Organic Chemistry Synthesis and Retrosynthetic Analysis Syllabus

Synthesis and Retrosynthetic Analysis syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Synthesis and Retrosynthetic Analysis in Organic Chemistry, not a summary of it.

1. 1

   Principles of Multistep Synthesis

   4 topics

   • Functional Group Interconversions
   • Carbon-Carbon Bond Forming Reactions
   • Protecting Group Strategy
   • Chemoselectivity and Regioselectivity
2. 2

   Retrosynthetic Analysis

   4 topics

   • Disconnection Approach
   • Synthons and Synthetic Equivalents
   • Identifying Strategic Bonds
   • Planning Convergent Syntheses
3. 3

   Key Synthetic Methods

   3 topics

   • Organometallic Reagents
   • Cross-Coupling Reactions
   • Oxidation and Reduction Toolbox
4. 4

   Pericyclic and Advanced Reactions

   3 topics

   • Cycloadditions and the Diels-Alder Reaction
   • Sigmatropic and Electrocyclic Reactions
   • Olefin Metathesis

Synthesis and Retrosynthetic Analysis flashcards for Organic Chemistry

22 of 49 cards from the Synthesis and Retrosynthetic Analysis deck — real questions with worked answers.

1. What is retrosynthetic analysis?

   A problem-solving technique for planning a synthesis by working backward from the target molecule (TM). The target is broken into simpler precursor structures through a series of transformations until commercially available starting materials are reached. The backward step is called a disconnection, shown with a double-lined retrosynthetic arrow $\Rightarrow$.

2. Define a 'disconnection' in retrosynthetic analysis.

   A disconnection is an imaginary bond-breaking operation that reverses a synthetic reaction, converting a target molecule into two (or more) simpler fragments. Each disconnection must correspond to a known, feasible forward reaction.

3. Define 'synthon' and 'synthetic equivalent.'

   A synthon is an idealized, often charged, fragment (a conceptual building block, e.g. an acyl cation or a carbanion) produced by a disconnection. A synthetic equivalent is the real reagent that delivers that synthon in the laboratory (e.g. an acyl chloride for an acyl cation $\ce{R-C+=O}$, or a Grignard reagent for a carbanion).

4. What is an acyl anion equivalent (umpolung) and give a common example.

   A reagent that provides a nucleophilic acyl synthon $\ce{R-C^-=O}$, reversing the normal electrophilic polarity of a carbonyl (umpolung). The classic example is a 1,3-dithiane, which after deprotonation acts as a masked acyl anion; another is the cyanide-derived benzoin condensation intermediate.

5. What is meant by 'natural' vs 'unnatural' (illogical) polarity of synthons?

   Natural polarity synthons follow the inherent electron distribution (e.g. carbonyl carbon = electrophilic $d^{+}$, alpha carbon = nucleophilic $d^{-}$). Unnatural (illogical) synthons reverse this, requiring umpolung reagents such as dithianes, nitroalkanes, or acetylides to access them.

6. What is a 'strategic bond' in synthetic planning?

   A bond whose disconnection maximally simplifies the target, typically one that (1) is in or connected to a ring, (2) joins two ring/functionality regions, (3) creates fragments of similar size (convergence), and (4) corresponds to a reliable, high-yielding forward reaction such as a C–C bond-forming step.

7. Contrast a linear synthesis with a convergent synthesis.

   In a linear synthesis fragments are added one at a time in sequence ($A \to AB \to ABC \to \dots$). In a convergent synthesis, separate fragments are built independently and then joined near the end. Convergence gives higher overall yield and less material loss because the longest step count is shorter.

8. For an n-step sequence where each step gives yield $y$, why does convergence improve overall yield?

   In a linear route the overall yield is $y^{n}$, which decays rapidly with the number of consecutive steps. Convergent routes shorten the longest linear sequence, so fewer steps multiply together. For example, at $80\%$ per step, a linear 6-step gives $0.8^{6}\approx 26\%$, whereas splitting into two 3-step branches joined once gives $\approx 41\%$ through the final product.

9. What is a functional group interconversion (FGI)?

   A retrosynthetic operation (arrow marked 'FGI') that changes one functional group into another without breaking the carbon skeleton, done to reveal a group that permits an easier disconnection. Example: an amine can be traced back to a nitro group or an amide.

10. Give the FGI relationships connecting alcohols, alkenes, and carbonyl compounds.

    Alcohol $\rightleftharpoons$ alkene (dehydration / hydration or hydroboration), alcohol $\rightleftharpoons$ carbonyl (oxidation / reduction), alkene $\rightleftharpoons$ carbonyl (ozonolysis forward; Wittig reverse). These interconversions let a chemist choose the group best suited for a target disconnection.

11. Define chemoselectivity.

    The preferential reaction of a reagent with one of two or more different functional groups present in a molecule. Example: $\ce{NaBH4}$ reduces aldehydes and ketones but generally leaves esters, amides, and carboxylic acids untouched, whereas $\ce{LiAlH4}$ reduces all of them.

12. Define regioselectivity and give the Markovnikov example.

    Regioselectivity is preference for bond formation/breaking at one position (constitutional site) over another. In HX addition to an unsymmetrical alkene, Markovnikov addition places H on the carbon with more H's, giving the more substituted carbocation; anti-Markovnikov (e.g. HBr/peroxides, radical) reverses this.

13. What is the purpose of a protecting group, and what are the three requirements of a good one?

    A protecting group temporarily masks a reactive functional group so a reaction can occur elsewhere. A good protecting group must: (1) be introduced easily and in high yield, (2) be stable to the reaction conditions used elsewhere, and (3) be removable selectively (orthogonally) under conditions that do not damage the rest of the molecule.

14. Name common protecting groups for alcohols, aldehydes/ketones, amines, and carboxylic acids.

    Alcohols: silyl ethers (TMS, TBS/TBDMS), benzyl (Bn), THP, acetate. Aldehydes/ketones: cyclic acetals/1,3-dioxolanes (from ethylene glycol). Amines: Boc, Cbz, Fmoc, acetamide. Carboxylic acids: methyl/ethyl/benzyl/tert-butyl esters.

15. How are 1,3-dioxolane acetals used as carbonyl protecting groups?

    A ketone/aldehyde reacts with ethylene glycol under acid catalysis with water removal to form a cyclic acetal, e.g. $\ce{R2C=O + HOCH2CH2OH ->[H+] R2C(OCH2CH2O)}$. The acetal is stable to base, nucleophiles, and hydrides, and is removed by aqueous acid hydrolysis.

16. What does 'orthogonal protection' mean?

    Using two or more protecting groups that are removed by independent, non-overlapping chemistries, so each can be cleaved selectively without affecting the others. Example in peptide synthesis: Fmoc (base-labile) and Boc/tBu (acid-labile) are orthogonal.

17. Give the general reaction and reactivity order for forming and using a Grignard reagent.

    $\ce{R-X + Mg ->[ether] R-MgX}$. The reactivity order of the halide is $\ce{R-I > R-Br > R-Cl}$. The reagent behaves as a carbanion nucleophile ($\ce{R^-}$ synthon) and reacts with electrophiles such as carbonyls, $\ce{CO2}$, and epoxides. It is destroyed by protic groups (–OH, –NH, –SH).

18. What products form when a Grignard reagent reacts with formaldehyde, other aldehydes, ketones, esters, and $\ce{CO2}$?

    Formaldehyde $\to$ primary alcohol; other aldehydes $\to$ secondary alcohol; ketones $\to$ tertiary alcohol; esters $\to$ tertiary alcohol (two R groups add); $\ce{CO2}$ $\to$ carboxylic acid after workup ($\ce{RMgX + CO2 -> RCO2MgX ->[H3O+] RCO2H}$).

19. Compare organolithium, Grignard, and organocuprate (Gilman) reagents in reactivity.

    Reactivity/basicity order: organolithium $\ce{RLi}$ > Grignard $\ce{RMgX}$ > cuprate $\ce{R2CuLi}$. RLi and RMgX add 1,2 to carbonyls; Gilman cuprates $\ce{R2CuLi}$ are softer, giving 1,4 (conjugate/Michael) addition to enones and coupling with alkyl/vinyl/aryl halides without attacking esters.

20. What C–C bond does the aldol reaction form, and what is the immediate product?

    The aldol reaction forms a bond between the nucleophilic alpha-carbon of one carbonyl's enol/enolate and the electrophilic carbonyl carbon of another, giving a beta-hydroxy carbonyl compound (an 'aldol'). Dehydration gives an alpha,beta-unsaturated carbonyl (aldol condensation). Retrosynthetically it disconnects the $C_\alpha - C_\beta$ bond.

21. What bond does the Claisen condensation form and what is the product?

    An ester enolate attacks the carbonyl of a second ester, forming a C–C bond and giving a beta-keto ester after loss of alkoxide: $\ce{2 CH3CO2Et ->[NaOEt] CH3COCH2CO2Et}$. It is the ester analogue of the aldol and requires a full equivalent of base because the acidic beta-keto ester product is deprotonated.

22. Describe the Wittig reaction and its regiochemical/stereochemical outcome.

    A phosphorus ylide ($\ce{R3P=CR'2}$) reacts with an aldehyde/ketone to form an alkene, placing the C=C exactly where the C=O was: $\ce{R2C=O + Ph3P=CHR' -> R2C=CHR' + Ph3P=O}$. Non-stabilized ylides favor Z-alkenes; stabilized (ester-conjugated) ylides favor E-alkenes.

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