Organic Chemistry - Reaction Mechanisms & Synthesis Flashcards

Question 1

Describe the SN2 reaction mechanism, including its key characteristics.

Accepted Answer

The SN2 (Substitution Nucleophilic Bimolecular) mechanism is a concerted (one-step) nucleophilic substitution. The nucleophile attacks the electrophilic carbon from the backside, simultaneously displacing the leaving group. It exhibits bimolecular kinetics ($rate = k[	ext{RX}][	ext{Nu}]$) and is favored by primary alkyl halides, strong nucleophiles, and aprotic polar solvents.

Question 2

Outline the electron flow, intermediate, and stereochemical outcome of an SN2 reaction.

Accepted Answer

*Electron flow:* A lone pair from the nucleophile forms a new bond with the electrophilic carbon, while the electron pair from the $	ext{C}-	ext{LG}$ bond moves to the leaving group.
*Intermediate:* No stable intermediate, but a high-energy pentavalent transition state where the carbon is partially bonded to both the nucleophile and the leaving group.
*Stereochemistry:* If the carbon undergoing substitution is chiral, an inversion of configuration (Walden inversion) occurs.

Question 3

What factors influence the rate of an SN2 reaction?

Accepted Answer

1.  **Substrate:** Methyl $>$ Primary $>$ Secondary (Tertiary is generally unreactive).
2.  **Nucleophile Strength:** Stronger nucleophiles increase the rate.
3.  **Leaving Group Ability:** Good leaving groups (weak bases) increase the rate.
4.  **Solvent:** Aprotic polar solvents (e.g., acetone, DMSO, DMF) enhance nucleophilicity and increase the rate.

Question 4

Describe the SN1 reaction mechanism, including its key characteristics.

Accepted Answer

The SN1 (Substitution Nucleophilic Unimolecular) mechanism is a two-step nucleophilic substitution. Step 1: The leaving group departs to form a carbocation intermediate (rate-determining step). Step 2: The nucleophile attacks the carbocation. It exhibits unimolecular kinetics ($rate = k[	ext{RX}]$) and is favored by tertiary alkyl halides, weak nucleophiles, and protic polar solvents.

Question 5

Outline the electron flow, intermediate, and stereochemical outcome of an SN1 reaction.

Accepted Answer

*Electron flow:* Step 1: The electron pair from the $	ext{C}-	ext{LG}$ bond moves to the leaving group, forming a carbocation. Step 2: A lone pair from the nucleophile attacks the empty p-orbital of the carbocation.
*Intermediate:* A planar carbocation, which is resonance-stabilized if possible.
*Stereochemistry:* If the carbocation is chiral, attack from either face is possible, leading to racemization (formation of a racemic mixture) or partial racemization.

Question 6

What factors influence the rate of an SN1 reaction?

Accepted Answer

1.  **Substrate:** Tertiary $>$ Secondary (Primary and methyl are generally unreactive).
2.  **Carbocation Stability:** More stable carbocations (e.g., tertiary, allylic, benzylic) increase the rate.
3.  **Leaving Group Ability:** Good leaving groups increase the rate.
4.  **Solvent:** Protic polar solvents (e.g., water, alcohols) stabilize the carbocation and increase the rate.

Question 7

Describe the E2 reaction mechanism, including its key characteristics.

Accepted Answer

The E2 (Elimination Bimolecular) mechanism is a concerted (one-step) elimination reaction. A strong base abstracts a $\beta$-hydrogen, while the leaving group simultaneously departs and a new $\pi$ bond forms between the $\alpha$ and $\beta$ carbons. It exhibits bimolecular kinetics ($rate = k[	ext{RX}][	ext{Base}]$) and requires an anti-periplanar arrangement of the $\beta$-hydrogen and the leaving group.

Question 8

Outline the electron flow, intermediate, and stereochemical outcome of an E2 reaction.

Accepted Answer

*Electron flow:* A lone pair from the base abstracts a $\beta$-hydrogen, the $	ext{C}-	ext{H}$ bond electrons form a new $\pi$ bond, and the $	ext{C}-	ext{LG}$ bond electrons move to the leaving group.
*Intermediate:* No stable intermediate, but a single, high-energy transition state involving the base, substrate, and leaving group.
*Stereochemistry:* The reaction is stereospecific, requiring an anti-periplanar arrangement of the $\beta$-hydrogen and the leaving group. This geometry dictates which stereoisomers can be formed.

Question 9

Explain Zaitsev's rule and Hofmann elimination in the context of E2 reactions.

Accepted Answer

*Zaitsev's Rule:* In E2 reactions, the major product is typically the most substituted (more stable) alkene, formed by removing a $\beta$-hydrogen from the carbon with fewer hydrogens.
*Hofmann Elimination:* When a bulky base is used (e.g., potassium tert-butoxide) or the leaving group is bulky, the less substituted (Hofmann) product may be favored due to steric hindrance, as the bulky base preferentially abstracts a more accessible $\beta$-hydrogen.

Question 10

Describe the E1 reaction mechanism, including its key characteristics.

Accepted Answer

The E1 (Elimination Unimolecular) mechanism is a two-step elimination reaction. Step 1: The leaving group departs to form a carbocation intermediate (rate-determining step). Step 2: A weak base abstracts a $\beta$-hydrogen, forming a $\pi$ bond. It exhibits unimolecular kinetics ($rate = k[	ext{RX}]$) and often competes with SN1 reactions, especially with tertiary alkyl halides and weak bases/acidic conditions.

Question 11

Outline the electron flow, intermediate, and stereochemical outcome of an E1 reaction.

Accepted Answer

*Electron flow:* Step 1: The electron pair from the $	ext{C}-	ext{LG}$ bond moves to the leaving group, forming a carbocation. Step 2: A lone pair from the base abstracts a $\beta$-hydrogen, and the $	ext{C}-	ext{H}$ bond electrons form a new $\pi$ bond.
*Intermediate:* A planar carbocation.
*Stereochemistry:* Not stereospecific regarding the $\beta$-hydrogen, as rotation around the $	ext{C}-	ext{C}$ bond is possible in the carbocation. Zaitsev's rule generally applies, favoring the most stable (most substituted) alkene product.

Question 12

Summarize the key factors for predicting whether SN1, SN2, E1, or E2 will be the major pathway.

Accepted Answer

1.  **Substrate:** Methyl/Primary (SN2, E2 with strong bulky base); Secondary (all four possible, depends on other factors); Tertiary (SN1/E1).
2.  **Nucleophile/Base:** Strong Nu/Base (SN2/E2); Weak Nu/Base (SN1/E1). Strong bulky bases favor E2.
3.  **Solvent:** Aprotic polar (SN2); Protic polar (SN1/E1).
4.  **Temperature:** High temperature favors elimination (E1/E2); Low temperature favors substitution (SN1/SN2).

Question 13

Describe the general mechanism for Electrophilic Aromatic Substitution (EAS), including intermediates.

Accepted Answer

EAS proceeds in two steps: 1. The electrophile ($	ext{E}^+$) attacks the electron-rich aromatic ring, forming a resonance-stabilized carbocation intermediate called a sigma complex or arenium ion. This step is slow and rate-determining. 2. A base abstracts a proton from the carbon bearing the electrophile, restoring aromaticity and forming the substituted aromatic product.

Question 14

Outline the electron flow and stereochemistry for the nitration of benzene (an EAS example).

Accepted Answer

*Electron flow:* Benzene's $\pi$ electrons attack the nitronium ion ($	ext{NO}_2^+$), forming the arenium ion. A base (e.g., $	ext{HSO}_4^-$ or $	ext{H}_2	ext{O}$) then abstracts a proton from the arenium ion, and the $	ext{C}-	ext{H}$ bond electrons restore aromaticity.
*Intermediate:* Arenium ion (sigma complex), a resonance-stabilized carbocation.
*Stereochemistry:* No chiral center is typically formed in the product, so stereochemistry is not a primary concern for the product itself, but the planar aromaticity is restored.

Question 15

How do activating/deactivating groups and their directing effects influence EAS?

Accepted Answer

*Activating groups* (electron-donating, e.g., $-	ext{OH}$, $-	ext{NH}_2$, alkyls) increase the electron density of the aromatic ring, making it more reactive towards electrophiles, and direct substitution to the *ortho* and *para* positions.
*Deactivating groups* (electron-withdrawing, e.g., $-	ext{NO}_2$, $-	ext{CN}$, $-	ext{COOH}$) decrease the electron density of the ring, making it less reactive, and direct substitution to the *meta* position. Halogens are deactivating but *ortho/para* directors.

Question 16

What is the Diels-Alder reaction, and what are its key characteristics?

Accepted Answer

The Diels-Alder reaction is a [4+2] cycloaddition reaction between a conjugated diene (contributing 4 $\pi$ electrons) and a dienophile (typically an alkene or alkyne, contributing 2 $\pi$ electrons) to form a cyclohexene ring. It is a concerted, pericyclic reaction, meaning all bond breaking and forming occurs simultaneously in a single transition state.

Question 17

Outline the electron flow and intermediate for a general Diels-Alder reaction.

Accepted Answer

*Electron flow:* Three curved arrows depict the cyclic movement of six electrons: two from one $\pi$ bond of the diene, two from the other $\pi$ bond of the diene, and two from the $\pi$ bond of the dienophile. This forms two new $\sigma$ bonds and one new $\pi$ bond.
*Intermediate:* No stable intermediate, but a single, cyclic transition state where the new $\sigma$ bonds are partially formed.

Question 18

What are the stereochemical considerations for the Diels-Alder reaction?

Accepted Answer

The reaction is *stereospecific*: the relative stereochemistry of substituents on the dienophile is retained in the product (e.g., a cis-dienophile gives a cis-substituted cyclohexene). For cyclic dienes and dienophiles with substituents, the *endo rule* often applies, favoring the endo product where substituents on the dienophile point towards the developing $\pi$ bond of the diene in the transition state.

Question 19

What is a Grignard reagent, how is it prepared, and what is its general reactivity?

Accepted Answer

A Grignard reagent is an organomagnesium halide ($	ext{RMgX}$), where $	ext{R}$ is an alkyl, aryl, or vinyl group. It is prepared by reacting an alkyl or aryl halide ($	ext{RX}$) with magnesium metal ($	ext{Mg}$) in an anhydrous ether solvent (e.g., diethyl ether or THF). Grignard reagents are extremely strong nucleophiles and strong bases due to the highly polarized $	ext{C}-	ext{Mg}$ bond, making the carbon carbanion-like.

Question 20

Outline the electron flow and intermediates for the reaction of a Grignard reagent with a ketone, followed by acidic workup.

Accepted Answer

*Electron flow:* The carbanion-like carbon of the Grignard reagent ($	ext{R}^\delta-	ext{MgX}^\delta+$) nucleophilically attacks the electrophilic carbonyl carbon of the ketone. The $\pi$ electrons of the carbonyl bond move to the oxygen, forming an alkoxide intermediate. Subsequent acidic workup (e.g., $	ext{H}_3	ext{O}^+$) protonates the alkoxide to yield a tertiary alcohol.
*Intermediate:* An alkoxide ion ($	ext{R}_2	ext{C}(	ext{R}')	ext{O}^-$) is formed after the initial nucleophilic attack, prior to protonation.

Question 21

What are the main limitations and precautions when working with Grignard reagents?

Accepted Answer

Grignard reagents are highly reactive with any source of acidic protons (e.g., water, alcohols, carboxylic acids, terminal alkynes, amines, even trace moisture from air) and will be protonated, destroying the reagent. Therefore, reactions must be performed under strictly anhydrous conditions. They also react with other electrophiles like epoxides, $	ext{CO}_2$, and nitriles, which can be exploited for synthesis but must be controlled.

Organic Chemistry - Reaction Mechanisms & Synthesis

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