Ch. 9 - Alkenes and AlkynesSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins

Dehydration reactions eliminate alcohols, yielding double bonds. 

Concept #1: General features of acid-catalyzed dehydration.

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.  

Concept #2: Dehydration of 1° alcohols: The E2 Mechanism

Protonation:

E2 Concerted β-Elimination:

Concept #3: Dehydration of 2° and 3° alcohols: The E1 Mechanism. 

Protonation:

Formation of a Carbocation (Slow Step):

E1 β-Elimination (Fast Step):

Concept #4: An extra note of caution with 1° alcohols.

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.

Practice: Predict the major product of the reaction

Practice: Predict the major product of the reaction

Additional Problems
Acid-catalyzed dehydration of 3-methyl-2-butanol gives three alkenes: 2-methyl-2-butene, 3-methyl-1- butene, and 2-methyl-1-butene. Propose a mechanism to account for the formation of each product.
Draw the mechanism for the following elimination reaction, without a carbocation rearrangement, and in the presence of sulfuric acid, H2SO4, as reagent. Indicate if it follows an E1 or an E2 mechanism.
Provide a detailed mechanism for the following transformation using a curved arrow notation.
Complete the mechanism for the following acid-catalyzed dehydration of a alcohol reaction. Draw all the arrows to indicate movement of electrons, write all lone pairs, all formal charges, and all the products for each step. In the dotted box write which mechanistic element is involved in each step. If a racemic mixture is formed you must draw both enantiomers by drawing dashes and wedges and write RACEMIC. Draw the expected reaction coordinate diagram for the above reaction. Assume the above reaction is overall exothermic.
Provide the following when a product is given. If an organic reactant is missing, supply a structural formula; if an inorganic reactant (reagent) or catalyst is missing, simply give a formula. 
Select the product(s) from the following reaction.
Predict the organic product of the following reaction. When appropriate, be sure to indicate stereochemistry. If more than one product is formed be sure to indicate the major product. Draw all answers in skeletal form.
Write structural formulas for all the alkene products that could reasonably be formed from each of the following compounds under the  indicated reaction conditions. Where more than one alkene is produced, specify the one that is the major product. (c) 3-Methyl-3-pentanol (sulfuric acid, 80°C)    
Write structural formulas for all the alkene products that could reasonably be formed from each of the following compounds under the indicated reaction conditions. Where more than one alkene is produced, specify the one that is the major product. (d) 2,3-Dimethyl-2-butanol (phosphoric acid, 120°C)    
Write structural formulas for all the alkene products that could reasonably be formed from each of the following compounds under the indicated reaction conditions. Where more than one alkene is produced, specify the one that is the major product. (f) 2,4-Dimethyl-3-pentanol (sulfuric acid, 120°C)      
Predict the major organic product of each of the following reactions. In spite of the structural complexity of some of the starting materials, the functional group transformations are all of the type described in this chapter.  
What is/are the major product(s) of the following reaction? What mechanism does it follow (E1, E2, SN1, or SN2)? (Hint: Note the temperature and reaction time).  
What would be the optimal conditions to effect the following transformation?  A)              Dilute H2SO4 B)              Concentrated H2SO4 C)              Dilute HBr D)             Concentrated HBr
Predict the organic product(s) of the following reaction. When appropriate, be sure to indicate stereochemistry. Be sure to indicate the major product if more than one product is formed. Draw all answers in skeletal form. 
Why would the concentrated hydrobromic acid be an inappropriate catalyst for the dehydration of alcohols? a) HBr is too weakly acidic to protonate the alcohol. b) The conjugate base, Br - , is a good nucleophile and it would attack the carbocation to form an alkyl bromide.  c) HBr is strongly acidic, so the water molecule would not be a good leaving group after protonation of the alcohol. d) HBr would be more likely to promote rearrangement of the carbocation intermediate. 
If this alcohol is dehydrated, which alkene is likely to be formed in the largest quantity?
What would be the first step in the dehydration of cyclohexanol in sulfuric acid? a) loss of OH - b) loss of H+ by the alcohol c) formation of a sulfite ester d) protonation of the alcohol
Which of the mechanistic steps shown is not a reasonable one in the mechanism to describe the formation of 2-methyl-1-butene by dehydration of 3-methyl-2-butanol?
Propose a mechanism for the following reaction
Predict the organic product of the following reaction. When appropriate, be sure to indicate stereochemistry. If more than one product is formed be sure to indicate the major product. Draw your answer in skeletal form. You will be graded on the product your draw from the reaction no other information is needed for this question.
Predict the major organic product of each of the following reactions. In spite of the structural complexity of some of the starting materials, the functional group transformations are all of the type described in this chapter.
Predict the major organic product of each of the following reactions. In spite of the structural complexity of some of the starting materials, the functional group transformations are all of the type described in this chapter.
DRAW THE MOLECULAR STRUCTURE OF THE FOLLOWING REACTION PRODUCT(S). Two products, both of molecular formula C10H18, from reaction of 1-(1,1-dimethylethyl)cyclohexanol (1-tert-butylcyclohexanol) with concentrated H2SO4.
What is the major product of the following alcohol dehydration?
Write a sequence of steps depicting the mechanisms of each of the following reactions. Use curved arrows to show electron flow.  
Write a sequence of steps depicting the mechanisms of each of the following reactions. Use curved arrows to show electron flow.  
Acid-catalyzed dehydration of 2,2-dimethyl-1-hexanol gave a number of isomeric alkenes including 2-methyl-2-heptene as shown in the following equation. (a) Write a stepwise mechanism for the formation of 2-methyl-2-heptene, using curved arrows to show the fl ow of electrons.
Two stereoisomers of 1-bromo-4-methylcyclohexane are formed when trans-4-methylcyclohexanol reacts with hydrogen bromide. Write structural formulas of:  (c) The two stereoisomers of 1-bromo-4-methylcyclohexane
As a method for the preparation of alkenes, a weakness in the acid-catalyzed dehydration of alcohols is that the initially formed alkene (or mixture of alkenes) sometimes isomerizes under the conditions of its formation. Write a stepwise mechanism showing how 2-methyl-1-butene might isomerize to 2-methyl-2-butene in the presence of sulfuric acid.
Provide a detailed mechanism for the following transformation. Show all charges and intermediates, and use curved arrows to indicate the flow of electrons. Do not draw transition states. Why does this process occur? Or, in other words, what is the thermodynamic driving force for this reaction?
Predict the organic product of the following reaction. When appropriate, be sure to indicate stereochemistry. If more than one product is formed be sure to indicate the major product. Draw your answer in skeletal form. You will be graded on the product your draw from the reaction no other information is needed for this question.
For the following dehydration:A. Predict the product of the following reaction.B. Construct a reaction mechanism that accounts for the product proposed.C. Construct a reaction coordinate diagram for the reaction. Label all axes. Note that the reaction has three steps: The 1st is fast and endothermic. The 2nd is slow and endothermic. The 3rd is fast and exothermic. The overall reaction is exothermic.D. Using complete sentences state how you have shown that the 1st and 3rd steps are fast and that the 2nd reaction is slow.
For each of the following, supply a structural formula for the major organic product(s) when the product(s) is/are not given; if no reaction occurs, write N.R. Give the best possible answers. Be sure to show stereoisomers properly when necessary.
Write a sequence of steps depicting the mechanisms of each of the following reactions. Use curved arrows to show electron flow.   
Draw the major organic product of the reaction shown below.You do not have to consider stereochemistryYou do not have to explicitly draw H atoms.In cases where there is more than one answer, just draw one.
Determine the alkene produced from the acid-catalyzed dehydration of 1-hexanol Which linear alkenes would you expect to be obtained from the acid-catalyzed dehydration of 1-hexanol? Draw all possible regio- and stereoisomers, including minor organic coproducts.  
Provide the name of the major organic product that results when (R)-2-methyl-3-pentanol is heated with conc. H2SO4. 
Predict the products of the following elimination reaction. 
The reaction in the figure below indicates the oxidation of C5H11OH with chromic acid. Write the systematic IUPAC name for the product in the space below.  
Draw the major organic product(s) of the following reaction (multiple products may be drawn in one box):
Draw the major organic product(s) of the following reaction (multiple products may be drawn in one box): 
Predict the when 1-butanol is dehydrated. Include all hydrogen atoms. Show both the organic product and the inorganic product formed in this reaction. 
Draw the major organic product formed in the following reaction.
Draw the organic and inorganic products for the following acid/base reaction. Include charges.
What is the expected product of the reaction below? 
Draw the major organic product of the reaction shown below. 
Draw the alkene product of the dehydration of the following alcohol: 1-butanol 
Draw the products formed when 2-propanol [(CH3)2CHOH], the main ingredient of rubbing alcohol, is treated with H2SO4. 
Draw the structure of the product that is expected when the following compound is treated draw the structure of the major product. 
Provide the structure of the major organic product in the reaction below. 
Predict the products of the following elimination reaction.
Provide the major organic product of the following.  
Give the major product for the following reaction. 
Dehydration of 2-methyl-2-pentanol forms one major and one minor organic product. Draw the structures of the two organic products of this reaction. 
Which of the following alcohols gives a rearranged carbocation when dehydrated?
Draw the major organic product formed in the following reaction.