4-6 Comprehensive Study Guide: Substitution, Elimination, Alkynes, and Carbonyl Chemistry

Fundamentals of Substitution and Elimination Reactions

• Nucleophiles and electrophiles exhibit distinct electronic characteristics:

  • A nucleophile is an electron-rich species containing a lone pair of electrons or a $\pi$ bond that donates electrons to an electron-deficient center.

  • An electrophile is an electron-poor species that accepts electron density from an electron donor.

• Periodic table trends dictate nucleophile strength across rows and down columns:

  • Across a row, nucleophile strength increases from right to left in both polar protic and polar aprotic solvents: $\text{C} > \text{N} > \text{O} > \text{F}$.

  • Polar protic solvents are capable of forming hydrogen bonds (e.g., $H_2O$, $CH_3OH$). In polar protic solvents, nucleophilicity increases down a group due to decreasing solvent cage strength around larger, more polarizable ions: $I^- > Br^- > Cl^- > F^-$.

  • Polar aprotic solvents cannot donate hydrogen bonds (e.g., acetone, tetrahydrofuran [$\text{THF}$, dimethyl sulfoxide [$\text{DMSO}$]). In polar aprotic solvents, nucleophilicity decreases down a group because nucleophiles are relatively unshielded by solvent shells and smaller, more basic anions attack more readily: $F^- > Cl^- > Br^- > I^-$.

• Leaving groups ($\text{LG}$) detach from a molecule during reaction, taking the bonding electron pair with them:

  • The best leaving groups are weak conjugate bases of strong acids.

  • Relative stability and leaving group ability follow the order: $H_2O > I^- > Br^- > Cl^- > F^-$.

• Carbocation stability is governed by hyperconjugation, induction, and resonance delocalization:

  • Relative stability order: $3^\circ\text{ allylic} \approx \text{benzylic} > 3^\circ > 2^\circ > 1^\circ > \text{methyl} > \text{vinyl} \approx \text{phenyl}$.

  • Carbocation rearrangements occur exclusively in unimolecular mechanisms ($S_N1$ and $E1$) to form a more stable intermediate:

  • Hydride shifts involve the migration of a hydrogen atom along with its electron pair ($H^-$) to an adjacent carbocation center.

  • Methyl shifts (or alkyl shifts) involve the migration of a methyl group with its bonding pair ($CH_3^-$) to an adjacent carbocation center.

Reaction Mechanisms: Substitution ($S_N1$ and $S_N2$)

• $S_N2$ mechanism (Bimolecular Nucleophilic Substitution):

  • Proceeds via a concerted, single-step backside attack of the nucleophile on the carbon bearing the leaving group.

  • Exhibits stereospecific inversion of stereochemistry at the reaction center, commonly referred to as an umbrella flip.

  • Follows second-order kinetics with the rate law: $\text{Rate} = k[\text{nucleophile}][\text{electrophile}]$.

  • Favored by:

  • Strong nucleophiles.

  • Primary ($1^\circ$) or secondary ($2^\circ$) carbon substrates; tertiary ($3^\circ$) carbons do not undergo $S_N2$ due to steric hindrance.

  • Polar aprotic solvents (such as $\text{DMSO}$ or acetone), which solvate cations while leaving nucleophilic anions bare and reactive.

• $S_N1$ mechanism (Unimolecular Nucleophilic Substitution):

  • Proceeds through a three-step sequence:

  • Step 1: Loss of the leaving group to generate a carbocation intermediate.

  • Step 2: Nucleophilic attack on the planar carbocation from either face.

  • Step 3: Deprotonation (when using neutral nucleophiles) to yield the neutral product.

  • Results in a racemic mixture (loss of stereospecificity) due to equal probability of attack on either face of the trigonal planar carbocation intermediate.

  • Follows first-order kinetics with the rate law: $\text{Rate} = k[\text{electrophile}]$.

  • The rate-determining step ($\text{RDS}$) is the unimolecular formation of the carbocation intermediate; more stable carbocations lead to faster $S_N1$ reactions.

  • Favored by:

  • Weak nucleophiles.

  • Highly substituted carbon substrates: tertiary ($3^\circ$) > secondary ($2^\circ$); primary ($1^\circ$) substrates never undergo $S_N1$.

  • Polar protic solvents (such as $H_2O$ or alcohols, $ROH$), which stabilize both the leaving group anion and the carbocation intermediate through dipole-dipole and hydrogen-bonding interactions.

Reaction Mechanisms: Elimination ($E1$ and $E2$)

• $E2$ mechanism (Bimolecular Elimination):

  • Operates via a concerted, one-step mechanism where a strong base removes a $\beta$-hydrogen simultaneously with double bond formation and displacement of the leaving group.

  • Follows second-order kinetics with the rate law: $\text{Rate} = k[\text{base}][\text{substrate}]$.

  • Favored by:

  • Strong bases (e.g., sodium ethoxide [$\text{NaOEt}$], sodium methoxide [$\text{NaOMe}$]).

  • Tertiary ($3^\circ$), secondary ($2^\circ$), or primary ($1^\circ$) substrates.

  • Polar aprotic solvents.

  • Stereoselectivity:

  • Occurs when two distinct $\beta$-protons are available on the adjacent carbon.

  • The $E$ (trans) alkene is the major product due to greater thermodynamic stability and reduced steric hindrance.

  • The $Z$ (cis) alkene is the minor product due to steric hindrance between bulky substituents.

  • Stereospecificity:

  • Occurs when only one $\beta$-proton is available.

  • The leaving group and the $\beta$-hydrogen must achieve an anti-periplanar conformation (a dihedral angle of $180^\circ$) to allow proper orbital overlap during elimination; free carbon-carbon single bond rotation is required to align them prior to the reaction.

• $E1$ mechanism (Unimolecular Elimination):

  • Operates through three steps:

  • Step 1: Departure of the leaving group to form a carbocation intermediate.

  • Step 2: Formation of the carbocation (rate-determining step).

  • Step 3: Removal of an adjacent $\beta$-proton by a weak base, collapsing electron density to generate an alkene.

  • Follows first-order kinetics with the rate law: $\text{Rate} = k[\text{substrate}]$.

  • Favored by:

  • Weak bases (e.g., $H_2O$, $ROH$).

  • Tertiary ($3^\circ$) or secondary ($2^\circ$) carbon substrates.

  • High reaction temperatures (heat).

Regioselectivity and Pathway Selection in Substitution vs. Elimination

• Zaitsev versus Hofmann product distribution:

  • Zaitsev product:

  • Represents the thermodynamic product (higher thermodynamic stability).

  • Forms the more substituted alkene.

  • Favored by higher temperatures and small, unhindered bases (e.g., ethoxide [$EtO^-$], methoxide [$MeO^-$]).

  • Hofmann product:

  • Represents the kinetic product, which forms faster with bulky, sterically hindered bases.

  • Forms the less substituted alkene due to minimized steric hindrance during $\beta$-proton abstraction.

  • Favored by sterically hindered bases such as potassium tert-butoxide ($t\text{-}BuO^-$) and lithium diisopropylamide ($\text{LDA}$).

  • Relative alkene stability order:

  • Tetrasubstituted > trisubstituted > disubstituted ($\text{trans} > \text{cis} > \text{geminal}$) > monosubstituted > unsubstituted ($H_2C=CH_2$).

• Acid-catalyzed dehydration of alcohols:

  • Reagents: Sulfuric acid ($H_2SO_4$) and heat.

  • Pathway: The hydroxyl group ($OH$) is protonated to form an oxonium ion ($H_2O^+$), creating a good leaving group ($H_2O$). Subsequent dehydration proceeds via an $E1$ mechanism for secondary and tertiary alcohols, or an $E2$ mechanism for primary ($1^\circ$) alcohols.

  • Yields the Zaitsev alkene as the major product.

• System for deciding between $S_N1$, $S_N2$, $E1$, and $E2$ pathways:

  • Degree of substitution on the carbon attached to the leaving group:

  • Primary ($1^\circ$): Only $S_N2$ or $E2$ are possible.

  • Secondary ($2^\circ$): All four mechanisms ($S_N1$, $S_N2$, $E1$, $E2$) are possible.

  • Tertiary ($3^\circ$): Only $S_N1$, $E1$, or $E2$ are possible; $S_N2$ is blocked.

  • Reagent nucleophilicity and basicity:

  • Strong nucleophile/base favors bimolecular pathways ($S_N2$ or $E2$).

  • Weak nucleophile/base favors unimolecular pathways ($S_N1$ or $E1$).

  • Steric profile of the base:

  • Bulky bases favor $E2$ and yield the Hofmann product.

  • Solvent effects:

  • Polar protic solvents favor unimolecular pathways ($S_N1$/$E1$) or $E2$ over $S_N2$.

  • Polar aprotic solvents favor $S_N2$.

  • Thermal conditions:

  • Heat promotes elimination pathways ($E1$/$E2$) over substitution pathways.

• Pathway determination matrix by substrate and reagent category:

  • Non-base nucleophiles (e.g., $I^-$, $Br^-$, $Cl^-$, $HS^-$, $RS^-$, $H_2S$, $RSH$):

  • Primary ($1^\circ$) substrate: $S_N2$

  • Secondary ($2^\circ$) substrate: $S_N2$ in polar aprotic solvents; $S_N1$ in polar protic solvents.

  • Tertiary ($3^\circ$) substrate: $S_N1$

  • Non-nucleophilic or bulky bases (e.g., $t\text{-}BuO^-$, $\text{DBU}$, $\text{DBN}$, diisopropylethylamine, triethylamine, $\text{LDA}$, $NaH$):

  • Primary ($1^\circ$) substrate: $E2$

  • Secondary ($2^\circ$) substrate: $E2$

  • Tertiary ($3^\circ$) substrate: $E2$

  • Strong nucleophile and strong base (e.g., $RO^-$, $HO^-$, $MeO^-$, $EtO^-$):

  • Primary ($1^\circ$) substrate: $S_N2$ is major; $E2$ is minor (unless a bulky base is used).

  • Secondary ($2^\circ$) substrate: $E2$ in polar protic solvents; $S_N2$ in polar aprotic solvents.

  • Tertiary ($3^\circ$) substrate: $E2$ only (due to steric hindrance blocking substitution).

  • Weak nucleophile and weak base (e.g., $ROH$, $H_2O$, $MeOH$, $EtOH$):

  • Primary ($1^\circ$) substrate: Reaction is not straightforward / generally unreactive.

  • Secondary ($2^\circ$) substrate: Mixture of $S_N1$ and $E1$; application of heat favors $E1$.

  • Tertiary ($3^\circ$) substrate: Mixture of $S_N1$ and $E1$; application of heat favors $E1$.

Structure, Properties, and Alkylation of Alkynes

• Core alkyne properties and definitions:

  • Alkyne: A hydrocarbon containing a carbon-carbon triple bond.

  • Terminal alkyne: An alkyne in which the triple bond is located at the terminus of a carbon skeleton ($\equiv C-H$).

  • Internal alkyne: An alkyne in which the triple bond is positioned within the internal carbon chain ($R-C\equiv C-R$).

  • Geminal dihalide: A compound containing two halogen atoms bonded to the identical carbon atom.

  • Acetylide ion: A strong carbon-centered nucleophile ($\equiv C^-$) generated by deprotonating a terminal alkyne.

  • Acidity: Terminal alkynes are acidic relative to other hydrocarbons because the terminal hydrogen is attached to an $sp$-hybridized carbon ($50\%$ $s$-character), which holds negative charge close to the nucleus and stabilizes the conjugate base.

• Alkylation of terminal alkynes:

  • Step 1: Acetylide ion formation:

  • Reagent: Sodium amide ($\text{NaNH}_2$).

  • Function: Deprotonates the terminal alkyne to produce the nucleophilic acetylide anion.

  • Step 2: Alkylation reaction:

  • Reagent: Alkyl halide ($R-X$), which must be methyl or primary ($1^\circ$) to avoid competing $E2$ elimination.

  • Mechanism: $S_N2$ nucleophilic displacement of the halide.

  • Product: Generates an internal alkyne, effectively extending the carbon chain.

Addition, Hydration, and Reduction Reactions of Alkynes

• Hydrohalogenation of alkynes:

  • Reagent: Hydrogen halide ($HX$), using either $1$ or $2$ equivalents.

  • Terminal alkynes:

  • Addition of $1$ equivalent of $HX$ yields a haloalkene via Markovnikov addition ($H$ adds to the less substituted carbon).

  • Addition of $2$ equivalents of $HX$ yields a geminal dihalide.

  • Anti-Markovnikov addition: Carried out using hydrogen bromide and peroxides ($HBr + \text{ROOR}$) to add the halogen to the less substituted carbon.

  • Internal alkynes:

  • Unsymmetrical internal alkynes reacted with $HX$ yield a mixture of isomeric haloalkenes or dihalides.

• Halogenation and tetrahalogenation:

  • Reagents: Excess diatomic halogen ($Cl_2$ or $Br_2$).

  • Product: A tetrachloro- or tetrabromo-substituted alkane.

  • Tetrahalogenation involves electrophilic addition of four halogen atoms across the carbon-carbon triple bond.

• Hydration of alkynes:

  • Acid-catalyzed hydration (Markovnikov):

  • Reagents: Sulfuric acid ($H_2SO_4$), mercuric sulfate ($\text{HgSO}_4$), and water ($H_2O$).

  • Mechanism: Hydration adds an $OH$ group to the more substituted carbon, forming an enol intermediate.

  • The enol rapidly tautomerizes (keto-enol tautomerization) to afford a ketone.

  • Hydroboration-oxidation (Anti-Markovnikov):

  • Reagents: Step 1: Dialkylborane ($R_2BH$) in tetrahydrofuran ($\text{THF}$); Step 2: Hydrogen peroxide ($H_2O_2$) and sodium hydroxide ($\text{NaOH}$).

  • Mechanism: Adds $OH$ anti-Markovnikov to the less substituted carbon, producing an enol intermediate.

  • Product: Tautomerization of the enol intermediate yields an aldehyde when starting with a terminal alkyne, or a ketone when starting with an internal alkyne.

• Reduction (hydrogenation) of alkynes:

  • Full reduction to alkane:

  • Reagents: Hydrogen gas ($H_2$) over palladium on carbon ($\text{Pd/C}$) or platinum ($\text{Pt}$).

  • Product: Alkane.

  • Stereochemistry: Syn addition of hydrogen across both $\pi$ bonds.

  • Partial reduction to cis ($Z$) alkene:

  • Reagents: Hydrogen gas ($H_2$) with Lindlar's catalyst.

  • Product: Cis ($Z$) alkene.

  • Stereochemistry: Syn addition across the triple bond.

  • Partial reduction to trans ($E$) alkene:

  • Reagents: Sodium ($\text{Na}$) or lithium ($\text{Li}$) in liquid ammonia ($\text{NH}_3\,(l)$) (dissolving metal reduction).

  • Product: Trans ($E$) alkene.

  • Stereochemistry: Anti addition across the triple bond.

• Ozonolysis of alkynes:

  • Terminal alkyne ozonolysis:

  • Reagents: Step 1: Ozone ($O_3$); Step 2: Water ($H_2O$).

  • Products: Carboxylic acid (from the internal fragment) and carbon dioxide ($\text{CO}_2$, from the terminal carbon).

  • Internal alkyne ozonolysis:

  • Reagents: Step 1: Ozone ($O_3$); Step 2: Water ($H_2O$).

  • Products: Cleavage of the triple bond yields two carboxylic acid molecules.

Oxidation, Reduction, and Deoxygenation of Aldehydes and Ketones

• Oxidation of alcohols to carbonyls and carboxylic acids:

  • Primary alcohol to aldehyde:

  • Mild oxidizing agents convert one $C-H$ bond into a $C-O$ bond, halting oxidation at the aldehyde stage.

  • Reagents: Pyridinium chlorochromate ($\text{PCC}$), Dess-Martin periodinane in dichloromethane ($\text{DMP}/\text{CH}_2\text{Cl}_2$), or Swern oxidation (Step 1: $\text{DMSO}$, oxalyl chloride [$(\text{COCl})_2$]; Step 2: Triethylamine [$\text{NEt}_3$]).

  • Primary alcohol to carboxylic acid:

  • Strong oxidizing agents replace two $C-H$ bonds with $C-O$ bonds, driving complete oxidation to the carboxylic acid.

  • Reagents: Potassium permanganate ($\text{KMnO}_4$), potassium dichromate ($K_2Cr_2O_7$), sodium dichromate ($\text{Na}_2Cr_2O_7$), or Jones reagent (chromium trioxide in sulfuric acid, $\text{CrO}_3/H_2SO_4$ / $H_2CrO_4$).

  • Secondary alcohol to ketone:

  • Oxidation of secondary alcohols halts at the ketone stage.

  • Reagents: $\text{PCC}$ or strong oxidants (e.g., Jones reagent).

• Reduction of carbonyl compounds to alcohols:

  • Aldehyde/ketone reduction:

  • Reagents: Sodium borohydride ($\text{NaBH}_4$, mild) or lithium aluminum hydride ($\text{LiAlH}_4$, stronger), followed by aqueous workup ($H_2O$).

  • Products: Ketones are reduced to secondary ($2^\circ$) alcohols; aldehydes are reduced to primary ($1^\circ$) alcohols.

  • Carboxylic acid and ester reduction:

  • $\text{LiAlH}_4$ followed by $H_2O$ workup cleanly reduces esters and carboxylic acids to primary ($1^\circ$) alcohols.

• Deoxygenation of carbonyls (conversion to alkanes):

  • Clemmensen reduction:

  • Reagents: Zinc amalgam ($\text{Zn(Hg)}$) and concentrated hydrochloric acid ($\text{HCl}$).

  • Product: Converts an aldehyde or ketone carbonyl group ($C=O$) directly into a methylene group ($CH_2$) under acidic conditions.

  • Wolff-Kishner reduction:

  • Reagents: Hydrazine ($H_2NNH_2$), potassium hydroxide ($\text{KOH}$), and heat.

  • Product: Converts an aldehyde or ketone carbonyl group ($C=O$) directly into a methylene group ($CH_2$) under strongly basic conditions.

Nucleophilic Addition of Nitrogen and Oxygen: Acetals, Imines, and Enamines

• Acetal and ketal formation:

  • Under acidic conditions:

  • Reagents: Aldehyde or ketone + alcohol ($ROH$) in the presence of a catalytic acid ($\text{cat. } H^+$).

  • Aldehydes react with two equivalents of alcohol to form acetals ($R-CH(OR)_2$).

  • Ketones react with two equivalents of alcohol to form ketals ($R_2C(OR)_2$).

  • The reaction is reversible; acidic hydrolysis ($H_3O^+$) restores the original carbonyl compound and alcohol.

  • Hemi-acetals and hemi-ketals:

  • Formed by reaction with $1$ equivalent of alcohol ($ROH$), serving as intermediates before full acetal or ketal construction.

  • Structural motifs: Hemi-acetal ($R-CH(OH)(OR)$), hemi-ketal ($R_2C(OH)(OR)$).

  • Carbonyl protection strategies:

  • Acetals as protecting groups: An aldehyde can be converted into an acetal using ethanol ($\text{EtOH}$) and catalytic sulfuric acid ($H_2SO_4$). This protects the aldehyde while an ester on the same molecule is selectively reduced with $\text{LiAlH}_4$. Subsequent acidic workup ($H_3O^+$) deprotects the acetal back to the aldehyde while protonating the reduced ester to an alcohol.

  • Diols as protecting groups: Ethylene glycol (or other diols) reacts with aldehydes or ketones under acid catalysis ($H_2SO_4$) to yield stable cyclic acetals or cyclic ketals.

• Imine and enamine formation:

  • Imine formation:

  • Reagents: Aldehyde or ketone + primary amine ($RNH_2$) in the presence of catalytic acid ($\text{cat. } H^+$).

  • Structure: Forms a carbon-nitrogen double bond ($C=N$, imine).

  • Enamine formation:

  • Reagents: Aldehyde or ketone + secondary amine ($R_2NH$) in the presence of catalytic acid ($\text{cat. } H^+$).

  • Structure: Forms an enamine containing a carbon-carbon double bond adjacent to the nitrogen ($C=C-N$).

  • Hydrolysis:

  • Both imines and enamines undergo reversible acidic hydrolysis with water ($H_3O^+$ or $\text{cat. } H^+/H_2O$) to regenerate the parent aldehyde or ketone and corresponding amine.

Carbon-Carbon Bond Formation: Organometallics and the Wittig Reaction

• The Wittig reaction:

  • Preparation of the Wittig reagent (phosphonium ylide):

  • Step 1: Triphenylphosphine ($\text{PPh}_3$) performs an $S_N2$ displacement on an alkyl halide to produce a phosphonium salt.

  • Step 2: A strong organolithium base, n-butyllithium ($n\text{-BuLi}$), deprotonates the phosphonium salt to generate the phosphorus ylide ($Ph_3P=CHR \leftrightarrow Ph_3P^+-C^-HR$).

  • Alkene synthesis:

  • The ylide attacks an aldehyde or ketone to form a carbon-carbon double bond ($C=C$), yielding an alkene and triphenylphosphine oxide ($\text{PPh}_3=O$) as a byproduct.

• Grignard reagents:

  • Synthesis: Formed by reacting an alkyl or aryl halide with elemental magnesium ($Mg$) in an anhydrous aprotic ether solvent (diethyl ether [$\text{Et}_2O$] or $\text{THF}$) to yield an organomagnesium halide ($RMgX$).

  • Electronic structure: Carbon is significantly more electronegative than magnesium, polarizing the bond so that the carbon behaves as a carbanion (a strong nucleophile and base).

  • Addition to carbonyls: The carbanionic carbon attacks the electrophilic carbonyl carbon of aldehydes or ketones to generate an alkoxide intermediate, which yields an alcohol upon acidic aqueous workup.

  • Acid-base side reactions: Grignard reagents are destroyed by moisture or protic sources (water, alcohols, carboxylic acids), acting as strong bases to produce alkanes and magnesium salts ($R-H + \text{HOMgCl}$).

• Organolithium reagents:

  • Synthesis: Formed by reacting an alkyl halide with elemental lithium ($Li$) in an anhydrous aprotic solvent (e.g., $CH_3CH_2Li$).

  • Reactivity: Organolithium reagents are stronger bases and more reactive nucleophiles than Grignard reagents.

  • Application: They undergo nucleophilic addition to carbonyl carbons, forming an alkoxide intermediate that is protonated during acidic workup ($\text{HCl}$, $H_2O$) to provide alcohols.