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Alcohols and Phenols notes

MDCAT Chemistry

This chapter covers the structure, classification, nomenclature, preparation, physical properties and chemical reactions of alcohols. It also explains the preparation, acidity, reactions and uses of phenol, including the reasons for the difference between alcohols and phenols.

Alcohols: Definition, Structure and Classification

Alcohols are organic compounds in which a hydroxyl group, OH, is attached to a saturated carbon atom. The carbon attached to the OH group is sp3 hybridised and forms single covalent bonds. The general formula of monohydric alcohols is CnH2n+1OH.

Alcohols are classified according to the number of hydroxyl groups and according to the type of carbon attached to the OH group. The classification of the carbon atom is based on the number of other carbon atoms bonded to it.

  • The functional group of alcohols is the hydroxyl group, OH.
  • In alcohols, the OH group is attached to a saturated carbon atom.
  • Monohydric alcohols contain one OH group, for example ethanol, C2H5OH.
  • Dihydric alcohols contain two OH groups, for example ethane-1,2-diol, HOCH2CH2OH.
  • Trihydric alcohols contain three OH groups, for example propane-1,2,3-triol, also called glycerol.
  • Primary alcohols have the OH group attached to a carbon bonded to one other carbon, for example propan-1-ol.
  • Secondary alcohols have the OH group attached to a carbon bonded to two other carbon atoms, for example propan-2-ol.
  • Tertiary alcohols have the OH group attached to a carbon bonded to three other carbon atoms, for example 2-methylpropan-2-ol.

Nomenclature and Isomerism of Alcohols

In the IUPAC system, the longest carbon chain containing the OH group is selected. The final e of the corresponding alkane is replaced by ol. Numbering starts from the end nearer to the OH group, so that the hydroxyl group receives the lowest possible number.

Alcohols may show chain isomerism, position isomerism and functional group isomerism. Their functional group isomeric compounds are ethers, which have the general structure ROR.

  • CH3OH is methanol.
  • CH3CH2OH is ethanol.
  • CH3CH2CH2OH is propan-1-ol.
  • CH3CHOHCH3 is propan-2-ol.
  • (CH3)3COH is 2-methylpropan-2-ol.
  • Alcohols and ethers are functional group isomers. For example, C2H6O may be ethanol or methoxymethane.
  • The common names of simple alcohols are formed by naming the alkyl group followed by alcohol, such as ethyl alcohol for ethanol.
  • When an alcohol loses the proton of its OH group, an alkoxide ion is formed, such as ethoxide ion, C2H5O−.

Preparation of Alcohols

Alcohols can be prepared by hydration of alkenes, hydrolysis of haloalkanes, reduction of aldehydes and ketones, and fermentation of carbohydrates. The method selected depends on the required alcohol.

Ethene gives ethanol by reaction with steam in the presence of phosphoric acid catalyst. Haloalkanes react with aqueous alkali hydroxides to form alcohols. Ethyl iodide reacts with dry silver oxide to give ethanol because dry Ag2O provides the oxide ion for hydrolysis.

  • Alkenes react with steam in the presence of an acid catalyst to form alcohols. CH2=CH2 + H2O gives CH3CH2OH.
  • Acid-catalysed hydration of propene gives mainly propan-2-ol because the reaction follows Markovnikov addition.
  • A haloalkane reacts with aqueous KOH or NaOH to form an alcohol. C2H5I + KOH(aq) gives C2H5OH + KI.
  • Ethyl iodide reacts with dry Ag2O to form ethanol: 2C2H5I + Ag2O gives 2C2H5OH + 2AgI.
  • Aldehydes are reduced to primary alcohols. Ketones are reduced to secondary alcohols.
  • Fermentation of glucose produces ethanol and carbon dioxide: C6H12O6 gives 2C2H5OH + 2CO2.
  • The optimum temperature for fermentation is about 25 to 35 degrees Celsius.
  • Methanol can be prepared from wood by destructive distillation, from synthesis gas containing CO and H2, and by suitable hydrolysis methods.

Physical Properties of Alcohols

Alcohol molecules form hydrogen bonds because oxygen is highly electronegative and contains lone pairs of electrons. Hydrogen bonding gives alcohols higher boiling points than hydrocarbons of similar molecular mass.

Lower alcohols are highly soluble in water because their OH group forms hydrogen bonds with water. Solubility decreases as the hydrocarbon part becomes larger. Branching generally increases solubility and lowers boiling point compared with a straight-chain isomer.

  • Alcohols have higher boiling points than corresponding hydrocarbons because of intermolecular hydrogen bonding.
  • Among isomeric alcohols of the same molecular formula, primary alcohols generally have the highest boiling point.
  • The usual boiling point order for isomeric alcohols is primary greater than secondary greater than tertiary.
  • Lower members of the alcohol series are colourless liquids and are soluble in water.
  • Water solubility decreases as the length of the hydrocarbon chain increases.
  • Branching lowers the surface area and usually lowers the boiling point.
  • Rectified spirit is ethanol containing about 95 percent ethanol and 5 percent water.
  • Absolute alcohol is nearly pure ethanol, containing very little water.

Chemical Reactions of Alcohols

Alcohols react through the O-H bond and the C-O bond. The O-H bond can break to form alkoxide ions, while the C-O bond can break during substitution or dehydration. Their reactions include reaction with active metals, esterification, oxidation and dehydration.

The acidity of alcohols is weak. Electron-releasing alkyl groups reduce acidity by destabilising the alkoxide ion. Therefore, the acidity generally decreases from primary to secondary to tertiary alcohols.

  • Alcohols react with sodium to produce sodium alkoxide and hydrogen gas: 2ROH + 2Na gives 2RONa + H2.
  • One mole of a diol reacts with excess sodium to produce one mole of hydrogen gas because it contains two OH groups.
  • Alcohols react with organic acids in the presence of concentrated H2SO4 to form esters and water. This reaction is called esterification.
  • Ethanol reacts with ethanoic acid to form ethyl ethanoate and water: C2H5OH + CH3COOH gives CH3COOC2H5 + H2O.
  • Oxidation of a primary alcohol gives an aldehyde and then a carboxylic acid.
  • Oxidation of a secondary alcohol gives a ketone.
  • Tertiary alcohols resist oxidation under ordinary conditions because the carbon bearing OH has no hydrogen atom attached to it.
  • A primary alcohol such as propan-1-ol can act as a reducing agent because it is oxidised to an aldehyde or acid.

Dehydration, Substitution and Lucas Test

On heating with concentrated sulphuric acid, alcohols may lose water. At about 140 degrees Celsius, ethanol gives diethyl ether. At about 170 degrees Celsius, ethanol gives ethene. Dehydration is easier for tertiary alcohols because the reaction involves formation of a relatively stable carbocation.

Alcohols also undergo substitution with hydrogen halides or reagents that convert the OH group into a better leaving group. The relative reaction rate depends on the type of alcohol and the mechanism involved.

  • At 140 degrees Celsius, excess ethanol with concentrated H2SO4 gives diethyl ether: 2C2H5OH gives C2H5OC2H5 + H2O.
  • At about 170 degrees Celsius, ethanol with concentrated H2SO4 gives ethene: C2H5OH gives CH2=CH2 + H2O.
  • The ease of dehydration is generally tertiary alcohol greater than secondary alcohol greater than primary alcohol.
  • 2-methylpropan-2-ol undergoes dehydration easily because it forms a stable tertiary carbocation.
  • Lucas reagent is concentrated HCl mixed with anhydrous ZnCl2.
  • Lucas test distinguishes primary, secondary and tertiary alcohols by the time taken to form turbidity.
  • Tertiary alcohols give turbidity immediately, secondary alcohols give turbidity after a few minutes, and primary alcohols react very slowly at room temperature.
  • 1-pentanol reacts slowest in the Lucas test among primary, secondary and tertiary alcohols because it is a primary alcohol.

Methanol, Ethanol and Glycerol

Methanol is a colourless, volatile and poisonous liquid. It is used as a solvent and as a fuel. Ingestion of methanol can cause blindness or death because it is oxidised in the body to toxic products.

Ethanol is prepared by fermentation and by hydration of ethene. Commercial ethanol may contain water and other impurities. Glycerol is a trihydric alcohol with three OH groups and has several reactions characteristic of polyhydric alcohols.

  • Methanol is also called wood spirit because it was historically obtained from wood by destructive distillation.
  • Methanol is poisonous and must not be consumed.
  • Ethanol obtained by fermentation is present in alcoholic beverages and can be used as a solvent and fuel.
  • Fermentation is carried out by enzymes in yeast in the absence of air.
  • Glycerol is propane-1,2,3-triol, HOCH2CHOHCH2OH.
  • Glycerol is a viscous, colourless liquid and is miscible with water because of its three OH groups.
  • Glycerol inhibits the decomposition of hydrogen peroxide and therefore acts as a negative catalyst or stabiliser.
  • Polyhydric alcohols react with freshly precipitated Cu(OH)2 to give a deep blue solution because of complex formation.

Phenol: Structure, Acidity and Preparation

Phenol is an aromatic hydroxy compound in which the OH group is directly attached to a benzene ring. Its formula is C6H5OH. Phenol is also called carbolic acid. It is different from an alcohol because its OH group is attached directly to an aromatic carbon.

Phenol is more acidic than alcohols. When phenol loses H+, the phenoxide ion is formed. The negative charge in phenoxide ion is delocalised over the benzene ring by resonance, making the ion more stable.

  • Phenol has the formula C6H5OH and is also known as carbolic acid.
  • The acidity of phenol is due to the greater stability of the phenoxide ion.
  • Phenoxide ion is stabilised by resonance or delocalisation of the negative charge.
  • Alcohols form alkoxide ions, in which the negative charge is mainly localised on oxygen.
  • Phenol is more acidic than ethanol, but it is much weaker than strong mineral acids.
  • Electron-withdrawing groups such as NO2 increase the acidity of phenol.
  • Electron-releasing groups such as CH3 decrease the acidity of phenol.
  • Phenol can be prepared from chlorobenzene by heating with aqueous NaOH at high temperature and pressure, followed by acidification of sodium phenoxide.

Reactions and Uses of Phenol

The OH group activates the benzene ring and directs incoming electrophiles mainly to the ortho and para positions. Phenol therefore undergoes electrophilic substitution more readily than benzene.

Phenol has important identification reactions. It gives a violet colour with neutral ferric chloride and a white precipitate of 2,4,6-tribromophenol with bromine water.

  • Phenol reacts with sodium to form sodium phenoxide and hydrogen gas.
  • Phenol reacts with sodium hydroxide to form sodium phenoxide and water: C6H5OH + NaOH gives C6H5ONa + H2O.
  • Phenol does not react with sodium bicarbonate in the usual test because it is not acidic enough to liberate CO2 from NaHCO3.
  • Phenol gives a violet or purple colour with neutral FeCl3 solution.
  • Phenol reacts with bromine water to give a white precipitate of 2,4,6-tribromophenol.
  • Kolbe reaction of sodium phenoxide with CO2 followed by acidification gives salicylic acid, mainly 2-hydroxybenzoic acid.
  • Reimer-Tiemann reaction of phenol with CHCl3 and NaOH introduces a formyl group mainly at the ortho position and gives salicylaldehyde.
  • Phenol is used as a disinfectant, antiseptic in suitable preparations, and in the manufacture of resins, plastics and dyes.

Key terms

Alcohol
An organic compound containing an OH group attached to a saturated carbon atom.
Hydroxyl group
The functional group, OH, present in alcohols and phenols.
Primary alcohol
An alcohol in which the carbon bearing OH is attached to one other carbon atom.
Secondary alcohol
An alcohol in which the carbon bearing OH is attached to two other carbon atoms.
Tertiary alcohol
An alcohol in which the carbon bearing OH is attached to three other carbon atoms.
Fermentation
The enzymatic conversion of glucose into ethanol and carbon dioxide by yeast.
Rectified spirit
Ethanol containing about 95 percent ethanol and about 5 percent water.
Esterification
The reaction of an alcohol with an organic acid to form an ester and water.
Lucas test
A test using concentrated HCl and anhydrous ZnCl2 to distinguish primary, secondary and tertiary alcohols.
Oxonium ion
An ion formed when an ether accepts a proton, with the general form R2OH+.
Alkoxide ion
An ion formed by removal of the proton from an alcohol, with the general form RO−.
Phenol
An aromatic compound in which OH is directly attached to a benzene ring.
Phenoxide ion
The ion formed when phenol loses H+, stabilised by resonance.
Carbolic acid
The common name for phenol, C6H5OH.
Dehydration
Removal of water from an alcohol, usually producing an alkene or ether.
Ester
An organic compound formed by reaction between an alcohol and an organic acid.
Resonance
Delocalisation of electrons or charge over several atoms in a molecule or ion.
Negative catalyst
A substance that decreases the rate of a reaction, such as glycerol in hydrogen peroxide decomposition.

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