Alcohol and Phenols

Hydroxy Compounds: Structural and Optical Isomerism

• Structural and optical isomerism in hydroxy compounds.

• Physical properties of hydroxy compounds.

• Classification of alcohols into primary, secondary, and tertiary alcohols.

• Preparation of alcohols (e.g., ethanol from fermentation and ethene hydration).

• Reactions of alcohols: oxidation, dehydration, reaction with Na, haloalkane formation, esterification, and acylation.

• Tests to determine alcohol class and type: Lucas test.

• Uses of alcohols: antiseptic, solvent, and fuel.

• Relative acidity of water, phenol, and alcohol (inductive and resonance effects).

• Preparation of phenols from the Cumene process.

• Reactions of phenols with Na, NaOH, acyl chloride, and electrophilic substitution.

• Tests for phenol: bromine water and aqueous iron(III) chloride.

• Use of phenol in cyclohexanol and nylon-6,6 manufacture.

Acidity of Alcohols

• Alcohols can donate or accept a proton under suitable conditions, similar to water.

• Alcohols are weak acids due to the polar character of the O-H bond.

• In a dilute aqueous solution, alcohol reacts as a very weak acid.

• The higher the $K_a$ value, the stronger the acid.

• $pK_a$ values are commonly used for convenience.

• The higher the $pK_a$ value, the weaker the acid.

• $K<em>a$/$pK</em>a$ depends on:

  • Stability of alkoxide ion, $R-O^{-}$

  • Polarity of O-H bond

• More stable alkoxide ion shifts the equilibrium towards the right, producing more $H_3O^{+}$ ions.

• More polar O-H bond makes alcohols more susceptible to water molecule attack.

• These factors depend on the nature of the alkyl group, R.

• Larger $K_a$ if R is an electron-withdrawing group. This increases the stability of the alkoxide ion and the polarity of the O-H bond.

• If R is an electron-donating group, the stability of the alkoxide ion and the polarity of the O-H bond decrease, thus decreasing the acidity of the alcohol.

• Less stable alkoxide ion implies less hydronium ion, smaller $K_a$ value, and less acidic; O-H is less polar.

• More stable alkoxide ion implies more hydronium ion, larger $K_a$ value, and more acidic; O-H is more polar.

• The bulkiness of R also decreases the acidity due to the reduced accessibility of the hydroxyl group, inhibiting solvation of the alkoxide ion.

• Selected $pK_a$ values for comparison:

  • Acetic acid ($CH_3COOH$): 4.8 (Stronger acid)

  • Methanol ($CH_3OH$): 15.5

  • Water ($H_2O$): 15.7

  • Ethanol ($CH<em>3CH</em>2OH$): 15.9

  • 2-propanol ($(CH<em>3)</em>2CHOH$): 17

  • 2-methyl-2-propanol ($(CH<em>3)</em>3COH$): 18 (Weaker acid)

• tert-butyl alcohol is less acidic than its isomer n-butyl alcohol due to the increased steric hindrance of the bulky tert-butyl group.

Alcohol as a Base

• In strong acids, alcohol acts as a weak base due to the lone pair on the oxygen atom.

• Alcohol can accept a proton from an acid and hence can react as a base.

• When this reaction occurs, the alcohol is said to be protonated.

• Protonation is the first important step in several reactions of alcohol.

Acidity of Phenol

• Phenols are stronger acids than alcohols.

• The $pK_a$ values of most alcohols are around 18, while those of phenols are smaller than 11.

• As weak acids, phenols dissociate slightly in dilute aqueous solution, forming a phenoxide ion $(ArO^{-})$ and generating $H_3O^{+}$.

• Phenols react with sodium metal and NaOH, unlike alcohols (alcohols do not react with NaOH).

• This reaction distinguishes phenols from ROH, which is insoluble in $NaOH_{(aq)}$.

• The greater acidity of phenols compared to alcohols is due to the stabilization of the phenoxide ion.

• The negative charge on the phenoxide ion is delocalized throughout the benzene ring.

• Ring substituents also affect the acidity of phenols.

• Electron-withdrawing groups stabilize the phenoxide ion, increasing acidity.

• Electron-donating groups destabilize the phenoxide ion, decreasing acidity.

Directing Groups

• Activating Groups (Electron-Donating):

  • Amino ($-NH_2$)

  • Substituted Amino ($-NHR$, $-NR_2$)

  • Hydroxy ($-OH$)

  • Alkoxy ($-OR$)

  • Phenyl ($-C<em>6H</em>5$)

  • Alkyl ($-R$)

• Deactivating Groups (Electron-Withdrawing):

  • Halogens ($X$)

  • Nitro ($-NO_2$)

  • Carboxyl ($-COOH$)

  • Cyano ($-CN$)

  • Ester ($-COOR$)

  • Sulfonic Acid ($-SO_3H$)

  • Aldehyde ($-CHO$)

  • Ketone ($-COR$)

Acidity Comparison Example

• Compounds in order of increasing acidity:

  • Benzyl alcohol ($CH_2OH$)

  • 4-methylphenol

  • Phenol

  • 4-fluorophenol

Benzyl alcohol has no resonance-stabilized structure; the negative charge is concentrated on the O atom, making it less stable.

• Benzyl alcohol – $pK_a$ 15.4

• 4-methylphenol – $pK_a$ 10.36

• Phenol – $pK_a$ 10.0

• 4-fluorophenol – $pK_a$ 9.9

Chemical Reactions of Alcohols (ROH)

• Oxidation

  • Strong

  • Mild

• Scission of O-H Bond

  • Reaction with Na

  • Esterification ($ROH + RCOOH$)

  • Acylation ($ROH + ROCl$)

• Scission of C-OH Bond

  • Preparation of alkene (dehydration)

  • Preparation of haloalkane (alkyl halide)

Oxidation of Alcohols

• Primary Alcohol:

  • With strong oxidizing agents ($[O] = KMnO<em>4/OH^- (H^+) \text{ or } Na</em>2Cr<em>2O</em>7/H^+$), a primary alcohol ($RCH<em>2OH$) is oxidized to an aldehyde ($RCH</em>2CHO$) and further to a carboxylic acid ($RCH_2COOH$).

  • Using mild oxidizing agents like pyridinium chlorochromate (PCC) in $CH<em>2Cl</em>2$ at 25°C, a primary alcohol ($RCH_2OH$) can be selectively oxidized to an aldehyde ($RCHO$).

• Secondary Alcohol:

  • A secondary alcohol ($R-CHOH-R'$) is oxidized to a ketone ($R-C=O-R'$) using oxidizing agents such as $Na<em>2Cr</em>2O<em>7$ and $H</em>2SO_4$, with the chromium changing from orange to a green $Cr^{3+}$ ion.

Scission of O-H Bond

• Reaction with Reactive Metals

  • $2 ROH + 2 Na \rightarrow 2 R-O^- Na^+ + H_2$

• Esterification (Condensation)

  • $ROH + R'COOH \rightleftharpoons R'COOR + H_2O$

Scission of C-OH Bond

• Dehydration (Preparation of Alkene)

  • $Alcohol \xrightarrow{heat} Alkene + H_2O$

• Reaction with Hydrogen Halides or Phosphorus Halides (Preparation of Alkyl Halide)

  • $R-OH + HX \text{ or } PX<em>3 \text{ or } SOX</em>2 \rightarrow R-X$