Alkanols (or alcohols) are a homologous series of organic compounds with the general formula CnH2n+1OH or simply ROH. Their molecules consist of a functional group — the hydroxyl group (—OH) joined to an alkyl group (R—). They can be used to fuel cars as they are flammable. Unlike alkanes, alkenes and alkynes, alkanols are not hydrocarbons as, in addition to carbon and hydrogen atoms, they contain oxygen atoms.
The two simplest alkanols are methanol and ethanol.
Other members of the alkanol family:
|Name||Molecular Formula||Shortened structural formula|
Classification of Alkanols
In naming the alkanols, the position of the hydroxyl group is given from the number of the carbon nearest it. The isomers of alcohols are numerous but they all fall into one of three groups Primary, Secondary or Tertiary.
A primary alcohol is one that contains two hydrogen atoms and one alkyl group bonded to the carbon atom that contains the alcohol group.
A secondary alcohol is one that contains one hydrogen atom and two alkyl groups bonded to the carbon atom containing the alcohol group.
A tertiary alcohol is one that contains three alkyl groups and no hydrogen atoms bonded to the carbon atom containing the alcohol group.
Nomenclature (Naming Alkanol)
Alkanols are similar in structure to alkanes, however one hydrogen has been replaced by an OH (hydroxyl) group. However, the ane is replaced with anol. The rules is as follow:
- Determine the parent chain. The parent chain must be the longest that includes the carbon holding the OH group.
- Number according to the end closest to the -OH group regardless of where alkyl substituents are.
- The format is as follows: (location of branch)-(branch name)-(location of OH group)-(parent chain)
- Change the parent chain -e ending and replace it with an -ol.
Note: Alchohols containing more than one hydroxyl group are also called polyalcohols. Polyalcohols are named similarly to alcohols, with the exception of the prefix di-, tri-, etc before the -ol ending.
Methanol, otherwise known as wood spirit or methyl alcohol, is manufactured by the destructive distillation of wood in limited supply of air or derived from the refining of crude oil (Petroleum). It is the first member of the alkanol series with the molecular formula, CH3OH
- Methanol is a colourless liquid with a boiling point of 65° C (149° F).
- It mixes completely with water and with other short-chain alkanol.
- It is very poisonous and toxic if drunk and small quantities of the alkanol could cause blindness while larger ones, death.
- Methylated spirits consists of ethanol to which about 10 per cent methanol has been added (along with a blue dye called pyridine) to make it undrinkable.
Reactions of Methanol
The chemical reactions of methanol include:
- oxidation to methanal and then methanoic acid;
CH3OH + [O] ——–>HCHO + [O] ——-> HCOOH
- Formation of esters with alkanoic acids;
CH3OH + HCOOH—>HCOOCH3 + H2O
- Methanol is used as a solvent,
- a fuel and additive for vehicle fuels, and
- to make methanal, from which plastics such as Bakelite are made
Sometimes called ethyl alcohol, Ethanol has the molecular formula C2H5OH. Often when the layman calls a substance alcohol, ethanol is being referred to. This is because ethanol, though the second member, is the most typical of the properties of alkanols (or alcohols).
Preparation by the hydration of ethene
Ethanol could be prepared in the laboratory by the hydrolysis of ethene. This is done by passing a mixture of ethene and steam over H2SO4 acid serving as catalyst at a temperature of above 500oC and pressure of 100 atm.
C2H4 (g) + H2O(l) ——-> C2H5OH(l)
Preparation by Fermentation
Fermentation is the slow breaking down of large molecules of starch to smaller molecules of alcohol by the enzymatic action of micro-organisms (usually yeast).
Ethanol can be prepared by extracting starch granules obtained by crushing starchy foodstuffs and then leaving it for some hours after inoculating with suitable enzymes(also found in yeasts).
The following reactions will take place with the gradual conversion of the starch to ethanol:
- Starch is hydrolyzed to maltose by the action of diastase (also called invertase)
- In the second reaction, maltose is hydrolyzed, by the action of maltase to glucose
- Lastly, glucose is converted to ethanol with carbon (IV) oxide gas given off by zymase enzyme
- Ethanol is a colourless, volatile liquid with a characteristics taste and smell
- It is readily soluble in water in all proportions (due to the presence of the hydroxyl group)
- It has a boiling point of 780C
- It has no action on litmus
Alcohols burns in oxygen to produce carbon dioxide and water. Alcohols burn cleanly and easily, and does not produce soot. It becomes increasingly more difficult to burn alcohols as the molecules get bigger.
e.g. combustion of ethanol: C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (g); (ΔHc = −1371 kJ/mol)
- Dehydration- alcohol to alkene
Dehydration of alcohols is done by heating with concentrated sulfuric acid, which acts as the dehydrating agent, at 180°C. This reaction uses alcohols to produce corresponding alkenes and water as byproduct.
e.g. dehydration of ethanol:
- Oxidation – alcohol to carboxylic acid
Alcohols can be oxidized into carboxylic acids.
e.g. oxidation of ethanol:
C2H5OH + [O] → CH3COOH + H2O
Oxidation can be done by using oxidizing agents such as acidified potassium dichromate (VI), acidified potassium manganate (VII). Or if left exposed to atmospheric air, can oxidize and become ethanoic acid. An example is wine turning sour as the alcohol content, which is ethanol, is oxidized by atmospheric oxygen.
Alcohols can be reacted with carboxylic acid to form esters.
- Ethanol is an important solvent used to dissolve resins, varnishes, lacquer, soaps, perfumes, dyes, drugs and flavouring extracts
- It is used as a fuel, either by itself or mixed with petrol, in racing cars and in rockets
- Ethanol is present in many alcoholic beverages such as beers, wines and spirits (e.g. whisky, gin, brandy, etc.)
- It is sometime used as an anti-freeze in automobile radiators.
Test for Alkanols
- To detect the presence of the OH group regardless there are several simple tests that can be done, however they all have the disadvantage of interference by water.
A small piece of sodium metal can be placed in the alcohol and a steady stream of hydrogen bubbles gives a positive indication. Remember that the presence of water will also cause hydrogen to be evolved.
- To differentiate between the three classes of alcohol, it is possible to use oxidation with sodium dichromate in dilute sulphuric acid and heat.
The 1º alcohol oxidizes to an aldehyde (alkanal) and then to a carboxylic acid (alkanoic acid)
The 2º alcohol oxidizes to a ketone (alkanone) and stops there.
The 3º alcohol cannot be oxidized under these conditions.
- Another test is Lucas’ test for 1º, 2º, 3º alcohols.
It involves shaking the alcohol with ZnCl2 and dilute HCl. The 3º alcohol goes cloudy almost at once, the 2º alcohol goes cloudy after a few minutes whereas the 1º alcohol needs concentrated HCl to go cloudy. The cloudiness is due to the formation of the haloalkane that is immiscible with the aqueous solution of the zinc chloride and forms tiny droplets of an organic phase within the aqueous phase.
Lets see how much you’ve learnt, attach the following answers to the comment below
- Ethanol is prepared from glucose by the process of (a) respiration (b)fermentation (c)catalytic hydration (d) photosynthesis
- What type of reaction takes place when propene is formed from propanol? (a) condensation (b) hydrolysis (c) dehydration (d) hydration
- When an alcohol is completely combust in air, the products are (a) carbon dioxide and hydrogen (b)carbon dioxide and water (c) carbon monoxide and hydrogen (d) carbon dioxide and water
- When acidified potassium dichromate is used to oxidise an alcohol the colour changes from (a) blue to red/orange (b) black to brown (c) green to orange (d) orange to green
- Which product(s) would be expected upon dehydration of the following alcohol?
(a) 2-methylbut-2-ene only (b) 2-methylbut-1-ene and 2 methylbut-2-ene (c) 2-methylbut-1-ene only (d) 2-methylbut-1-ene and 3-methylbut-1-ene