Structures of Alcohols, Phenols, Thiols and Ethers Alcohols, phenols, thiols and ethers consist of a hydrocarbon singly bonded to an oxygen or a sulfur Alcohols have an -OH group attached to an alkane, phenols have an -OH group attached to a benzene, thiols have an -SH group attached to an alkane and ethers have an O bonded to two C’s

Naming Alcohols Parent name ends in -ol Find longest chain containing the C to which the OH group is attached Number C’s starting at end nearest OH group Locate and number substituents and give full name - use a number to indicate position of OH group - cyclic alcohols have cyclo- before the parent name; numbering begins at the OH group, going in direction that gives substituents lowest possible numbers - use a prefix (di-, tri-) to indicate multiple OH groups in a compound

Classification of Alcohols Alcohols can be classified as methyl, primary, secondary or tertiary Classification is based on the number of alkyl groups attached to the carbon to which the OH group is attached If OH is attached to a 1 C, it’s a 1 alcohol, etc.

Naming Phenols Phenol is the common name for an OH group attached to a benzene, and is accepted by IUPAC Compounds with additional substituents are named as substituted phenols Ortho, meta and para are used when there is only one other substituent If there are two or more additional substituents, each must be numbered, beginning at the OH and going in direction that gives substituents lowest numbers (or alphabetical if same in both directions)

Many phenols have pleasant odors, and some are bioactive - Euganol (from cloves) is a topical anesthetic - Thymol (from thyme) is an antiseptic

Naming Thiols Parent name ends in -thiol Find longest chain containing the C to which the SH group is attached Number C’s starting at end nearest SH group Parent name is alkane name of carbon portion of longest chain, followed by thiol Locate and number substituents and give full name - use a number to indicate position of SH group - cyclic thiols have cyclo- before the parent name; numbering begins at the SH group, going in direction that gives substituents lowest possible numbers - use a prefix (di-, tri-) to indicate multiple SH groups in a compound

Naming Ethers Simple ethers are named by their common names For common names: name each alkyl group attached to the oxygen followed by ether For complex ethers IUPAC names are used For IUPAC names: 1. Name as an alkane, with larger alkyl group being the parent chain 2. The smaller alkyl group and the O are named together as an alkoxy group (replace -yl with -oxy) 3. Number chain starting at end nearest alkoxy group 4. Use a number to give location of alkoxy group

Naming Cyclic Ethers Cyclic ethers are generally named by their common names (we will not study the IUPAC names) A cyclic ether containing two carbons is called ethylene oxide (generally known as epoxides) A cyclic ether containing 4 carbons (with 2 double bonds) is called a furan A cyclic ether containing 5 carbons (with 2 double bonds) is called a pyran A cyclic ether containing 4 carbons and 2 oxygens is called a dioxane

Naming Examples

Physical Properties of Alcohols, Phenols, Thiols and Ethers All of these types of compounds have a bent geometry around the O or the S, and are polar compounds Alcohols and phenols contain a very polarized O-H bond, and they can H-bond with themselves and with other alcohols or water - Small alcohols (4 or less C’s) are soluble in water - Phenol is soluble in water (even with 6 C’s) because it partially ionizes in water (it’s a weak acid) - Alcohols and phenols have relatively high boiling points Thiols are much less polar than alcohols because the electronegativity of S is the same as that of C (2.5), much less than that of O (3.5), so C-S and S-H bonds are not polar - thiols do not H-bond and have relatively low boiling points Ethers do not H-bond with themselves, so have boiling points similar to hydrocarbons -ethers are only slightly soluble in water and are highly flammable

Acidity and Basicity of Alcohols and Phenols Alcohols and phenols, like water, can act as either weak acids or weak bases (although phenol is more acidic) Phenols are more acidic because the anion that forms upon loss of the proton is stabilized by resonance

Combustion Reactions of Alcohols and Ethers Both alcohols and ethers can burn with oxygen to produce water, carbon dioxide and heat (just like hydrocarbons) However, ethers are much more flammable than alcohols and care should be taken when working with ethers in the laboratory (just a spark from static electricity can set off ether fumes) Examples: CH3CH2OH + 3O2  2CO2 + 3H2O + Heat CH3-O-CH3 + 3O2  2CO2 + 3H2O + Heat

Dehydration of Alcohols to Form Alkenes An alcohol can lose a water molecule to form an alkene using an acid catalyst such as H2SO4 and heat (an “elimination reaction”) This is the reverse of the addition of H2O to an alkene Dehydration is favored by using heat (endothermic reaction) and a solvent other than water (lower concentration of H2O) When more than one alkene can be formed, Zaitsev’s rule states that the more substituted alkene will be the major product Order of reactivity = 3 > 2 > (1 > methyl) - In fact this reaction only works with 3 and 2 alcohols

Mechanism of Acid-Catalyzed Dehydration of an Alcohol First, the acid catalyst protonates the alcohol Next, H2O is eliminated to form a carbocation Finally, a proton is removed to form an alkene + H3O+