Organic chemistry Rearrangement of carbonium ions Very often

Organic chemistry

Rearrangement of carbonium ions: Very often, dehydration gives alkenes that do not fit the mechanism as we have so far seen it. The double bond appears in unexpected places; sometimes the carbon skeleton is even changed. For example: Take the formation of 2 -butene from n-butyl alcohol. Loss of water from the protonated alcohol gives the n-butyl carbonium ion. Loss of the proton from the carbon adjacent to the positive carbon could give 1 -butene but not the 2 -butene that is the major product.

The other examples are similar. In each case we conclude that if, indeed, the alkene is formed from a carbonium ion, it is not the same carbonium ion that is initially formed from the alcohol.

The idea of intermediate carbonium ions accounts for the facts only if we add this to theory: a carbonium ion can rearrange to form a more stable carbonium ion. n-Butyl alcohol, for example, yields the n-butyl cation; this rearranges to the sec-butyl cation, which loses a hydrogen ion to give (predominantly) 2 -butene:

l. In a similar way, the 2 -methyl-l -butyl cation rearranges to the 2 methyl-2 -butyl cation, and the 3, 3 -dimethyl-2 -butyl cation rearranges to the 2, 3 dimethyl-2 -butyl cation.

l We notice that in each case rearrangement occurs in the way that yields the more stable carbonium ion: primary to a secondary, primary to a tertiary, or secondary to a tertiary. l Rearrangement as taking place in this way: a hydrogen atom or alkyl group migrates with a pair of electrons from an adjacent carbon to the carbon bearing the positive charge. The carbon that loses the migrating group acquires the positive charge.

l l A migration of hydrogen with a pair of electrons is known as a hydride shift; a similar migration of an alkyl group is known as an alkyl shift. These are just two examples of the most common kind of rearrangement, the 1, 2 -shifts: rearrangements in which the migrating group moves from one atom to the very next atom. l

We can account for rearrangements in dehydration in the following way. A carbonium ion is formed by the loss of water from the protonated alcohol. If a 1, 2 -shift of hydrogen or alkyl can form a more stable carbonium ion, then such a rearrangement takes place. The new carbonium ion now loses a proton to yield an alkene. In the case of the n-butyl cation, a shift of hydrogen yields the more stable sec-butyl cation; migration of an ethyl group would simply form a different n-butyl cation.

l In the case of the 2 -methyl-l-butyl cation, a hydride shift yields a tertiary cation, and hence is preferred over a methyl shift, which would only yield a secondary cation.

In the case of the 3, 3 -dimethyl-2 -butyl cation, on the other hand, a methyl shift can yield a tertiary cation and is the rearrangement that takes place.

If isomeric alkenes can be formed in this step, which, if any, will predominate? The examples we have already encountered give us the answer:

l Reduction of an alkyne to the double-bond stage can yield either a cis -alkene or a trans-alkene. Just which isomer predominates depends upon the choice of reducing agent, except when the triple bond is at the end of a chain. Predominantly trans-alkene is obtained by reduction of alkynes with sodium or lithium in liquid ammonia. Almost entirely cis-alkene (as high as 98%) is obtained by hydrogenation of alkynes with several different catalysts: a specially prepared palladium or a nickel boride.

Reactions of the carbon-carbon double bond: addition l The double bond consists of a strong σ bond a weak π bond; therefore, the reaction would involve the breaking of this weaker bond. The typical reactions of the double bond are: Where the π bond is broken and two strong σ bonds are formed in its place. A reaction in which two molecules combine to yield a single molecule of product is called an addition reaction.

The typical reaction of an alkene is electrophilic addition, or, in l other words, addition of acidic reagents. In many of its reactions the carbon-carbon double bond serves as a source of electrons: that is, it acts as a base. The compounds with which it reacts are those that are deficient in electrons, that is, are acids. These acidic reagents that are seeking a pair of electrons are called electrophilic reagents (Greek: electron-loving). The typical reaction of an alkene is electrophilic addition, or, in other words, addition of acidic reagents. Reagents of another kind, free radicals, seek electrons or, rather, seek an electron. And so we find that alkenes also undergo freeradical addition. Besides the addition reactions characteristic of the carbon-carbon double bond, alkenes may undergo the free-radical substitution characteristic of alkanes.

l. Addition reactions: We have already encountered hydrogenation as the most useful method for preparing alkanes. The greater the number of alkyl groups attached to the doublybonded carbon atoms, the more stable the alkene.

l 2 - Addition of halogens: Alkenes are readily converted by chlorine or bromine into saturated compounds that contain two atoms of halogen attached to adjacent carbons; iodine generally fails to react.

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3 -l. Addition of hydrogen halides. Markovnikov's rule: An alkene is converted by hydrogen chloride, hydrogen bromide, or hydrogen iodide into the corresponding alkyl halide.

l Propylene could yield either of two products, the n-propyl halide or the isopropyl halide, depending upon the orientation of addition, that is, depending upon which carbon atoms the hydrogen and halogen become attached to. Actually, it is found that the isopropyl halide greatly predominates.

l In the same way, isobutylene could yield either of two products, isobutyl halide or tert-butyl halide; here the orientation of addition is such that the tert-butyl halide greatly predominates.

In the ionic addition of an acid to the carbon-carbon double bond of an alkene, the hydrogen of the acid attaches itself to the carbon atom that already holds the greater number of hydrogens. This statement is generally known as Markovnikov's rule. Using Markovnikov's rule, we can correctly predict the principal product of many reactions. For example:

l In 2 -pentene each of the doubly-bonded carbons holds one hydrogen, so that according to the rule we should expect neither product to predominate. Here again the prediction is essentially correct, roughly equal quantities of the two isomers actually being obtained.

l The examples have involved the addition of hydrogen iodide; exactly similar results are obtained in the addition of hydrogen chloride, except for special conditions indicated in the following section, of hydrogen bromide.

l Addition of hydrogen bromide. Peroxide effect: Addition of hydrogen chloride and hydrogen iodide to alkenes follows Markovnikov's rule. Addition of hydrogen bromide to a particular alkene yields a product in agreement with Markovnikov's rule; by others, a product in contradiction to Markovnikov's rule; and by still others, a mixture of both products. The orientation of addition of hydrogen bromide to the carbon double bond is determined by the presence or absence of peroxides.

l Organic peroxides are compounds containing the ─O─O─ linkage. Certain peroxides are deliberately synthesized, and used as reagents, if one deliberately puts peroxides into the reaction system, HBr adds to alkenes in exactly the reverse direction. This reversal of the orientation of addition caused by the presence of peroxides is known as the peroxide effect. Of the reactions we are studying, only the addition of hydrogen bromide shows the peroxide effect. The presence or absence of peroxides has no effect on the orientation of addition of hydrogen chloride, hydrogen iodide, sulfuric acid, water, etc.

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4 -l. Addition of sulfuric acid: Alkenes react with cold, concentrated sulfuric acid to form compounds of the general formula ROSO 3 H, known as alkyl hydrogen sulfates. These products are formed by addition of hydrogen ion to one side of the double bond and bisulfate ion to the other. It is important to notice that carbon is bonded to oxygen and not to sulfur.

l If the sulfuric acid solution of the alkyl hydrogen sulfate is diluted with water and heated, there is obtained an alcohol bearing the same alkyl group as the original alkyl hydrogen sulfate. The alkyl hydrogen sulfate has been cleaved by water to form the alcohol and sulfuric acid, and is said to have been hydrolyzed.

l Because the addition of sulfuric acid follows Markovnikov's rule, certain alcohols cannot be obtained by this method. For example, isopropyl alcohol can be made but not n-propyl alcohol; tert-butyl alcohol, but not isobutyl alcohol.

5 - Addition of water. Hydration: l Water adds to the more reactive alkenes in the presence of acids to yield alcohols, this addition follows Markovnikov's rule.

l Electrophilic addition: mechanism: Addition of the acidic reagent, HZ, is believed to proceed by two steps: Step (1) involves transfer of hydrogen ion from : Z to the alkene to form a carbonium ion; this is a transfer of a proton from one base to another. Step (2) is the union of the carbonium ion with the base : Z.

l. Step (1) is the difficult step, and its rate largely or entirely controls the overall rate of addition. This step involves attack by an acidic, electron-seeking reagent that is, an electrophilic reagent and hence the reaction is called electrophilic addition.

l The electrophile need not necessarily be a Lowry-Bronsted acid transferring a proton, as shown here, but can be almost any kind of electron-deficient molecule (Lewis acid).

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