Chapter 5 ALCOHOLS , ETHERS AND THIOLS

1. Chapter 5 ALCOHOLS, ETHERS AND THIOLS

2. Alcohols, Ethers and Thiols Alcohol – has an –OH group (hydroxy) attached to a saturated alkane. General formula R-OH. Ether – a compound that has the functional group Thiols-similar to alcohols except with a –SH instead of -OH

3. Alcohols • Nomenclature 1.Select the longest carbon chain that contains the -OH group as the parent alkane and number it from the end that gives the -OH the lower number. 2.Change the ending of the parent alkane from -e to -ol and use a number to show the location of the -OH group; for cyclic alcohols, the carbon bearing the -OH group is carbon-1. 3.Name and number substituents and list them in alphabetical order.

4. CH3 | OH | CH3 | Example: CH3—CH2—CH2—CH—CH—CH3 6 5 4 3 2 1 2,4-dimethyl-3-hexanol

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8. Classification of Alcohols

10. Physical Properties • Alcohols are polar molecules • The C-O and O-H bonds are both polar covalent.

11. Physical Properties • In the liquid state, alcohols associate by hydrogen bonding.

12. Physical Properties • bp increases as MW increases. • Solubility in water decreases as MW increases.

13. Physical Properties of Alcohols The –OH group is polar and capable of hydrogen bonding. This makes low molecular weight alcohols highly soluble in water. Hydrogen bonding in a water-methanol solution:

14. Alcohol Reactions The removal of water (dehydration) from an alcohol is an elimination reaction and produces an alkene.

15. Dehydration

16. Dehydration • When isomeric alkenes are obtained, the alkene having the greater number of alkyl groups on the double bond generally predominates. • examples:

18. Dehydration-Hydration • Acid-catalyzed hydration of alkenes to give alcohols (Chapter 3) and acid-catalyzed dehydration of alcohols to give alkenes are competing reactions. • The following equilibrium exists:

19. Oxidation – the removal of hydrogen atoms.

20. Alcohol Reactions Primary alcohol oxidation Secondary alcohol oxidation

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23. The measure of the percentage of alcohol in the 18th century, when alcohol and water would be mixed together: if the combination lit when a flame was applied, it would be considered to have "proved" it was a strong alcohol. Originally that was considered 100 proof (e.g. "100% proof") and was actually 50% alcohol.

25. Nomenclature • Although ethers can be named according to the IUPAC system, chemists almost invariably use common names for low-molecular-weight ethers. • common names are derived by listing the alkyl groups bonded to oxygen in alphabetical order and adding the word "ether”.

27. Physical Properties • Ethers are polar compounds in which oxygen bears a partial negative charge and each carbon bonded to it bears a partial positive charge. • However, only weak forces of attraction exist between ether molecules in the pure liquid. • Consequently, boiling points of ethers are close to those of hydrocarbons of similar molecular weight. • Ethers have lower boiling points than alcohols of the same molecular formula.

28. Hydrogen bonding of dimethyl ether: (a) with water and (b) no hydrogen bonding in the pure state:

29. Public demonstration of ether anaesthesia in Boston, Massachusetts on 16 October 1846

30. Thiol:a compound containing an -SH(sulfhydryl)group. • The most outstanding property of low-molecular-weight thiols is their stench. • They are responsible for smells such as those from rotten eggs and sewage. • The scent of skunks is due primarily to two thiols.

31. Thiols - Nomenclature • IUPAC names are derived in the same manner as are the names of alcohols. • To show that the compound is a thiol, the final -e of the parent alkane is retained and the suffix -thiol added.