Organic Chemistry Reactions & Concepts

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Alcohols: Reactions & Properties Acid-catalyzed dehydration of 3-methylbutan-2-ol: Reactant: $\text{CH}_3\text{CH}(\text{OH})\text{CH}(\text{CH}_3)_2$ Mechanism: E1 elimination. Protonation of -OH, loss of water to form carbocation, then deprotonation to form alkene. Major product (Zaitsev's rule): 2-methylbut-2-ene ($\text{CH}_3\text{C}(\text{CH}_3)=\text{CHCH}_3$) Boiling points of alcohols vs. alkanes: Alcohols have higher boiling points due to the presence of hydrogen bonding between alcohol molecules. The -OH group allows for strong intermolecular hydrogen bonds, which require more energy to break compared to the weaker London dispersion forces (and dipole-dipole interactions for polar alkanes) in alkanes of comparable molecular weight. Classification of Alcohols ($1^\circ, 2^\circ, 3^\circ$): $1^\circ$ (Primary): -OH group attached to a carbon bonded to only one other carbon atom. E.g., Ethanol. $2^\circ$ (Secondary): -OH group attached to a carbon bonded to two other carbon atoms. E.g., Isopropanol. $3^\circ$ (Tertiary): -OH group attached to a carbon bonded to three other carbon atoms. E.g., Tert-butanol. Preparation of $1^\circ, 2^\circ, 3^\circ$ Alcohols using Grignard Reagent: $1^\circ$: Formaldehyde ($\text{HCHO}$) + $\text{RMgX} \rightarrow \text{RCH}_2\text{OH}$ $2^\circ$: Aldehyde ($\text{R'CHO}$, not formaldehyde) + $\text{RMgX} \rightarrow \text{RR'CHOH}$ $3^\circ$: Ketone ($\text{R'COR''}$) + $\text{RMgX} \rightarrow \text{RR'R''COH}$ Reactivity of Alcohols with Lucas Reagent ($\text{HCl}/\text{ZnCl}_2$): Order of reactivity: $3^\circ > 2^\circ > 1^\circ$. $3^\circ$ alcohols react immediately (turbidity appears instantly) due to the stability of the tertiary carbocation formed. $2^\circ$ alcohols react within 5-10 minutes. $1^\circ$ alcohols react very slowly or not at all at room temperature. Reaction of Methyl alcohol with $\text{CH}_3\text{MgI}$: $\text{CH}_3\text{OH} + \text{CH}_3\text{MgI} \rightarrow \text{CH}_4 \uparrow + \text{CH}_3\text{OMgI}$ Methyl alcohol acts as an acid, and the Grignard reagent acts as a strong base, leading to the formation of methane gas. Reduction of $\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2-\text{CHO}$: (i) With $\text{H}_2$ in presence of catalyst (e.g., $\text{Ni}$, $\text{Pd}$, or $\text{Pt}$): Reduces both alkene and aldehyde. $\text{CH}_3-\text{CH}_2-\text{CH}_2-\text{CH}_2-\text{CH}_2\text{OH}$ (Pent-1-ol) (ii) With $\text{LiAlH}_4$: Selectively reduces aldehyde to alcohol, leaves alkene intact. $\text{CH}_3-\text{CH}=\text{CH}-\text{CH}_2-\text{CH}_2\text{OH}$ (Pent-3-en-1-ol) Phenols: Acidity & Reactions Acidic strength of Phenols: Electron-withdrawing groups (EWG) increase acidity, electron-donating groups (EDG) decrease acidity. Order of increasing acidic strength for given phenols: $p\text{-CH}_3\text{C}_6\text{H}_4\text{OH}$ (p-Cresol) < $\text{C}_6\text{H}_5\text{OH}$ (Phenol) < $p\text{-OCH}_3\text{C}_6\text{H}_4\text{OH}$ (p-Methoxyphenol) < $p\text{-ClC}_6\text{H}_4\text{OH}$ (p-Chlorophenol) < $p\text{-NO}_2\text{C}_6\text{H}_4\text{OH}$ (p-Nitrophenol) Reason: $-\text{CH}_3$ and $-\text{OCH}_3$ are electron-donating groups (EDG), stabilizing the parent phenol, destabilizing the phenoxide ion, thus decreasing acidity. $-\text{OCH}_3$ has a stronger resonance donation effect than $-\text{CH}_3$'s inductive effect, making p-methoxyphenol less acidic than phenol itself. $-\text{Cl}$ and $-\text{NO}_2$ are electron-withdrawing groups (EWG). They stabilize the phenoxide ion by delocalizing the negative charge, thus increasing acidity. $-\text{NO}_2$ is a stronger EWG than $-\text{Cl}$ due to strong resonance and inductive withdrawal, making p-nitrophenol the most acidic. Directive influence of -OH group in phenol towards electrophilic substitution: The -OH group is an ortho, para-directing and activating group. The lone pair on the oxygen atom of the -OH group donates electron density to the benzene ring via resonance. This increases the electron density at the ortho and para positions, making them more susceptible to electrophilic attack. The resonance structures for phenol show partial negative charges at the ortho and para positions. Resonance stabilization of phenoxide ion: When phenol loses a proton, it forms a phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$). The negative charge on the oxygen atom can be delocalized into the benzene ring through resonance. Resonance structures: The negative charge moves from the oxygen to the ortho and para positions of the ring. This delocalization stabilizes the phenoxide ion, making phenol more acidic than alcohols. $\text{C}_6\text{H}_5\text{OH} \rightleftharpoons \text{C}_6\text{H}_5\text{O}^- + \text{H}^+$ Stability of phenoxide ion vs. phenol: Phenoxide ion is more stable than phenol. Reason: Phenoxide ion is stabilized by resonance (delocalization of negative charge). While phenol also exhibits resonance, the resonance structures involve charge separation (positive charge on oxygen and negative charge on ring carbons), which is less stabilizing than the delocalization of a full negative charge in the phenoxide ion. Named Reactions Reimer-Tiemann Reaction: Definition: The ortho-formylation of phenols (or naphthols) under alkaline conditions with chloroform ($\text{CHCl}_3$) to yield salicylaldehyde (or substituted salicylaldehyde). Mechanism: Phenol reacts with strong base ($\text{NaOH}$) to form phenoxide ion. Chloroform reacts with base to form dichlorocarbene ($\text{:CCl}_2$), a highly electrophilic species. Dichlorocarbene attacks the electron-rich ortho position of the phenoxide ion. Intermediate undergoes hydrolysis and tautomerization to yield salicylaldehyde. Reaction: C6H5OH + CHCl3 + 3 NaOH -> o-HOC6H4CHO + 3 NaCl + 2 H2O Gattermann Synthesis (Gattermann Aldehyde Synthesis): Mechanism: The reaction involves the treatment of phenols or phenol ethers with hydrogen cyanide ($\text{HCN}$) and hydrogen chloride ($\text{HCl}$) in the presence of a Lewis acid catalyst (e.g., $\text{AlCl}_3$ or $\text{ZnCl}_2$). This forms an imine intermediate, which upon hydrolysis yields an aromatic aldehyde. The electrophile is usually $\text{HCN} \cdot \text{HCl}$ adduct, which forms an iminium ion. Reaction (Example with Phenol): C6H5OH + HCN + HCl --(AlCl3)--> C6H5O-CH=NH --(H2O, H+)--> HOC6H4CHO (Salicylaldehyde) Fries Rearrangement: Mechanism: The rearrangement of a phenolic ester to a hydroxyaryl ketone by heating with a Lewis acid catalyst (e.g., $\text{AlCl}_3$). The acyl group migrates from the phenolic oxygen to an ortho or para position on the aromatic ring. It proceeds via an intramolecular or intermolecular mechanism depending on the conditions. Reaction: C6H5OCOCH3 --(AlCl3, heat)--> o-HOC6H4COCH3 + p-HOC6H4COCH3 Glycerol Reactions Reaction of glycerol with excess ammonia: Glycerol reacts with excess ammonia at high temperatures to form allylamine and acrolein. $\text{CH}_2(\text{OH})\text{CH}(\text{OH})\text{CH}_2(\text{OH}) + \text{NH}_3 \rightarrow \text{CH}_2=\text{CH}-\text{CH}_2\text{NH}_2$ (Allylamine) + complex products. Alternatively, under specific conditions, it can form 2-aminopropan-1,3-diol. Reaction of glycerol with $\text{KHSO}_4/\Delta$: Glycerol undergoes dehydration when heated with $\text{KHSO}_4$ (or other dehydrating agents like $\text{P}_2\text{O}_5$). The product is acrolein (propenal). $\text{CH}_2(\text{OH})\text{CH}(\text{OH})\text{CH}_2(\text{OH}) \xrightarrow{\text{KHSO}_4, \Delta} \text{CH}_2=\text{CH}-\text{CHO}$ (Acrolein) $+ 2\text{H}_2\text{O}$ Reaction of glycerol with oxalic acid: (i) At 383 K: Glycerol reacts with oxalic acid to form glyceryl monoformate. This intermediate then decomposes to form formic acid ($\text{HCOOH}$) and allyl alcohol. CH2(OH)CH(OH)CH2(OH) + (COOH)2 --(383 K)--> HCOOH + CH2=CH-CH2OH (ii) At 503 K: Glycerol reacts with oxalic acid at higher temperatures to form allyl alcohol and carbon dioxide. The initial product is glyceryl monooxalate, which then decarboxylates and dehydrates. CH2(OH)CH(OH)CH2(OH) + (COOH)2 --(503 K)--> CH2=CH-CH2OH + CO2 + H2O Reaction of glycerol with $\text{HIO}_4$ (Periodic Acid): Glycerol undergoes oxidative cleavage with periodic acid. It cleaves C-C bonds between adjacent carbons bearing hydroxyl groups. $\text{CH}_2(\text{OH})\text{CH}(\text{OH})\text{CH}_2(\text{OH}) + 2\text{HIO}_4 \rightarrow 2\text{HCHO}$ (Formaldehyde) $+ \text{HCOOH}$ (Formic acid) $+ 2\text{HIO}_3 + \text{H}_2\text{O}$ Conversions & Preparations Phenol to Salicylaldehyde: Can be achieved by Reimer-Tiemann reaction. $\text{C}_6\text{H}_5\text{OH} + \text{CHCl}_3 + 3\text{NaOH} \rightarrow o\text{-HOC}_6\text{H}_4\text{CHO} + 3\text{NaCl} + 2\text{H}_2\text{O}$ Phenol to Phenolphthalein: Phenolphthalein is prepared by the condensation of phenol with phthalic anhydride in the presence of a dehydrating agent like concentrated sulfuric acid or anhydrous zinc chloride. $\text{2 C}_6\text{H}_5\text{OH} + \text{C}_8\text{H}_4\text{O}_3 \text{ (Phthalic Anhydride)} \xrightarrow{\text{conc. H}_2\text{SO}_4} \text{Phenolphthalein} + \text{H}_2\text{O}$ Miscellaneous Dipole moment of Phenol vs. Ethanol: Phenol has a higher dipole moment than ethanol. Reason: In phenol, the oxygen atom is directly attached to the benzene ring. The electronegativity difference between oxygen and carbon, combined with the resonance effect of the -OH group with the aromatic ring, leads to a significant dipole moment. In ethanol, the -OH group is attached to an alkyl group, and while there is a dipole moment due to the C-O and O-H bonds, the overall effect is less pronounced than in phenol due to the delocalization and aromatic nature.