Organic Chemistry - Advanced Reaction Mechanisms & Synthesis Flashcards

Question 1

What factors favor an $S_N1$ reaction over an $S_N2$ reaction?

Accepted Answer

$S_N1$ is favored by: 1. **Substrate**: Tertiary > Secondary (due to carbocation stability). 2. **Solvent**: Polar protic solvents (stabilize carbocation). 3. **Nucleophile**: Weak nucleophiles (nucleophile not involved in rate-determining step). $S_N2$ is favored by: 1. **Substrate**: Methyl > Primary > Secondary (steric hindrance). 2. **Solvent**: Polar aprotic solvents. 3. **Nucleophile**: Strong nucleophiles.

Question 2

What factors favor an $E1$ reaction over an $E2$ reaction?

Accepted Answer

$E1$ is favored by: 1. **Substrate**: Tertiary > Secondary (carbocation stability). 2. **Solvent**: Polar protic solvents. 3. **Base**: Weak bases (often the solvent acts as a base). $E2$ is favored by: 1. **Substrate**: Tertiary > Secondary > Primary (requires $eta$-hydrogen). 2. **Solvent**: Polar aprotic or protic (less critical than base). 3. **Base**: Strong, bulky bases.

Question 3

Describe the stereochemical outcomes of $S_N2$ and $E2$ reactions.

Accepted Answer

$S_N2$ reactions proceed with **inversion of configuration** at the reacting carbon (Walden inversion), as the nucleophile attacks from the backside. $E2$ reactions require an **anti-periplanar** geometry between the leaving group and the $eta$-hydrogen, leading to specific stereoisomers (e.g., *trans*-alkenes from certain diastereomers).

Question 4

Explain why carbocation rearrangements are common in $S_N1$ and $E1$ reactions.

Accepted Answer

Both $S_N1$ and $E1$ reactions proceed via a carbocation intermediate. Carbocations can undergo **hydride shifts** or **alkyl shifts** (1,2-shifts) to form a more stable carbocation (e.g., secondary to tertiary), leading to rearranged products.

Question 5

Outline the general mechanism for Electrophilic Aromatic Substitution (EAS).

Accepted Answer

EAS involves two main steps: 1. **Attack by Electrophile**: The $\pi$ electrons of the aromatic ring attack an electrophile ($E^+$), forming a resonance-stabilized carbocation intermediate (a $\sigma$-complex or arenium ion), which temporarily breaks aromaticity. 2. **Proton Abstraction**: A base removes a proton from the carbon bearing the electrophile, restoring aromaticity.

Question 6

How do activating and deactivating groups influence the regioselectivity of EAS?

Accepted Answer

**Activating groups** (e.g., $-OH$, $-NH_2$, $-OCH_3$, alkyls) are electron-donating, stabilize the $\sigma$-complex, and direct incoming electrophiles to the **ortho** and **para** positions. **Deactivating groups** (e.g., $-NO_2$, $-COOH$, $-CN$, halogens) are electron-withdrawing. Most deactivating groups direct to the **meta** position, except for halogens which are deactivating but *ortho/para* directors due to resonance effects.

Question 7

What are the key limitations of Friedel-Crafts alkylation and acylation reactions?

Accepted Answer

**Alkylation limitations**: 1. Polyalkylation (product is more activated than starting material). 2. Carbocation rearrangements. 3. Cannot be used with strongly deactivating groups or anilines. **Acylation limitations**: 1. Requires an acyl halide or anhydride. 2. Cannot be used with strongly deactivating groups or anilines. Acylation does *not* suffer from rearrangements or polyalkylation.

Question 8

Describe the general mechanism for nucleophilic acyl substitution.

Accepted Answer

Nucleophilic acyl substitution proceeds via a **tetrahedral intermediate**. A nucleophile attacks the carbonyl carbon, forming a tetrahedral intermediate. The carbonyl reforms by expelling a leaving group (e.g., $-Cl$, $-OR$, $-NR_2$).

Question 9

Rank the common carboxylic acid derivatives by their reactivity towards nucleophilic acyl substitution.

Accepted Answer

The reactivity order is determined by the leaving group ability: **Acyl Halides** ($RCOCl$) > **Acid Anhydrides** ($(RCO)_2O$) > **Esters** ($RCOOR'$) $\approx$ **Carboxylic Acids** ($RCOOH$) > **Amides** ($RCONR_2$). Better leaving groups (weaker bases) lead to higher reactivity.

Question 10

What is the Wittig reaction, and what is its primary product?

Accepted Answer

The Wittig reaction is a powerful method for forming **carbon-carbon double bonds** (alkenes) from aldehydes or ketones using a phosphorus ylide (Wittig reagent). The reaction involves the formation of a betaine intermediate, followed by elimination of triphenylphosphine oxide ($Ph_3P=O$) to yield the alkene.

Question 11

How are phosphorus ylides (Wittig reagents) typically prepared?

Accepted Answer

Phosphorus ylides are prepared in two steps: 1. **$S_N2$ reaction**: An alkyl halide reacts with triphenylphosphine ($Ph_3P$) to form a phosphonium salt. 2. **Deprotonation**: A strong base (e.g., $n$-butyllithium, $NaH$) deprotonates the carbon adjacent to phosphorus, forming the neutral phosphorus ylide ($R_3P^+-C^-R_2$).

Question 12

How are Grignard reagents synthesized, and what are their reactions with carbonyl compounds?

Accepted Answer

Grignard reagents ($RMgX$) are synthesized by reacting an alkyl or aryl halide ($RX$) with magnesium metal in an ethereal solvent (e.g., $Et_2O$, THF). They are strong nucleophiles and bases. With carbonyls:
*   **Formaldehyde**: Primary alcohols.
*   **Aldehydes**: Secondary alcohols.
*   **Ketones**: Tertiary alcohols.
*   **Esters/Acid Chlorides**: Tertiary alcohols (after two additions).
*   **$CO_2$**: Carboxylic acids (after acidification).

Question 13

What are the main limitations and precautions when using Grignard reagents?

Accepted Answer

Grignard reagents are extremely strong bases and nucleophiles. They react violently with **protic solvents** (water, alcohols, carboxylic acids, amines) and any compound containing **acidic protons** (e.g., terminal alkynes). Therefore, reactions must be conducted under anhydrous conditions.

Question 14

Describe the Diels-Alder reaction, identifying the key components.

Accepted Answer

The Diels-Alder reaction is a **[4+2] cycloaddition** between a conjugated **diene** (4 $\pi$ electrons) and a **dienophile** (2 $\pi$ electrons, typically an alkene or alkyne with electron-withdrawing groups). It forms a six-membered ring, often with high stereospecificity and regioselectivity. It is a concerted, pericyclic reaction.

Question 15

Explain the "endo rule" and regioselectivity in Diels-Alder reactions.

Accepted Answer

The **endo rule** states that the *endo* product (where the electron-withdrawing group on the dienophile points *under* the newly forming ring) is kinetically favored over the *exo* product, due to secondary orbital interactions in the transition state. **Regioselectivity** is observed when both the diene and dienophile are unsymmetrical, typically leading to the *ortho* or *para* product (1,2- or 1,4-addition) based on electronic factors (e.g., resonance stabilization of transition state).

Question 16

Define pericyclic reactions and list their main types.

Accepted Answer

Pericyclic reactions are concerted reactions that proceed through a **cyclic transition state** involving a reorganization of electrons, without any intermediate. They are often highly stereospecific. Main types include: **Cycloadditions** (e.g., Diels-Alder), **Electrocyclic reactions** (ring-opening/closing of conjugated systems), **Sigmatropic rearrangements** (migration of a $\sigma$-bond across a $\pi$-system), and **Ene reactions**.

Question 17

Briefly explain the significance of the Woodward-Hoffmann rules for pericyclic reactions.

Accepted Answer

The Woodward-Hoffmann rules predict the stereochemical outcome and feasibility of pericyclic reactions based on the **symmetry of the molecular orbitals** involved and whether the reaction is **thermal** or **photochemical**. They state that a pericyclic reaction can occur if the symmetry of the reactant orbitals correlates with the symmetry of the product orbitals in the transition state. This often involves **conrotatory** or **disrotatory** modes for electrocyclic reactions.

Question 18

Describe the three main steps of a radical chain reaction.

Accepted Answer

Radical chain reactions proceed through three stages: 1. **Initiation**: Formation of radicals from non-radical species (e.g., homolytic cleavage of a weak bond by heat or light). 2. **Propagation**: Radicals react with non-radicals to form new radicals, perpetuating the chain. 3. **Termination**: Two radicals combine to form a non-radical product, ending the chain.

Question 19

Explain the regioselectivity observed in the radical halogenation of alkanes (e.g., with $Br_2$ or $NBS$).

Accepted Answer

Radical halogenation (especially bromination with $Br_2$ or $NBS$) shows high regioselectivity, favoring substitution at the most stable radical intermediate. The order of radical stability is **tertiary > secondary > primary > methyl**. Therefore, bromination typically occurs at the most substituted carbon, due to the stability of the intermediate alkyl radical.

Question 20

What is retrosynthesis, and why is it a crucial tool in organic synthesis?

Accepted Answer

Retrosynthesis is a problem-solving technique in organic synthesis where the target molecule is mentally **disassembled** step-by-step into simpler, readily available starting materials. It involves working backward from the product to reactants, using **retrosynthetic transforms** (the reverse of known reactions), to identify a viable synthetic pathway. It simplifies complex multi-step syntheses.

Question 21

List common strategies employed in multi-step organic synthesis.

Accepted Answer

Key strategies include: 1. **Functional Group Interconversion (FGI)**: Converting one functional group into another. 2. **Carbon-Carbon Bond Formation**: Using reactions like Grignard, Wittig, Diels-Alder, aldol, Claisen, etc. 3. **Control of Stereochemistry**: Using stereoselective reactions, chiral auxiliaries, or protecting groups. 4. **Protecting Groups**: Temporarily masking reactive functional groups. 5. **Strategic Disconnections**: Breaking C-C bonds in a retrosynthetic analysis.

Question 22

Propose a multi-step synthesis for converting 1-bromobutane to 3-heptanone.

Accepted Answer

Target: $CH_3CH_2C(=O)CH_2CH_2CH_2CH_3$.
1.  **Form Butyl Grignard**: $CH_3CH_2CH_2CH_2Br + Mg \xrightarrow{Et_2O} CH_3CH_2CH_2CH_2MgBr$.
2.  **Form a nitrile**: $CH_3CH_2Br + NaCN ightarrow CH_3CH_2CN$ (Propanenitrile).
3.  **Grignard addition to nitrile**: $CH_3CH_2CH_2CH_2MgBr + CH_3CH_2CN \xrightarrow{1. Et_2O, 2. H_3O^+} CH_3CH_2C(=O)CH_2CH_2CH_2CH_3$ (3-heptanone). (Grignard adds to nitriles to form ketones after hydrolysis).

Question 23

Differentiate between enantioselective and diastereoselective reactions.

Accepted Answer

**Enantioselective reactions** produce one enantiomer in preference to the other from an achiral starting material, typically using a chiral catalyst or reagent. **Diastereoselective reactions** produce one diastereomer in preference to another, often from a chiral starting material or via a chiral auxiliary, or through specific transition state geometries (e.g., Diels-Alder *endo* rule).

Question 24

What is a chiral auxiliary, and how is it used in asymmetric synthesis?

Accepted Answer

A chiral auxiliary is a temporary chiral group that is covalently attached to a substrate to induce stereoselectivity in a subsequent reaction. After the desired stereoselective transformation, the auxiliary is removed, leaving the product with the desired stereochemistry. This allows for the synthesis of enantiomerically pure compounds from achiral precursors.

Organic Chemistry - Advanced Reaction Mechanisms & Synthesis

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