Question

A reaction in which an alkene is formed is (a) 1 -bromo-1-methylcyclohexane is treated with \(\mathrm{NaOH}\) in the presence of acetone. (b) 1 -bromo- 1 -methylcyclohexane is treated with \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3} \mathrm{~N}\). (c) Chlorocyclohexane is heated with \(\left(\mathrm{CH}_{3}\right)_{3}\) CONa in t-butanol (d) 1 -bromo-1-phenylbutane is treated with aqueous \(\mathrm{NaOH}\)

Short answer

#Short Answer# In each case, we will analyze the reaction mechanism and predict the products as follows: (a) 1-bromo-1-methylcyclohexane with NaOH in acetone undergoes an E2 elimination reaction, resulting in the formation of 1-methylcyclohexene. (b) 1-bromo-1-methylcyclohexane with \((C_2H_5)_3N\) undergoes an SN2 nucleophilic substitution reaction, and no alkene is formed in this case. (c) Chlorocyclohexane heated with \((CH_3)_3\)CONa in t-butanol undergoes an E2 elimination reaction, resulting in the formation of cyclohexene. (d) 1-bromo-1-phenylbutane treated with aqueous NaOH undergoes an E2 elimination reaction, resulting in the formation of 1-phenyl-1-butene.

Step-by-step solution

Case (a) 1 -bromo-1-methylcyclohexane with NaOH in acetone

The reaction proceeds via an elimination (E2) mechanism with a strong base, \(\mathrm{NaOH}\), removing the \(\beta\)-hydrogen from the carbon atom adjacent to the carbon bonded to the bromine atom in the 1-bromo-1-methylcyclohexane. The elimination creates a double bond in the ring. 1. Draw the starting molecule: 1-bromo-1-methylcyclohexane 2. Identify the \(\beta\)-hydrogen that will be removed by the strong base 3. Show the E2 mechanism where the hydroxide ion extracts the \(\beta\)-hydrogen and the bromine atom leaves as Br-, forming a double bond in the ring 4. Draw the final product: 1-methylcyclohexene

Case (b) 1 -bromo- 1 -methylcyclohexane with \(\left(\mathrm{C}_{2}\mathrm{H}_{5}\right)_{3} \mathrm{~N}\)

In this case, the reaction is a nucleophilic substitution (SN2) reaction with the nucleophile being the \(\mathrm{N}\) in \(\left(\mathrm{C}_{2}\mathrm{H}_{5}\right)_{3} \mathrm{~N}\). As this is not an elimination reaction, no alkene will be formed in this case. 1. Draw the starting molecule: 1-bromo-1-methylcyclohexane 2. Identify the nucleophile: \(\mathrm{N}\) in \(\left(\mathrm{C}_{2}\mathrm{H}_{5}\right)_{3} \mathrm{~N}\) 3. Show the SN2 reaction where the nucleophile attacks the carbon with the bromine, causing the bromine to leave as Br- 4. Write the product: No alkene is formed in this case

Case (c) Chlorocyclohexane heated with \(\left(\mathrm{CH}_{3}\right)_{3}\) CONa in t-butanol

The reaction proceeds via an E2 elimination reaction in which the base, \(\left(\mathrm{CH}_{3}\right)_{3}\) CONa, donates an electron pair to a \(\beta\)-hydrogen on the carbon adjacent to the chlorine atom in chlorocyclohexane, forming a double bond and expelling the chlorine. 1. Draw the starting molecule: chlorocyclohexane 2. Identify the \(\beta\)-hydrogen that will be removed by the strong base 3. Show the E2 mechanism in which the base removes the \(\beta\)-hydrogen, and the chlorine atom leaves as Cl-, forming a double bond in the ring 4. Draw the final product: cyclohexene

Case (d) 1 -bromo-1-phenylbutane is treated with aqueous NaOH

In this case, the starting molecule can undergo an E2 elimination reaction with the strong base (\(\mathrm{NaOH}\)). The base removes the \(\beta\)-hydrogen from the carbon adjacent to the carbon bonded to the bromine atom in 1-bromo-1-phenylbutane. The elimination creates a double bond in the chain. 1. Draw the starting molecule: 1-bromo-1-phenylbutane 2. Identify the \(\beta\)-hydrogen that will be removed by the strong base 3. Show the E2 mechanism where the hydroxide ion extracts the \(\beta\)-hydrogen and the bromine atom leaves as Br-, forming a double bond in the chain 4. Draw the final product: 1-phenyl-1-butene

Key concepts

E2 Elimination Mechanism

The E2 elimination mechanism is a type of reaction where a molecule loses atoms or groups from adjacent carbons, resulting in the formation of a double bond – essentially creating an alkene. This mechanism is bimolecular, meaning it involves two molecular entities, the substrate (the starting organic molecule) and the base.

During this process, a base abstracts a proton (a hydrogen atom) from a carbon atom that is adjacent to the one bearing the leaving group (often a halogen like Br or Cl). As this happens, the leaving group is expelled and a pi bond is formed between the two adjacent carbon atoms, resulting in the alkene.

In an E2 reaction, the proton abstraction and the leaving group's departure occur simultaneously, which means the reaction has a single, concerted step. This is important because one can predict the outcome based on the stereochemistry of the starting material. E2 reactions often occur when a strong base is present and conditions favor a quick, one-step elimination.

SN2 Nucleophilic Substitution

SN2 stands for bimolecular nucleophilic substitution, and it's another very common reaction in organic chemistry. The 'S' denotes substitution, 'N' for nucleophilic, and '2' indicates that the rate of reaction is dependent on the concentration of two reactants: the nucleophile and the substrate.

The SN2 mechanism involves the nucleophile attacking the electrophilic center of the substrate from the opposite side of the leaving group, leading to a backside attack. This attack pushes the leaving group off the other side and the nucleophile takes its place.

A critical aspect of SN2 reactions is that they proceed with inversion of configuration at the stereocenter being attacked due to the backside approach. They are typically seen with primary and sometimes secondary substrates, because steric hindrance (crowding around the reactive center) in tertiary substrates can prevent the nucleophile from approaching.

Alkene Formation

Alkenes are a class of hydrocarbons characterized by at least one carbon-carbon double bond. These are unsaturated compounds, as they have fewer hydrogen atoms compared to alkanes. Alkene formation is crucial in various organic synthesis routes and can be achieved through several reactions, including E1 and E2 eliminations.

The E2 mechanism, in particular, is a favored pathway for synthesizing alkenes, especially when a strong base is present and immediate elimination is desired. Reaction conditions, such as temperature and the presence of certain solvents, can help steer a reaction towards E2 elimination and thus alkene formation.

Understanding the factors that affect alkene formation, such as the structure of the substrate and the strength of the base, is key to effectively synthesizing these compounds. Furthermore, the stereochemistry of the formed alkene – whether it's cis or trans – can be predicted if the geometry of the starting material is known, as E2 reactions have specific stereochemical requirements.