Organic Chemistry - Functional Groups & Key Reactions Flashcards

Question 1

Carbocation Stability Order

Accepted Answer

Carbocation stability increases with alkyl substitution: $3^\circ > 2^\circ > 1^\circ > 	ext{methyl}$. Stabilized by **hyperconjugation** (sigma donation from adjacent $C-H$ or $C-C$ bonds) and **inductive effects**.

Question 2

$S_N1$ Reaction Mechanism & Kinetics

Accepted Answer

**Unimolecular Nucleophilic Substitution**. A two-step process involving carbocation intermediate formation (rate-determining step).
- **Rate Law**: $	ext{Rate} = k[RX]$
- **Favored by**: $3^\circ$ alkyl halides, weak nucleophiles, polar protic solvents
- **Stereochemistry**: **Racematization** (loss of chirality)

Question 3

$S_N2$ Reaction Mechanism & Kinetics

Accepted Answer

**Bimolecular Nucleophilic Substitution**. A one-step concerted process with a pentacoordinate transition state.
- **Rate Law**: $	ext{Rate} = k[RX][Nu^-]$
- **Favored by**: Methyl and $1^\circ$ substrates, strong nucleophiles, polar aprotic solvents
- **Stereochemistry**: **Inversion of configuration** (Walden inversion)

Question 4

Polar Protic vs. Polar Aprotic Solvents

Accepted Answer

- **Polar Protic** ($H_2O$, $EtOH$, $MeOH$): Contain $O-H$ or $N-H$ bonds. Solvate both cations and anions via hydrogen bonding. Favors $S_N1$ and $E1$.
- **Polar Aprotic** ($DMSO$, Acetone, $DMF$): Lack hydrogen bond donors. Solvate cations well, leaving nucleophiles 'naked' and highly reactive. Favors $S_N2$.

Question 5

$E1$ vs. $E2$ Elimination Mechanisms

Accepted Answer

- **$E1$**: Unimolecular, 2-step via carbocation. $	ext{Rate} = k[RX]$. Favored by weak bases and $3^\circ$ substrates.
- **$E2$**: Bimolecular, 1-step concerted. $	ext{Rate} = k[RX][B^-]$. Favored by strong bases. Requires **anti-periplanar** geometry between proton and leaving group.

Question 6

Zaitsev's Rule vs. Hofmann's Rule

Accepted Answer

- **Zaitsev's Rule**: Predicts the major product is the most substituted, thermodynamically stable alkene (favored by small unhindered bases like $NaOCH_3$).
- **Hofmann's Rule**: Predicts the major product is the least substituted, kinetically favored alkene (favored by bulky bases like $t-BuOK$).

Question 7

Chirality & Chiral Centers

Accepted Answer

A molecule is **chiral** if it is non-superimposable on its mirror image. A **chiral (stereogenic) center** is an $sp^3$-hybridized carbon atom bonded to four distinct chemical groups.

Question 8

Enantiomers vs. Diastereomers

Accepted Answer

- **Enantiomers**: Non-superimposable mirror images; inverted configuration at **all** stereocenters. Identical physical properties (except optical rotation direction).
- **Diastereomers**: Non-superimposable non-mirror images; configuration differs at **some**, but not all, stereocenters. Different physical and chemical properties.

Question 9

Assigning $R/S$ Configuration (Cahn-Ingold-Prelog)

Accepted Answer

1. Assign priority (1 to 4) based on atomic number ($Z$).
2. Orient group priority 4 to the back (dashed wedge).
3. Trace path from priority $1 \rightarrow 2 \rightarrow 3$:
   - **Clockwise** = $R$ (*Rectus*)
   - **Counterclockwise** = $S$ (*Sinister*)

Question 10

Meso Compounds

Accepted Answer

Molecules containing two or more chiral centers that also possess an **internal plane of symmetry** (or center of inversion). They are **achiral** and optically inactive overall.

Question 11

Nucleophile vs. Electrophile

Accepted Answer

- **Nucleophile**: Electron pair donor (Lewis base); electron-rich (contains negative charge, lone pairs, or $\pi$ bonds).
- **Electrophile**: Electron pair acceptor (Lewis acid); electron-deficient (contains positive charge, partial positive $\delta^+$, or open octet).

Question 12

Electrophilic Aromatic Substitution (EAS)

Accepted Answer

A reaction where an electrophile ($E^+$) replaces a hydrogen on an aromatic ring. Proceeds via a resonance-stabilized **arenium ion** (sigma complex) intermediate, followed by deprotonation to restore aromaticity.

Question 13

EAS Directing Groups: Activators vs. Deactivators

Accepted Answer

- **Activators (ortho/para directors)**: Electron-donating groups (e.g., $-OH$, $-NH_2$, $-OCH_3$, alkyls) that increase ring reactivity.
- **Deactivators (meta directors)**: Electron-withdrawing groups (e.g., $-NO_2$, $-CN$, $-COOH$) that decrease ring reactivity.
- *Exception*: **Halogens** are deactivating due to induction, but **ortho/para-directing** due to lone pair resonance.

Question 14

Nucleophilic Addition to Carbonyls

Accepted Answer

A nucleophile attacks the electrophilic carbonyl carbon ($sp^2$), converting it to a **tetrahedral intermediate** ($sp^3$). Aldehydes are more reactive than ketones due to **less steric hindrance** and **less inductive stabilization**.

Question 15

Grignard Reagent ($R-MgX$) Reactions

Accepted Answer

Organometallic reagents that act as strong nucleophiles/carbanions ($R^-$). React with carbonyls followed by acidic workup ($H_3O^+$):
- Formaldehyde $\rightarrow 1^\circ$ alcohol
- Aldehyde $\rightarrow 2^\circ$ alcohol
- Ketone $\rightarrow 3^\circ$ alcohol

Question 16

Fischer Esterification

Accepted Answer

An acid-catalyzed reversible condensation of a **carboxylic acid** and an **alcohol** to yield an **ester** and water:
$$R-COOH + R'-OH \xrightarrow{H^+} R-COOR' + H_2O$$
Shifted to completion using excess alcohol or water removal.

Question 17

Oxidation of Primary ($1^\circ$) and Secondary ($2^\circ$) Alcohols

Accepted Answer

- **$1^\circ$ Alcohol**:
  - PCC (mild) $\rightarrow$ **Aldehyde**
  - $H_2CrO_4$ / Jones (strong) $\rightarrow$ **Carboxylic Acid**
- **$2^\circ$ Alcohol**:
  - PCC or $H_2CrO_4 \rightarrow$ **Ketone**

Question 18

Epoxide Ring Opening (Acidic vs. Basic Conditions)

Accepted Answer

- **Acidic Conditions**: Nucleophile attacks the **more substituted** carbon (which carries more partial positive character).
- **Basic Conditions**: Nucleophile attacks the **less substituted** carbon via an $S_N2$ mechanism (steric control).

Question 19

Alkene Halogenation & Halohydrin Formation

Accepted Answer

- **Halogenation** ($X_2$): *Anti*-addition of halogen atoms via a cyclic **halonium ion** intermediate.
- **Halohydrin Formation** ($X_2/H_2O$): Regioselective addition where $-OH$ binds to the **more substituted** carbon and $-X$ to the less substituted carbon with *anti* stereochemistry.

Question 20

Key Functional Groups Summary

Accepted Answer

- **Ester**: $R-COO-R'$
- **Amide**: $R-CO-NR_2$
- **Acid Anhydride**: $R-CO-O-CO-R$
- **Acyl Chloride**: $R-CO-Cl$
- **Acetal**: $R_2C(OR')_2$
- **Enamine**: $R_2N-C=C$

Organic Chemistry - Functional Groups & Key Reactions

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