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Chapter 6. Organic Reaction Commons

Recommended Article: 【Organic Chemistry】 Table of Contents

1. Names of Carbon

2. Acids and Bases

3. Nucleophiles

4. Leaving Groups

5. Solvents

6. EDG, EWG

1. Names of Carbon

⑴ Names of Hydrogens and Carbons Depending on Functional Group Position

Figure 1. Names of Hydrogens and Carbons Depending on Functional Group Position

① α carbon: Carbon directly attached to the functional group (e.g., X-).

② α hydrogen: Hydrogen attached to the α carbon.

③ β carbon: Carbon adjacent to the α carbon.

④ β hydrogen: Hydrogen attached to the β carbon

⑵ Carbon’s Order

① Primary Carbon: Carbon with only one other carbon bonded to it

② Secondary Carbon: Carbon with two other carbons bonded to it

③ Tertiary Carbon: Carbon with three other carbons bonded to it

④ Quaternary Carbon: Carbon with four other carbons bonded to it

⑤ Applications: n-degree alcohols, n-degree alkyl halides, n-degree amines

⑶ Carbon’s Position

① Vinyl Carbon: Carbon with a double bond

② Allyl Carbon: Carbon next to a vinyl carbon

③ Benzyl Carbon: Carbon next to a phenyl group (i.e., benzene functional group)

○ Benzylic = Arylic

2. Acids and Bases

⑴ Thermodynamic Concepts.

⑵ Strong Acids

① Major pK_a

○ R-CH₃: pK_a = 50

○ R₂C=CH₂: pK_a = 44

○ H-H: pK_a = 35

○ R-NH₂: pK_a = 30

○ RC≡CH: pK_a = 25

○ RCOR: pK_a = 20

○ Secondry alcohol: pK_a = 16

○ H-OH: pK_a = 15.7

○ Primary alcohol: pK_a = 15.5

○ RCHO: pK_a = 13

○ NH₄⁺: pK_a = 10

○ RCOOH: pK_a = 5

○ HN₃: pK_a = 5

○ H₃O⁺: pK_a = -1.7

② TsOH > HI > HBr > HCl > HF

③ CF₃SO₃H (superacid): A very strong acid that protonates key functional groups to facilitate subsequent reactions.

○ Main functional groups that accept H⁺: olefins (i.e., alkenes), alcohols, ethers, carbonyl compounds, amines, etc.

○ Donates H+ to all of these functional groups.

○ Creates strong electrophiles that react with less reactive aromatic compounds: superacid-catalyzed Friedel-Crafts reaction.

○ Can even induce reactions that disrupt aromaticity.

Figure 2. Reaction of superacid breaking aromaticity.

⑶ Strong Bases

① NaOEt, NaOMe

② NaNH₂: E2 reaction dominant

③ t-BuOK

④ CN^-

⑤ n-BuLi

⑥ KOCEt₃

⑦ LDA(lithium diisopropylamide)

⑷ Weak Acids

① EtOH

② BF₃

③ FeCl₃

⑸ Weak Bases

① NaHCO₃

3. Nucleophiles

⑴ Reaction Kinetics Concepts

⑵ Comparison of Nucleophilicities of Ions with the Same Group but Different Periods: Larger period implies stronger nucleophilicity.

① Example: CH₃O^- < CH₃S^-

⑶ Comparison of Nucleophilicities of Neutral Nucleophiles: Larger atoms have greater polarizability, leading to stronger nucleophilicity

① Strong Nucleophiles: RMgX, R₂CuLi, NaNH₂, NaOR, KC≡CH

② Weak Nucleophiles: t-BuOK, NaCN, KN₃, RNH₂, RCOONa, NaSR, NaBR, KI

○ t-BuOK is a strong base but a weak nucleophile due to steric hindrance

③ Strong Nucleophiles

④ Weak Nucleophiles

4. Leaving Groups

⑴ Leaving ability increases with leaving group stability

⑵ Major Leaving Groups Comparison

① TfO^- > TsO^- > MsO^- > I^- > Br^- > Cl^- > H₂O ≫ F^- > OH^- > OR^- > NH₂^-

② H₂O is a major leaving group in alcohol reactions.

③ Halogens: Leaving ability inversely related to basicity

⑶ Solvent Effects on the Reaction Rate of 1-Phenylethyl Sulfonate Esters Depending on Leaving Group Type

① CF₃SO₃^-: 1.4 × 10⁸

② p-Nitrobenzenesulfonate: 4.4 × 10⁵

③ p-Toluenesulfonate: 3.7 × 10⁴

④ CH₃SO₃^-: 3.0 × 10⁴

5. Solvents

⑴ Criteria for Solvents

① Should dissolve reactants

○ Acid-base reactions should not be conducted in nonpolar solvents.

② Should not participate in the reaction

○ Toluene solvent shouldn’t be used in halogen addition reactions since it participates

○ Solvent for making Grignard reagents should not be acidic: In this case, anhydrous ether is often used

③ Should have low boiling point: To facilitate separation by evaporation

④ Polar aprotic solvents needed for S_N2 reactions.

⑤ Organic solvents with higher density than water needed in certain cases

○ Organic solvents with higher density than water: CS₂, CCl₄, CHCl₃, CH₂Cl₂, CH₃Cl, DMSO

○ Purpose 1. Separatory funnel used together

⑵ Category 1. Polar Protic Solvents (PPS): Solvents with hydrogen bonding

① Ionizing nucleophiles are less nucleophilic in these solvents

② Example: Water (H₂O), Ethanol (EtOH), Methanol (MeOH)

③ S_N2: Nucleophile’s charge stabilized by solvent’s hydrogen ion, reducing reaction rate

⑶ Category 2. Polar Aprotic Solvents (PAS): Solvents without hydrogen bonding

① Interaction between ionizing nucleophiles and solvent is weaker

② Example: Acetone, THF, DMSO, DMF, Acetonitrile (CH₃CN), HMPA

Figure 3. Acetone

Figure 4. Tetrahydrofuran (THF)

Figure 5. Dimethyl Sulfoxide (DMSO)

Figure 6. Dimethylformamide (DMF)

Figure 7. CH₃CN

Figure 8. Hexamethylphosphoramide (HMPA)

② There are many solvents where cations are shielded, exposing only the anions.

○ DMSO

○ Weak π bond between 3rd-period S and 2nd-period O due to d orbitals, causing O to be negative and S to be positive

○ S concealed by hydrophobic methyl groups, exposing only O’s negative charge

○ Easier solvent interaction for cations than anions

○ DMF

○ Anion exposed

○ Easier solvent interaction for cations than anions

③ S_N2: Reaction rate not decreased

○ Faster reaction rate than polar protic solvents

○ S_N2 rate comparison: Acetone < THF < DMSO < DMF < CH3CN < HMPA

⑷ Category 3. Aprotic Solvents: Ionizing nucleophiles are insoluble, inhibiting reaction

① Ethers used in reactions with hydrogen halides and Claisen rearrangements, as they only perform a few reactions

② Examples of ethers as solvents

○ Diethyl ether: Also called ether

○ Tetrahydrofuran (THF)

○ Tetrahydropyran (THP)

○ 1,4-dioxane

○ 1,2-dimethoxyethane (DME)

○ tert -butyl methyl ether (MTBE)

6. EDG, EWG

⑴ EDG, EWG are vital in aromatic compound multisubstitution effects, foundational in organic chemistry

⑵ Substituent Groups

① Strong Activating Groups: -NH₂, -NHR, -NR₂, -OH, -O^-

○ Groups with lone pairs of electrons

② Moderately Activating Groups: -NHCOCH₃, -NHCOR, -OCOCH₃, -OCH₃, -OR

○ Groups with lone pairs of electrons and substantial electron-withdrawing effects

③ Weak Activating Groups: -R, -Phe

○ Groups with electron-donating inductive effects

④ -H: Benzene

⑤ Weak Deactivating Groups: -X

○ Halogen groups

⑥ Moderately Deactivating Groups: -C≡N, -SO₃H, -CO₂H, -CO₂R, -CHO, -COR

⑦ Strong Deactivating Groups: -CF₃, -CCl₃, -NO₂, -NR₃

○ Groups with positive charge or -CF₃ group

○ -CF₃ and -NR₃ have weak EWG capability but strong deactivating effects

⑶ EDG (Electron Donating Group, Activating Substituents)

① Strong/Moderate Activating Substituents: Electron-donating resonance effect > Electron-withdrawing inductive effect

② Weak Activating Substituents

⑷ EWG (Electron Withdrawing Group, Deactivating Substituents)

① Halogen Group

○ F: High electronegativity causes the electron-withdrawing inductive effect to exceed the electron-donating resonance effect → leads to deactivation

○ Cl, Br, I: The difference in periods from second-period carbon reduces the effectiveness of non-bonding electron pair resonance

○ Electron-withdrawing inductive effect > Electron-donating resonance effect

② Strong/Moderate Deactivating Substituents

⑸ Even an EDG can function as an EWG if it cannot resonate

① Example: In ROCH, RO- group acts as an EDG, decreasing hydrogen acidity

② Example: In ROCH₂CH, RO- group acts as an EWG, increasing hydrogen acidity

Input: 2019.03.15 21:56