Alcohols, Phenols and Thiols Structure Alcohols contain an hydroxyl (OH) group connected to sp3 hybridized carbon; considered as "alkyl hydroxides“ Phenols contain an OH group directly bonded to a benzene ring.

Alcohols are classified as 1⁰, 2⁰, or 3⁰. Not used for phenols.

Nomenclature IUPAC rules for naming alcohols: 1. select the longest carbon chain containing the hydroxyl group and derive the parent name by replacing the -e ending of the corresponding alkane with -ol 2. number the chain from the end nearer the hydroxyl group 3. number substituents according to position on chain, listing the substituents in alphabetical order 4. compounds containing more than one OH group are named diols, triols, etc.   Many alcohols have common names: Name the alkyl group and add "alcohol" as a separate word Naming phenols: 1. named on basis of phenol as parent 2. name substituents on ring in alphabetical order by their position from OH

Physical Properties Alcohols and phenols can participate in intermolecular hydrogen bonds.

This strong force can increase boiling points (melting points).

It can also increase solubility in water: C1-C3 highly soluble, C4-C5 moderately, C6 slightly soluble / insoluble

Density

Important Alcohols Methanol CH3OH, also known as wood alcohol made by catalytic hydrogenation of carbon monoxide uses: common solvent, perfumes, other smelly stuff, important industrial starting material toxic   Ethanol CH3CH2OH, also known as grain alcohol made by fermentation of sugars, made by acid catalyzed hydration of ethene uses: common solvent in flavors, medicines; important industrial starting material acts as a depressant, disables (kills) people 2-propanol isopropyl alcohol or rubbing alcohol disinfectant, solvent; highly toxic, if ingested 1,2-ethanediol ethylene glycol, antifreeze and toxic

Compounds of phenol are the active ingredients in some essential oils

Acidity and Basicity Brønsted-Lowry definition: an acid is a proton donor, a base is a proton acceptor.   A strong acid is one that is completely ionized in water. A weak acid is one that ionizes in water to the extent of less than 100%. Acid strength is measured by Ka or pKa Consider: HA (aq) ⇌ H+ (aq) + A− (aq) Ka = [A−][[H+] / [HA] Ka = ∞, strong acid pKa = −log Ka pKa = small value, strong acid

Preparation of Alcohols A. From alkenes: acid-catalyzed hydration Addition of water across a double bond in presence of H+. Stepwise reaction with a carbocation intermediate. Proceed through Markovnikov addition. Rearrangement possible.

B. From alkenes: hydroboration-oxidation Addition of water across a double bond. Stepwise reaction with a borane intermediate. Syn addition, anti-Markovnikov. No rearrangement. Borane dimerizes but it is possible to stabilize BH3 by using a solvent such as tetrahydrofuran. 2

The mechanism:

C. From carbonyl compounds: catalytic hydrogenation This is a reduction.   Reduction = a decrease in oxygen content = an increase in hydrogen content Oxidation = an increase in oxygen content = a decrease in hydrogen content

D. From carbonyl compounds: reduction using metal hydrides   They are reducing agents. They act as hydride donors (nucleophilic H:−). NaBH4 is not sensitive to moisture. The solvent can be ethanol, methanol, or water. LAH is a much stronger reagent. It reacts violently with water. Diethyl ether can be used as solvent.

They do not reduce isolated double bonds. Both NaBH4 and LAH will reduce aldehydes or ketones.

They do not reduce isolated double bonds. Both NaBH4 and LAH will reduce aldehydes or ketones. LAH will reduce carboxylic acids or esters, but NaBH4 will not.

The mechanism:

The mechanism:

E. From carbonyl compounds: using Grignard reagents   It is formed by the reaction between an alkyl halide and magnesium. The difference in electronegativity between C and Mg is so large that the bond can be treated as ionic They are carbon nucleophiles capable of attacking a wide range of electrophiles. They will react with ketones or aldehyde to produce alcohols. They will react with esters to produce alcohols, with introduction of two R groups.

The mechanism (ketone and aldehyde):

The mechanism (ester):

They will not add to carboxylic acids. They undergo acid-base reactions. They cannot be prepared if there are reactive functional groups in the same molecule, including proton donors. (−OH, −NH, −SH, −COOH)

F. From alkyl halides   Via nucleophilic substitution reactions. Careful, elimination reactions are competing.

Reactions of Alcohols Two general classes of reaction: cleavage of either the C–O bond or the O–H bond.

A. Acid-Base Reactions   Alcohols and phenols can act as Bronsted acids. Alcohols are very weak acids and react with strong bases or alkali metals.

A. Acid-Base Reactions   Alcohols and phenols can act as Bronsted acids. Alcohols are very weak acids and react with strong bases or alkali metals. Phenols are stronger acids than alcohols, can be converted to phenoxides using aq. NaOH.

B. Dehydration   3° alcohols are readily dehydrated with acid. 2° alcohols require severe conditions. 1° alcohols require very harsh conditions. Zaitsev’s product (more highly substituted) . Rearrangement is likely. E1 mechanism.

C. Conversion to Alkyl Halides with Gaseous or Concentrated Hydrogen Halides   Mechanism is primarily SN1 for 2° and 3° alcohols and rearranged products are observed. Mechanism is SN2 for 1 alcohols.

D. Conversion to Alkyl Halides with Halides of Phosphorus or with Thionyl Chloride   Best for 1° and 2° alcohols. Rearrangement is not generally observed. Via SN2 mechanism. 

E. Oxidation of the Alcohol Functional Group   The outcome of an oxidation process depends on whether the starting alcohol is primary, secondary, or tertiary.

E. Oxidation of the Alcohol Functional Group   The outcome of an oxidation process depends on whether the starting alcohol is primary, secondary, or tertiary.

Common oxidizing reagent is chromic acid (H2CrO4), which can be formed either from chromium trioxide (CrO3) or from sodium dichromate (Na2Cr2O7) in aqueous acidic solution.

More selective oxidizing reagents exist, including pyridinium chlorochromate (PCC) and pyridinium dichromate (PDC). To produce the aldehyde as the final product, PCC or PDC is used. Methylene chloride (CH2Cl2) is typically the solvent used

Spectroscopy A. Mass spectrometry Alcohols:   Alcohols: Molecular ion peak is usually weak of absent. Fragmentation involves loss of an alkyl group or loss of H2O from the molecular ion (M - 18). Phenols: Molecular ion peak is stong. C6H5+ at m/z = 77

B. UV-Visible spectroscopy   not specific to –OH C. IR spectroscopy Broad peak ~3500 cm–1 due to O–H stretch C–O stretch at ~1200 cm–1

  D. 1H NMR spectroscopy Chemical shift of O—H proton is variable; depends on temperature and concentration. O—H proton can be identified by adding D2O; signal for O—H disappears (converted to O—D) Usually singlet due to exchange reactions (with moisture or acids). E. 13C NMR spectroscopy Chemical shift of C—OH is 60-75 ppm

Sulphur Analogs Thiols (RSH) are sulfur analogs of alcohols. They are named with the suffix −thiol When the −SH group is a substituent, it is called a mercapto or a sulfanyl group.   Their preparations and reactions can be understood by referring to the analogous alcohols. Thiols are most notorious for their pungent, unpleasant odors.