Alcohol and Phenol: Structure and Classification

Alcohol and Phenol Structure and Classification

• Both alcohols and phenols contain a hydroxy (-OH) group.
• Alcohols have the -OH group attached to an sp3 hybridized carbon atom, as shown in the general formula R-OH.
• Phenols have the -OH group attached directly to a benzene ring.
• Phenols are distinct from alcohols and are not considered aromatic alcohols.
• General formula for alcohol is CnH2n+2O.
• This formula does not apply to compounds with more than one -OH group per molecule or those with cyclic structures.

Classifying Alcohols

• Alcohol classification depends on the position of the R groups attached to the carbon
• Primary (1°) alcohols have one R group attached to the carbon bonded to the -OH group.
• Secondary (2°) alcohols have two R groups attached to the carbon bonded to the -OH group.
• The carbon bonded to the -OH group has three R groups attached to it in tertiary (3°) alcohols.
• Amine classification hinges on the amount of R groups bound to the nitrogen (N) atom, unlike alcohols.

Nomenclature for Alcohols

• Find the longest chain with the hydroxyl group to determine the alcohol name
• Begin numbering the carbon atoms closest to the hydroxy group.
• The placement of the -OH group is indicated by the corresponding number.
• Numbering starts at the carbon atom that has the -OH group bound to it in cyclic alcohols.
• Alcohols that have above 1 -OH groups, is called ~diol, ~triol, ~tetraol etc.

Nomenclature for Phenols

• Named as phenol derivatives, substituted phenols

Isomers

• Isomers are different chemicals with the same molecular formula.
• Constitutional and structural isomers are of two types:
  • Stereoisomers
• Arrangement differs in chemicals with differing connectivity.
• Chain isomerism is when there contains a different carbon chain.
• Positional isomerism is when there is a difference in the location of functional group.
• Functional group isomerism is when there is a difference in functional group.
• Compounds that share connectivity but have different spatial arrangements are optical isomers.
• Presence of stereocenters distinguishes stereoisomers.
• Enantiomers are stereoisomers differing only in plane-polarized light rotation.
• For instance, (+)-butan-2-ol rotates clockwise while (-)-butan-2-ol rotates counter-clockwise by the same measure.

Physical Properties

• Lower molecular weight alcohols are often free-flowing, volatile liquids identified by their distinct fruity scents.
• Boiling point:
  • The necessary energy to overcome attractive intermolecular forces is a key influence.
  • Relative Molar Mass (RMM): Higher RMM leads to greater van der Waals forces, elevating boiling point.
  • Exposed Surface Area: Branched molecules have lower boiling points compared to linear ones.
  • Primary ROH > Secondary ROH > Tertiary ROH
  • Dipole-Dipole Attraction: Higher dipole moment results in a higher boiling point.
  • Hydrogen Bonding: H-bond attraction > dipole-dipole attraction > vdW force attraction
• Attractive force is the total of vdW + μ + H-bonding
• Ethanol (MW 46, bp 78 °C) has H-bonding and dipole-dipole interactions, this contrasts propane (MW 44, bp -42 °C) having solely vdW and a trace dipole.
• Compared with dimethyl ether, which has H-bonding and a greater dipole moment (1.30 D), ethanol has a boiling point that is 103 °C higher and cannot engage in H-bonding since it lacks -OH hydrogens.
• Greater vdW forces are a result of bigger size, thus boiling point rises with rising molar mass and number of -OH groups.

Solubility

• Presence of the -OH group makes alcohols and water have similarities.
• Intermolecular H-bonding with one another and with water molecules gives them this property.
• Alcohols are given a hydrophilic, water-loving nature by the -OH group.
• Water readily dissolves alcohols containing one to three carbon atoms.
• Alcohols that range from 4 to 10 carbon atoms are oily liquids.
• Alcohols that have greater than eleven carbon atoms are solids nearly insoluble in water.
• Longer hydrophobic areas, shown by rising RMM, reduce water solubility.
• Solubility increases with number of -OH groups because of the capacity to generate greater hydrogen bonds with water molecules, for alcohols that have same molar mass.
• Water at less than 66°C only partially dissolves phenol.
• Compact shape and strong hydrogen generate between phenolic (-OH) groups and water molecules account for the increased solubility of phenol, six-carbon alcohol.

Acidity of Alcohols

• Weak acids are a result of the alcohols' O-H bond's polarity.
• A strong base may eliminate the hydroxyl proton and provide an alkoxide ion.
• The acidity varies, the acid-dissociation constant (Ka) identifies the equilibrium.
• Higher Ka values indicate stronger acids.
  • pKa is commonly expressed as pKa = -log(Ka), indicating acidity is weaker when pKa values are higher.
  • Alkoxide ion stability and O-H bond polarity determine the Ka (or pKa) value.
  • Unstable ionic nature, +ve & -ve, causes strongly charged cationic and anionic species made on RHS of equilibrium, to reform original species on LHS.
  • The -NH2 group tends to be basic (pKa is estimated around 5), thus the molecule gains H atom abstraction from the -OH group, making creation of phenoxide much more easy.
• An -OH group that is directly attached to the benzene ring is a strong activator and dictates ortho-para for subsequent electrophilic replacements on the benzene ring.
• Water with a pKa of 14 serves as the baseline for acidity comparison.
  • For reference, tert-butyl alcohol has a pKa of 18.
• The solution gets more acidic as more H+ ions are freed as the alkoxide ion gets stabilised due to the electron-withdrawing effect.
• Alkyl groups are electron donating groups, and their presence lowers the alcohols’ acidic qualities.

Basicity of Alcohols

• Alcohols serve as a weak base in the face of strong acids.
• This results from lone electron pairs presence on the oxygen atom.
• Alcohols accept protons from acids and produces as base.

Acidity of Phenols

• Although phenols are weak acids similar to alcohols, but also stronger acid.
• Phenols have pKa measures or 10, this contrast that alcohols are in the order of 18.
• Phenol experiences nearly 100 million times greater acidic conditions as cyclohexanol.
• When dilute water is added to weak acids it produces a phenoxide ion (ArO-) that generates H3O+.
• Resonance causes phenoxide ions be more stabled than typical alkoxide ions as the negative charge expands through 3 nitrogen atoms & oxygen, within the ring.