Alcohol formation [H2O]
Alcohol formation (H2O) Definition:
Alkyl halides will react with water to form alcohols through an SN1 mechanism.
Alcohol formation (H2O) Explained:
Unimolecular nucleophilic substitution abbreviated SN1, in which Unimolecular or number 1 indicates that only one molecule participates in the rate-determining step. This means the rate of the reaction does not depend on the concentration of the nucleophile. The rate is dependent only on the concentration of the substrate. This means that if we would increase or decrease a concentration of a nucleophile, there will be no change in reaction rate. Thus, the rate-determining step is the step where nucleophile does not occur.
When the solvent is water, the term hydrolysis is applied. But if the solvent is not water, and also participates in the reaction as a nucleophile then we use term – solvolysis.
In these conditions, even secondary and tertiary halides do undergo substitution reactions. The relative reactivity of different substrates in SN1 is opposite in SN2 which means that the tertiary halides are the most reactive, and the methyl and primary halides are the least reactive. This also applies to carbocations. Alkyl group are electron donor groups that stabilize a positive charge of the carbocation.
The positively charged carbon has a trigonal planar geometry which is a result of sp² hybridization. So that the carbocation can be attacked from either side, leading to both inversions of configuration and retention of configuration. Although starting with an optically active secondary or tertiary alkyl halide, under condition favorable for SN1 reaction we should obtain racemic products (example Ether formation [ROH]).
This mechanism consists of three steps:
STEP 1. This is the rate-determining step in which comes to dissociation of the alkyl halide to the carbocation and the leaving group.
STEP 2. Carbocation formed in the previous step is powerful electrophile which immediately trapped by the surrounding water. Alkyloxonium ion is obtained.
STEP 3. All alkyloxonium ions are strong acids and they are deprotonated by the water to give the final alcohol.