General Mechanism of the E₁ Reaction
The first step of the E₁ mechanism is the same as the first step of the SN₁ reaction. The bond between the leaving group and the sp³ carbon bonded to the leaving group breaks to generate a carbocation.   An adjacent sp³ carbon then gives up electrons from one of its C-H bonds to form a new pi bond between the carbocation carbon and the adjacent carbon.  For diene formation, the mechanism occurs so that one pi bond is completely formed and then the second pi bond is formed in the two step sequence.

Alkenes

Dienes

For alcohols, the OH group must first be activated with acid before it will become a good leaving group (LG).  This is frequently done using a strong acid (catalyst) with a conjugate base that is not nucleophilic.  Sulfuric and phosphoric acids are good activators for E₁ reactions. The oxygen atom of the alcohol becomes protonated to form an oxonuim ion.  Water is then the leaving group.  Loss of the water leaving group results in the formation of a carbocation which then undergoes the E1 elimination as described above. Hydrohalic acids (i.e., HX) are not typically used as catalysts for elimination reactions because the resulting halide ion (conjugate base) may serve as a nucleophile. As a result products of SN₁ reactions will be generated along with the E₁ products. 

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Rearrangements in E1 Reactions
Whenever carbocations are generated, there is the possibility that rearrangement to an equivalently stable or more stable carbocation may occur.  The consequences of carbocation rearrangements in some E1 elimination reactions is that multiple alkene products may be generated.  Consider the examples of 1-methylcyclohexanol and 2-methylcyclohexanol.  In the case of the tertiary alcohol, 1-methylcyclohexanol, the hydroxyl group of the alcohol is protonated by the acid catalyst to form the oxonium ion. Water is lost in the rate determining step of the reaction to form a tertiary carboction.  No rearrangement occurs in this case since a 1,2-methyl or 1,2-hydride shift would result in a less stable secondary carbocation.  The tertiary carbocation goes on to react to form two alkenes, A(through loss of the Ha proton) and B (via loss of the Hb proton). 

By contrast, rearrangements may occur in the E1 elimination of the secondary alcohol, 2-methylcyclohexanol.  As described above, the hydroxyl group is protonated and after loss of water, a secondary carbocation is generated.  However, this carbocation CAN undergo rearrangement to a more stable tertiary carbocation.  The original secondary carbocation can undergo elimination to give two products (A and C) and the rearranged tertiary carbocation can undergo elimination to give two products (A and B).  Both the secondary and tertiary carbocations provide the same product (A) and each produces a unique product B or C.  So 2-methylcyclohexanol generated three unique alkene products whereas 1-methylcyclohexanol provides only two alkene products. 

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Kinetically-Controlled Reactions
The rate determining step of the E1 elimination is formation of the carbocation, and the  alkene product derived from the most stable carbocation will be generated as the major product.  A reaction energy diagram showing a general E₁ reaction for generation of an alkene from an alcohol is given below.  Because E₁ reactions involve a carbocation, and because the stability of the carbocation will dictate what the activation energy of the rate determining step of the reaction is, alcohols that generate more stable carbocations, (i.e., tertiary) will react faster than less stable carbocations (secondary or primary).  The rate determining step (slowest step) in the reaction energy diagram shown is step 2 (conversion of the oxonium to the carbocation).  This step has the highest activation energy and highest energy transition state. 

In this experiment, cyclohexenes are prepared from cyclohexanols.  Each of these compounds have unique functional groups and should therefore be easily distinguished from one another by IR spectroscopic analysis.  Most characteristically, the starting material, methylcyclohexanol, will have a broad absorbance in the range between 3200- 3600 cm^-1 corresponding to its alcohol O-H bond.  This absorbance should be absent in the IR spectrum of the cyclohexene product.  Some caution must be exercised however, since specific reaction conditions and by-products of the reaction must be taken into account before interpreting the IR spectra.  Water is a major by-product of this reaction and as discussed above, may very likely contaminate the product, even after a fractional distillation is done.  If water does in fact contaminate the product, an absorbance at 3200-3600cm^-1 may appear in the IR spectrum of the product due to the O-H bond of water.  Alternatively, an absorbance in this region in the IR spectrum of the product might also be interpreted as contamination by unreacted cyclohexanol.  Consideration of the reaction procedure/conditions should allow you to rule this out.  Why?

Reaction of bromine with alkenes or alkynes occurs through an electrophilic addition mechanism to generate dihaloalkanes.  The brown colored bromine reagent becomes incorporated into the organic hydrocarbon, producing a colorless product.  If the bromine reagent is used in excess, or if no reaction occurs, the solution will remain brownish in color. Carbon tetrachloride (CCl₄) serves as solvent.  Cyclohexenes, the products in the E1 elimination would be expected to react readily with Br₂ in CCl₄ to form the dihaloalkane.   Bromine is a dark brown liquid, and even solutions of bromine (such as Br₂ in CCl₄) are brown, light brown or yellow, depending on the concentration.  Dihaloalkanes, are colorless compounds.  When Br₂ in CCl₄ reacts with an alkene, the colored bromine becomes incorporated into the colorless organic dihaloalkane, and the reaction solution goes from brown (or yellow) to colorless.  Br₂ in CCl₄ does not react with alcohols, and the solution of Br₂ in CCl₄ would be expected to remain brown, light brown or yellow upon reaction with methylcyclohexanols.    Bromine may also react with benzylic atoms through a radical halogenation mechanism.  Even without a specific initiator, products from benzylic bromination are often detected in this test.  In the reaction of bromine with toluene, the pi bonds of the benzene ring do not react, however, the benzylic atom of toluene will be brominated in this reaction.

Jones Oxidation Test
Jones reagent is a powerful oxidizing agent (CrO₃/H₂SO₄) that is used primarily to convert primary alcohols to carboxylic acids, and secondary alcohols to ketones.  Tertiary alcohols do not react with Jones reagent.  The carbon atom directly bonded to the OH group of the alcohol becomes oxidized (increases its bonds to oxygen and decreases its bonds to hydrogen) and the chromium (VI) atom of the Jones reagent is reduced to Cr³⁺.

 

Chromium trioxide is an orange-colored or red-orange reagent which upon reduction (reaction with primary or secondary alcohols) turns green or blue-green.  Over the course of the organic oxidation, the Cr is reduced to Cr (III).  The first two steps of the reaction mechanism help to explain why tertiary alcohols do not undergo oxidation with the Jones reagent.  In step 2, water reacts with a proton of the chromate ester that is bonded to the carbon atom of the former alcohol functional group.  Tertiary alcohols do not have a hydrogen atom bonded to this carbon, therefore the reaction could not proceed beyond this point

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