Chapter 4: Alcohols and Alkyl Halides

Alcohols

Nomenclature:
Functional group suffix = -ol
Functional group prefix = hydroxy-

Review alcohol nomenclature ?

Primary, secondary or tertiary ? Alcohols are described as being primary (1^o), secondary (2^o) or tertiary (3^o) depending on how many alkyl substiutents are attached to the carbon that carries the -OH unit.

Physical Properties:

• The polar nature of the O-H bond (due to the electonegativity difference of  the atoms ) results in the formation of hydrogen bonds with other alcohol molecules or other H-bonding systems (e.g. water). The implications of this are:
• high melting and boiling points compared to analogous alkanes
• high solubility in aqueous media

• The alcohol functional group consists of an O atom bonded to a C atom and a H atom via σ bonds.
• Both the C-O and the O-H bonds are polar due to the high electronegativity of the O atom.

The image shows the electrostatic potential for methanol.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.  The alcohol O atom are a region of high electron density (red) due to the lone pairs. (red)  Alcohol oxygen atoms are Lewis bases. So alcohols can react as either bases or nucleophiles at the oxygen. There is low electron density (blue) on H atom of the -OH group alcohol, i.e. H+ character. So alcohols are acidic (pKa ~ 16). Removal of the proton generates the alkoxide. The -OH group is a poor leaving group and needs to be converted to a better leaving group before substitution can occur.

Acidity:

• Due to the electronegativity of the O atoms, alcohols are slightly acidic (pK_a 16-18)
• The anion dervived by the deprotonation of an alcohol is the alkoxide.
• Alkoxides are important bases in organic chemistry.
• Alcohols react with Na (or K) like water to give the alkoxide:

© Dr. Ian Hunt, Department of Chemistry