Chapter 8 Alcohols, Ehters and Thiols

Hydroxyl (OH) functional group Oxygen is sp 3 hybridized.

Alcohols Primary: carbon with –OH is bonded to one other carbon. Secondary: carbon with –OH is bonded to two other carbons. Tertiary: carbon with –OH is bonded to three other carbons.

Naming OH- Find the longest carbon chain containing the carbon with the -OH group. Drop the -e from the alkane name, add -ol. Number the chain, starting from the end closest to the -OH group. Number and name all substituents

Name these:

2-methyl-1-propanol 2-methyl-2-propanol 2-butanol 3-bromo-3-methylcyclohexanol

Unsaturated Alcohols Hydroxyl group takes precedence. Assign that carbon the lowest number. Use alkene or alkyne name. 4-penten-2-ol (old) pent-4-ene-2-ol (1997 revision of IUPAC rules)

Naming Priority Acids Esters Aldehydes Ketones Alcohols Amines Alkenes Alkynes Alkanes Ethers Halides

Hydroxy Substituent When -OH is part of a higher priority class of compound, it is named as hydroxy. Example: 4-hydroxybutanoic acid

Common Names Alcohol can be named as alkyl alcohol. Useful only for small alkyl groups. Examples: isobutyl alcohol sec-butyl alcohol

Naming Diols Two numbers are needed to locate the two -OH groups. Use -diol as suffix instead of -ol. 1,6-hexanediol

Physical Properties Unusually high boiling points due to hydrogen bonding between molecules. Small alcohols are miscible in water, but solubility decreases as the size of the alkyl group increases.

Solubility in Water Solubility decreases as the size of the alkyl group increases.

Acidity of Alcohols pK a range: (water: 15.7) Acidity decreases as alkyl group increases. Halogens increase the acidity. Phenol is 100 million times more acidic than cyclohexanol!

Basicity Of Alchols – In the presence of a strong acid, alcohols can act as weak bases Reaction with active metals – They can react with Li, Na, K, Mg and other active metals to liberate hydrogen and to form metal alkoxides

Conversion to Haloalkanes – The conversion of an alcohol to an alkyl halide involves substituting the halogen for –OH at a saturated carbon Tertiary alcohols react rapidly at room temperature with HCl, HBr and HI to give a haloalkane

Acid Catalyzed Dehydration to Alkenes – Alcohols can be converted to an alkene by dehydration – In other words By elimination of a molecule of water from adjacent carbon atoms – Primary –OH are the most difficult to dehydrate and generally require heating in concentrated sulfuric acid and very high temperatures (180° C) – Secondary –OH undergo acid-catalyzed dehydration at somewhat lower temperatures – Tertiary -OH only require temperatures slightly above room temperature

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Oxidation of Primary Alcohols A synonym for the oxidation of alcohols, is dehydrogenation. The OH group must be attached to a carbon atom that is bonded to at least one hydrogen atom. Give an aldehyde or a carboxylic acid, this is dependant on the conditions.

The two reagents that are most commonly used for the oxidation of a primary alcohol to carboxylic acid are chromic acid and PCC – H 2 Cr 2 O 7 (chromic acid) – Pyridinium chlorochromate (PCC) The second one is preferred because no PCC will remain once all the alcohol has been converted to aldehydes and it will not affect the double bonds.

Oxidation of Secondary Alcohols Give ketones Both PCC and chromic acid are used to oxidize

Oxidation of Tertiary Alcohols They resist oxidation. The carbon that has the OH- is bonded to 3 carbon atoms, this means that it cannot form a carbon-oxygen double bond.

Ethers Is an atom of an oxygen bonded to two carbon atoms Dimethyl ether is the simplest ether

Naming Ethers Select the longest carbon chain as the parent alkane Name the –OR group bonded to it (parent alkane) as alkoxy (alkyl+oxygen) Common names were given by listing the alkyl groups bonded to the oxygen in alphabetical order and adding the word ether.

Physical properties of Ehters They are polar compounds – Oxygen a partial negative charge Each carbon bonded to it bears a positive charge Steric hindrance caused poor attraction between ether molecules in pure liquid Boiling points are lower than –OH of comparable molecular weight

More soluble in water than hydrocarbons of comparable weight From hydrogen bonds with water The are resistant to chemical reactions – No reaction with oxidizing agents – Not affected by most acids or bases at moderate temperatures

Epoxides They are considered to be a subclass of ethers They are cyclic ethers in which the oxygen is one atom of the three-membered ring But they are extremely reactive, contrary to ethers.

Naming Epoxides Common names are derived by giving the common name of the alkene from which the epoxide might have been derived followed by the word oxide ethylene oxide

The root name is based on the longest chain with the two C-O bonds attached. The chain is numbered so as to give the epoxide unit the lowest possible locant (again like alkenes) The epoxide prefix is inserted prior to the root name along with both locants e.g. 1,2-epoxypropane. Both locants are included since this method is also used for naming other cyclic ethers.

Thiols The functional group of a thiol is an –SH (sulfhydryl) Methaethiol (CH 3 SH) is the simplest thiol They are responsible for unpleasant odors – Skunk, rotten eggs, natural gas and sewage

Naming Thiols Select the parent alkane – The longest chain of carbon atoms that contains the –SH group To show that a compound is a thiol we add – -thiol to the name of the parent chain – and number the parent chain in the direction that gives the –SH group the lower number

butanethiol

Common names for thiols are given by naming the alkyl group bonded to –SH and adding the word mercaptan (mercury capturing) Ethyl Mercaptan In the presence of other functional groups the presence of an –SH group is indicated by the prefix mercapto- 3-mercaptocyclohexanol

Properties Non polar, small difference in electronegativity between H and S. No hydrogen bonding Low boiling points and not that soluble in water (than other –OH of similar molecular weight)