four challenging organic chemistry questions along with detailed step-by-step solutions

**Question 1: Challenging Synthesis Problem**

**Problem:** Design a synthetic route to convert benzene into 2,4-dimethyl-3-ethylhexane. Provide a step-by-step reaction sequence with the necessary reagents and mechanisms for each step.

**Solution:**

This synthesis involves multiple steps and requires careful planning. Here's a step-by-step solution:

**Step 1: Bromination of Benzene**

**Reaction:**

Benzene + Br₂/FeBr₃ → Bromobenzene

In this reaction, bromination of benzene occurs using Br₂/FeBr₃ as the catalyst.

**Step 2: Friedel-Crafts Acylation**

**Reaction:**

Bromobenzene + CH₃COCl/AlCl₃ → 1-Methyl-2-(bromophenyl)ethanone

Friedel-Crafts acylation replaces one of the bromine atoms with an acyl group (CH₃CO) using CH₃COCl/AlCl₃ as the catalyst.

**Step 3: Reduction of Ketone to Alcohol**

**Reaction:**

1-Methyl-2-(bromophenyl)ethanone + NaBH₄ → 1-Methyl-2-(bromophenyl)ethanol

The ketone is reduced to an alcohol using NaBH₄.

**Step 4: Alkylation of Alcohol**

**Reaction:**

1-Methyl-2-(bromophenyl)ethanol + 2-Bromoethylbenzene + NaOH → 2,4-Dimethyl-3-ethylhexane

Alkylation of the alcohol with 2-Bromoethylbenzene in a strong base (NaOH) environment results in the formation of 2,4-dimethyl-3-ethylhexane.

**Question 2: Stereochemistry and Reaction Mechanism**

**Problem:** Explain the stereochemistry and reaction mechanism involved in the bromination of (E)-2-butene. Include the formation of any possible stereoisomers.

**Solution:**

The bromination of (E)-2-butene involves the addition of bromine to the double bond. Here's the step-by-step mechanism:

1. **Formation of the Bromonium Ion:** The π-bond in (E)-2-butene attacks one bromine atom, creating a bromonium ion intermediate.

2. **Nucleophilic Attack:** In the next step, a bromide ion attacks the positively charged carbon in the bromonium ion. This attack can occur from either side of the bromonium ion, resulting in the formation of two enantiomers: (2R,3S)-2,3-dibromobutane and (2S,3R)-2,3-dibromobutane.

3. **Final Product:** Both enantiomers can be obtained as a racemic mixture, as they have equal probability of formation.

**Question 3: Spectroscopy Problem**

**Problem:** Given the proton NMR spectrum of a compound, C₆H₄O₂, determine the structure of the compound based on the spectrum provided:

- A singlet at δ 7.2 ppm (intensity: 2H)

- A singlet at δ 7.8 ppm (intensity: 1H)

**Solution:**

The given proton NMR spectrum indicates the presence of two distinct types of hydrogen atoms. The chemical shifts suggest the presence of aromatic protons. 

- The singlet at δ 7.2 ppm (2H) corresponds to two chemically equivalent protons in an aromatic ring.

- The singlet at δ 7.8 ppm (1H) corresponds to a single proton in another aromatic ring.

Based on this information, the compound is likely to be **phthalic acid** (1,2-benzenedicarboxylic acid), which contains two aromatic rings with the specified chemical shifts.

**Question 4: Reaction Mechanism and Stereochemistry**

**Problem:** Explain the reaction mechanism and stereochemistry involved in the **Addition of HBr to 1-methylcyclohexene**. Provide a step-by-step mechanism and identify the stereochemical outcome.

**Solution:**

The addition of HBr to 1-methylcyclohexene involves electrophilic addition. Here's the step-by-step mechanism:

1. **Formation of Carbocation:** The double bond in 1-methylcyclohexene attacks the proton (H⁺) in HBr, forming a carbocation intermediate.

2. **Nucleophilic Attack:** Br⁻ ion acts as a nucleophile and attacks the carbocation. There are two possible carbocations formed, one on a primary carbon and the other on a secondary carbon. However, the more stable secondary carbocation is favored.

3. **Stereochemistry:** The addition occurs from both sides of the planar carbocation, leading to the formation of a racemic mixture of 1-bromomethylcyclohexane. This racemic mixture contains both (R)- and (S)-enantiomers.

These challenging questions and solutions cover a range of advanced organic chemistry topics, including synthesis, stereochemistry, reaction mechanisms, and spectroscopy. They are designed to challenge and expand the knowledge of university-level students in the field of organic chemistry.