What happens to the halogen atom when a halogenoalkane undergoes nucleophilic substitution?
The halogen atom is replaced by a nucleophile.
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Describe, and give the ionic equation for, the reaction of halogenoalkanes (RX) with aqueous alkali (OH-).
Halogenoalkanes are warmed with aqueous alkali to produce an alcohol. A hydrolysis reaction occurs.
RX + OH- ➔ ROH + X-
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Explain the role of OH- in the nucleophilic substitution reaction mechanism of halogenoalkanes.
The OH- ion acts as the nucleophile, donating a pair of electrons to the carbon atom bonded to the halogen.
This breaks the carbon-halogen bond and forms a new carbon-oxygen bond.
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What type of bond fission (homolytic or heterolytic) occurs in a nucleophilic substitution reaction of a halogenoalkane?
Heterolytic fission
The bond breaks to give a halide ion and a carbocation.
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What does the notation SN1 represent?
SN1 represents a nucleophilic substitution reaction mechanism where the reaction rate depends only on the concentration of the halogenoalkane. It involves the formation of a carbocation intermediate.
The reaction between (CH3)3CBr and OH- proceeds via an SN1 mechanism:
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What does the notation SN2 represent?
SN2 represents a nucleophilic substitution reaction mechanism where the reaction rate depends on both the concentration of the halogenoalkane and of the nucleophile. It involves a one-step substitution process.
The reaction between CH3CH2Br and OH- proceeds via an SN2 mechanism:
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How does halogenoalkane strucutre affect whether SN1 or SN2 reactions predominate?
Primary halogenoalkanes tend to react via an SN2 mechanism.
Tertiary halogenoalkanes tend to react via an SN1 mechanism.
Secondary halogenoalkanes react via both SN2 and SN1 mechanisms.
The is due to differences in carbocation stability.
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