Organic Synthesis — Class 11 Chemistry Solved Exercises

Q1. Choose the Correct Answer

i.. Retrosynthetic analysis works:

ii.. PCC is typically used to oxidise primary alcohols to:

iii.. Alcoholic KOH with a haloalkane favours:

iv.. A Grignard reagent is generally written as:

v.. Anti-Markovnikov hydration of alkenes is achieved by:

vi.. LiAlH₄ can reduce carboxylic acids to:

vii.. Overall yield of a 3-step synthesis with 80% each step is about:

viii.. Conversion of RBr to ROH is typically:

ix.. Dry ether is essential when preparing Grignard reagents because:

x.. Secondary alcohols on oxidation generally give:

Q2. Give Short Answers

2(i). What is organic synthesis? Why are functional-group interconversions important?

Organic synthesis is the planned construction of organic molecules from simpler starting materials. It may aim at a natural product, a medicine, or a simple laboratory target. Functional-group interconversions change one functional group into another using known reagents. They allow the chemist to install reactive sites where needed. A synthetic plan works backwards from the target (retrosynthesis) or forwards from available reagents. Yield, selectivity and safety guide practical choices.

2(ii). Give reagents for converting an alcohol to an alkyl halide and an alkyl halide to an alcohol.

Alcohols react with HX, PCl₃, PCl₅ or SOCl₂ to give alkyl halides (conditions depend on the alcohol class). Primary alcohols with PBr₃ or similar reagents also give bromoalkanes. Alkyl halides undergo nucleophilic substitution with aqueous KOH or moist Ag₂O routes conceptually to alcohols. In practice, SN1/SN2 conditions must match substrate structure. These interconversions link haloalkane and alcohol chemistry. Always consider competing elimination under strong base and heat.

2(iii). How can a primary alcohol be oxidised to an aldehyde or a carboxylic acid?

Controlled oxidation of a primary alcohol with PCC or distillation of product with acidified dichromate can give an aldehyde. Strong oxidation with excess acidified K₂Cr₂O₇ or KMnO₄ converts a primary alcohol all the way to a carboxylic acid. Secondary alcohols oxidise to ketones; tertiary alcohols resist ordinary oxidation. Reaction conditions and stopping points determine the oxidation level. These oxidations are central synthetic steps. Green methods increasingly replace chromium reagents where possible.

2(iv). State Markovnikov’s rule and its use in alkene to alcohol synthesis via hydrohalogenation then substitution, or hydration.

Markovnikov’s rule: in addition of HX to an unsymmetrical alkene, H adds to the carbon already bearing more hydrogens. Acid-catalysed hydration of alkenes likewise gives the more substituted alcohol as the major product. This allows planned synthesis of secondary or tertiary alcohols from alkenes. Anti-Markovnikov hydration needs special reagents such as hydroboration–oxidation. Understanding regioselectivity is essential in synthesis design. Carbocation stability explains the classical Markovnikov outcome.

2(v). What is a Grignard reagent? Give one synthetic use.

A Grignard reagent has the formula RMgX and forms from an alkyl/aryl halide with Mg in dry ether. The carbon attached to Mg is strongly nucleophilic. Addition to aldehydes or ketones followed by acid work-up yields alcohols. Reaction with CO₂ followed by acidification yields carboxylic acids. Water, alcohols and acids destroy Grignard reagents, so apparatus must be dry. Grignard chemistry builds C–C bonds, a key synthetic goal.

2(vi). Differentiate addition, substitution and elimination in synthetic planning.

Addition increases saturation by adding atoms across multiple bonds (e.g. Br₂ to alkenes). Substitution replaces an atom or group (e.g. OH⁻ displacing Br from RBr). Elimination removes atoms to form multiple bonds (e.g. alcoholic KOH giving alkenes from alkyl halides). The same substrate may follow substitution or elimination depending on base strength, heat and structure. Choosing conditions controls which path dominates. Synthetic routes string these reaction types into sequences toward a target.

3. Explain retrosynthetic analysis with a simple example converting an alcohol target back to an alkene or carbonyl compound.

Retrosynthetic analysis imagines breaking the target into simpler precursors by reversing known reactions. Disconnections focus on strategic bonds, often next to functional groups. For example, a secondary alcohol might be disconnected to a ketone plus a Grignard reagent, or to an alkene via reverse hydration thinking.

Each precursor must be commercially available or easily made. Forward synthesis then specifies reagents, order of steps, and protecting groups if needed. Selectivity problems (regio- and stereo-) are anticipated at the planning stage.

Even school-level multi-step conversions (alkene → alkyl halide → alcohol → carbonyl) train retrosynthetic thinking. Clear arrow pushing and condition notes turn a plan into a laboratory procedure.

4. Describe important functional-group interconversions among alkanes, alkenes, alkyl halides, alcohols and carbonyl compounds.

Alkanes halogenate under radical conditions to alkyl halides (limited selectivity). Alkenes add HX, X₂, or H₂O (acid) to give alkyl halides, dihalides or alcohols. Alkyl halides substitute to alcohols, ethers or amines, or eliminate to alkenes. Alcohols dehydrate to alkenes, convert to alkyl halides, or oxidise to aldehydes/ketones/acids.

Aldehydes and ketones reduce to alcohols or add Grignard reagents to higher alcohols. Carboxylic acids form esters, acid chlorides and amides. Mapping these conversions creates a synthetic toolkit.

Students should memorise key reagents and the structural class each transformation requires.

5. Discuss oxidation and reduction methods commonly used in organic synthesis at this level.

Oxidising agents: acidified K₂Cr₂O₇, KMnO₄, PCC, Tollens’ and Fehling’s reagents (tests as well as reactions). Primary alcohols → aldehydes → acids; secondary alcohols → ketones. Alkenes cleaved by hot concentrated KMnO₄ to carbonyl fragments in vigorous oxidations.

Reducing agents: H₂/Ni (catalytic hydrogenation of alkenes/alkynes/carbonyls under suitable conditions), LiAlH₄ and NaBH₄ (carbonyl reductions; LiAlH₄ also reduces acids/esters). Choice depends on functional-group tolerance and desired stopping point.

Balancing oxidation level across a synthesis avoids unnecessary redox steps. Safety and waste disposal are important with chromium and strong hydride reagents.

6. Explain the synthetic importance of Grignard reagents and constraints on their use.

Grignard reagents create new C–C bonds by nucleophilic addition to electrophilic carbons. With methanal they give primary alcohols; other aldehydes give secondary alcohols; ketones give tertiary alcohols. With CO₂ they give carboxylic acids after hydrolysis. They also react with esters (addition–elimination sequence) to tertiary alcohols under appropriate conditions.

Constraints: absolute dryness; no acidic hydrogens in the substrate before the intended work-up; incompatible with many oxidising or protic functional groups unless protected. Alternative organometallics may be used when Grignard fails.

Despite limitations, RMgX chemistry remains a cornerstone of classical synthesis teaching.

7. Outline factors affecting yield and selectivity in multi-step organic synthesis.

Each step has a yield less than 100%; overall yield is the product of step yields, so short routes are preferred. Side reactions—elimination versus substitution, over-oxidation, polymerisation—reduce selectivity. Temperature, solvent, concentration and catalysts steer pathways.

Protecting groups temporarily mask reactive sites but add steps. Purification between steps removes impurities that would spoil later reactions. Green chemistry seeks atom economy, safer solvents and catalytic rather than stoichiometric reagents.

Good notebook records and thin-layer monitoring improve practical success. Synthesis is as much optimisation as it is reaction knowledge.

Q1. Choose the Correct Answer

i.. Retrosynthetic analysis works:

ii.. PCC is typically used to oxidise primary alcohols to:

iii.. Alcoholic KOH with a haloalkane favours:

iv.. A Grignard reagent is generally written as:

v.. Anti-Markovnikov hydration of alkenes is achieved by:

vi.. LiAlH₄ can reduce carboxylic acids to:

vii.. Overall yield of a 3-step synthesis with 80% each step is about:

viii.. Conversion of RBr to ROH is typically:

ix.. Dry ether is essential when preparing Grignard reagents because:

x.. Secondary alcohols on oxidation generally give:

Q2. Give Short Answers

Q1. Choose the Correct Answer

i.. Retrosynthetic analysis works:

ii.. PCC is typically used to oxidise primary alcohols to:

iii.. Alcoholic KOH with a haloalkane favours:

iv.. A Grignard reagent is generally written as:

v.. Anti-Markovnikov hydration of alkenes is achieved by:

vi.. LiAlH₄ can reduce carboxylic acids to:

vii.. Overall yield of a 3-step synthesis with 80% each step is about:

viii.. Conversion of RBr to ROH is typically:

ix.. Dry ether is essential when preparing Grignard reagents because:

x.. Secondary alcohols on oxidation generally give:

Q2. Give Short Answers

2(i). What is organic synthesis? Why are functional-group interconversions important?

2(ii). Give reagents for converting an alcohol to an alkyl halide and an alkyl halide to an alcohol.

2(iii). How can a primary alcohol be oxidised to an aldehyde or a carboxylic acid?

2(iv). State Markovnikov’s rule and its use in alkene to alcohol synthesis via hydrohalogenation then substitution, or hydration.

2(v). What is a Grignard reagent? Give one synthetic use.

2(vi). Differentiate addition, substitution and elimination in synthetic planning.

Long Questions

3. Explain retrosynthetic analysis with a simple example converting an alcohol target back to an alkene or carbonyl compound.

4. Describe important functional-group interconversions among alkanes, alkenes, alkyl halides, alcohols and carbonyl compounds.

5. Discuss oxidation and reduction methods commonly used in organic synthesis at this level.

6. Explain the synthetic importance of Grignard reagents and constraints on their use.

7. Outline factors affecting yield and selectivity in multi-step organic synthesis.

Q1. Choose the Correct Answer

i.. Retrosynthetic analysis works:

ii.. PCC is typically used to oxidise primary alcohols to:

iii.. Alcoholic KOH with a haloalkane favours:

iv.. A Grignard reagent is generally written as:

v.. Anti-Markovnikov hydration of alkenes is achieved by:

vi.. LiAlH₄ can reduce carboxylic acids to:

vii.. Overall yield of a 3-step synthesis with 80% each step is about:

viii.. Conversion of RBr to ROH is typically:

ix.. Dry ether is essential when preparing Grignard reagents because:

x.. Secondary alcohols on oxidation generally give:

Q2. Give Short Answers

2(i). What is organic synthesis? Why are functional-group interconversions important?

2(ii). Give reagents for converting an alcohol to an alkyl halide and an alkyl halide to an alcohol.

2(iii). How can a primary alcohol be oxidised to an aldehyde or a carboxylic acid?

2(iv). State Markovnikov’s rule and its use in alkene to alcohol synthesis via hydrohalogenation then substitution, or hydration.

2(v). What is a Grignard reagent? Give one synthetic use.

2(vi). Differentiate addition, substitution and elimination in synthetic planning.

Long Questions

3. Explain retrosynthetic analysis with a simple example converting an alcohol target back to an alkene or carbonyl compound.

4. Describe important functional-group interconversions among alkanes, alkenes, alkyl halides, alcohols and carbonyl compounds.

5. Discuss oxidation and reduction methods commonly used in organic synthesis at this level.

6. Explain the synthetic importance of Grignard reagents and constraints on their use.

7. Outline factors affecting yield and selectivity in multi-step organic synthesis.