Mechanism for 3º Alcohol (1º and 2º function the same way):

Temperatures for Types of Alcohol Synthesis

Heat is used to catalyze the elctrophilic hydration, and since it is in equilbrium with the dehydration of an alcohol which requires higher temperatures to form an alkene, lower temperatures are required to form an alcohol.  The exact temperatures used are highly variable and depend on the product being formed.

• Primary Alcohol:  Less than 170ºC
• Secondary Alcohol:  Less than 100ºC
• Tertiary Alcohol:  Less than 25ºC

But...Why Does Electrophilic Hydration Work?

• An alkene placed in an aqueous non-nucleophilic strong acid immediately "reaches out" with its double bond and attacks one of the acid's hydrogens in red (while the bond between the oxygen and hydrogen performs heterolytic cleavage towards the oxygen- in other words, both electrons that were in the oxygen/hydrogen single bond move onto the oxygen). 
• A carbocation is formed on the original alkene (now alkane) in the more substituted position where the oxygen end of water will attack with its 4 non-bonded valence electrons (it has 6 total valence electrons because it is found in group 6 on the periodic table and the second row down- 2 in a 2S orbital and 4 in 2P orbitals.  For each bond oxygen has made, oxygen has donated 1 valence electron to that bond.  This leaves oxygen with 4 non-bonded valence electrons). 
• After the blue oxygen forms its 3rd bond with the more substituted carbon, it develops a positive charge (3 bonds and 2 valence electrons make the blue oxygen have a formal charge of +1 since it does not have enough electron density around it). 
• The bond between the green hydrogen and the blue oxygen undergoes heterolytic cleavage, and both the electrons from the bond move onto the blue oxygen.   The now negatively charged strong acid picks up the green electrophilic hydrogen. 
• Now that the reaction is complete, it is clear that the non-nucleophilic strong acid is regenerated as a catalyst and an alcohol has formed on the most substituted carbon of the current alkane.  With lower temperatures, more alcohol product can be formed.

What is Regiochemistry and How Does It Apply?

Regiochemistry deals with where the substituent will bond on the product. Zaitsev's and Markovnikov's rules address regiochemistry, but Zaitsev's Rule applies to making an alkene while Markovnikov's Rule describes where the substituent will bond onto the product. Since we are synthesizing alcohol by electrophilic hydration, Markovnikov's Rule is the only rule that directly applies. See here for another in-depth explanation of regiochemistry Markovnikov explanation: Radical Additions--Anti-Markovnikov Product Formation

In the mechanism for a 3º alcohol drawn above, the red H was added to the least substituted carbon that was connected to the nucleophilic double bonds (it has less carbons attached to it).  This means that the carbocation forms on the 3º carbon, causing it to be highly stabilized by hyperconjugation- electrons in nearby sigma (single) bonds will help fill the empty p orbital of the carbocation which lessens the positive charge.  More substitution on a carbon means more sigma bonds are available to "help out" (by using overlap) with the positive charge, which means more carbocation stability.  This translates into: carbocations will form on the most substituted carbon that was connected to the double bond.  Carbocations are also stabilized by resonance, but resonance is not a large factor here since any carbon-carbon double bonds are used to initiate the reaction and other double bonded molecules can cause a completely different reaction.

If the carbocation does originally form on the less substituted part of the alkene, carbocation rearrangements will occur to form more substituted products by:

• Hydride shifts- a hydrogen bonded to a carbon next to the carbocation will leave that carbon to bond with the carbocation (after the hydrogen has taken both electrons from the single bond, it is known as a hydride).  This changes the once neighboring carbon to a carbocation, and the former carbocation changes into a neighboring carbon.

• Alkyl shifts-  when there is no hydrogen available for a hydride shift, an entire methyl group will perform the same shift.

The nucleophile will attack the positive charge that has been formed on the most substituted carbon connected to the double bond, since the nucleophile is seeking that positive charge.  In the mechanism for a 3º alcohol drawn above, water is the nucleophile.  When the green H is removed from the water molecule, it is clear the alcohol has added to the most substituted carbon that was connected to the double bond. Hence, electrophilic hydration follows Markovnikov's Rule.

What is Stereochemistry and How Does It Apply?

Stereochemistry is how the substituent will bond on the product directionally. Dashes and wedges denote stereochemistry- whether the molecule or atom is going into or out of the plane of the board.  Whenever the bond is a simple single straight line, the molecule that is bonded is equally likely to be found going into the plane of the board as it is out of the plane of the board. This means the product is a racemic mix.

Electrophilic hydration follows a stereochemistry where the substituent will be equally likely to bond going into the plane of the board as it is going out of the plane of the board. The 3º alcohol product could look like either of these:

Note:  Whenever a straight line is used along with dashes and wedges on the same molecule, it could be denoting that the straight line bond is in the same plane as the board. Practice with a molecular model kit and attempting the practice problems at the end would help solve any unclarities about what a straight line is denoting. 

Is this a Reversible Synthesis?

Yes, it is because an alkene in water is in equilibrium with the alcohol product. To sway the equilibrium one way or another, the concentration of the non-nucleophilic, strong acid and the temperature can be changed. For example:

• Less sulfuric or phosphoric acid and an excess of water help synthesize more alcohol product.
• Lower temperatures help synthesize more alcohol product.

Is There a Better Way to Add Water to Synthesize an Alcohol From an Alkene?

A better way does exist!  How about Oxymercuration - Demercuration: A Special Electrophilic Addition?  Oxymercuration does not allow for rearrangements, but it does require the use of mercury, which is highly toxic. Detractions for using electrophilic hydration to make alcohols include:

• Allowing for carbocation rearrangements
• Poor yields due to the reactants and products being in equilibrium
• Allowing for product mixtures (such as an (R)-enantiomer and an (S)-enantiomer)
• Using sulfuric or phosphoric acid (is a safety precaution)