Alcohol Elimination Reaction

7 Key excerpts on "Alcohol Elimination Reaction"

• 

  eBook - PDF

  Organic Chemistry

  A Mechanistic Approach

  • Penny Chaloner(Author)
  • 2014(Publication Date)
  • CRC Press

    (Publisher)

  381 10.1 INTRODUCTION Elimination reactions are those in which we remove two atoms or groups from a molecule to generate a multiple bond. Most of the examples we will discuss involve making a carbon–carbon double bond, but we will also make some triple bonds and a few carbon–heteroatom bonds. We previously saw elimination processes as a “side reaction” of substitution—now, we turn the tables and see substitution as a side reaction of a desired elimination. Although we have good method- ologies for inducing reactions to go in one direction or the other, we should recognize that there may always be some competition. In this chapter, we will concentrate initially on eliminations to give alkenes, turning later to alkynes, and multiple bonds involving heteroatoms. 10.2 MECHANISMS As with substitution, we have two mechanisms that are relatively common and one that is much rarer. The two common mechanisms, E1 and E2, have strong similarities to the S N 1 and S N 2 pro- cesses described in the previous chapter and are generally favored by similar conditions to these analogues. The third mechanism, E1cB, is rather different, with no direct analogy in substitution chemistry. We will initially exemplify all the reactions by removal of hydrogen halides, HX, in the presence of base and then explore the scope of the reactions. 10.2.1 E1 ELIMINATION, UNIMOLECULAR This mechanism (Figure 10.1) has strong similarities to the S N 1 process—indeed, the first RDS is identical. A leaving group is lost, taking the electrons from the bond being broken with it, in a slow step, to give a carbocation. However, the next step is not capture of the carbocation by a nucleophile but a rapid loss of a proton to give an alkene. The observed kinetics of the reaction are first order, with the rate proportional only to the concentration of the alkyl halide, not to any added base. The RDS is unimolecular, hence the name E1. The analogy to S N 1 is clear.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Comprehensive Organic Chemistry Experiments for the Laboratory Classroom

  • Carlos A M Afonso, Nuno R Candeias, Dulce Pereira Simão, Alexandre F Trindade, Jaime A S Coelho, Bin Tan, Robert Franzén, Carlos A M Afonso, Nuno R Candeias, Dulce Pereira Simão, Alexandre F Trindade, Jaime A S Coelho, Bin Tan, Robert Franzén(Authors)
  • 2020(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  Keywords Carbocations, dehydration, distillation, GC-MS, IR, NMR

  Background

  Eliminations are one of the most important types of reactions within organic chemistry. In these reactions, a single molecule splits into two molecules. Typically, one of these molecules is a small (diatomic or triatomic) compound, while in the other fragment a π-bond is formed. The general form is shown in Scheme 9.1.3.1 , in which X represents a leaving group. A hydrogen atom from the carbon atom adjacent to the leaving group is abstracted.

  Scheme 9.1.3.1 General elimination reaction.

  Elimination reactions often lead to the formation of multiple compounds, as hydrogen atoms can be abstracted from chemically different carbons atoms. The product composition is determined by the stability of the resulting alkenes, the mechanism involved and the conformational dependence of the elimination process. For instance, dehydrohalogenation of menthyl chloride under E2 conditions gives 100% 2-menthene, while under E1 conditions 32% 2-menthene and 68% 3-menthene are obtained (Scheme 9.1.3.2 ).1

  Scheme 9.1.3.2 Possible dehydrohalogenation pathways of menthyl chloride.

  In this experiment, you will perform an acid-catalysed dehydration of a methylcyclohexanol. The dehydration agent is concentrated (85%) phosphoric acid. You will do the reaction with one of three possible starting compounds: 2-methylcyclohexanol, 3-methylcyclohexanol, or 4-methylcyclohexanol. Other members of your group will start with another methylcyclohexanol. Note that mixtures of the cis - and trans -isomers of the methylcyclohexanols will be used. You will determine the product distribution by the use of GC-MS and 1 H NMR spectroscopy, while infrared spectroscopy also will be used. By comparing the results obtained for different starting alcohols, together you will gain surprising insights into the mechanism of the dehydration reactions.

  Before You Start

  In your organic chemistry textbook, study the theory of elimination reactions in general, and dehydration of alcohols in particular. Read about E1 and E2 mechanisms, the stability of carbocations, rearrangements and Zaitsev’s rule.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  ΔE 1 is the energy difference between two transition states and ΔE 2 is the energy difference between the olefins. 3.1.1.1 Dehydration of Alcohols Elimination of a water molecule from alcohols is a dehydration reaction. This reaction requires an acid as a catalyst to convert the hydroxyl groups into a good leaving group. Mineral acids, as well as Lewis acids, are used for H 2 O elimination. t-Alcohols easily undergo elimination and form alkenes. If there is more than one adjacent carbon atom, a mixture of alkenes is formed. Regioselectivity is observed in the products. As seen from the example below, thermodynamically stable olefin is produced as the major product in 95% yield. 92 3 Elimination Reactions R OH BF 3 ·OEt 2 –H 2 O R R 95% Zaitsev product 5% Alkenes formed under E1 conditions can further rearrange under the reaction conditions. For example, when 2-cyclohexyl-2-propanol is treated with BF 3 ⋅OEt 2 , the endocyclic product is formed in 90% yield as the major product, whereas the expected Zaitsev product, an exocyclic product, is formed in a yield of 10% [2]. OH BF 3 ·OEt 2 + + 10% 90% 0% 2-Cyclohexylpropan-2-ol BF 3 BF 3 If both the double-bond carbons are within the ring, the bond is called endocyclic, while if only one of them is within a ring, the bond is said to be exocyclic. It is known that 1-methylcyclohexene is 2 kcal/mol more stable than methylene cyclohexene. Endocyclic double bond Exocyclic double bond The major product in this reaction cannot arise from a β-elimination process. Furthermore, the carbon atom to which the leaving group was bonded does not appear in the double bond. When any of these compounds formed is separately reacted under the same reaction conditions, equal product distribution is observed irrespective of the starting product. Based on these results, we can conclude that the main product is thermodynamically the most stable one.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Reaction Mechanisms in Organic Chemistry

  • Metin Balcı(Author)
  • 2021(Publication Date)
  • Wiley-VCH

    (Publisher)

  On the other hand, if the β-carbon from which a proton is to be removed is bonded to two hydrogens, two configurational isomers of an alkene can be formed. The carbocation created in the first step has a planar structure. Now, elimination can proceed with one of these two β-protons forming both the E and Z products. For example, (2-chloropentan-2-yl)benzene reacts with the base to eliminate hydrogen chloride to create two different olefins. Elimination in which cis - and a trans double bonds are formed is called stereogenic elimination. trans -Alkenes are more stable than their cis -isomers because of the nonbonded interaction strain between alkyl groups on the same side of the double bond in the cis - isomers. The ratio of cis - and trans -products depends on the reaction mechanism. In E1 reactions, this ratio is determined by the transition state. The first step is the formation of a carbocation. For elimination of the β-proton, the C—H bond from which the proton will be eliminated and the empty p orbital must align parallel to form the C=C double bond. The cis - transition state always has higher energy than the trans - transition state because of steric repulsion between the substituents. The double bond formation takes place “preferentially via the lower energy pathway,” which leads to the formation of the trans - isomer (Figure 3.3). Figure 3.3 Energy profile of cis - and trans -transition states in E1 elimination reactions. ΔE 1 is the energy difference between two transition states and ΔE 2 is the energy difference between the olefins. 3.1.1.1 Dehydration of Alcohols Elimination of a water molecule from alcohols is a dehydration reaction. This reaction requires an acid as a catalyst to convert the hydroxyl groups into a good leaving group. Mineral acids, as well as Lewis acids, are used for H 2 O elimination. t- Alcohols easily undergo elimination and form alkenes. If there is more than one adjacent carbon atom, a mixture of alkenes is formed

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Organic Chemistry

  • David R. Klein(Author)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  Alcohols Chapter 12 covers reactions of alcohols (ROH), and in Section 12.9, we will explore a number of reac- tions that involve substitution and elimination processes. We will now preview two such reactions: 1) the reaction between ROH and HBr; and 2) the reaction of ROH in concentrated sulfuric acid. This current discussion is meant to reinforce the importance of the mechanisms covered in this chapter. Unlike alkyl halides and alkyl sulfonates, alcohols do not undergo S N 2 reactions directly when treated with a strong nucleophile. For example, no reaction is observed when an alcohol is treated with sodium bromide: NaBr No reaction R OH 326 CHAPTER 7 Alkyl Halides: Nucleophilic Substitution and Elimination Reactions Alcohols are not suitable substrates for nucleophilic substitution reactions, because hydroxide (HO − ) is a poor leaving group. However, under strongly acidic conditions (such as HBr), a substitution reac- tion is indeed observed: HBr R OH H 2 O R Br + Under strongly acidic conditions, the OH group can be protonated, thereby converting a bad leaving group (HO − ) into a good leaving group (H 2 O). This allows for an S N 2 reaction to occur, giving an alkyl bromide: R OH H 2 O R Br + H Br R OH 2 Br S N 2 ⊕ ⊝ A similar reaction is also observed for secondary and tertiary alcohols, although tertiary alcohols presumably react via an S N 1 process (rather than S N 2), as shown here: R OH R R H Br R Br S N 1 R R R R –H 2 O R R Br R R OH 2 ⊕ ⊝ ⊕ In the examples above, we see that strongly acidic conditions can activate an alcohol towards substi- tution (either S N 2 or S N 1, depending on the substrate). Similarly, strongly acidic conditions can also activate an alcohol towards elimination. Consider the following example: conc. H 2 SO 4 heat OH + H 2 O Under these strongly acidic conditions, the OH group can be protonated, thereby converting it into a good leaving group.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Survival Guide to Organic Chemistry

  Bridging the Gap from General Chemistry

  • Patrick E. McMahon, Bohdan B. Khomtchouk, Claes Wahlestedt(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  leaving group .
• The identities for the possible X -leaving groups, in most cases, are the same as for nucleophilic substitution: F − , CI − , Br − , I −

  , H2 O, HOR

  (or OTs ).
  1. If the leaving group is a halogen , the substrate for the elimination reaction is thus identified as an
     alkyl halide;
     halogens always leave as the anion, and not as HX.
  2. If the leaving group is water , the elimination substrate is an
     alcohol;
     (OH) may never leave as the anion but only as neutral H2 O. A leaving group of HOR, where R = carbon group, corresponds to an ether substrate (a less common reaction).
• The other atom removed in an elimination for alkyl halides and alcohols is hydrogen, specifically as H + : hydrogen is removed without its bonding electron.
  1. Removal of
     H+
     from carbon allows the carbon to keep both of the original bonding electrons from the C—H bond. These electrons are ultimately transferred to the neighboring electrophilic carbon to form the carbon–carbon pi-bond .
  2. Acid/base conditions for elimination differ significantly for alcohols versus alkyl halides; these reactions are described separately.
• The general mechanisms for elimination are classified based on the number of molecules in the rate-determining step:
  1. If the r.d.s . for elimination is unimolecular , the mechanism is E1 .
  2. If the r.d.s . for elimination is bimolecular , the mechanism is E2 .

12.4.2 IDENTIFICATION OF MAJOR ISOMER ALKENE PRODUCT IN ELIMINATIONS (NON-REARRANGEMENT)

1. Elimination reactions may undergo rearrangements . However, in the absence of rearrangements , the determination of the correct major product requires an analysis of the substitution pattern