Alcohols, Phenols and Ethers · Study notes

CHEAlcohols: Preparation & Reactions

Classification, Nomenclature & Basic Concepts

The foundation of the chapter: classifying alcohols as $1^\circ$ , $2^\circ$ or $3^\circ$ , writing correct IUPAC names, enumerating structural isomers, and explaining physical properties from hydrogen bonding. These short, accuracy-driven questions appear in almost every paper.

General formula of saturated monohydric alcohol

C_nH_{2n+1}OH \quad (C_nH_{2n+2}O)

Boiling-point trend (same mass)

\text{alcohol} > \text{ether} \approx \text{alkane}

only alcohols form intermolecular H-bonds

Classification by the carbon bearing $\text{-}OH$ : $1^\circ$ (carbon attached to one C), $2^\circ$ (two C), $3^\circ$ (three C); e.g. $(CH_3)_3COH$ is $3^\circ$ , $CH_3CH(OH)CH_3$ is $2^\circ$ .
IUPAC: replace the $\text{-}e$ of the alkane with $\text{-}ol$ and number the chain to give the $\text{-}OH$ the lowest locant; e.g. $CH_3CH(OH)CH_2CH_3$ is butan-2-ol.
$C_4H_9OH$ has 4 monohydric isomers: butan-1-ol ( $1^\circ$ ), butan-2-ol ( $2^\circ$ ), 2-methylpropan-1-ol ( $1^\circ$ ) and 2-methylpropan-2-ol ( $3^\circ$ ).
Ethers are named as alkoxyalkanes: $C_6H_5OCH_2CH_3$ is ethoxybenzene (phenetole); $CH_3OCH_3$ is methoxymethane.
Alcohols have far higher boiling points than ethers/alkanes of similar mass because of intermolecular hydrogen bonding via $\text{-}OH$ .
Ethanol (b.p. $78^\circ\text{C}$ ) boils much higher than its isomer dimethyl ether (b.p. $-24^\circ\text{C}$ ) because only the alcohol can H-bond between molecules.
Boiling point of an alcohol rises and water-solubility falls as the hydrocarbon chain lengthens, since the hydrophobic part outweighs the H-bonding $\text{-}OH$ .
Alcohols are classified by number of $\text{-}OH$ groups too: monohydric (one $\text{-}OH$ ), dihydric (e.g. ethylene glycol), trihydric (glycerol); also as alkyl, allylic, benzylic or aromatic (phenols) by the carbon type.
In the lowest-locant rule the principal characteristic group ( $\text{-}OH$ ) takes priority over chain length when choosing the parent chain and over alkyl substituents when numbering.
When the $\text{-}OH$ is not the senior group it is named as a 'hydroxy' substituent (e.g. 2-hydroxybenzoic acid), and the longest chain bearing the $\text{-}OH$ is chosen as parent for an alcohol.
Phenol ( $C_6H_5OH$ ) is an aromatic hydroxy compound with the $\text{-}OH$ bonded directly to the ring; it is a distinct class from aliphatic alcohols and from benzyl alcohol ( $C_6H_5CH_2OH$ , a primary alcohol).
Lower alcohols (methanol to propanol) are fully miscible with water; solubility falls sharply from butanol onward as the alkyl chain dominates.
Ethers, $R\text{-}O\text{-}R'$ , are symmetrical (both groups identical, e.g. diethyl ether) or unsymmetrical/mixed (different groups, e.g. methoxybenzene/anisole); common names simply state both alkyl/aryl groups plus 'ether' (dimethyl ether, diethyl ether).
IUPAC of ethers: name the smaller $\text{-}OR$ group as the 'alkoxy' prefix on the longer parent chain, e.g. $CH_3\text{-}O\text{-}C_2H_5$ is methoxyethane and $C_6H_5\text{-}O\text{-}CH_3$ is methoxybenzene (anisole).
Quick property comparison across the chapter: reaction with NaOH — alcohols no, phenols yes, ethers no; reaction with Na — alcohols slow yes, phenols faster yes, ethers no; $FeCl_3$ test — only phenol gives a colour (violet); $NaHCO_3$ — none of the three reacts; bromine water — only phenol gives a white precipitate; hydrogen bonding (and hence relatively high b.p.) — alcohols and phenols yes, ethers no (no $O\text{-}H$ ).
Odour/appearance: lower alcohols have a characteristic pleasant smell and are volatile liquids; phenol is a colourless crystalline solid (m.p. $\approx 43^\circ C$ ) that turns pink on standing in air due to slow oxidation; diethyl ether is a highly volatile, flammable liquid (b.p. $35^\circ C$ ).

Where the marks go

Mis-numbering the chain — the $\text{-}OH$ must get the lowest locant, so $CH_3CH(OH)CH_2CH_3$ is butan-2-ol, not butan-3-ol.
Calling benzyl alcohol ( $C_6H_5CH_2OH$ ) a phenol — it is a primary alcohol because the $\text{-}OH$ is on a $CH_2$ , not on the ring.
Missing isomers of $C_4H_9OH$ — there are exactly four monohydric alcohols (not three); forgetting 2-methylpropan-1-ol is common.
Attributing ethanol's high boiling point to higher molar mass — it is identical in mass to dimethyl ether; the cause is hydrogen bonding.
Confusing degree of substitution (carbon type) with the number of $\text{-}OH$ groups — ' $2^\circ$ ' refers to the carbon, 'dihydric' to two $\text{-}OH$ groups.

How the board asks it

Structure / naminglowest-locant rule and alkoxyalkane naming
Give the IUPAC name of $CH_3CH(OH)CH_2CH_3$ and of $C_6H_5OCH_2CH_3$ .
Structure / namingstructural isomers of $C_4H_9OH$
Draw the structures of all the monohydric alcohols having the molecular formula $C_4H_9OH$ and label each as $1^\circ$ , $2^\circ$ or $3^\circ$ .
Give reasonsintermolecular hydrogen bonding
Account for the fact that ethanol ( $b.p.\ 78^\circ C$ ) boils at a much higher temperature than its isomer dimethyl ether ( $b.p.\ -24^\circ C$ ), although both have the same molar mass.
Distinguishdegree of substitution vs number of $\text{-}OH$ groups
Distinguish between a secondary alcohol and a dihydric alcohol, giving one example of each.
Define / stateclassification by the carbon bearing $\text{-}OH$
Define a tertiary alcohol and state why benzyl alcohol ( $C_6H_5CH_2OH$ ) is classified as a primary alcohol and not as a phenol.

Practise this topic

NextPreparation of Alcohols

Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.