1. Alcohols Lec.8

2. Introduction Alcohols have the general formula R-OH and are characterized by the presence of a hydroxyl group, -OH. They are structurally similar to water, but with one of the hydrogen's replaced by an alkyl group. Alcohols are important because they can be converted into many other types of compounds, including alkenes, halo-alkanes, aldehydes, ketones, carboxylic acids, and esters. Not only can alcohols be converted to these compounds, but these compounds can also be converted to alcohols. Thus, alcohols play a central role in the inter conversion of organic functional groups.

3. Nomenclature of Alcohols In the IUPAC system, the hydroxyl group in alcohols is indicated by the ending -ol. In common names, the separate word alcohol is placed after the name of the alkyl group. Examples:

4.  With unsaturated alcohols, two endings are needed: one for the double or triple bond and one for the hydroxyl group. In these cases, the -ol suffix comes last and takes priority in numbering.(i.e: The parent alkane is numbered to give the -OH group the lowest possible number)  For examples: 3-pentyne-1-ol (not 2-pentyne-5-ol) BrCH2CH2CH2OH 3-bromopropan-1-ol cyclobutanol

5.  In the IUPAC system, a compound containing two hydroxyl groups is named as a diol, one containing three hydroxyl groups as a triol, and so on.

6. Classification of Alcohols  Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°), depending on whether one, two, or three organic groups are connected to the hydroxyl- bearing carbon atom.

7. Example: Classify each alcohol as primary, secondary, or tertiary. Solution (a)Secondary (2°) (b) Tertiary (3°) (c) Primary (1°)

8. Physical Properties of Alcohols 1.Because of the presence of the polar -OH group, alcohols are polar compounds, with partial positive charges on carbon and hydrogen and a partial negative charge on oxygen 2. The alcohols have the higher boiling points because more energy is needed to overcome the attractive forces of hydrogen bonding between their polar -OH groups. 3. The presence of additional hydroxyl groups in a molecule further increases the extent of hydrogen bonding, so increase the B.pt.

9. Note: When the positive end of one of the dipoles is a hydrogen atom bonded to O or N (atoms of high electronegativity) and the negative end of the other dipole is an O or N atom, the attractive interaction between dipoles is particularly strong and is given the special name of hydrogen bonding. 4. Because of increased dispersion forces between larger molecules, boiling points of all types of compounds, including alcohols, increase with increasing molecular weight.

10. 5-Because alcohols can interact by hydrogen bonding with water, they are more soluble in water than alkanes and alkenes of comparable molecular weight. 6. Higher-molecular-weight alcohols are much less soluble in water because of the increase in size of the hydrocarbon portion of their molecules.  See the following table and notice the difference in b.pt and solubility in water.

11. Conclusion : as M.wt increase, the b.pt increases and the solubility decreases.

12. Solution Hexane, C 6 H 14, a nonpolar hydrocarbon, has the lowest solubility in water. Both 1-pentanol and 1,4-butanediol are polar compounds due to the presence of –OH groups, and each interacts with water molecules by hydrogen bonding. Because 1,4-butanediol has more sites within its molecules for hydrogen bonding than 1-pentanol, the diol is more soluble in water than is 1-pentanol. Example: Arrange these compounds in order of increasing solubility in water.