Aromatic Aldehydes, Ketones, and Phenols Notes

Benzaldehyde Overview: - Benzaldehyde is a benzene derivative. - It is classified under aromatic aldehydes.
Phenol Structure and Properties: - Phenol can be written as the chemical formula: $C_6H_5-OH$ . - The functional group is the hydroxyl group ( $-OH$ ). - Non-bonding electrons (lone pairs) on the oxygen atom in the phenol group are drawn into the benzene ring system. - This lone pair becomes part of the delocalization, which significantly increases the electron density within the ring.
Role of the Hydroxyl Group: - The hydroxyl group is an Electron-Donating Group (EDG). - It acts as an "activating" group toward Electrophiles ( $E^+$ ). - It acts as a "deactivating" group toward Nucleophiles ( $Nu^-$ ). - The hydroxyl group orients incoming Electrophiles ( $E^+$ ) to the ortho and para positions on the benzene ring. - Results of the donation of the lone pair into the ring: - Makes the ring much more reactive than benzene itself. - Makes the hydrogen of the $-OH$ group significantly more acidic compared to hydrogens in aliphatic alcohols.

Classification: - Phenol is an aromatic hydroxyl-substituted compound. - Alcohol is an aliphatic hydroxyl-substituted compound.
Comparative Acidity: - Both alcohols and phenols are acidic compounds. - However, phenols are a lot more acidic than aliphatic alcohols. - The Reason: The resonance stabilization of the conjugate-base anion (the phenoxide ion).
Substituent Effects on Acidity: - Whether a phenol derivative is more or less acidic than phenol depends on the nature and position of the substituent. - Electron-Donating Groups (EDG): These groups increase the electron density of the benzene ring, making the phenol less acidic. - Electron-Withdrawing Groups (EWG): These groups decrease the electron density of the benzene ring, making the phenol more acidic.

General Reactivity: - Phenols are considered great substrates for all electrophilic aromatic substitution reactions. - The hydroxyl group is strongly activating, making the aromatic ring highly electron-rich. - Orientation is directed to the para and ortho positions. - The para position is favored due to the steric hindrance associated with the ortho position.
Halogenation (Bromination): - At room temperature, halogenation cannot be stopped at monobromination; the reaction proceeds further. - To obtain monobromination, the reaction must be performed at lower temperatures (e.g., $< 5\,^{\circ}C$ ).
Friedel-Crafts Reactions: - These reactions utilize a typical Lewis acid catalyst, such as $FeBr_3$ . - A complication occurs because the oxygen atom binds to the Lewis acid catalyst. - Because of this binding, the reactions may require higher temperatures to proceed.
Nitration and Sulfonation: - These reactions produce a mixture of products at the ortho and para positions. - Nitration Specifics: - Using dilute nitric acid ( $HNO_3$ ) results in a monosubstituted product (ortho and para mixture). - Using concentrated nitric acid ( $HNO_3$ ) forms polysubstituted products, specifically $2,4,6$ -trinitrobenzene.

Nucleophilicity: - The oxygen of the $-OH$ group in phenol is not as nucleophilic as the oxygen in an aliphatic alcohol. - To activate phenol for nucleophilic reactions, it must be deprotonated to prepare a phenoxide ion. - The phenoxide ion is a much better nucleophile than neutral phenol.
Reactions of Phenoxide Ion: - It reacts with electrophiles including: - Alkyl halides. - Acyl halides. - Acid anhydrides. - The phenoxide ion can also act as a carbon nucleophile because of the resonance activation of the ring.
Synthesis Applications: - Phenoxide ions are used in the preparation of salicylic acid. - Salicylic acid is then converted into aspirin (acetylsalicylic acid).
Azo Dyes: - Azo dyes represent a large and very important group of synthetic dyes. - The Azo group is represented as $-N=N-$ or $-N=N=$ . - They involve substituent groups (SG) such as $-OH, -OR, -NH_2,$ and $-NR_2$ .
Oxidation: - Oxidation of phenol forms $1,4$ -conjugated diketones known as quinones. - This typically requires oxidizing agents such as chromic acid ( $Na_2Cr_2O_7$ ).

Question: Does benzene react with a base to obtain phenol directly?
Answer: NO.
Question: Give one method to obtain phenol.
Answer: Use Electrophilic Aromatic Substitution (E.A.S.) followed by Nucleophilic Aromatic Substitution (N.A.S.): 1. Chlorination of benzene to produce chlorobenzene ( $Ph-Cl$ ). 2. React chlorobenzene with sodium hydroxide ( $NaOH$ ).

Nitrobenzene Reactions (Meta-Directing): - Bromination: Forms meta-bromo nitrobenzene. - Nitration: Forms meta-nitrobenzene (or meta-dinitrobenzene). - Sulfonation: Forms Nitrobenzene- $3$ -sulfonic acid. - Alkylation: Forms meta-methyl nitrobenzene. - Acylation: Forms $3$ -Nitroacetophenone.
Aniline / Aminobenzene Reactions (Ortho/Para-Directing): - Bromination: Forms $2,4,6$ -tribromoaniline (shown as multiple Br substitutions on aniline). - Sulfonation: - Forms a mixture of ortho-nitro aniline and para-nitro aniline (or Aniline- $2$ -sulfonic acid and Aniline- $4$ -sulfonic acid depending on reaction conditions). - At room temperature ( $r.t$ ) with $H_2SO_4$ , it forms the meta-product (anilinium salt side) $NH_3^+.HSO_4^-$ (meta-directing as salt). - At high temperature ( $190\,^{\circ}C$ ), it rearranges to the para-substituted product ( $SO_2OH$ at para). - Alkylation: Forms $2$ -methylaniline or $1,4$ -Dimethylbenzene. - Acylation: Substitution on the nitrogen or the ring.

Reduction: - Nitrobenzene undergoes reduction using $H_2/Ni$ and heat ( $\Delta$ ) to form Aniline (Aminobenzene).
Hydrohalogenation and Hydration: - Hydration: Production of $2$ -phenyl- $1$ -ethanol. - Hydrohalogenation: Involves reactions producing products such as $CH_2CH_2Br$ ; follows Markovnikov's rule (Markovnikov's product) unless reagents like $HBr/H_2O_2$ are specify otherwise.