materials, which cause less damage to the environment. The compound 1,1,1,2-tetrafluoroethane is now used as an alternative to CFCs. the presence of the hydrogen atoms increases the reactivity of the compound and so it undergoes reaction lower in the atmosphere and so does not produce damaging free radicals. This type of compound is called an HCFC. Carbon dioxide is an alternative blowing agent for making expanded polymers now. Hydrocarbons are used as alternative solvents for organic chemicals.
Student activity 3 Show the reagents necessary to make the following products.
1