Question

(a) The compound 2-bromopropane \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHBr}\right]\) can undergo both substitution and elimination when treated with \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-},\) which is a strong base. Predict the organic product in each case, and write a separate chemical equation for each reaction. (b) The compound 1,2 -dibromoethane \(\left(\mathrm{BrCH}_{2} \mathrm{CH}_{2} \mathrm{Br}\right)\) was formerly used in large amounts as an agricultural chemical. Write a chemical equation showing how this compound could be prepared from ethylene by an addition reaction. (c) Certain reactions of aldehydes and ketones begin with isomerization of the aldehyde or ketone to an \(e n o l\) isomer. Enols contain an \(-\) OH group attached to a carbon-carbon double bond. Write a chemical equation for the isomerization of acetone \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}\right]\) to its enol isomer.

Short answer

Substitution with ethoxide gives 2-ethoxypropane; elimination gives propene. 1,2-dibromoethane forms from ethylene and bromine. Acetone isomerizes to its enol form.

Step-by-step solution

Substitution product prediction and equation

When 2-bromopropane (\[\text{(CH}_3\text{)}_2\text{CHBr}\]) undergoes a substitution reaction with a strong base like (\[\text{CH}_3\text{CH}_2\text{O}^{-}\]),the bromine atom is replaced by the ethoxide ion, producing 2-ethoxypropane (\[\text{(CH}_{3}\text{)}_2\text{CHOCH}_2\text{CH}_3\]).The reaction can be written as:\[\text{(CH}_3\text{)}_2\text{CHBr} + \text{CH}_3\text{CH}_2\text{O}^{-} \rightarrow \text{(CH}_3\text{)}_2\text{CHOCH}_2\text{CH}_3} + \text{Br}^{-}\]

Elimination product prediction and equation

For the elimination reaction, the base abstracts a proton, leading to the removal of a bromine atom, which results in the formation of propene(\[ ext{CH}_3 ext{CH}= ext{CH}_2\]).The mechanism involves the formation of a double bond between carbons 1 and 2.The equation for this reaction is:\[\text{(CH}_3\text{)}_2\text{CHBr} + \text{CH}_3\text{CH}_2\text{O}^{-} \rightarrow \text{CH}_3\text{CH}= ext{CH}_2 + \text{Br}^{-} + \text{CH}_3\text{CH}_2\text{OH}\]

Addition reaction of ethylene with bromine

To prepare 1,2-dibromoethane from ethylene (\[ ext{CH}_2= ext{CH}_2\]),undergoes an addition reaction with bromine (\[ ext{Br}_2\])that results in the formation of a saturated compound.The chemical equation for this reaction is:\[\text{CH}_2= ext{CH}_2 + \text{Br}_2 \rightarrow \text{BrCH}_2\text{CH}_2\text{Br}\]

Isomerization of acetone to enol form

Acetone, (\[ ext{(CH}_3 ext{)}_2 ext{C}= ext{O}\]),can undergo isomerization to form its enol isomer. This involves a keto-enol tautomerism where the carbonyl group converts to an alkene with an alcohol group at the adjacent carbon.The chemical equation for this tautomerization is:\[\text{(CH}_3 ext{)}_2 ext{C}= ext{O} \rightleftharpoons \text{CH}_3 ext{C(OH)}= ext{CH}_2\]

Key concepts

Substitution Reaction

Substitution reactions in organic chemistry are fundamental processes where one group in a compound is replaced by another group. In the context of 2-bromopropane reacting with a strong base like ethoxide ion (\[\text{CH}_3\text{CH}_2\text{O}^{-}\]),the bromine atom in 2-bromopropane (\[\text{(CH}_3\text{)}_2\text{CHBr}\]) is substituted by the ethoxide ion.This results in the formation of 2-ethoxypropane (\[\text{(CH}_{3}\text{)}_2\text{CHOCH}_2\text{CH}_3\]),accompanied by the release of a bromide ion (\[\text{Br}^{-}\]).This type of reaction is typical of haloalkanes and involves nucleophilic substitution mechanisms.A key aspect is the involvement of a nucleophile, in this case, the ethoxide ion.The strong base plays a crucial role in facilitating the substitution by making the leaving group (bromine) exit.Additionally, substitution reactions tend to occur in polar protic solvents, which help stabilize the transition state and the leaving group.Substitution reactions are classified into two primary mechanisms:

• \(\text{S}_\text{N}2\) mechanism occurs in a single concerted step and is characterized by a bimolecular rate equation.
• \(\text{S}_\text{N}1\) mechanism involves the formation of an intermediate carbocation, leading to the substitution process in two distinct steps.

In our context, the reaction proceeds most likely via an \(\text{S}_\text{N}2\) mechanism due to the presence of the strong base and the nature of the substrate.

Elimination Reaction

Elimination reactions are another crucial aspect of organic chemistry, particularly in transforming haloalkanes into alkenes. During an elimination reaction, an atom or group is removed from a molecule, resulting in the formation of a double bond. In the case of 2-bromopropane reacting with ethoxide ion, it's a classic example of a beta-elimination or \(E2\) reaction.The base abstracts a proton from the beta-carbon, and simultaneously, the bromide ion leaves the molecule.This results in the formation of propene (\[\text{CH}_3\text{CH}=\text{CH}_2\]) as the product.The formation of the double bond is a defining characteristic of elimination reactions.This process often requires a strong base and typically occurs in aprotic solvents.Elimination mechanisms are divided into two types:

• \(\text{E}2\) mechanism, which is a one-step concerted process.
• \(\text{E}1\) mechanism, which involves the formation of a carbocation intermediate.

In our example, the \(\text{E}2\) mechanism is favored due to the strong base and the secondary carbon structure of 2-bromopropane.This results in a more straightforward, single-step reaction where the base simultaneously abstracts the proton as the leaving group departs.

Addition Reaction

Addition reactions are simple yet powerful reactions in organic chemistry where atoms are added to a double or triple bond, converting unsaturated molecules to saturated ones. These reactions are typical for alkenes and alkynes.For instance, ethylene (\[\text{CH}_2=\text{CH}_2\]) undergoes an addition reaction with bromine (\[\text{Br}_2\]), resulting in 1,2-dibromoethane (\[\text{BrCH}_2\text{CH}_2\text{Br}\]). The pi-bond of the alkene breaks, and the two bromine atoms add to the carbon atoms, saturating the molecule.Addition reactions can be classified into several types depending on the reagent:

• Hydrogenation: Addition of \(\text{H}_2\).
• Halogenation: Addition of halogens like \(\text{Br}_2\) or \(\text{Cl}_2\).
• Hydration: Addition of \(\text{H}_2\text{O}\).

These reactions are generally exothermic, releasing energy in the process.The halogenation of ethylene, as shown, is a typical example where bromine adds across the double bond, resulting in a vicinal dibromide.The reaction proceeds through the formation of a cyclic bromonium ion intermediate, which helps the second bromide ion to attack from the opposite side, giving the final three-dimensional product.

Keto-Enol Tautomerism

Keto-enol tautomerism is a fascinating concept in organic chemistry involving the equilibrium between a ketone or aldehyde and its corresponding enol form. This process involves the rearrangement of the molecular structure without a loss of atoms, thereby forming isomers known as tautomers.For acetone (\[(\text{CH}_3)_2\text{C}=\text{O}\]),the keto form is the most stable, and it can tautomerizes to its enol form (\[\text{CH}_3\text{C(OH)}=\text{CH}_2\]).During this conversion, a hydrogen atom is transferred from the alpha-carbon to the oxygen, forming a hydroxyl group, while the double bond shifts from \(C=O\) to \(C=C\).This tautomerism is important in many biological and chemical processes:

• The balance of both forms in a solution depends on the solvent and pH conditions.
• Enol forms are often more reactive in chemical reactions, serving as intermediates or starting points for further transformations.

Keto-enol tautomerism is essential in understanding the behavior of carbonyl compounds in various reactions, such as aldol condensations or halogenation reactions under specific conditions.In practice, the tautomerization may be acid or base-catalyzed, enhancing the feasibility of the conversion by stabilizing the transition state or intermediate structures.