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Halogenoalkanes (3.3.3)
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Halogenoalkanes have a polar carbon–halogen () bond.
Due to the higher electronegativity of halogens compared to carbon, electrons in the covalent bond are drawn towards the halogen. This creates a carbon and a halogen.
The polarity of the bond makes the carbon electrophilic and susceptible to nucleophilic attack.
Substitution reactions involve the replacement of an atom or group in a molecule by another atom or group.
Halogenoalkanes commonly undergo substitution reactions, where the halogen atom is replaced by a nucleophile, such as , , or .
Primary halogenoalkanes react via the generic nucleophilic substitution mechanism shown below.
In an exam, remember to include the partial charges on the bond.
The reaction is initiated by the attack of the 𝛿+ carbon in the halogenoalkane by an electron pair from the nucleophile. Note that the arrow starts from the lone pair drawn on the nucleophile.
bond formation and bond breaking are simultaneous. The reaction proceeds via a transition state. This is a theoretical state of the molecule that cannot be isolated and represents the maximum in the energy profile.
The final product is inverted, as the nucleophile attacks from behind the bond.
When all other conditions are matched, the trend for the rate of hydrolysis for halogenoalkanes is as follows:
This directly correlates with the decrease in bond enthalpy of bonds down group , shown in the table.
is also shown to show bond strength in the alkyl group, explaining why fluoroalkanes are highly unreactive.
The conversion of a halogenoalkane to an alcohol via nucleophilic substitution is called hydrolysis. Halide ions are produced during the reaction.
Ethanolic can be used to identify the type of halogen present, and to study how fast the reaction progresses.
The addition of is a key qualitative test in A-level chemistry, as forms a precipitate with halide anions.
Halide ions are formed as the reaction progresses. The reaction rate is taken as the time taken for a silver halide precipitate to become visible.
Generally, the rate of hydrolysis of haloalkane in pure water is slow.
Aqueous alkaline conditions can increase the rate of hydrolysis of a halogenoalkane. The ion is a more nucleophilic species than water and is present at higher concentration in alkaline conditions.
Water and halogenoalkanes have low miscibility with each other, but both of them are miscible in ethanol. Adding ethanol to the reaction mixture allows better mixing of reactants, resulting in more collisions per unit time and higher reaction rates.
Nucleophiles can be negatively charged (e.g. ) or neutral with available electron lone pairs (e.g. )
Nucleophiles are electron pair donors.
The term nucleophile comes from the words ‘nucleus’ and ‘philos’ (friend in Greek).
The electrons in a nucleophile are attracted to positively charged areas; areas of low electron density, exposing the positive nuclear charge.
Elimination reactions will occure under hot, ethanolic conditions with a strong base like .
In elimination, acts as a base, abstracting a proton () from a -carbon, leading to the formation of a double bond and the elimination of a halide ion.
This contrasts with the role of as a nucleophile in the conditions for nucleophillic substitution, where it directly attacks the carbon.
The double bond formed in elimination reactions can result from the abstraction of a proton from any -carbon. As a result, a mixture of isomers is expected in the product.
It is important to consider the possible positions of the double bond, as well as the E/Z orientation of the groups in the product.
Remember to exclude any products that are superimposable when stating the number of isomers formed.
When halogenoalkanes react with hot ethanolic potassium hydroxide, an elimination reaction occurs. Hydroxide ions are nucleophilic.
In elimination reactions, the hydroxide ion also acts as a base; it abstracts an acidic proton from the -carbon.
The ozone layer absorbs and blocks a portion of the ultraviolet (UV) radiation emitted by the sun. This minimises the damage to human skin, as well as to other living organisms, caused by overexposure to this damaging radiation.
Ozone () forms naturally in the upper atmosphere through a series of reactions involving oxygen molecules and UV radiation.
High-energy UV light splits an oxygen molecule () into two oxygen atoms (), which are highly reactive.
Each oxygen atom then reacts with another oxygen molecule to form ozone.
The oxygen atoms formed in the initial splitting of the oxygen molecule are diradicals – they have two unpaired electrons.
Some gaseous halogenoalkanes, like chlorofluorocarbons (CFCs), can cause damage when allowed to enter the atmosphere. In the upper atmosphere, UV radiation causes homolytic fission of the bond, producing free radicals.
These radicals go on to react with ozone (), reducing its atmospheric abundance.
Fission in a CFC occurs at the bond which is significantly weaker than the bond.
The Montreal Protocol is a treaty signed to reduce the production and release of substances associated with depletion of the ozone layer.
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were developed as propellants and refrigerants but have since been regulated through the Montreal Protocol and subsequent legislation.
Hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs) were developed as alternatives to CFCs and HCFCs. HFCs and HFOs do not contain the weak bond linked to the production of chlorine radicals. They are not harmful to the ozone layer but are potent greenhouse gases.
Optimising use of natural refrigerants, such as ammonia, carbon dioxide, and hydrocarbons, is increasingly popular due to their reduced environmental impact.
The overall reaction for the breakdown of ozone is:
The degradation of ozone by radicals in the upper atmosphere is a radical chain reaction.
Initiation: Formation of a halogen radical from the CFC. UV light is required for this step.
Propagation: Reaction between a radical species and a non radical species resulting in a radical and non radical product.
Note that the chlorine radical, which is a reactant in the first propagation step, is a product following the second propagation step, making it catalytic. One radical can destroy many molecules of ozone.
Termination: Reaction between two radical species to form a non radical product.
e.g.