Alcohols & Thiols: Structure/Properties

Types of Alchols

• Primary

• Secondary

• Tertiary

• Phenols (benzene with -OH)

Alcohol Properties

• Highly polar and can hydrogen bond with other molecules of alcohols

• Large EN difference between H and O creates a dipole moment

• Intermolecular force (between the H of one molecule and O of another

Alcohol Boiling Points

High because they can hydrogen bond, have dipole-dipole, and LDFs

• Pentane (just C’s): 36 deg

• Butanol (C chain w/ OH): 118 deg

• Range of bp’s within alcohols: the more linear the chain, the higher the boiling point

  • I.e. tert-butanol = 82 deg; butanol = 118 deg

Alcohol pKa

• pKa depends on how soluble the conjugate base can be

• More soluble = a better acid = lower pKa

Alcohol Inductive Effect

Electron density is more spread out → creates a more stable conjugate base → better acid

• Proximity and EN matter

Alcohol Resonance

• More resonance = better stabilizing effect (- charge is more spread out) = lower pKa

Alcohol Deprotonation

• Alkali metals (Na, K, Li) can deprotonate the H; efficient but dangerous to handle

• Strong bases (NaH or NH₂Na) can deprotonate using A/B chemistry

  • The conjugate acid of that base must have a higher pKa than the alcohol

  • Regular alcohol: must be > ~16

  • Phenol: must be > ~10 (can use water to deprotonate)

Thiol Properties

• Cannot H bond b/c they have a weaker dipole compared to alcohols

Thiol pKa

• Have lower pKa than alcohols because S is larger than O (it can spread the negative charge better)

• R-S-H: ~10

• Thiophenol: ~6.6

Thiol Boiling Points

• Much lower than alcohols

• -OH: 65 deg; -SH: 6 deg

Thiol Deprotonation

• Can be deprotonated by -OH

• Thiol and thiophenol have lower pKas than water so rxn proceeds to the right