Q3.1 Write three reagents and conditions, including the name/type of the reactions, which could be used to perform the following transformation:

(Cyclohexanone to cyclohexane)

Background

Topic: Reduction of Ketones to Alkanes

This question tests your knowledge of different reduction methods that convert a ketone (cyclohexanone) to an alkane (cyclohexane), including the reagents, conditions, and the type of reaction involved.

Key Terms and Formulas

• Reduction: A chemical reaction that involves the gain of electrons or hydrogen, or the loss of oxygen.

• Ketone: An organic compound with a carbonyl group (C=O) bonded to two carbon atoms.

• Alkane: A saturated hydrocarbon with only single bonds.

• Common reduction methods: Clemmensen reduction, Wolff-Kishner reduction, catalytic hydrogenation.

Step-by-Step Guidance

1. Identify the functional group in the starting material (cyclohexanone) and the product (cyclohexane).

2. Recall reduction reactions that can remove the carbonyl group and fully reduce it to a methylene group (CH2).

3. List three different reagents and their conditions that can achieve this transformation, and name the type of reaction for each.

4. For each method, briefly describe the reaction conditions (acidic, basic, or catalytic hydrogenation).

5. Set up your answer by listing each reagent, its conditions, and the reaction type, but do not write the final list yet.

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Q3.2 Propose a plausible mechanism for the following reaction:

(Reduction of a conjugated ketone with Na/NH3 (liq), then NH4Cl)

Background

Topic: Dissolving Metal Reduction (Birch Reduction)

This question tests your understanding of the stepwise mechanism for the reduction of conjugated carbonyl compounds using sodium in liquid ammonia, followed by quenching with ammonium chloride.

Key Terms and Formulas

• Birch Reduction: A reaction where aromatic or conjugated systems are partially reduced using alkali metals in liquid ammonia.

• Electron transfer: Sodium donates electrons to the substrate, forming radical anions.

• Protonation: Ammonium chloride provides protons to neutralize the intermediate anions.

Step-by-Step Guidance

1. Draw the starting conjugated ketone and identify the positions where reduction will occur.

2. Show the first electron transfer from sodium to the substrate, forming a radical anion.

3. Indicate the protonation of the radical anion by ammonia, forming a neutral radical.

4. Show the second electron transfer from sodium, converting the radical to an anion.

5. Describe the final protonation step by ammonium chloride to yield the reduced product, but do not draw the final structure yet.

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Q3.3 Briefly discuss the important application of tris(triphenylphosphine)chlororhodium in organic chemistry, illustrating your answer with a suitable example.

Background

Topic: Homogeneous Catalysis – Wilkinson's Catalyst

This question tests your knowledge of the use of Wilkinson's catalyst (tris(triphenylphosphine)chlororhodium(I)) in organic synthesis, especially in catalytic hydrogenation reactions.

Key Terms and Formulas

• Wilkinson's Catalyst: [RhCl(PPh3)3], a homogeneous catalyst for hydrogenation of alkenes.

• Homogeneous catalysis: The catalyst is in the same phase as the reactants (usually solution).

• Hydrogenation: Addition of hydrogen (H2) across a double bond to form an alkane.

Step-by-Step Guidance

1. State the main use of Wilkinson's catalyst in organic chemistry (hydrogenation of alkenes).

2. Describe the advantages of homogeneous catalysis (selectivity, mild conditions).

3. Give a general example of an alkene being hydrogenated to an alkane using this catalyst.

4. Briefly mention the reaction conditions (H2 gas, room temperature, solvent).

5. Set up your answer with a reaction equation, but do not write the full example yet.

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