General Chemistry - Alkanols/Alcohols

Alkanols/Alcohols

• Homologous series with hydroxyl (-OH) group attached to an alkyl group.
• Can be regarded as derivatives of alkanes with general formula $C<em>nH</em>{2n+1}OH$ or $R-OH$.
• Hydroxyl groups attached to aromatic rings are called phenols.

Classes of Alcohols

• Primary ($1^{\circ}$): Carbon atom with -OH group attached to one other carbon atom.
  • E.g., $CH<em>3CH</em>2OH$, $CH<em>3CH</em>2CH<em>2OH$, $(CH</em>3)<em>2CHCH</em>2OH$
• Secondary ($2^{\circ}$): Carbon atom with -OH group attached to two other carbon atoms.
  • E.g., $CH<em>3CH(OH)CH</em>3$, $CH<em>3CH(OH)CH(CH</em>3)_2$
• Tertiary ($3^{\circ}$): Carbon atom with -OH group attached to three other carbon atoms.
  • E.g., $CH<em>3CH</em>2C(CH<em>3)</em>2(OH)$, $CH<em>3C(OH)(C</em>2H<em>5)C</em>3H_7$
• Alcohols can be mono-, di-, tri-, or polyhydric based on the number of hydroxyl groups.
  • E.g., Ethan-1,2-diol, propan-1,2-diol, propan-1,2,3-triol, glucose (polyhydric).

Functional Groups Containing OH

• Carboxylic acid: OH attached to a carbonyl (C=O).
• Enol: OH attached to an alkene.
• Hydrates (“geminal diol”): Separate category from alcohols.

Nomenclature (IUPAC System)

• Replace ‘e’ in alkane name with ‘ol’.
• Include the carbon bearing the hydroxyl group in the parent chain.
• Indicate the position of the ‘OH’ group with a number; terminal carbon gets number 1, use the lowest number if not terminal.
• Consider the order of seniority if other functional groups are present; see later tables.

Functional Group Priorities (Highest to Lowest)

• Carboxylic acid, sulfonic acid, ester, acid halide, amide, nitrile, aldehyde, ketone, alcohol, thiol, amine (continues to Part 2)
• Groups That Are Always Prefixes And Ordered Alphabetically: bromo, chloro, fluoro, iodo, alkoxy, azido, nitro

Examples

• 1-butanol (butyl alcohol): a $1^{\circ}$-alcohol.
• 2-butanol (sec-butyl alcohol): a $2^{\circ}$-alcohol.
• 2-methyl-1-propanol (isobutyl alcohol): a $1^{\circ}$-alcohol.
• 2-methyl-2-propanol (tert-butyl alcohol): a $3^{\circ}$-alcohol.
• 2-propen-1-ol (allyl alcohol).
• 3-cyclohexen-1-ol.
• 1,2-ethanediol (ethylene glycol).
• 1,2,3-propanetriol (glycerol).

Unsaturated Alcoholic Molecules

• Alcohol has priority; use suffix “-ol”.
• Note the double bond with locant and prefix “en-“ (e.g., pent-4-en-1-ol).

Common Names System

• Used for simple alcohols.
• Name of alkyl group followed by “alcohol” or “glycol” (if two hydroxyl groups are present).
• E.g., Ethyl alcohol ($CH<em>3CH</em>2OH$), Isopropyl alcohol ($CH<em>3CHCH</em>3$/OH), Tert butyl alcohol, Ethylene glycol

Common Substituents

• Alkyl Groups: methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, tert-butyl
• Nonalkyl Groups: iodo, fluoro, chloro, bromo, nitro, amino

Physical Properties

• Lower members are liquids with distinct odor and burning taste.
• Higher members are solid and almost odorless.
• Simple alcohols have relatively low boiling points.
• Properties primarily depend on the hydroxy group and hydrogen bonding.

Hydrogen Bonding

• Oxygen is more electronegative than hydrogen, leading to a polarized O-H bond.
• Polarized O-H groups interact, forming hydrogen bonds.

Boiling Point

• Boiling points are higher than alkanes of comparable molecular weight due to hydrogen bonding.
• Increased hydroxyl groups lead to higher boiling points.

Water Solubility

• Hydroxyl group imparts greater water solubility due to hydrogen bonding with water.
• Ethanol and water are miscible in all proportions.
• Butanol starts showing interference with water solubility due to the alkyl chain.

Boiling Points of Primary, Secondary, and Tertiary Alcohols

• More “exposed” hydroxyl groups lead to higher boiling points.
• Primary alcohols generally have higher boiling points than secondary, which are higher than tertiary alcohols.

General Preparation of Alcohols

1. Hydrolyses of alkyl halides

• Alkyl halide treated with boiling aqueous alkali or a suspension of Silver oxide in boiling water
• Halide functional group replaced by a hyphomy group

2. Reduction of aldehydes, ketones, esters and acids

• Treated with appropriate reducing agents, the Corresponding alcohols are formed
• $LiAlH<em>4$ and $NaBH</em>4$ reduce Carbonyl groups without affecting the Carbon double bonds
• Aldehydes and acids give primary alcohols
• Ketones give Secondary alcohols
• Fsters give two alcohols