examples of double replacement reactions
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The oxymercuration reaction is an electrophilic additionorganic reaction that transforms an alkene into a neutral alcohol. In oxymercuration, the alkene reacts with mercuric acetate (AcO-Hg-OAc) in aqueous solution to yield the addition of an acetoxymercuri (HgOAc) group and a hydroxy (OH) group across the double bond. Carbocations are not formed in this process and thus rearrangements are not observed. The reaction follows Markovnikov's rule (the hydroxy group will always be added to the more substituted carbon) and it is an anti addition (the two groups will be trans to each other) .
Oxymercuration followed by demercuration is called an oxymercuration-reduction reaction. This reaction, which is almost always done in practice instead of oxymercuration, is treated at the conclusion of the article.
Oxymercuration can be fully described in three steps(the whole process is sometimes called deoxymercuration), which is illustrated in stepwise fashion to the right. In the first step, the nucleophilic double bond attacks the mercury ion, ejecting an acetoxy group. The electron pair on the mercury ion in turn attacks a carbon on the double bond, forming a mercurinium ion in which the mercury atom bears a positive charge. The electrons in the highest occupied molecular orbital of the double bond are donated to mercury's empty dz2 orbital and the electrons in mercury's dxz orbital are donated in the lowest unoccupied molecular orbital of the double bond.
In the second step, the nucleophilic water molecule attacks the more substituted carbon, liberating the electrons participating in its bond with mercury. The electrons collapse to the mercury ion and neutralizes it. The oxygen in the water molecule now bears a positive charge.
In the third step, the negatively charged acetoxy ion that was expelled in the first step attacks a hydrogen of the water group, forming the waste product HOAc. The two electrons participating in the bond between oxygen and the attacked hydrogen collapse into the oxygen, neutralizing its charge and creating the final alcohol product.
Regioselectivity and stereochemistry
Oxymercuration is very regioselective and is a textbook Markovnikov reaction; ruling out extreme cases, the water nucleophile will always preferentially attack the more substituted carbon, depositing the resultant hydroxy group there. This phenomenon is explained by examining the three resonance structures of the mercurinium ion formed at the end of the step one.
By inspection of these structures, it is seen that the positive charge of the mercury atom will sometimes reside on the more substituted carbon (approximately 4% of the time). This forms a temporary tertiary carbocation, which is a very reactive electrophile. The nucleophile will attack the mercurinium ion at this time. Therefore, the nucleophile attacks the more substituted carbon because it retains more positive character than the lesser substituted carbon.
Stereochemically, oxymercuration is an anti addition. As illustrated by the second step, the nucleophile cannot attack the carbon from the same face as the mercury ion because of steric hindrance. There is simply insufficient room on that face of the molecule to accommodate both a mercury ion and the attacking nucleophile. Therefore, when free rotation is impossible, the hydroxy and acetoxymercuri groups will always be trans to each other.
Shown below is an example of regioselectivity and stereospecificity of the oxymercuration reaction with substituted cyclohexenes. A bulky group like t-butyl locks the ring in a chair conformation and prevents ring flips. With 4-t-butylcyclohexene, oxymercuration yields two products - where addition across the double bond is always anti - with slight preference towards acetoxymercury group trans to the t-butyl group, resulting in slightly more cis product. With 1-methyl-4-t-butylcyclohexene, oxymercuration yields only one product - still anti addition across the double bond - where water only attacks the more substituted carbon. The reason for anti addition across the double bond is to maximize orbital overlap of the lone pair of water and the empty orbital of the mercurinium ion on the opposite side of the acetoxymercury group. Regioselectivity is observed to favor water attacking the more subsituted carbon, but water does not add syn across the double bond which implies that the transition state favors water attacking from the opposite side of the acetomercury group.
In practice, the mercury adduct product created by the oxymercuration reaction is almost always treated with sodium borohydride (NaBH4) in aqueous base in a reaction called demercuration. In demercuration, the acetoxymercury group is replaced with a hydrogen in a stereochemically insensitive reaction known as reductive elimination. The combination of oxymercuration followed immediately by demercuration is called an oxymercuration-reduction reaction.
Therefore, the oxymercuration-reduction reaction is the net addition of water across the double bond. Any stereochemistry set up by the oxymercuration step is scrambled by the demercuration step, so that the hydrogen and hydroxy group may be cis or trans from each other. Oxymercuration-reduction is a popular laboratory technique to achieve alkene hydration with Markovnikov selectivity while avoiding carbocation intermediates and thus the rearrangement which can lead to complex product mixtures.
Other Applications of the Oxymercuration Reaction
Oxymercuration is not limited to an alkene reacting with water. Using an alkyne instead of an alkene yields an enol, which tautomerizes into a ketone. Using an alcohol instead of water yields an ether. In both cases, Markovnikov's rule is observed.
Using a vinyl ether in the presence of an alcohol allows the transfer of the alkoxy group (RO-) from the alcohol to the ether. An allyl alcohol and a vinyl ether under the conditions of oxymercuration reaction can give R-CH=CH-CH2-O-CH=CH2, which is suitable for a Claisen Rearrangement.
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Answers:For a double replacement reaction, or a metathesis( more technical term) reacton to take place, at least 1 of three conditions must be met: 1) a precipitate must form 2) a non dissociating product must form - water is a good example 3) gas is generated. Now one of the old definitions of a base is that it is a substance that reacts with an acid to form a salt and water. (there are more modern and scientific definitions, but you have not learnt these yet). Let us consider the following acid/base reaction: NaOH + HCl NaCl + H2O. This is possibly the classic acid/base reaction.In addition, has this satisfied 1 of the requirements for a metathesis reaction? Yes - No 2 above - water has been formed. Therefore this is a double replacement reaction or a metathesis reaction. In your case you have a bicarbonate reacting with an acid. Write out the balanced equation. Does it meet the requiremets of a double replacement reaction? Yes - a gas is liberated, in this case CO2. It is a double replkacement reaction. The critical thing for you to grasp, and many students do not understand this, is that a mixture of the following, as an exa
Answers:Suppose you mix the following solutions: (a) NaCl(aq) + AgNO3(aq) ------> (b) NaCl(aq) + Fe(NO3)2(aq) -------> Which one gives a double replacement (displacement) reaction? Answer is (a). Why? Complete the equations. (a) NaCl(aq) + AgNO3(aq) ------> NaNO3(aq) + AgCl(s) (b) NaCl(aq) + Fe(NO3)2(aq) -------> Na^+(aq) + Cl^-(aq) + Fe^2+(aq) + 2NO3^-(aq) (in other words no reaction) Can you recognize the difference. In (a), a slightly soluble salt AgCl is formed, that is a precipitation occurs. But in (b), even if the anions or cations are displaced, the formed salts NaNO3 and FeCl2 are also soluble and there will not be a separation of substance occur. All 4 ions will be present in the solution. Similarly an acid-base reaction is also a displacement reaction. But, why a neutralization reaction occurs, although it does not produce an insoluble salt? This time, instead of a slightly soluble salt, a slightly ionizable substance (WATER) is formed. Therefore 4 ions are not present. For example: NaOH(aq) + HCl(aq) -----> NaCl(aq) + H2O(l) But not; NaOH(aq) + HCl(aq) -----> Na^+(aq) + Cl^-(aq) + H^+(aq) + OH(aq) Therefore a neutralization takes place. Displacement reaction: a slightly soluble salt formed Neutralization reaction: a slightly ionizable water formed But in general both of them are double replacement reactions.
Answers:Combination: the rusting of iron (4Fe + 3O2 2Fe2O3) Decomposition: the production of quicklime (Ca(OH)2 CaO + H2O) Single displacement: the polishing of silverware by soaking with aluminium (2Al + 3Ag2S 6Ag + Al2S3) Double displacement: the extraction of magnesium from seawater (MgCl2 + 2NaOH Mg(OH)2 + 2NaCl) Combustion: burning natural gas for heat (CH4 + 2O2 CO2 + 2H2O)
Answers:Yes, a reaction can occur between an aqueous compound and an insoluble compound, but it occurs only on the surface, unless the precipitate does not adhere to the reactant, and therefore falls off, exposing more solid to the solution. This is therefore usually not a useful method for preparing an insoluble solid. One example is the reaction between calcium carbonate and sulfuric acid. Fizzing occurs initially, but calcium sulfate builds up on the surface, and calcium carbonate stops dissolving. CaCO3(s) + H2SO4(aq) -----> CaSO4(s) + CO2(g) + H2O(l) (This is actually an acid-base reaction.) Another example is Ag2O(s) + 2HI(aq) -----> 2AgI(s) + H2O(l) This reaction occurs because AgI is more insoluble than Ag2O.