The Addition of Water to an Alkene

An alkene does not react with water, because there is no electrophile present to start the reaction by adding to the alkene. The O — H bonds of water are too strong—water is too weakly acidic—to allow the hydrogen to act as an electrophile. 

If an acid (the acid used most often is H2SO4) is added to the solution, then a reaction wiU occur because the acid provides the electrophile. The product of the reaction is an alcohol. 

The addition of water to a molecule is called hydration, so we can say that an alkene will be hydrated in the presence of water and acid. 

H2SO4 = -5) is a strong acid, so it dissociates almost completely (Section 2.2). The acid that participates in the reaction, therefore, is most apt to be a hydrated proton. This is called a hydronium ion, which we will write as H30.  

When you look at the mechanism for the acid-catalyzed addition of water to an alkene, notice that the first two steps are the same (except for the nucleophile employed) as the two steps of the mechanism for the addition of a hydrogen halide to an alkene (Section 6.1). 

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Hydration is the addition of water to an alkene to form an alcohol. HiO itself is too weak an acid to protonate an alkene, but with added H2SO4, H30 is formed and addition readily occurs. 

Although alcohols can be relatively easily dehydrated to form an alkene, the reverse is not as easily done alcohols cannot be synthesised in a single simple step in an ordinary laboratory by the addition of water to an alkene. However, in industry it is very different. Most ethanol these days is manufactured by hydration of ethene using a catalyst at high temperature and pressure ... 

The overall result of the sequence hydroboration -I- oxidation is a regioselective and-Markownikoff-addition of water to an alkene. This reaction is an important method in organic synthesis, since it can be made stereoselective and even enantioselective. 

The addition of halogens, halogen acids, water and hypohalides proceeds according to the trans scheme. As an example we take the addition of bromine to an alkene which proceeds in the following steps. 

Mechanism of the Markovnikov addition of water to an alkene to yield an alcohol. 

From the encouraging results obtained in the reactions of a series of gold(III) oxo complexes with olefins [56], Cinellu et al. tried to achieve the supposed oxametalla-cyclic intermediate, which had never been isolated before [25]. In the reaction of 8 and norbornene 56, if the - atoms were considered to be equivalents of coordinated water, and it was therefore possible to talk about the gold-catalyzed addition of water to an alkene. The metallaoxetane 58 was separated from the gold-alkene complex 57 and characterized by X-ray crystal structure analysis. The subsequent stoichiometric reaction yielded epoxide 59 (Scheme 8.5). 

The overall result of the hydroboration-oxidation sequence is addition of water to an alkene with the opposite regiochemistry to that expected for a conventional acid-catalysed hydration. The usual way to do such a hydration is by oxymercuration-reduction. 

Hydrogen peroxide, H202 Oxidizes organoboranes to yield alcohols. Used in conjunction with addition of borane to alkenes, the overall transformation effects syn Markovnikov addition of water to an alkene (Section 7.5). 

As a simple case, the chlorination of a C—H bond, converting it to C—Cl, is certainly an oxidation. The chloride can be solvolyzed to an alcohol, which is an oxidation product of the original C— H bond, and no one would think of the solvolysis as the oxidation step. Thus we will consider as an oxidation any conversion of a C—H bond to a derivative in which the hydrogen has been replaced by a more electronegative element. Furthermore, conversion of a saturated carbon to an unsaturated one will be considered an oxidation of that carbon. In principle the double bond could be hydrated so as to leave the hydroxy group on that carbon, and addition of water to an alkene is not an oxidation step. 

The addition of bromine to an alkene in the presence of water can lead to the formation of a 1,2-bromo-alcohol (or bromohydrin) in addition to a 1,2-dibromide. This is because water can act as a nucleophile and compete with the bromide ion for ring-opening of the bromonium ion. Even though Br is a better nucleophile than H20, if H20 is present in excess, then the 1,2-bromo-alcohol will be formed. 

At this point, you know only one way to synthesize a ketone—the addition of water to an alkyne (Section 6.6). The alkyne can be prepared from two successive E2 reactions of a vicinal dihalide, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction. 

Fermentation ceases when the ethanol content of the mixture reaches about 14% (v/v) higher concentrations of ethanol kill the yeast. If a higher concentration of alcohol is required, the mixture is fractionally distilled. Spirits such as whisky or brandy may contain about 40% ethanol. Although fermentation is used to produce alcoholic beverages, it is rarely used to synthesize the chemical ethanol on a large scale. Alcohols, such as ethanol, can be produced commercially by the addition of water across an alkene double bond (a hydration reaction). The reaction is catalysed by phosphoric acid ... 

Hydration of alkenes is the addition of the elements of water (H and OH) across the carbon-carbon double bond. There is substantial evidence that acid-catalyzed addition of water to an alkene involves a cationic intermediate. Rate constants for hydration increase with the electron-donating ability of the substituents on the double bond, and rate constants for hydration of im-symmetrical alkenes with the general formula RiR2C=CH2 give a good correlation with cr values. 

The addition of an acid such as HX to an alkene leads to formation of an alkyl halide. If the acid catalyzed addition of water or an alcohol is examined, the product is an alcohol or an ether. Oxymercuration also leads to addition of water to an alkene. The functional group exchange can be generalized as shown, where X = Cl, Br, I, OH, OR. When an alkene reacts with a halogen, the product is a vicinal dihalide. The functional group exchange is shown where X = Cl, Br, I. When bromine in water or chlorine in water is reacted with an... 

Since the ionic addition of hydrogen bromide and the acid-catalyzed addition of water to an alkene proceed via planar carbocations, the formation of products derived from rearranged carbocations is relatively common. In contrast, the bromi-nation of alkenes, which is illustrated by the addition of bromine to cyclopentene (Eq. 10.18), occurs via a cyclic bromonium ion 38 related to 33, so skeletal rearrangements of such cations are not generally observed. Moreover, the intervention of this cyclic ion dictates that the stereochemistry of the addition is completely in the anti sense, as shown. The reddish color of bromine is discharged upon addition to an alkene, making this reaction a useful qualitative test for unsaturation (Sec. 25.8A). 

In the second step of the reaction, water acts as a nucleophile and adds to the Lewis acid (the protonated carbonyl) to generate a protonated diol (Fig. 16.25). In the final step of this acid-catalyzed reaction, the protonated diol is deprotonated by a water molecule to generate the neutral diol and regenerate the add catalyst, H30. A diol with both OH groups on the same carbon is called a em-diol (gem stands for geminal) or a hydrate. Note how closely related are the acid-catalyzed additions of water to an alkene and to a carbonyl group. Each reaction is a sequence of three steps protonation, addition of water, and deprotonation. 

MECHANISM FOR THE ACID-CATALYZED ADDITION OF WATER TO AN ALKENE... 

At this point in your study of organic chemistry, you know only three ways to synthesize a ketone (1) the addition of water to an alkyne (Section 7.7), (2) hydroboration-oxi tion of an alkyne (Section 7.8), and (3) ozonolysis of an alkene (Section 6.12). Because the target molecule has the same number of carbons as the starting material, we can rule out ozonolysis. Now we know that the precursor molecule must be an alkyne. The alkyne needed to prepare the ketone can be prepared from two successive E2 reactions of a vicinal dihahde, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction, using a bulky base to maximize the elimination product... 

Notice that the acid-catalyzed dehydration of an alcohol is the reverse of the acid-catalyzed addition of water to an alkene (Section 6.5). 

The hydroboration-oxidation product has a hydroxyl group at the CH, site and a hydrogen atom at the ring carbon atom. This process is equivalent to Markovnikov addition of water to an alkene. 

Methanol (H and OCH3) is added across the alkene in a Markovnikov fashion. The reaction is extremely similar to the addition of water across an alkene under acid-catalyzed conditions, so we expect the mechanism to have three steps 1) proton transfer, 2) nucleophilic attack, and 3) proton transfer. In the first step, a proton is transferred from CH30H2 to the alkene, which requires two curved arrows, as shown below. The resulting tertiary carbocation is then captured by a... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

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