Alcohols, Ethers and Thiols Bettelheim, Brown, Campbell and Farrell Chapter 14

Review of Carbon Classification Type of C Attached to 1o (primary) C one other carbon 2o (secondary) C two other carbons 3o (tertiary) C three other carbons 4o (quaternary) C four other carbons

Alcohols Alcohol: Has an -OH (hydroxyl) group bonded to a tetrahedral carbon methanol, CH3OH, is the simplest alcohol

Classification of Alcohols Depends on the C which has the –OH group attached 1o Alcohol—attached to one C 2o Alcohol— attached to two Cs 3o Alcohol-- attached to three Cs

Alcohol Nomenclature 1.Find longest carbon chain that contains the -OH group (parent chain) 2.Number chain from end that gives the -OH the lower number 3.Change the ending -e to -ol 4.Use a number to show the location of the -OH group For cyclic alcohols, the carbon with the -OH group is C-1 5.Name and number substituents and list them in alphabetical order

Nomenclature

Nomenclature Write the IUPAC name for each alcohol

Nomenclature

Nomenclature Alcohols containing more than one hydroxyl groups are named as a diol, a triol, and so on IUPAC names for diols, triols, and so on retain the final "-e" in the name of the parent alkane Compounds containing two hydroxyl groups on adjacent carbons are commonly called glycols

Name the following

Name the following trans-1,3-cyclohexanediol 1,3-propanediol

Physical Properties Alcohols are polar molecules the C-O and O-H bonds are both polar covalent

Physical Properties Alcohols can also form hydrogen bonds with water Alcohol molecules can form hydrogen bonds with each other Alcohols can also form hydrogen bonds with water

Physical Properties of Alcohols Boiling point increases as MW increases Solubility in water decreases as MW increases

Chemical Properties of Alcohols Combustion Reactions (they burn in O2) Acid-Base Chemistry Dehydration Oxidation

Acidity of Alcohols Alcohols have about the same pKa values as water Aqueous solutions of alcohols have the same pH as that of pure water Phenols have lower pKa values than normal alcohols do (are more acidic)

Acidity of Alcohols Alcohols and phenols both contain an OH group Phenols are weak acids and react with strong bases such as NaOH to form water-soluble salts pKa = 9.89 for phenol Alcohols are much weaker acids and do not react in this manner

Why are phenols more acidic than alcohols Why are phenols more acidic than alcohols? Aromatic ring and O have delocalization of electrons. Less pull on H, so it is more acidic.

Dehydration Dehydration: Removal of water from adjacent carbons to form an alkene most often occurs by heating an alcohol with either 85% H3PO4 or concentrated H2SO4

Dehydration essentially the reverse of hydration (addition of water to double bond) Conditions required depend on kind of alcohol

Acid-Catalyzed Dehydration 1o ca 180oC needed Hard 2o lower temperatures Moderate 3o slightly above room temperature Easy

Dehydration Zaitsev’s Rule: When two alkene products are possible, the alkene having the greater number of alkyl groups (i.e., C atoms) on the double bond generally predominates (forms major product) Major product favored. (minor product)

Dehydration-Hydration Related Acid-catalyzed hydration of alkenes and acid-catalyzed dehydration of alcohols are competing reactions Equilibrium between alkene and alcohol exists Large amounts of water favor alcohol formation, while removal of water from the equilibrium mixture favors alkene formation (Le Chatelier's principle)

Dehydration vs Oxidation Dehydration: Remove –OH and –H OH from one C: H from adjacent C Oxidation: Remove two –H atoms One H from OH group: Other H from C that has the –OH attached

Oxidation of Alcohols Oxidation of a 1° alcohol gives an aldehyde or a carboxylic acid, depending on the experimental conditions Oxidation of a 1° alcohol to a carboxylic acid is carried out using an oxidizing agent such as potassium dichromate, K2Cr2O7, in aqueous sulfuric acid

Oxidation of Alcohols Can sometimes stop the oxidation at the aldehyde stage by distilling the mixture (Aldehydes usually have lower boiling points than either the 1° alcohol or the carboxylic acid) Alcohol Aldehyde Carboxylic acid

Oxidation of Alcohol Oxidation of a 2° alcohol gives a ketone Tertiary alcohols are resistant to oxidation

Oxidation of Alcohols Draw a structural formula for the product formed by oxidation of each of the following alcohols with potassium dichromate. a) 1-Hexanol b) 2-Hexanol Strategy Oxidation of 1-hexanol, a primary alcohol, gives either an aldehyde or a carboxylic acid, depending on the experimental conditions. Oxidation of 2-hexanol, a secondary alcohol, gives a ketone.

Solution (a) or Hexanal Hexanoic acid (b) 2- Hexanone

Common Alcohols Ethanol Ethyl alcohol, “grain alcohol” Produced by fermentation Natural fermentation of sugars and starches in grains, corn, molasses, fruits, vegetables Found in beer, wine, and other “spirits” C6H12O6 → 2 CH3CH2OH + 2 CO2 glucose ethanol carbon dioxide

Common Alcohols Methanol Methyl alcohol or “wood alcohol” Distilled from wood pulp originally Very toxic, even in small amounts Reacts to make formaldehyde (a fixative) CH3OH → CH2=O methanol formaldehyde

Common Alcohols Isopropyl alcohol 2-propanol “rubbing alcohol” Evaporates quickly—used to reduce fever Solvent in many cosmetics, perfumes, etc.

Common Alcohols 1,2-ethanediol Glycerol Ethylene glycol Very soluble in water Used as antifreeze Glycerol 1,2,3-propanetriol Sweet taste Part of fats and oils

Preparation of Important Alcohols Many alcohols (or their derivatives) can be formed by reacting smaller molecules.

Preparation of Important Alcohols

Ethers The functional group of an ether is an oxygen atom bonded to two carbon atoms Simplest ether is dimethyl ether Most common ether is diethyl ether Often called just “ether”

Ether Nomenclature Generally use common names for small (low-molecular-weight) ethers common names are derived by listing the alkyl groups bonded to oxygen in alphabetical order and adding the word "ether” alternatively, name one of the groups on oxygen as an alkoxy group

Ether Nomenclature Cyclic ether: an ether in which one of the atoms in a ring is oxygen Cyclic ethers also known by their common names ethylene oxide is an important building block for the organic chemical industry; it is also used as a fumigant in foodstuffs and textiles, and in hospitals to sterilize surgical instruments tetrahydrofuran is a useful laboratory and industrial solvent

Physical Properties of Ethers Ethers are polar compounds O has a partial negative charge Each C bonded to it has a partial positive charge Only weak forces of attraction exist between ether molecules in the pure liquid Boiling points of ethers are close to those of hydrocarbons of similar molecular weight Have lower boiling points than alcohols of the same molecular formula

Reactions of Ethers Ethers not very reactive (like hydrocarbons) Do not react with oxidizing agents such as potassium dichromate Do not react with reducing agents such as H2 in the presence of a transition metal catalyst Not affected by most acids or bases at moderate temperatures Good solvents for carrying out organic reactions, because of their general inertness and good solvent properties Diethyl ether and THF (tetrahydrofuran)

Thiols Thiol: a compound containing an -SH (sulfhydryl) group Also called mercaptans Low-molecular-weight thiols STINK Responsible for smells such as those from rotten eggs and sewage the scent of skunks is due primarily to these two thiols

Thiols - Nomenclature IUPAC names are derived in the same manner as are the names of alcohols Retain the final -e of the parent alkane and add the suffix -thiol Common names for simple thiols are derived by naming the alkyl group bonded to -SH and adding the word "mercaptan"

Thiol Physical Properties S-H bonds are nonpolar covalent Small difference in electronegativity between sulfur and hydrogen (2.5 - 2.1 = 0.4) No hydrogen bonding Lower boiling points than comparable alcohols Less soluble in water and other polar solvents than comparable alcohols

Reactions of Thiols Thiols are weak acids (pKa~10), and are comparable in strength to phenols thiols react with strong bases such as NaOH to form water-soluble thiolate salts

Reactions of Thiols The most important thiol reaction in biological systems is their oxidation to disulfides Functional group is a disulfide (-S-S-) bond Disulfide bonds important in protein structure Thiols are readily oxidized to disulfides by O2 -SH group very susceptible to oxidation and must be protected from contact with air during storage disulfides, in turn, are easily reduced to thiols by several reducing agents.

Reactions of Thiols Oxidation of –SH groups to form S-S bonds Reduction of S-S bonds to form –SH groups