Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds
Acid catalyzed hydration
1. Electrophilic addition at c=c
ACID CATALYZED HYDRATION:
Notice that the base used to deprotonate the oxonium ion is H2O rather than a hydroxide ion (HO-).
Why? In acidic conditions, the concentration of hydroxide ions is extremely low, but the
concentration of H2O is quite large.
Reaction proceeds via a Markovnikov addition, just as we saw for hydrohalogenation, because
there is a strong preference for the reaction to proceed via the more stable carbocation
intermediate.
Reaction rates for substituted alkenes can be justified by comparing the carbocation intermediates
in each case. Reactions that proceed via tertiary carbocations will generally occur more rapidly than
reactions that proceed via secondary carbocations.
Controlling the Position of Equilibrium for acid-catalyzed hydration:
The equilibrium is sensitive not only to temperature but also to the concentration of water that is
present. By controlling the amount of water present (using either concentrated acid or dilute acid),
one side of the equilibrium can be favored over the other:
COnCEpTUAL CHECKpOInT:Identify whether you would use dilute sulfuric acid or concentrated
sulfuric acid to achieve each of the following transformations. In each case, explain your choice.
Stereochemistry of Acid-Catalyzed Hydration:
The stereochemical outcome of acid-catalyzed hydration is similar to the stereochemical outcome
of hydrohalogenation. Once again, the intermediate carbocation can be attacked from either side
with equal likelihood (Figure ). Therefore, when a new chirality center is generated, a racemic
mixture of enantiomers is expected:
2. FiguRE :
In the second step of acidcatalyzed hydration,the carbocation intermediateis planar and can beattacked from either face,
leadingto a pair of mirror-imageproducts (enantiomers).
In this reaction, water is added across an alkene in a Markovnikov fashion
under acidcatalyzed conditions. As a result, the OH is positioned at the more substituted carbon. To draw a
mechanism for this process, recall that the proposed mechanism for acid-catalyzed hydration has three
steps: (1) protonation to give a carbocation, (2) nucleophilic attack of water to give an oxonium ion, and
(3) deprotonation to generate a neutral product.
STEP 1
Using two curved arrows, protonate the alkene to form the more stablecarbocation.
3. STEP 2
Using one curved arrow, draw a water molecule attacking the carbocation.
STEP 3
With two curved arrows, deprotonate the oxonium ion using water as a base.
