Alcohols and phenols may be classified as mono–, di–, tri- or polyhydric compounds depending on whether they contain one, two, three or many hydroxyl groups respectively in their structures as given below:
Monohydric alcohols may be further classified according to the hybridisation of the carbon atom to which the hydroxyl group is attached.
(i) Compounds containing bond: In this class of alcohols, the –OH group is attached to an sp3 hybridised carbon atom of an alkyl group. They are further classified as follows:
Primary, secondary and tertiary alcohols: In these three types of alcohols, the –OH group is attached to primary, secondary and tertiary carbon atom, respectively as depicted below:
Allylic alcohols: In these alcohols, the —OH group is attached to a sp3 hybridised carbon adjacent to the carbon-carbon double bond, that is to an allylic carbon. For example
Benzylic alcohols: In these alcohols, the —OH group is attached to a sp3—hybridised carbon atom next to an aromatic ring. For example.
Allylic and benzylic alcohols may be primary, secondary or tertiary.
(ii) Compounds containing bond: These alcohols contain —OH group bonded to a carbon-carbon double bond, i.e., to a vinylic carbon or to an aryl carbon. These alcohols are also known as vinylic alcohols.
Vinylic alcohol: CH2 = CH – OH
IUPAC naming system:
(i) Select the longest possible chain that contains the α-carbon
(ii) Number the carbon atoms in the chain in such a way that α-carbon gets the minimum number
(iii) Then identify the hydrocarbon group and name it as per the IUPAC naming system.
(iv) Now replace the ‘e’ in the end of the parent hydrocarbon name by ‘ol’.
Common naming system:
o The common name of alcohol is alkyl alcohol.
o Depending upon the upon the structure of the alkyl group, prefix n, iso, sec, tert or neo is added to the common name.
‘n’ is added when α-carbon and other carbon atoms make a straight chain.
‘Iso’ is added when the ─OH group is attached to
‘sec’ is added when the α-carbon is attached to two other carbon atoms.
‘tert’is added when the α-carbon is attached to three other carbon atoms.
‘Neo’ is used when quaternary carbon is present.
Physical properties of alcohol
- The boiling points of alcohols and phenols increase with an increase in the number of carbon atoms due to an increase in van der Waals forces.
- In alcohols, the boiling points decrease with the increase of branching in the carbon chain because of a decrease in van der Waals forces with a decrease in surface area.
- The presence of intermolecular hydrogen bonding in alcohols and phenols is responsible for their higher boiling points compared to other classes of compounds of comparable molecular masses.
- Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules.
- The solubility of alcohols and phenols decreases with an increase in the size of the alkyl/aryl group as it becomes more hydrophobic.
- Lower molecular mass alcohols are miscible with water in all proportions.
The properties of alcohols and phenols are mainly due to the hydroxyl group. The presence of this group enables alcohols and phenols to form intermolecular hydrogen bonding with themselves and with water molecules. The boiling points of alcohols and phenols increase with an increase in the number of carbon atoms due to an increase in van der Waals forces. However, the boiling points decrease with an increase in branching in the carbon chain due to a decrease in van der Waals forces. The solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules. The solubility decreases with an increase in the size of the alkyl/aryl group, as the hydrophobic nature of the group increases.
Preparation of Alcohols
Alcohols from alkenes
By acid catalysed hydration of alkenes in accordance with Markownikoffs rule.
The mechanism of the reaction involves the following three steps:
Step 1: Protonation of alkene to form carbocation by electrophilic attack of H3O+.
H2O + H+ → H3O+
Step 2: Nucleophilic attack of water on carbocation.
Step 3: Deprotonation to form an alcohol.
Diborane (BH3)2 reacts with alkenes to give trialkyl boranes as addition product. This is oxidised to alcohol by hydrogen peroxide in the presence of aqueous sodium hydroxide.
The addition of borane to the double bond takes place in such a manner that the boron atom gets attached to the sp2 carbon carrying greater number of hydrogen atoms. The alcohol so formed looks as if it has been formed by the addition of water to the alkene in a way opposite to the Markovnikov’s rule. In this reaction, alcohol is obtained in excellent yield.
Alcohols from carbonyl compounds
By reduction of aldehydes and ketones
Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4). Aldehydes yield primary alcohols whereas ketones give secondary alcohols.
By reduction of carboxylic acids and esters
Carboxylic acids are reduced to primary alcohols in excellent yields by lithium aluminium hydride, a strong reducing agent.
However, LiAlH4 is an expensive reagent, and therefore, used for preparing special chemicals only. Commercially, acids are reduced to alcohols by converting them to the esters (Section 11.4.4), followed by their reduction using hydrogen in the presence of catalyst (catalytic hydrogenation).
Alcohol synthesis from Grignard
It gives R group as a nucleophile to attack at the partially positive carbon atom of carbonyl (–C=O) group.
Alcohol oxidation reaction
Alcohol oxidation is a reaction in which an alcohol molecule loses hydrogen atoms, and oxygen atoms are added to the carbon chain resulting in the formation of carbonyl compounds.
Primary alcohols undergo oxidation to form aldehydes and then further to carboxylic acids. The oxidation of primary alcohols can be carried out by various oxidizing agents such as potassium dichromate (K2Cr2O7), potassium permanganate (KMnO4), and nitric acid (HNO3).
Acidity and basicity of alcohol
The acidity of alcohols is determined by the strength of their corresponding conjugate base, i.e. the alkoxide ion. The strength of alkoxide ion depends on two factors - steric and electronic.
1. Steric Factors: Steric hindrance caused by bulky substituents attached to the carbon atom bearing the hydroxyl group decreases the stability of the alkoxide ion. More substituted alkyl groups make the alkoxide ion bulkier, and it becomes harder for the solvent to stabilize the ion. Thus, the conjugate base becomes more reactive and has higher basic strength, leading to weaker acidity.
2. Electronic Factors: Alcohols containing electron-donating groups attached to the carbon bearing the hydroxyl group destabilize the alkoxide ion, making it less stable and more reactive. This results in the alkoxide ion being a strong conjugate base and a weaker acid.
Based on these factors, we can conclude that primary alcohols are more acidic than secondary alcohols, which are more acidic than tertiary alcohols. This is because the primary alkoxide ion is the most stable among the three, while the tertiary alkoxide ion is the least stable. Thus, primary alcohols have the strongest acidity, while tertiary alcohols have the weakest acidity.
Therefore, the decreasing order of acidity of alcohols is: Primary alcohol (1∘) > Secondary alcohol (2∘) > Tertiary alcohol (3∘).
Alcohols are versatile compounds. They react both as nucleophiles and electrophiles. The bond between O–H is broken when alcohols react as nucleophiles.
Alcohols as nucleophiles
(ii) The bond between C–O is broken when they react as electrophiles. Protonated alcohols react in this manner.
Protonated alcohols as electrophiles
Based on the cleavage of O–H and C–O bonds, the reactions of alcohols and phenols may be divided into two groups:
Reaction of alcohols with metals
Alcohols and phenols react with active metals such as sodium, potassium and aluminium to yield corresponding alkoxides/phenoxides and hydrogen.
Acidity of alcohols
The acidic character of alcohols is due to the polar nature of O–H bond. An electron-releasing group (–CH3, –C2H5) increases electron density on oxygen tending to decrease the polarity of O-H bond. This decreases the acid strength. For this reason, the acid strength of alcohols decreases in the following order:
Alcohols are, however, weaker acids than water. This can be illustrated by the reaction of water with an alkoxide.
This reaction shows that water is a better proton donor (i.e., stronger acid) than alcohol. Also, in the above reaction, we note that an alkoxide ion is a better proton acceptor than hydroxide ion, which suggests that alkoxides are stronger bases (sodium ethoxide is a stronger base than sodium hydroxide).
Alcohols act as Bronsted bases as well. It is due to the presence of unshared electron pairs on oxygen, which makes them proton acceptors.
Esterification of alcohols
- Alcohols and phenols can react with carboxylic acids, acid chlorides, and acid anhydrides to form esters.
- The reaction with carboxylic acid and acid anhydride is carried out in the presence of a small amount of concentrated sulfuric acid, and the reaction is reversible.
- The reaction with acid chloride is carried out in the presence of a base (such as pyridine) to neutralize the HCl formed during the reaction and to shift the equilibrium to the right-hand side.
- Water is removed as soon as it is formed during the reaction with carboxylic acid and acid anhydride.
- The introduction of the acetyl (CH3CO) group in alcohols or phenols is known as acetylation.
- Acetylation of salicylic acid produces aspirin.
Reaction of alcohol with hydrogen halides
Alcohols react with hydrogen halides to form alkyl halides.
ROH + HX → R–X + H2O
The difference in reactivity of three classes of alcohols with HCl distinguishes them from one another (Lucas test). Alcohols are soluble in Lucas reagent (conc. HCl and ZnCl2) while their halides are immiscible and produce turbidity in solution. In case of tertiary alcohols, turbidity is produced immediately as they form the halides easily. Primary alcohols do not produce turbidity at room temperature.
Reaction of alcohol with phosphorus trihalides
Dehydration of alcohols with conc. H2SO4 at different temperatures yields different products.
The ease of dehydration of three alcohols increases in the order
1o ROH < 2o ROH < 3o ROH
Test for distinguishing 1o, 2o and 3o alcohols
Lucas reagent which s a mixture of conc. HCI and anhyd. ZnCl2 is used to distinguish and alcohols.
• 3o alcohol gives cloudiness or turbidity with Lucas reagent immediately.
• 2o alcohol gives cloudiness or turbidity after 5 – 10 min.
• 1o alcohol does not give cloudiness or turbidity at room temperature.
The mechanism of dehydration of ethanol involves the following steps:
Step 1: Formation of protonated alcohol.
Step 2: Formation of carbocation: It is the slowest step and hence, the rate determining step of the reaction.
Step 3: Formation of ethene by elimination of a proton.
The acid used in step 1 is released in step 3. To drive the equilibrium to the right, ethene is removed as it is formed.
Oxidation of alcohol
Depending upon the structure of the alcohol and the type of oxidising agent used, oxidation of alcohol gives different products.
Some Commercially Important Alcohols
Methanol is mainly produced by catalytic hydrogenation of carbon monoxide at high pressure and temperature and in the presence of ZnO – Cr2O3 catalyst.
• It is a colourless liquid and highly poisonous.
• It is completely soluble in water.
• It is used as a solvent for paints, varnishes, and for making formaldehyde.
Ethanol is mainly obtained commercially by fermentation of sugars. The sugar sugarcane or fruits such as grapes is converted to glucose and fructose, in the presence of an enzyme, invertase. Glucose and fructose on fermentation in the presence of another enzyme, zymase, yield ethanol.
• Ethanol is a colourless liquid.
• The boiling point of ethanol is higher than methanol.
• It is used as s solvent in paint industry.
• It is also used in the preparation of a number of carbon compounds.