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When heated, the pure 100% acid loses sulfur trioxide gas, SO3, until a constant-boiling solution, or azeotrope, containing about 98.5% H2SO4 is formed at 337°C. Concentrated sulfuric acid is a weak acid (see acids and bases) and a poor electrolyte because relatively little of it is dissociated into ions at room temperature. When cold it does not react readily with such common metals as iron or copper. When hot it is an oxidizing agent, the sulfur in it being reduced; sulfur dioxide gas may be released. Hot concentrated sulfuric acid reacts with most metals and with several nonmetals, e.g., sulfur and carbon. Because the concentrated acid has a fairly high boiling point, it can be used to release more volatile acids from their salts, e.g., when sodium chloride (NaCl), or common salt, is heated with concentrated sulfuric acid, hydrogen chloride gas, HCl, is evolved. Concentrated sulfuric acid has a very strong affinity for water. It is sometimes used as a drying agent and can be used to dehydrate (chemically remove water from) many compounds, e.g., carbohydrates. It reacts with the sugar sucrose, C12H22O11, removing eleven molecules of water, H2O, from each molecule of sucrose and leaving a brittle spongy black mass of carbon and diluted sulfuric acid. The acid reacts similarly with skin, cellulose, and other plant and animal matter. When the concentrated acid mixes with water, large amounts of heat are released; enough heat can be released at once to boil the water and spatter the acid. To dilute the acid, the acid should be added slowly to cold water with constant stirring to limit the buildup of heat. Sulfuric acid reacts with water to form hydrates with distinct properties. Sections in this article:The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2022, Columbia University Press. All rights reserved. See more Encyclopedia articles on: Compounds and Elements
By James Ashenhurst Last updated: November 9th, 2022 | All About Elimination Reactions of Alcohols (With Acid)
Table of Contents 1. Hydrohalic acids (HX) plus alcohols give substitution products…Previously (See post: Making Alkyl Halides from Alcohols) we saw that treating an alcohol with a strong hydrohalic acid – think HCl, HBr, or HI – resulted in the formation of alkyl halides. With a tertiary alcohol like the one drawn below, this proceeds through an SN1 mechanism. [Protonation of alcohol, then loss of H2O to form a carbocation, then attack of nucleophile on carbocation]. 2. …But H2SO4, H3PO4, and TsOH Give Elimination Products!You’d be forgiven for thinking that if we treated an alcohol with H2SO4 (sulfuric acid) the same type of thing would occur, and the carbocation would be attacked by the (-)OSO3H anion to make the product below. Very reasonable to propose. In practice, however, it doesn’t work that way! So what happened here? First, look at what bonds formed and broke. We formed C-C (π) and broke C-OH and C-H. (We also formed H-O , in that molecule of water that forms as a byproduct). This is the pattern of an elimination reaction. Now let’s ask: How could this have formed? If you look closely, note that we’ve broken a C-H bond on the carbon adjacent to the carbocation and formed a new C-C π bond at that spot. It’s reasonable to propose that instead of attacking the carbocation to form a new substitution product, a base removed a proton adjacent to the carbocation and formed the alkene. [That carbon adjacent to the carbocation is often referred to as the “β (beta) carbon”. The carbocation itself is the “α (alpha) carbon”]. 3. Elimination of Tertiary Alcohols Proceeds Through an E1 MechanismWe’ve seen this type of process before actually! This is an E1 process [elimination (E) , unimolecular (1) rate determining step]. You might also remember that elimination reactions tend to follow “Zaitsev’s rule” – we always form the most substituted alkene [or to put it another way, we remove a proton from the carbon with the fewest attached hydrogens] because alkene stability increases as we increase the number of attached carbons. [By the way, you might ask – why “heat” ? Heat generally tends to favour elimination reactions.] By no means is H2SO4 the only acid that does this. Phosphoric acid (H3PO4) as well as “tosic acid” (p-toluenesulfonic acid) also tend to form elimination products. 4. Why Does H2SO4 (Or H3PO4 or TsOH) Give Elimination Products But HCl, HBr, HI give Substitution Products?So why do we get elimination reactions with H2SO4 as acid (or H3PO4, or TsOH) whereas we get substitution reactions with HCl, HBr, and HI? The answer is that the HSO4– anion is a very poor nucleophile , being quite stabilized by resonance. In the diagram below, note how that negative charge is delocalized over three different oxygens [the same is true for the TsO– and H2PO4– anions]. Compare that to halide anions, where the negative charge cannot be spread over more than one atom. The upshot is that delocalization of charge results in a slower reaction of HSO4– as a nucleophile compared to deprotonation of C-H by a base, and the alkene product dominates. So the bottom line here is that heating tertiary alcohols with these acids will result in loss of water [“dehydration”] and formation of an alkene [elimination]. 5. What About Elimination Reactions Of Secondary Alcohols?Heating a secondary alcohol with sulfuric acid or phosphoric acid? Same deal as with tertiary alcohols: expect an alkene to form. As with all elimination reactions, there are two things to watch out for: first, the “most substituted” alkene (Zaitsev) will be the dominant product, and also, don’t forget that trans alkenes will be favoured (more stable) than cis alkenes due to less steric strain. There is one last thing to watch out for with secondary alcohols, though… like a bad nightmare, they keep coming back. 6. Elimination Reactions With Carbocation RearrangementsAs we saw with the reactions of HCl, HBr, and HI with secondary alcohols, we have to watch out for carbocation rearrangement reactions. If a more stable carbocation can be formed through migration of an adjacent hydride (H- ) or an alkyl group, then that migration will occur. For example, treatment of the alcohol below with H2SO4 leads to formation of a secondary carbocation, followed by a hydride shift to give a tertiary carbocation, followed by deprotonation at whichever β carbon leads to the most substituted alkene. 7. Ring Expansion Followed By EliminationIt’s also possible for alkyl shifts to occur to give a more stable carbocation. A classic example of this are expansions of strained rings (like cyclobutanes) to give less strained rings (like cyclopentanes). For example in the case below the key step is where the C3-C4 bond breaks to form the C2-C4 bond, resulting in a new (tertiary) carbocation on C-3 as well as a less strained ring. Since there isn’t a good nucleophile around, elimination occurs in such a way that the most substituted alkene is formed. 8. Primary Alcohols And H2SO4 Can Form Alkenes (E2)The final class of alcohols to be concerned about is primary alcohols. You might ask: if we treat a primary alcohol (say, 1-butanol) with a strong acid like H2SO4, will also get elimination to an alkene? Yes, alkenes can be formed this way (along with some formation of symmetrical ethers [see this previous post]). Here’s an example. There is a catch however: the E1 pathway (formation of a primary carbocation) is not the most likely pathway here. Primary carbocations tend to be extremely unstable, and it’s more likely that the reaction passes through an E2 mechanism where the transition state will be lower in energy. Notice what happens here: first we protonate the alcohol to give the good leaving group OH2+ , and then a weak base (which I’m leaving vague, but could be H2O, (-)OSO3H, or another molecule of the alcohol) could then break C-H, leading to formation of the alkene. 9. Summary: Elimination Reactions of Alcohols (E1)If you see a tertiary or secondary alcohol with H2SO4, TsOH, or H3PO4 (and especially if you see “heat”) think: carbocation formation followed by elimination reaction (E1). And if you see that a more stable carbocation could be formed through migration of an adjacent H or alkyl group, expect that to happen. If you see a primary alcohol with H2SO4, TsOH, or H3PO4, expect symmetrical ether formation accompanied by elimination to form the alkene. Next Time…So far we’ve learned two ways to convert alcohols to alkenes:
Ideally, we’d like to just use one step. But strong acid can lead to complications (carbocation rearrangements, cough cough) and we might ask: “isn’t there an easier way”? Next Post: Elimination Of Alcohols To Alkenes With POCl3 Notes(Advanced) References and Further ReadingA variety of conditions are possible for this transformation (alcohol -> alkene), all of which involve converting the -OH into a better leaving group. The use of acid is the simplest method to achieve this, as protonation of -OH gives -OH2+, an excellent leaving group (water). Elimination with POCl3:
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