Alcohols

media type="youtube" key="nQ7QSV4JRSs" height="315" width="560" Named by changing “-e” at end of the alkane to “-ol”. The chain is numbered from the end giving the OH the lowest number, regardless of the position of any other substituents. Alcohols are classed as primary, secondary and tertiary in the same way as the haloalkanes ie. depending on the number of C atoms that are attached to the C carrying the OH group.
 * Alcohols (alkanols) - ROH, where functional group is OH. **

 Primary RCH2OH (and methanol), secondary R2CHOH, tertiary R3COH.

Small alcohol molecules are polar and the presence of the OH group means they are able to undergo intermolecular hydrogen bonding. The large difference in electronegativity between the O and H atoms means the O-H bond is very polar and the slightly positive charge on this H atom is attracted to the non-bonding electron pairs of the oxygen on another molecule. This means small alcohol molecules are highly soluble in water. However as the length of the non-polar hydrocarbon chain increases this solubility in water decreases.

Aqueous solutions are **neutral.** The presence of the OH group in this molecule is **NOT** the same as the OH- in sodium hydroxide, NaOH (an ionic compound). **Alcohols are formed** by a) **(nucleophilic) substitution **of OH // - // for X // - // on haloalkanes b) **addition **of H2O to alkenes. c) **reduction ** of aldehydes (see page 49) and carboxylic acids (forming primary alcohols) and ketones (forming secondary alcohols). The reagent used is NaBH4 or LiAlH4.


 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Reactions of alcohols **<span style="font-family: Arial,sans-serif; font-size: 11pt;"> are:
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">(a) elimination (or dehydration) - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">forming an alkene and water

CH3CH(OH)CH3 conc H2SO4/heat ® CH3CH=CH2 + H2O

<span style="font-family: Arial,sans-serif; font-size: 11pt;"> propan-2-ol propene

<span style="font-family: Arial,sans-serif; font-size: 11pt;">This substitution is faster for tertiary alcohols than for secondary, and slowest for primary alcohols. It is the basis of the **Lucas test** for distinguishing between small molecules of primary, secondary and tertiary alcohols. The reagent used is conc HCl and anhydrous ZnCl2 (called Lucas Reagent), and it is shaken with alcohol in a test tube at room temperature. The haloalkane formed is nonpolar and insoluble in the aqueous solution so forms a cloudy emulsion that separates out as two layers.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;"> (b) Substitution - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">of the OH - <span style="font-family: Arial,sans-serif; font-size: 11pt;"> by a Cl // - //<span style="font-family: Arial,sans-serif; font-size: 11pt;"> to form a chloroalkane.

<span style="font-family: Arial,sans-serif; font-size: 11pt;"> (i) For tertiary alcohols - solution rapidly goes cloudy and two layers form. <span style="font-family: Arial,sans-serif; font-size: 11pt;">(ii) For secondary alcohols - solution slowly goes cloudy as the chloroalkane slowly forms and separates. <span style="font-family: Arial,sans-serif; font-size: 11pt;">(iii) For primary alcohols - reaction is so slow a single layer containing unreacted alcohol remains.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The rate of substitution of primary alcohols is increased by heating the reaction mixture under **reflux.** Reflux is a system of heating the solution with a condenser attached to the reaction vessel so that any organic substance which evaporates will be condensed and returned to the container. This way the reaction can be heated for a period of time without the organic substance (reactant, product or solvent) evaporating away.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Substitution of alcohols can also be carried out using **PCl5, PCl3 and SOCl2.**

<span style="font-family: Arial,sans-serif; font-size: 11pt;">The type of product formed depends on whether the alcohol used in the oxidation reaction is primary or secondary.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">(c) Oxidation - **<span style="font-family: Arial,sans-serif; font-size: 11pt;">using acidified KMnO4 or acidified K2Cr2O7

<span style="font-family: Arial,sans-serif; font-size: 11pt;"> <span style="font-family: Arial,sans-serif; font-size: 10pt;">Cr2O72 - <span style="font-family: Arial,sans-serif; font-size: 10pt;">/H+ Cr2O72 - <span style="font-family: Arial,sans-serif; font-size: 10pt;">/H+ <span style="font-family: Arial,sans-serif; font-size: 11pt;"> CH3CH2OH ® <span style="font-family: Arial,sans-serif; font-size: 14px;">CH <span style="font-family: Arial,sans-serif;">3 <span style="font-family: Arial,sans-serif; font-size: 14px;">CHOH ® <span style="font-family: Arial,sans-serif; font-size: 14px;">CH <span style="font-family: Arial,sans-serif;">3 <span style="font-family: Arial,sans-serif; font-size: 14px;">COOH
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">(i) ** **<span style="font-family: Arial,sans-serif; font-size: 11pt;">Primary alcohols **<span style="font-family: Arial,sans-serif; font-size: 11pt;">(RCH2OH) are oxidised to form **aldehydes** (RCHO), which are then easily oxidised further to form **carboxylic acids** (RCO2H)**.**

<span style="font-family: Arial,sans-serif; font-size: 11pt;">ethanol ethanal ethanoic acid

<span style="font-family: Arial,sans-serif; font-size: 11pt;">When using acidified dichromate in this redox reaction, the Cr2O72 - <span style="font-family: Arial,sans-serif; font-size: 11pt;"> is reduced to Cr3+, and the colour changes from orange to green. This colour change was the basis for the chemical reaction in the old “blow in the bag” breathalyser test.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">MnO4 - <span style="font-family: Arial,sans-serif; font-size: 10pt;">/H+ ®
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">(ii) Secondary alcohols **<span style="font-family: Arial,sans-serif; font-size: 11pt;">(R2CHOH) are oxidised to **ketones** (R2CO)

<span style="font-family: Arial,sans-serif; font-size: 11pt;">propan-2-ol propanone (also called acetone) <span style="font-family: Arial,sans-serif; font-size: 11pt;">When using acidified permanganate in this reaction, the purple MnO4- ion is reduced to the colourless Mn2+ ion.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">This means these oxidation reactions can be used to distinguish tertiary alcohols from primary and secondary alcohols. If an alcohol is heated with either acidified potassium dichromate or acidified potassium permanganate, and a colour change is observed in the oxidising agent (orange to green, or purple to colourless) then the alcohol must be either primary or secondary.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">(iii) Tertiary alcohols - DO NOT REACT **<span style="font-family: Arial,sans-serif; font-size: 11pt;">with oxidising agents.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">If the aldehyde is the desired product in the oxidation of a primary alcohol, the oxidising agent must be added slowly and the aldehyde distilled off as it forms. This works because the aldehyde has a lower boiling point than the alcohol and carboxylic acid as both the latter two compounds are not only polar but are capable of intermolecular hydrogen bonding. The aldehyde is a polar molecule but as it does **not** have an H atom attached to an O atom, it therefore does not have hydrogen bonding between its molecules.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">If the carboxylic acid is the preferred product then the oxidising agent is refluxed with the alcohol to ensure complete reaction.