Now we're going to talk about a way that we can make double bonds out of alcohols. And this is through a mechanism called acid catalyzed dehydration. Okay? So how does this work? Well, remember back to when I've talked about leaving groups in the past. Okay? And a leaving group is even a concept from acid-base chemistry. All it means is that it's something that really doesn't want to leave, something that really doesn't want to get a negative charge. So another word for leaving group is just conjugate base. Okay? So if you maybe don't really remember exactly what a leaving group is, just think conjugate base, back to acids and bases. And basically, alcohols are really bad leaving groups or they're really bad conjugate bases. They hate to become OH-. Why? Because OH- is actually a really strong base. Remember that you always want to go from stronger to weaker? If we're making OH- hydroxide, that's a really strong base. Okay? So this is not going to be very favored to just leave a molecule. But it turns out that there is one thing we can do to make alcohol a better leaving group and that is to use. And if we can use some kind of acid, we can actually convert alcohol into an awesome leaving group. That awesome leaving group would just be that we add an H to it, we protonate it, so it turns into water. And water is an awesome leaving group because it's neutral. It loves to leave and it loves to just be by itself in solution. Okay? So here's the general formula. I'm not going to show you the mechanism just yet. But basically what we have is we have some kind of alcohol and we have some kind of acid over water. Now in this case, I just put the general H8. I mean, really any acid, but your common acids are going to be sorry sorry about that. Any of the strong acids. So H2SO4 is seen all the time. Any of the hydrogen halide strong acids. I've also seen phosphoric acid. This is not really a strong acid, but it's still strong enough to make the reaction go. These are all very frequently used acids for acid catalyzed dehydration. Okay? And what we're basically doing is we're taking an alcohol. We're going to be removing 2 sigma bonds. We're going to be removing the alcohol and a beta hydrogen. Okay. So this is my alpha. This is my beta carbon. Alpha and beta relative to the alcohol. We're going to be taking away 2 different sigma bonds and we're going to be making 1 pi bond instead. Now that general reaction of taking away 2 sigmas and making 1 pi is an elimination. So this is actually going to be an elimination reaction. Okay? Hope that's making sense so far. Now there's another reaction that you may have already learned or that you will learn soon and that's actually the opposite. It's called acid catalyzed hydration. Acid catalyzed hydration is a reaction where we go from the double bond and we go back to the alcohol. Okay? This is actually what's called an addition reaction. Okay? So if you remember back to the general types of reactions we talked about, that when you take one bond and you make it into 2, that's actually addition. When you take 2 bonds and you make it into 1, that's elimination. So hopefully that makes sense. So basically, the addition part is gonna be the hydration. The elimination part is the dehydration. Okay? How do you know which one it is? How do you know if it's going to be a hydration or a dehydration? You just look at what you're starting with. So in this case, since I have my alcohol, I know that I'm starting with an alcohol and I'm going to try to eliminate it with an acid to become a double bond. However, if I was starting with a double bond and I used acid, then I could add water to it and that would become a hydration. In this step, in this video, we're just gonna learn about dehydration, but I'm just asking you to keep this in the back of your mind later on for when you have to do practice problems with hydration. That this is the way that we tell the two reactions apart. Okay? So let's just talk more in-depth about dehydration. There's a few facts I want you to know. First of all, the more R groups on that alcohol, the easier it's gonna be to dehydrate. This is just a fact that might come up on maybe a conceptual part of your exam or your professor might even ask you, give you 4 different alcohols. Which one's the easiest to dehydrate? Tertiary would be the easiest. Secondary, primary, and then is the worst and then, Oops, sorry. I don't know what I'm trying to draw here. But basically, the methyl can't even happen because if it's a methyl alcohol then or methanol, that can't even eliminate cause it's only got one carbon. Okay? So basically, the easiest one is tertiary, the worst one is primary. That's the first thing. The second thing is that the specific elimination mechanism that we use is going to depend on how easily the molecule is going to form a carbocation. Okay? And the understanding of carbocations is kind of essential to these two mechanisms. All that means is that if you guys remember back to the trend of carbocation stability, that tertiary carbocations are the most stable and primary carbocations are the worst. Okay? And then obviously, methyl is even worse than that, but like I said, methyl doesn't even get counted because we can't use it.
- 1. A Review of General Chemistry5h 5m
- Summary23m
- Intro to Organic Chemistry5m
- Atomic Structure16m
- Wave Function9m
- Molecular Orbitals17m
- Sigma and Pi Bonds9m
- Octet Rule12m
- Bonding Preferences12m
- Formal Charges6m
- Skeletal Structure14m
- Lewis Structure20m
- Condensed Structural Formula15m
- Degrees of Unsaturation15m
- Constitutional Isomers14m
- Resonance Structures46m
- Hybridization23m
- Molecular Geometry16m
- Electronegativity22m
- 2. Molecular Representations1h 14m
- 3. Acids and Bases2h 46m
- 4. Alkanes and Cycloalkanes4h 19m
- IUPAC Naming29m
- Alkyl Groups13m
- Naming Cycloalkanes10m
- Naming Bicyclic Compounds10m
- Naming Alkyl Halides7m
- Naming Alkenes3m
- Naming Alcohols8m
- Naming Amines15m
- Cis vs Trans21m
- Conformational Isomers13m
- Newman Projections14m
- Drawing Newman Projections16m
- Barrier To Rotation7m
- Ring Strain8m
- Axial vs Equatorial7m
- Cis vs Trans Conformations4m
- Equatorial Preference14m
- Chair Flip9m
- Calculating Energy Difference Between Chair Conformations17m
- A-Values17m
- Decalin7m
- 5. Chirality3h 39m
- Constitutional Isomers vs. Stereoisomers9m
- Chirality12m
- Test 1:Plane of Symmetry7m
- Test 2:Stereocenter Test17m
- R and S Configuration43m
- Enantiomers vs. Diastereomers13m
- Atropisomers9m
- Meso Compound12m
- Test 3:Disubstituted Cycloalkanes13m
- What is the Relationship Between Isomers?16m
- Fischer Projection10m
- R and S of Fischer Projections7m
- Optical Activity5m
- Enantiomeric Excess20m
- Calculations with Enantiomeric Percentages11m
- Non-Carbon Chiral Centers8m
- 6. Thermodynamics and Kinetics1h 22m
- 7. Substitution Reactions1h 48m
- 8. Elimination Reactions2h 30m
- 9. Alkenes and Alkynes2h 9m
- 10. Addition Reactions3h 18m
- Addition Reaction6m
- Markovnikov5m
- Hydrohalogenation6m
- Acid-Catalyzed Hydration17m
- Oxymercuration15m
- Hydroboration26m
- Hydrogenation6m
- Halogenation6m
- Halohydrin12m
- Carbene12m
- Epoxidation8m
- Epoxide Reactions9m
- Dihydroxylation8m
- Ozonolysis7m
- Ozonolysis Full Mechanism24m
- Oxidative Cleavage3m
- Alkyne Oxidative Cleavage6m
- Alkyne Hydrohalogenation3m
- Alkyne Halogenation2m
- Alkyne Hydration6m
- Alkyne Hydroboration2m
- 11. Radical Reactions1h 58m
- 12. Alcohols, Ethers, Epoxides and Thiols2h 42m
- Alcohol Nomenclature4m
- Naming Ethers6m
- Naming Epoxides18m
- Naming Thiols11m
- Alcohol Synthesis7m
- Leaving Group Conversions - Using HX11m
- Leaving Group Conversions - SOCl2 and PBr313m
- Leaving Group Conversions - Sulfonyl Chlorides7m
- Leaving Group Conversions Summary4m
- Williamson Ether Synthesis3m
- Making Ethers - Alkoxymercuration4m
- Making Ethers - Alcohol Condensation4m
- Making Ethers - Acid-Catalyzed Alkoxylation4m
- Making Ethers - Cumulative Practice10m
- Ether Cleavage8m
- Alcohol Protecting Groups3m
- t-Butyl Ether Protecting Groups5m
- Silyl Ether Protecting Groups10m
- Sharpless Epoxidation9m
- Thiol Reactions6m
- Sulfide Oxidation4m
- 13. Alcohols and Carbonyl Compounds2h 17m
- 14. Synthetic Techniques1h 26m
- 15. Analytical Techniques:IR, NMR, Mass Spect6h 50m
- Purpose of Analytical Techniques5m
- Infrared Spectroscopy16m
- Infrared Spectroscopy Table31m
- IR Spect:Drawing Spectra40m
- IR Spect:Extra Practice26m
- NMR Spectroscopy10m
- 1H NMR:Number of Signals26m
- 1H NMR:Q-Test26m
- 1H NMR:E/Z Diastereoisomerism8m
- H NMR Table21m
- 1H NMR:Spin-Splitting (N + 1) Rule17m
- 1H NMR:Spin-Splitting Simple Tree Diagrams11m
- 1H NMR:Spin-Splitting Complex Tree Diagrams8m
- 1H NMR:Spin-Splitting Patterns8m
- NMR Integration18m
- NMR Practice14m
- Carbon NMR4m
- Structure Determination without Mass Spect47m
- Mass Spectrometry12m
- Mass Spect:Fragmentation28m
- Mass Spect:Isotopes27m
- 16. Conjugated Systems6h 13m
- Conjugation Chemistry13m
- Stability of Conjugated Intermediates4m
- Allylic Halogenation12m
- Reactions at the Allylic Position39m
- Conjugated Hydrohalogenation (1,2 vs 1,4 addition)26m
- Diels-Alder Reaction9m
- Diels-Alder Forming Bridged Products11m
- Diels-Alder Retrosynthesis8m
- Molecular Orbital Theory9m
- Drawing Atomic Orbitals6m
- Drawing Molecular Orbitals17m
- HOMO LUMO4m
- Orbital Diagram:3-atoms- Allylic Ions13m
- Orbital Diagram:4-atoms- 1,3-butadiene11m
- Orbital Diagram:5-atoms- Allylic Ions10m
- Orbital Diagram:6-atoms- 1,3,5-hexatriene13m
- Orbital Diagram:Excited States4m
- Pericyclic Reaction10m
- Thermal Cycloaddition Reactions26m
- Photochemical Cycloaddition Reactions26m
- Thermal Electrocyclic Reactions14m
- Photochemical Electrocyclic Reactions10m
- Cumulative Electrocyclic Problems25m
- Sigmatropic Rearrangement17m
- Cope Rearrangement9m
- Claisen Rearrangement15m
- 17. Ultraviolet Spectroscopy51m
- 18. Aromaticity2h 31m
- 19. Reactions of Aromatics: EAS and Beyond5h 1m
- Electrophilic Aromatic Substitution9m
- Benzene Reactions11m
- EAS:Halogenation Mechanism6m
- EAS:Nitration Mechanism9m
- EAS:Friedel-Crafts Alkylation Mechanism6m
- EAS:Friedel-Crafts Acylation Mechanism5m
- EAS:Any Carbocation Mechanism7m
- Electron Withdrawing Groups22m
- EAS:Ortho vs. Para Positions4m
- Acylation of Aniline9m
- Limitations of Friedel-Crafts Alkyation19m
- Advantages of Friedel-Crafts Acylation6m
- Blocking Groups - Sulfonic Acid12m
- EAS:Synergistic and Competitive Groups13m
- Side-Chain Halogenation6m
- Side-Chain Oxidation4m
- Reactions at Benzylic Positions31m
- Birch Reduction10m
- EAS:Sequence Groups4m
- EAS:Retrosynthesis29m
- Diazo Replacement Reactions6m
- Diazo Sequence Groups5m
- Diazo Retrosynthesis13m
- Nucleophilic Aromatic Substitution28m
- Benzyne16m
- 20. Phenols55m
- 21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
- Naming Aldehydes8m
- Naming Ketones7m
- Oxidizing and Reducing Agents9m
- Oxidation of Alcohols28m
- Ozonolysis7m
- DIBAL5m
- Alkyne Hydration9m
- Nucleophilic Addition8m
- Cyanohydrin11m
- Organometallics on Ketones19m
- Overview of Nucleophilic Addition of Solvents13m
- Hydrates6m
- Hemiacetal9m
- Acetal12m
- Acetal Protecting Group16m
- Thioacetal6m
- Imine vs Enamine15m
- Addition of Amine Derivatives5m
- Wolff Kishner Reduction7m
- Baeyer-Villiger Oxidation39m
- Acid Chloride to Ketone7m
- Nitrile to Ketone9m
- Wittig Reaction18m
- Ketone and Aldehyde Synthesis Reactions14m
- 22. Carboxylic Acid Derivatives: NAS2h 51m
- Carboxylic Acid Derivatives7m
- Naming Carboxylic Acids9m
- Diacid Nomenclature6m
- Naming Esters5m
- Naming Nitriles3m
- Acid Chloride Nomenclature5m
- Naming Anhydrides7m
- Naming Amides5m
- Nucleophilic Acyl Substitution18m
- Carboxylic Acid to Acid Chloride6m
- Fischer Esterification5m
- Acid-Catalyzed Ester Hydrolysis4m
- Saponification3m
- Transesterification5m
- Lactones, Lactams and Cyclization Reactions10m
- Carboxylation5m
- Decarboxylation Mechanism14m
- Review of Nitriles46m
- 23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
- 24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
- Tautomerization9m
- Tautomers of Dicarbonyl Compounds6m
- Enolate4m
- Acid-Catalyzed Alpha-Halogentation4m
- Base-Catalyzed Alpha-Halogentation3m
- Haloform Reaction8m
- Hell-Volhard-Zelinski Reaction3m
- Overview of Alpha-Alkylations and Acylations5m
- Enolate Alkylation and Acylation12m
- Enamine Alkylation and Acylation16m
- Beta-Dicarbonyl Synthesis Pathway7m
- Acetoacetic Ester Synthesis13m
- Malonic Ester Synthesis15m
- 25. Condensation Chemistry2h 9m
- 26. Amines1h 43m
- 27. Heterocycles2h 0m
- Nomenclature of Heterocycles15m
- Acid-Base Properties of Nitrogen Heterocycles10m
- Reactions of Pyrrole, Furan, and Thiophene13m
- Directing Effects in Substituted Pyrroles, Furans, and Thiophenes16m
- Addition Reactions of Furan8m
- EAS Reactions of Pyridine17m
- SNAr Reactions of Pyridine18m
- Side-Chain Reactions of Substituted Pyridines20m
- 28. Carbohydrates5h 53m
- Monosaccharide20m
- Monosaccharides - D and L Isomerism9m
- Monosaccharides - Drawing Fischer Projections18m
- Monosaccharides - Common Structures6m
- Monosaccharides - Forming Cyclic Hemiacetals12m
- Monosaccharides - Cyclization18m
- Monosaccharides - Haworth Projections13m
- Mutarotation11m
- Epimerization9m
- Monosaccharides - Aldose-Ketose Rearrangement8m
- Monosaccharides - Alkylation10m
- Monosaccharides - Acylation7m
- Glycoside6m
- Monosaccharides - N-Glycosides18m
- Monosaccharides - Reduction (Alditols)12m
- Monosaccharides - Weak Oxidation (Aldonic Acid)7m
- Reducing Sugars23m
- Monosaccharides - Strong Oxidation (Aldaric Acid)11m
- Monosaccharides - Oxidative Cleavage27m
- Monosaccharides - Osazones10m
- Monosaccharides - Kiliani-Fischer23m
- Monosaccharides - Wohl Degradation12m
- Monosaccharides - Ruff Degradation12m
- Disaccharide30m
- Polysaccharide11m
- 29. Amino Acids3h 20m
- Proteins and Amino Acids19m
- L and D Amino Acids14m
- Polar Amino Acids14m
- Amino Acid Chart18m
- Acid-Base Properties of Amino Acids33m
- Isoelectric Point14m
- Amino Acid Synthesis: HVZ Method12m
- Synthesis of Amino Acids: Acetamidomalonic Ester Synthesis16m
- Synthesis of Amino Acids: N-Phthalimidomalonic Ester Synthesis13m
- Synthesis of Amino Acids: Strecker Synthesis13m
- Reactions of Amino Acids: Esterification7m
- Reactions of Amino Acids: Acylation3m
- Reactions of Amino Acids: Hydrogenolysis6m
- Reactions of Amino Acids: Nihydrin Test11m
- 32. Lipids 2h 50m
- 34. Nucleic Acids1h 32m
- 35. Transition Metals5h 33m
- Electron Configuration of Elements45m
- Coordination Complexes20m
- Ligands24m
- Electron Counting10m
- The 18 and 16 Electron Rule13m
- Cross-Coupling General Reactions40m
- Heck Reaction40m
- Stille Reaction13m
- Suzuki Reaction25m
- Sonogashira Coupling Reaction17m
- Fukuyama Coupling Reaction15m
- Kumada Coupling Reaction13m
- Negishi Coupling Reaction16m
- Buchwald-Hartwig Amination Reaction19m
- Eglinton Reaction17m
Dehydration Reaction - Online Tutor, Practice Problems & Exam Prep
Acid-catalyzed dehydration transforms alcohols into alkenes through elimination reactions. Primary alcohols typically undergo the E2 mechanism, while secondary and tertiary alcohols follow the E1 mechanism, involving carbocation formation. Protonation of the alcohol enhances its leaving group ability, converting it to water. The stability of carbocations influences the reaction pathway, with tertiary carbocations being the most stable. Understanding these mechanisms is crucial for predicting reaction outcomes and applying concepts like Zaitsev's rule in organic synthesis.
Dehydration reactions eliminate alcohols, yielding double bonds.
General features of acid-catalyzed dehydration.
Video transcript
Recall that for elimination to take place, you need a good leaving group. Alcohols are terrible leaving groups, but in the presence of acid, they can be converted into water, which is an amazing leaving group.
If an alcohol can form a stable carbocation, the E1 mechanism will be favored. If it can’t, then the mechanism will follow an E2 pathway. Let’s start off 1° ROH, which usually follow E2.
Dehydration of 1° alcohols:The E2 Mechanism
Protonation:
E2 Concerted β-Elimination:
Dehydration of 2° and 3° alcohols:The E1 Mechanism.
Protonation:
Formation of a Carbocation (Slow Step):
E1 β-Elimination (Fast Step):
An extra note of caution with 1° alcohols.
Video transcript
Let's do some cumulative practice based on everything we've learned from acid-catalyzed dehydration and make sure to be mindful of all the different details I taught you about which mechanism you would use and what would happen in terms of how many steps they would have and stuff like that. Now there is one instruction that I want to give you, and that's remember that I told you that primaries would do an E2 reaction or mechanism, and then secondaries and tertiaries would perform an E1. Okay? Now that is almost always true, but there is one exception to that. That is going to be if you have a primary alcohol that can rearrange to a tertiary carbocation through a shift, then it's actually going to do a carbocation mediated E1 mechanism instead. Okay? So you're going to have to use that information to maybe determine this first one. Notice that it is primary. Figure out, okay, this would usually be E2. If it could shift to a tertiary position, then you should actually use E1. But I'm going to let you guys figure that out.
The second one, same thing. You have to figure out what mechanism and everything that would happen in between. Alright? So go ahead and get started on this first one and then I'll show you guys how to do the second.
Remember how I mentioned that 1° alcohols usually follow E2?
This isn’t the case of 1° alcohols that can rearrange to 3° alcohols. Since the 1,2-rearrangement creates a super stable carbocation, the reaction will follow the E1 pathway.
Predict the major product of the reaction
Predict the major product of the reaction
Do you want more practice?
More setsHere’s what students ask on this topic:
What is the mechanism of acid-catalyzed dehydration of alcohols?
Acid-catalyzed dehydration of alcohols involves converting an alcohol into an alkene through an elimination reaction. The mechanism depends on the type of alcohol. Primary alcohols typically undergo the E2 mechanism, which is a concerted process where protonation of the alcohol enhances its leaving group ability, converting it to water. Secondary and tertiary alcohols follow the E1 mechanism, involving carbocation formation. The steps include protonation of the alcohol, formation of a carbocation (for secondary and tertiary alcohols), and beta elimination to form the double bond. The stability of the carbocation influences the reaction pathway, with tertiary carbocations being the most stable.
Why are tertiary alcohols easier to dehydrate than primary alcohols?
Tertiary alcohols are easier to dehydrate than primary alcohols because they form more stable carbocations. In the E1 mechanism, which is common for secondary and tertiary alcohols, the formation of a carbocation intermediate is a key step. Tertiary carbocations are more stable due to hyperconjugation and inductive effects from the surrounding alkyl groups. This stability lowers the activation energy required for the reaction, making the dehydration process more favorable. In contrast, primary alcohols form less stable carbocations and typically follow the E2 mechanism, which is less favorable for dehydration.
What role does the acid play in the dehydration of alcohols?
The acid in the dehydration of alcohols serves as a catalyst that protonates the hydroxyl group of the alcohol, converting it into a better leaving group, water. This protonation step is crucial as it transforms the poor leaving group (OH-) into a neutral and stable molecule (H2O), facilitating its departure from the molecule. The acid also helps stabilize the carbocation intermediate in the E1 mechanism for secondary and tertiary alcohols. Common acids used include sulfuric acid (H2SO4), phosphoric acid (H3PO4), and hydrochloric acid (HCl).
How does the E1 mechanism differ from the E2 mechanism in acid-catalyzed dehydration?
The E1 and E2 mechanisms differ primarily in their steps and the types of alcohols they involve. The E1 mechanism, common for secondary and tertiary alcohols, involves two steps: protonation of the alcohol to form a better leaving group, followed by the formation of a carbocation intermediate and subsequent beta elimination to form the double bond. The E2 mechanism, typical for primary alcohols, is a concerted process where protonation and elimination occur simultaneously without forming a carbocation intermediate. The E1 mechanism relies on carbocation stability, while the E2 mechanism does not.
What is Zaitsev's rule and how does it apply to dehydration reactions?
Zaitsev's rule states that in an elimination reaction, the more substituted alkene is usually the major product. This rule applies to dehydration reactions of alcohols, where the formation of the more stable, more substituted alkene is favored. During the beta elimination step, the hydrogen is typically removed from the beta carbon with the fewest hydrogens, leading to the formation of the more substituted double bond. This results in the major product being the alkene with the greater number of alkyl groups attached to the double-bonded carbons.
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- A student was studying terpene synthesis, and she wanted to make the compound shown here. First she converted ...
- b. What is the product of the following reaction?
- Which of the following alcohols dehydrates the fastest when heated with acid?
- Propose mechanisms for the following reactions. Additional products may be formed, but your mechanism only nee...
- Predict the major products of acid-catalyzed dehydration of the following alcohols.(c) 2-methylcyclohexanol(d)...
- A vicinal diol has OH groups on adjacent carbons. The dehydration of a vicinal diol is accompanied by a rearra...
- Propose mechanisms for the following reactions. In most cases,more products are formed than are shown here. Yo...