Alkanols Nomenclature and Classification

• Alkanols, also known as alcohols, are organic compounds characterized by the presence of one or more hydroxyl (-OH) groups attached to a saturated carbon atom.

Nomenclature

• IUPAC nomenclature for alkanols involves identifying the longest carbon chain containing the hydroxyl group and naming it as the parent alkane
• Replace the "-e" at the end of the alkane name with "-ol"
• Number the carbon chain to give the carbon bearing the hydroxyl group the lowest possible number
• Indicate the position of the hydroxyl group with this number placed immediately before the "-ol" suffix; for example, propan-1-ol
• If multiple hydroxyl groups are present, use prefixes like di-, tri-, etc., before "-ol" to indicate the number of hydroxyl groups
• Retain the "-e" of the parent alkane name in this case; for example, ethane-1,2-diol
• Substituents are named and numbered as with alkanes
• Cyclic alcohols are named by numbering the carbon atom bearing the -OH group as 1

Classification

• Alkanols are classified based on the number of carbon atoms attached to the carbon bearing the hydroxyl group
• Primary (1°) alkanols have the hydroxyl-bearing carbon attached to one other carbon atom
• Secondary (2°) alkanols have the hydroxyl-bearing carbon attached to two other carbon atoms
• Tertiary (3°) alkanols have the hydroxyl-bearing carbon attached to three other carbon atoms

Physical Properties

• The presence of the hydroxyl group allows alkanols to form hydrogen bonds, leading to higher boiling points and water solubility compared to alkanes of similar molecular weight
• Shorter chain alkanols (e.g., methanol, ethanol, propanol) are miscible with water due to the strong hydrogen bonding between the alkanol and water molecules
• As the length of the carbon chain increases, the solubility in water decreases due to the increasing hydrophobic character of the alkyl chain, for example, butanol is only slightly soluble
• Boiling points of alkanols increase with increasing molecular weight due to increased van der Waals forces and the increasing strength of hydrogen bonding
• Alkanols have higher boiling points than ethers, haloalkanes, and alkanes of comparable molecular mass because alkanols can form intermolecular hydrogen bonds whereas the others cannot

Acidity and Basicity

• Alkanols are weakly acidic due to the polarization of the O-H bond
• They can donate a proton to form an alkoxide ion
• The acidity of alkanols is less than that of water
• Alkanols can also act as weak bases due to the lone pairs of electrons on the oxygen atom
• They can accept a proton from strong acids to form an oxonium ion

Preparation of Alkanols

• Hydration of Alkenes: Alkenes react with water in the presence of an acid catalyst (e.g., sulfuric acid) to form alkanols, following Markovnikov’s rule
• Hydroboration-Oxidation: Alkenes react with borane (BH3) followed by oxidation with hydrogen peroxide in basic medium to yield alkanols, following anti-Markovnikov's rule
• Grignard Reaction: Reaction of Grignard reagents (RMgX) with aldehydes or ketones followed by hydrolysis yields secondary or tertiary alkanols
• Reduction of Aldehydes and Ketones: Aldehydes are reduced to primary alkanols, and ketones are reduced to secondary alkanols using reducing agents like sodium borohydride (NaBH4) or lithium aluminum hydride (LiAlH4)
• From Haloalkanes: Haloalkanes react with aqueous sodium hydroxide to produce alcohols through nucleophilic substitution

Reactions of Alkanols

• Dehydration: Alkanols undergo dehydration in the presence of a strong acid catalyst (e.g., sulfuric acid, phosphoric acid) at high temperatures to form alkenes
• The reaction follows Zaitsev’s rule, favoring the formation of the more substituted alkene
• Oxidation: Primary alkanols are oxidized to aldehydes or carboxylic acids, depending on the oxidizing agent used
• Secondary alkanols are oxidized to ketones
• Tertiary alkanols are resistant to oxidation due to the absence of a hydrogen atom on the carbon bearing the hydroxyl group
• Esterification: Alkanols react with carboxylic acids in the presence of an acid catalyst to form esters
• This reaction is known as Fischer esterification
• Reaction with Hydrogen Halides: Alkanols react with hydrogen halides (HX) to form haloalkanes
• The reactivity order of hydrogen halides is HI > HBr > HCl
• Lucas Test: Used to differentiate between primary, secondary, and tertiary alkanols
• It involves reacting an alkanol with Lucas reagent (anhydrous zinc chloride in concentrated hydrochloric acid)
• Tertiary alkanols react immediately, secondary alkanols react within 5-10 minutes, and primary alkanols do not react at room temperature

Important Alkanols

• Methanol (CH3OH): Also known as wood alcohol, is a common solvent and a precursor for many chemical products; it is toxic
• Ethanol (C2H5OH): Also known as grain alcohol, is used in alcoholic beverages, as a solvent, and as a fuel additive
• Isopropanol (CH3CHOHCH3): Commonly known as rubbing alcohol, is used as a disinfectant and solvent
• Ethylene glycol (HOCH2CH2OH): Used as antifreeze in vehicles and as a precursor to polymers
• Glycerol (HOCH2CHOHCH2OH): Also known as glycerin, is used in cosmetics, pharmaceuticals, and as a humectant

Learn about alkanols, also known as alcohols, which are organic compounds characterized by the presence of one or more hydroxyl (-OH) groups attached to a saturated carbon atom. This lesson covers IUPAC nomenclature rules, including identifying the longest carbon chain, numbering, and handling multiple hydroxyl groups. Also learn about the classification of alkanols.