Alcohols, Phenols and Ethers

• After studying this unit, you will be able to:

  • Name alcohols, phenols, and ethers according to the IUPAC system of nomenclature
  • Discuss the reactions involved in the preparation of alcohols from alkenes, aldehydes, ketones, and carboxylic acids
  • Discuss the reactions involved in the preparation of phenols from haloarenes, benzene sulphonic acids, diazonium salts, and cumene
  • Discuss the reactions for the preparation of ethers from alcohols and alkyl halides and sodium alkoxides/aryloxides
  • Correlate physical properties of alcohols, phenols, and ethers with their structures
  • Discuss chemical reactions of the three classes of compounds based on their functional groups
• Alcohols, phenols, and ethers are the basic compounds for the formation of detergents, antiseptics, and fragrances respectively
• Alcohols and phenols are formed when a hydrogen atom in a hydrocarbon, aliphatic and aromatic respectively, is replaced by –OH group
• An alcohol contains one or more hydroxyl (OH) group(s) directly attached to carbon atom(s) of an aliphatic system, while a phenol contains –OH group(s) directly attached to carbon atom(s) of an aromatic system
• Alcohols and phenols may be classified as mono-, di-, tri- or polyhydric compounds depending on the number of hydroxyl groups they contain in their structures
• Ethers are compounds formed by substituting the hydrogen atom of the hydroxyl group of an alcohol or phenol by an alkyl or aryl group
• Alcohols may be classified based on the hybridization of the carbon atom to which the hydroxyl group is attached
• Alcohols containing sp3–OH bond are classified as primary, secondary, tertiary, allylic, and benzylic alcohols
• Alcohols containing sp2–OH bond are known as vinylic alcohols
• Ethers can be classified as simple or symmetrical if the alkyl or aryl groups attached to the oxygen atom are the same, and mixed or unsymmetrical if the two groups are different
• In naming alcohols, the common name is derived from the common name of the alkyl group with the word alcohol added to it. In the IUPAC system, the name is derived from the alkane name by substituting 'e' with 'ol'
• Phenols are named using common names or IUPAC names, with ortho, meta, and para used in substituted compounds
• Dihydroxy derivatives of benzene are known as 1, 2-, 1, 3- and 1, 4-benzenediol
• Common names for these derivatives are:

  • Catechol (Benzene-1,2-diol)
  • Resorcinol (Benzene-1,3-diol)
  • Hydroquinone or quinol (Benzene-1,4-diol)
• Common names of ethers are derived from the names of alkyl/aryl groups written as separate words in alphabetical order, with the word ‘ether’ added at the end
• In alcohols, the oxygen of the –OH group is attached to carbon by a sigma (σ) bond formed by the overlap of a sp3 hybridized orbital of carbon with a sp3 hybridized orbital of oxygen
• In phenols, the –OH group is attached to sp2 hybridized carbon of an aromatic ring
• If both alkyl groups in an ether are the same, the prefix ‘di’ is added before the alkyl group
• Examples of common and IUPAC names of ethers:

  • CH3OCH3 (Dimethyl ether / Methoxymethane)
  • C2H5OC2H5 (Diethyl ether / Ethoxyethane)
  • CH3OCH2CH2CH3 (Methyl n-propyl ether / 1-Methoxypropane)
  • C6H5OCH3 (Methyl phenyl ether / Methoxybenzene - Anisole)
  • C6H5OCH2CH3 (Ethyl phenyl ether / Ethoxybenzene - Phenetole)
  • C6H5O(CH2)6 – CH3 (Heptyl phenyl ether / 1-Phenoxyheptane)
• Ethers are named in the IUPAC system by replacing a hydrogen atom with an –OR or –OAr group, where R and Ar represent alkyl and aryl groups, respectively
• The bond angle in alcohols is slightly less than the tetrahedral angle (109°-28°) due to repulsion between the unshared electron pairs of oxygen
• The carbon–oxygen bond length in phenol is slightly less than in methanol due to partial double bond character from the conjugation of unshared electron pair of oxygen with the aromatic ring
• In ethers, the four electron pairs on oxygen are arranged approximately in a tetrahedral arrangement
• Alcohols can be prepared by various methods:

  1. From alkenes:

  • By acid-catalyzed hydration
  • By hydroboration-oxidation
• The C–O bond length in ethers is almost the same as in alcohols
• Alcohols can also be prepared:

  2. From carbonyl compounds:

  • By reduction of aldehydes and ketones
  • By reduction of carboxylic acids and esters
• Alcohols can additionally be prepared:

  3. From Grignard reagents by reacting them with aldehydes and ketones
• Phenols can be prepared from benzene derivatives through various methods:

  • Reaction of Grignard reagents with methanal produces primary alcohols, with other aldehydes secondary alcohols, and with ketones tertiary alcohols
• Methods of preparing phenols:

  1. From haloarenes:

  • Chlorobenzene is fused with NaOH at 623K and 320 atmospheric pressure. Phenol is obtained by acidification of sodium phenoxide produced.

  2. From benzenesulphonic acid:

  • Benzene is sulphonated with oleum, and benzene sulphonic acid formed is converted to sodium phenoxide by heating with molten sodium hydroxide. Acidification of the sodium salt gives phenol.

  3. From diazonium salts:

  • A diazonium salt is formed by treating an aromatic primary amine with nitrous acid at 273-278 K. Diazonium salts are hydrolyzed to phenols by warming with water or treating with dilute acids.

  4. From cumene:

  • Phenol is manufactured from cumene. Cumene is oxidized in the presence of air to cumene hydroperoxide, which is then converted to phenol and acetone by treating it with dilute acid
• Alcohols and phenols consist of an alkyl/aryl group and a hydroxyl group. The properties of alcohols and phenols are mainly due to the hydroxyl group
• Boiling points of alcohols and phenols increase with the number of carbon atoms due to increased van der Waals forces. In alcohols, boiling points decrease with increased branching in the carbon chain
• Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules. Solubility decreases with an increase in the size of alkyl/aryl groups
• Alcohols act as both nucleophiles and electrophiles. The bond between O-H is broken when alcohols act as nucleophiles
• Reactions of alcohols and phenols involving cleavage of O-H bond:

  • Acidity of alcohols and phenols: They react with active metals to yield corresponding alkoxides/phenoxides and hydrogen. Phenols react with aqueous sodium hydroxide to form sodium phenoxides
• In substituted phenols, the presence of electron-withdrawing groups like the nitro group enhances the acidic strength of phenol
• This effect is more pronounced when such a group is at the ortho and para positions
• Phenol is a million times more acidic than ethanol
• Electron-releasing groups, such as alkyl groups, do not favor the formation of phenoxide ion, resulting in a decrease in acid strength
• Cresols are less acidic than phenol
• The greater the pKa value, the weaker the acid