Aromatic Compounds 4

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Questions and Answers

What is the primary role of a Brönsted base in a chemical reaction?

  • It donates protons to acids.
  • It acts as an electron pair donor.
  • It accepts protons from acids. (correct)
  • It forms water exclusively.

Which compound behaves as a Lewis base in the context of acidity?

  • Hydrochloric acid (HCl)
  • Water (H2O)
  • Ammonia (NH3) (correct)
  • Sodium hydroxide (NaOH)

Which equation represents the reaction involving a primary amine with water?

  • NH3 + HCl → NH4Cl
  • RNH2 + H2O ↔ NH4+ + OH-
  • RNH2 + HCl → RNH3+Cl-
  • RNH2 + H2O ↔ RNH3+ + HO- (correct)

What is the relationship between pKa and pKb?

<p>pKa + pKb = 14 (B)</p> Signup and view all the answers

What does a lower pKb value indicate about a base?

<p>It is a stronger base. (B)</p> Signup and view all the answers

What type of compound forms salts as a result of acid-base reactions, unlike sodium hydroxide?

<p>Amines (A)</p> Signup and view all the answers

In the process of determining basicity, which is now more commonly measured?

<p>The acidity of its conjugate acid. (B)</p> Signup and view all the answers

What does the reaction CH3NH3+ ↔ CH3NH2 + H+ illustrate?

<p>It represents the dissociation of a conjugate acid. (A)</p> Signup and view all the answers

What is the boiling point of ortho-nitrophenol at 70 mm Hg?

<p>100 °C (C)</p> Signup and view all the answers

Which nitro-phenol is most soluble in water?

<p>Meta-Nitrophenol (B)</p> Signup and view all the answers

Why do meta- and para-nitrophenols have high boiling points?

<p>Due to intermolecular hydrogen bonding (D)</p> Signup and view all the answers

What type of hydrogen bonding occurs in ortho-nitrophenol?

<p>Intramolecular hydrogen bonding (D)</p> Signup and view all the answers

What is the solubility of para-nitrophenol in water?

<p>1.69 g/100 g (D)</p> Signup and view all the answers

Which of the following factors contributes to the solubility of meta- and para-nitrophenols in water?

<p>Hydrogen bonding with water molecules (B)</p> Signup and view all the answers

Ortho-nitrophenol has low solubility compared to meta- and para-nitrophenols mainly because:

<p>It primarily participates in intramolecular hydrogen bonding (D)</p> Signup and view all the answers

Which nitro-phenol decomposes rather than boiling?

<p>Para-Nitrophenol (C)</p> Signup and view all the answers

Which statement about the pKa value of drugs is accurate?

<p>Aspirin is fully dissociated at pH 7.4 due to its low pKa. (D)</p> Signup and view all the answers

What distinguishes phenols from alcohols?

<p>Phenols have the -OH group directly attached to an aromatic ring. (D)</p> Signup and view all the answers

Which statement regarding the solubility of phenols is correct?

<p>Phenol dissolves in water due to hydrogen bonding. (A)</p> Signup and view all the answers

What is the boiling point of m-cresol primarily attributed to?

<p>The presence of hydrogen bonding. (A)</p> Signup and view all the answers

Which structural feature is common in the nomenclature of phenols?

<p>They are numbered to indicate the position of substituents. (A)</p> Signup and view all the answers

What is a characteristic property of most phenols?

<p>They tend to oxidize easily. (B)</p> Signup and view all the answers

What is the pKa value of most phenols indicative of regarding their acidity?

<p>They are weak acids. (A)</p> Signup and view all the answers

Which of the following is a method for naming phenols?

<p>Hydroxy- compound nomenclature. (C)</p> Signup and view all the answers

Which nitrophenol has the lowest boiling point?

<p>o-nitrophenol (D)</p> Signup and view all the answers

How does the solubility of o-nitrophenol compare to m- and p-nitrophenols?

<p>Lower than m- and p-nitrophenols (D)</p> Signup and view all the answers

What compound is formed when phenols react with aqueous hydroxides?

<p>Phenoxide salt (D)</p> Signup and view all the answers

Which statement correctly describes the acidity of phenols compared to alcohols?

<p>Phenols are significantly more acidic than alcohols. (D)</p> Signup and view all the answers

What is the approximate difference in pKa values between phenol and cyclohexanol?

<p>107 (B)</p> Signup and view all the answers

Which of the following compounds is stabilized by resonance effects?

<p>Phenoxide anion (C)</p> Signup and view all the answers

What is a known side effect of salicylic acid due to its free phenolic group?

<p>Gastric bleeding (A)</p> Signup and view all the answers

How can the side effect of gastric bleeding caused by salicylic acid be mitigated?

<p>By converting it to an ester (B)</p> Signup and view all the answers

What is the primary reason for the lower boiling point of o-nitrophenol compared to m- and p-nitrophenols?

<p>The lower boiling point is due to weaker intermolecular forces in o-nitrophenol.</p> Signup and view all the answers

Why are phenols more acidic than alcohols?

<p>Phenols are more acidic because their conjugate base (phenoxide anion) is stabilized by resonance.</p> Signup and view all the answers

What is the effect of resonance stabilization on the pKa values of phenols?

<p>Resonance stabilization lowers the pKa values of phenols, making them more acidic.</p> Signup and view all the answers

How do aqueous hydroxides react with phenols?

<p>Aqueous hydroxides convert phenols into their corresponding salts.</p> Signup and view all the answers

What is the typical pKa value of phenol compared to cyclohexanol?

<p>The pKa of phenol is approximately 9.95, while for cyclohexanol it is about 17.</p> Signup and view all the answers

In what way does the structure of salicylic acid contribute to its side effects?

<p>The free phenolic group in salicylic acid leads to gastric bleeding.</p> Signup and view all the answers

What compounds can convert phenoxide anions back into free phenols?

<p>Aqueous mineral acids, carboxylic acids, or carbonic acid can convert phenoxide anions back to phenols.</p> Signup and view all the answers

Why does the acidity of phenols increase compared to alcohols?

<p>The acidity of phenols increases due to the resonance stabilization of their conjugate base.</p> Signup and view all the answers

How does the pKa value of aspirin affect its ability to cross cell membranes?

<p>The pKa value of aspirin is 3.5, which means it is fully dissociated at pH 7.4, preventing it from easily diffusing across cell membranes.</p> Signup and view all the answers

What advantage does morphine have in terms of cell membrane permeability compared to aspirin?

<p>Morphine has a pKa of 7.9, which means it is partially ionised at pH 7.4, allowing for easier diffusion across cell membranes.</p> Signup and view all the answers

In what way do phenols differ structurally from alcohols?

<p>Phenols have the -OH group directly attached to an aromatic ring, while alcohols have the -OH group attached to aliphatic carbon chains.</p> Signup and view all the answers

What is the effect of hydrogen bonding on the boiling points of phenols?

<p>Hydrogen bonding in phenols leads to relatively high boiling points compared to non-hydrogen bonded compounds.</p> Signup and view all the answers

Why are most phenols considered to be slightly acidic?

<p>Most phenols have Ka values around $10^{-10}$, indicating they can donate protons, thus exhibiting acidic behavior.</p> Signup and view all the answers

How does the solubility of phenol in water compare to that of most other phenols?

<p>Phenol is somewhat soluble in water (9g per 100g) due to hydrogen bonding, whereas most other phenols are insoluble.</p> Signup and view all the answers

What structural feature is commonly used in the nomenclature of phenols?

<p>Numbering is often used to denote the relative positions of substituents on the aromatic ring in phenols' nomenclature.</p> Signup and view all the answers

What happens to phenols upon oxidation, and how does this relate to their color?

<p>Phenols are easily oxidized to form colored substances, which can result in their appearance changing dramatically.</p> Signup and view all the answers

What type of hydrogen bonding occurs in ortho-nitrophenol and what is its effect on solubility?

<p>Ortho-nitrophenol exhibits intramolecular hydrogen bonding, which reduces its solubility in water compared to meta- and para-nitrophenols.</p> Signup and view all the answers

Explain why meta- and para-nitrophenols have higher boiling points compared to ortho-nitrophenol.

<p>Meta- and para-nitrophenols have higher boiling points due to intermolecular hydrogen bonding, which is not present in ortho-nitrophenol due to intramolecular bonding.</p> Signup and view all the answers

How does the solubility of ortho-nitrophenol compare to that of meta- and para-nitrophenols?

<p>Ortho-nitrophenol has lower solubility than meta- and para-nitrophenols due to intramolecular bonding which hinders hydrogen bonding with water.</p> Signup and view all the answers

What is the reason for the decomposition of para-nitrophenol at its boiling point?

<p>Para-nitrophenol decomposes instead of boiling due to instability at elevated temperatures.</p> Signup and view all the answers

Why are meta- and para-nitrophenols soluble in water?

<p>They are soluble in water due to their ability to form hydrogen bonds with water molecules.</p> Signup and view all the answers

Identify the compound with the highest boiling point among the nitro-phenols and justify why.

<p>Meta-nitrophenol has the highest boiling point due to significant intermolecular hydrogen bonding.</p> Signup and view all the answers

Discuss how the positions of -NO2 and -OH groups in ortho-nitrophenol affect its physical properties.

<p>The positions of -NO2 and -OH in ortho-nitrophenol facilitate intramolecular hydrogen bonding, affecting both its boiling point and solubility.</p> Signup and view all the answers

What roles do hydrogen bonds play in determining the physical properties of nitro-phenols?

<p>Hydrogen bonds influence boiling points and solubility, with stronger hydrogen bonding resulting in higher boiling points and greater solubility in water.</p> Signup and view all the answers

How does ammonia act as a base in acid-base reactions, and what does it form when it reacts with a mineral acid?

<p>Ammonia acts as a Brönsted base by accepting a proton to form ammonium chloride (NH4+Cl-). This reaction does not produce water, distinguishing it from reactions involving other bases like NaOH.</p> Signup and view all the answers

In the context of basicity, what is the significance of the pKa and pKb values?

<p>pKa and pKb values provide a measure of the strength of acids and bases, respectively, with the equation pKa + pKb = 14. A lower pKb indicates a stronger base.</p> Signup and view all the answers

What is the equation representing the dissociation of a primary amine in water?

<p>The equation is RNH2 + H2O ↔ RNH3+ + OH-, showing the formation of a conjugate acid and hydroxide ion.</p> Signup and view all the answers

What indicates the strength of a base when measuring its conjugate acid's pKa?

<p>The ability of the conjugate acid to hold onto the proton indicates the strength of the base; a strong base has a weak conjugate acid.</p> Signup and view all the answers

What role does the -OH group play in the basicity of compounds like phenols?

<p>The -OH group can act as both a proton acceptor and an electron pair donor, allowing it to function as both a Brönsted and Lewis base.</p> Signup and view all the answers

What characterizes the basicity of amines in relation to their dissociation in water?

<p>The basicity of amines affects the extent of their dissociation in water, which correlates with their structural properties as primary, secondary, or tertiary amines.</p> Signup and view all the answers

Why is it essential to understand the pKb scale in evaluating basicity?

<p>The pKb scale quantifies how readily a base accepts protons, with higher pKb values indicating weaker bases. Hence, it provides a direct comparison for base strength.</p> Signup and view all the answers

In what way can the strength of a Brönsted base be determined indirectly?

<p>The strength of a Brönsted base can be determined indirectly by measuring the pKa of its conjugate acid; a lower pKa indicates a stronger base.</p> Signup and view all the answers

Flashcards

Brønsted Base

A substance that accepts a proton (H+).

Lewis Base

A substance that donates an electron pair.

pKb

A measure of the basicity of a substance, inverse log of Kb

pKa

Measure of acidity of a substance's conjugate acid; related to pKb by the equation pKa + pKb = 14.

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Conjugate Acid

The substance formed when a base accepts a proton (H+).

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Kb

Equilibrium constant for a base reacting with water.

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Ammonia Basicity

Ammonia (NH3) forms a salt rather than water when reacting with an acid, even though it acts as a proton acceptor and electron pair donor.

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Amines

Organic compounds similar to ammonia that act as bases.

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Aspirin's pKa

Aspirin is a weak acid with a pKa of 3.5, meaning it's fully dissociated at physiological pH (7.4).

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Morphine's pKa

Morphine is a weak base with a pKa of 7.9, making it partially ionized at physiological pH (7.4).

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Phenol's Structure

Phenols are compounds with a hydroxyl group (-OH) directly attached to an aromatic ring.

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Phenol's Nomenclature

Phenols are often named as derivatives of the simplest member, phenol, or as hydroxy-compounds.

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Phenol's Physical Properties

Simple phenols are liquids or low-melting solids with high boiling points due to hydrogen bonding. They are soluble in water due to H-bonding.

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Phenol's Acidity

Phenols are acidic, with a pKa around 10, because the aromatic ring stabilizes the negative charge of the phenoxide ion.

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Phenol vs. Alcohol

The key difference is the -OH group's position: directly attached to an aromatic ring in phenols, but to an aliphatic carbon in alcohols.

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Phenol's Reactivity

Phenols are easily oxidized, leading to colored compounds. They are also more reactive than alcohols due to the aromatic ring.

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What affects nitrophenol's boiling point?

The strength of intermolecular hydrogen bonding between nitrophenol molecules determines its boiling point. Stronger hydrogen bonding leads to a higher boiling point.

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Why are meta- and para-nitrophenols soluble in water?

Meta- and para-nitrophenols can form hydrogen bonds with water molecules due to the presence of the hydroxyl group (-OH) and the nitro group (-NO2). This interaction allows them to dissolve in water.

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What makes ortho-nitrophenol different from meta- and para-isomers?

Ortho-nitrophenol exhibits intramolecular hydrogen bonding between its hydroxyl group (-OH) and the nitro group (-NO2) within the same molecule. This prevents the formation of intermolecular hydrogen bonds.

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Intramolecular hydrogen bonding

A hydrogen bond formed between atoms within the same molecule. This type of interaction is crucial in ortho-nitrophenol.

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Intermolecular hydrogen bonding

A hydrogen bond formed between atoms of different molecules. This interaction is important in meta- and para-nitrophenols.

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Boiling point trend

Ortho-nitrophenol has the lowest boiling point due to intramolecular hydrogen bonding, followed by meta- and para-nitrophenols which have higher boiling points due to intermolecular hydrogen bonding.

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Solubility trend

Meta- and para-nitrophenols are more soluble in water than ortho-nitrophenol. This is because they can form hydrogen bonds with water molecules.

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Why does ortho-nitrophenol have lower solubility?

Ortho-nitrophenol's intramolecular bonding reduces the ability to form intermolecular hydrogen bonds with water molecules, leading to lower solubility.

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o-Nitrophenol's Boiling Point

o-Nitrophenol has a lower boiling point than its m- and p- isomers. This is due to intramolecular hydrogen bonding, which prevents strong intermolecular interactions.

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o-Nitrophenol's Solubility

o-Nitrophenol has poor water solubility compared to its m- and p- isomers. This is because the intramolecular hydrogen bonding reduces its ability to form hydrogen bonds with water.

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Phenoxide Ion Stability

The phenoxide ion is stabilized through resonance, which delocalizes the negative charge over the aromatic ring.

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Phenol's Reaction with NaOH

Phenols react with aqueous NaOH to form their salts (phenoxides).

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Phenol's Reaction with H2CO3

Phenol salts react with carbonic acid (H2CO3) or other weak acids to regenerate the free phenols.

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Salicylic Acid's Acidity

Salicylic acid, a precursor to aspirin, is acidic due to its phenolic group.

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Aspirin Synthesis

Salicylic acid is reacted with acetic anhydride to make aspirin, masking the phenolic group as an ester and preventing gastric bleeding.

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pKb Scale

A measure of the strength of a base; the higher the pKb value, the weaker the base.

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pKa and pKb Relationship

The pKa of a conjugate acid and the pKb of its corresponding base are related by the equation pKa + pKb = 14.

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Amines as Bases

Ammonia (NH3) and similar organic compounds (amines) act as bases by accepting protons, but form salts instead of water when reacting with acids.

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Salicylic Acid Reactivity

Salicylic acid is a phenol that reacts with acetic anhydride to form aspirin, masking the phenolic group.

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Nitrophenol Boiling Points

Meta- and para-nitrophenols have higher boiling points than ortho-nitrophenol due to stronger intermolecular hydrogen bonding.

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Nitrophenol Solubility

Meta- and para-nitrophenols are more soluble in water than ortho-nitrophenol because they can form hydrogen bonds with water molecules.

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Why is ortho-nitrophenol less soluble?

Ortho-nitrophenol forms an intramolecular hydrogen bond, preventing it from forming hydrogen bonds with water molecules, making it less soluble.

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Why is ortho-nitrophenol's boiling point lower?

Ortho-nitrophenol has a lower boiling point because the intramolecular hydrogen bond reduces intermolecular attractions.

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What is the relationship between hydrogen bonding and boiling point?

Stronger intermolecular hydrogen bonding leads to higher boiling points.

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What makes ortho-nitrophenol unique?

Ortho-nitrophenol exhibits intramolecular hydrogen bonding between its hydroxyl (-OH) and nitro (-NO2) group within the same molecule, while meta and para nitrophenols form intermolecular hydrogen bonds.

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Aspirin's Dissociation

Aspirin, a weak acid with a pKa of 3.5, is fully dissociated at physiological pH (7.4).

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Morphine's Ionization

Morphine, a weak base with a pKa of 7.9, is partially ionized at physiological pH (7.4).

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Intermolecular Hydrogen Bonding (Phenols)

Hydrogen bonds form between the hydroxyl groups of different phenol molecules.

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Intramolecular Hydrogen Bonding (Phenols)

Hydrogen bonds occur within the same phenol molecule, usually between the -OH and a nearby electronegative group.

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What causes the difference in boiling point of nitrophenols?

The strength of hydrogen bonding between nitrophenol molecules determines their boiling point. Ortho-nitrophenol has weaker intermolecular hydrogen bonding due to intramolecular hydrogen bonding, leading to a lower boiling point compared to meta- and para-nitrophenols.

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Why is ortho-nitrophenol less soluble in water?

Ortho-nitrophenol forms intramolecular hydrogen bonds within the molecule, which reduces its ability to form intermolecular hydrogen bonds with water molecules, leading to lower solubility.

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Compare the acidity of phenols and alcohols

Phenols are significantly more acidic than alcohols due to resonance stabilization in the phenoxide anion. The resonance delocalizes the negative charge, making the phenoxide ion more stable and increasing phenol's acidity.

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What is the effect of resonance in phenoxide ions?

The phenoxide anion is stabilized by resonance. This delocalization of the negative charge across the benzene ring makes the phenoxide ion more stable, contributing to the acidity of phenols.

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How do phenols react with NaOH?

Phenols react with aqueous NaOH to form their salts, called phenoxides. This reaction is driven by the formation of the more stable phenoxide ion.

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How are phenols converted back to their neutral form?

Phenol salts react with carbonic acid (H2CO3) or other weak acids to regenerate the free phenols. This is a reversible reaction.

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What makes salicylic acid a good analgesic?

Salicylic acid, a precursor to aspirin, is a good analgesic due to its phenolic group. However, this group also causes gastric bleeding.

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How does aspirin overcome salicylic acid's side effects?

Aspirin is synthesized by reacting salicylic acid with acetic anhydride, masking the phenolic group as an ester. This prevents the gastric bleeding caused by the free phenolic group.

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Study Notes

Pharmaceutical Chemistry: Aromatic Compounds - Lecture 4

  • Lecture Content: Theory of basicity, Phenol and substituted phenols, Synthesis of Aspirin.

Theory of Basicity

  • Brønsted-Lowry Bases: Proton acceptors. NaOH + HCl → NaCl + H₂O. OH⁻ acts as the base, accepting a proton from HCl.
  • Lewis Bases: Electron-pair donors. OH⁻ donates a pair of electrons to the electron-deficient H⁺.
  • Ammonia (NH₃) and Amines: Act as bases differently from NaOH. Reaction with an acid forms a salt instead of water (e.g., NH₃ + HCl → NH₄Cl). Still act as proton acceptors and electron-pair donors. Amines behave similarly.
  • Basicity Measurement: Extent of dissociation measured by pKₐ scale. CH₃NH₂ + H₂O → CH₃NH₃⁺ + OH⁻. K₂ = [CH₃NH₃⁺] [OH⁻] / [CH₃NH₂]. pK₂ = -log₁₀ K₂. More commonly, basicity is defined in terms of pKₐ using the equation: pKₐ + pK₂ = 14.
  • Application: Drug pKₐ values critical for determining water solubility. Example: Aspirin (pKₐ 3.5) is fully dissociated at pH 7.4, and thus can't easily diffuse across cell membranes. Morphine (pKₐ 7.9), partially ionised at pH 7.4, crosses cell membranes more easily.

Phenols

  • Definition: Compounds of the general formula ArOH, where Ar is phenyl or substituted phenyl. Differ from alcohols in having the -OH group directly attached to an aromatic ring.
  • Nomenclature: Generally named as derivatives of phenol. Sometimes trivial names used (e.g., cresol). Numbering used for positions of substituents.
  • Physical Properties: Simplest phenols are liquids or low-melting solids. Phenol has high boiling points due to hydrogen bonding. Phenol slightly soluble in water, while most other phenols are insoluble. Generally colourless, easily oxidised to form coloured substances. Acidic in nature (pKₐ values ~ 10).
  • Nitro-phenols: Specific physical properties of ortho-, meta-, and para-nitrophenols are detailed, including boiling points, solubility in water, and factors impacting intramolecular hydrogen bonding affecting these properties
  • Acidity: Phenols are more acidic than alcohols (pKₐ phenol ~9.95, pKₐ cyclohexanol ~ 17). Phenolate anions are stabilised by resonance.
  • Acidity Applications: Aqueous hydroxides (like NaOH) convert phenols to their salts, but not with bicarbonates. Aqueous mineral acids, carboxylic acids, or carbonic acid convert the salts back into phenols.

Reactions of Phenols

  • Salicylic Acid and Aspirin: Salicylic acid, a precursor to aspirin, causes gastric bleeding due to a free phenolic group. Aspirin masks this group as an ester using acetic anhydride. The ester is then hydrolysed in the body to yield active salicylic acid. Aspirin itself is also an anti-inflammatory drug.

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