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Halogenoalkanes (Topic 6D)
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Substitution reactions involve the replacement of an atom or group in a molecule by another atom or group.
Haloalkanes commonly undergo substitution reactions, where the halogen atom is replaced by a nucleophile, such as , , or .
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.
When haloalkanes 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.
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.
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.
Halogenoalkanes are classified as primary, secondary, or tertiary depending on the number of alkyl groups attached to the carbon.
Primary halogenoalkanes have the general formula .
Secondary halogenoalkanes have the general formula .
Tertiary halogenoalkanes have the general formula .
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.
The conversion of a haloalkane 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 qualitative test in chemistry, as forms different coloured precipitates 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 haloalkane. The ion is a more nucleophilic species than water and is present at higher concentration in alkaline conditions.
Water and haloalkanes 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.
When all other conditions are matched, the trend for the rate of hydrolysis for haloalkanes 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.
Tertiary carbocations are the most stable due to the charge-stabilising effect of the three alkyl groups.
The reactivity of haloalkanes follows the stability trend of the carbocations that would be formed following the loss of a halide ion.