Name the alcohol that has been produced and classify the reaction type?

Question

water is added to but-2-ene -->

type of reaction: (addition, substitution, elimination?)

Answer 1

The reaction type is an electrophilic (hydration) addition that usually proceeds under acid-catalyzed conditions. **Note, there are mechanistic similarities to the second-step of S_N1 substitution reactions that involve the nucleophilic-attack addition to a carbocation-intermediate.

It is critical to consider the stereoselectivity and regioselectivity aspects of any reaction, when applicable. The case for the reactant starting material - but-2-ene - demonstrates cis- / trans- configurational (stereo)isomerism; and, further, the E / Z isomerism notation can also used as a symmetry-heuristic to denote the diastereomeric relationship that exists in the absolute stereogenic orientation (geometric configuration) of substituent groups on carbon-carbon double-bonding motifs as seen in unsaturated hydrocarbons, ex., alkenes. —Remember, stereogenic priority is assigned using the Cahn-Ingold-Prelog sequence rules that are based on atomic-# designations.

Electrophilic addition reactions tend to proceed with Markovnikov (regiospecific) orientation and loss of stereochemistry. It is my understanding that due to the (1) symmetry of the but-2-ene molecule and the (2) implicated reaction mechanism—involving the nucleophilic-attack at an sp²-hybridized carbon site—, that the cis/trans stereochemistry of the reactant material is not necessarily relevant in this specific hydration reaction and results in randomization of stereoselectivity in the alcohol products formed (racemization). However, the case is otherwise so in the halogenation addition (X₂) of (E/Z-)-but-2-ene or for alkene groups in cyclic ring structures.

Remember that the general mechanism for electrophilic addition involves π-bonding electrons as a quasi-nucleophile to add an electrophilic-hydrogen at the π-MO site resulting in the formation of a carbocation intermediate (rate-limiting step). Then, an H₂O molecule attacks as a nucleophile, which is then deprotonated (-H) to form an alcohol product (fast step). The reaction induces chirality at the carbon-2 site from the addition of the hydroxyl group. The enantiomeric hydration products are expected to be:

• (R)-(–)-butan-2-ol

• (S)-(+)-butan-2-ol

These are secondary alcohols; non-IUPAC nomenclature includes sec-butyl alcohol.

How is the case regioselectivity &/or stereoselectivity different for the acid-catalyzed electrophilic hydration-addition of 1,2-dimethylcyclohexane?

Compare other reaction mechanisms, like oxymercuration-reduction or hydroboration-oxidation, that produce more-optimal yields for alcohol preparation from an alkene!