Alcohols
Read our general notes on Risk Assessment
Learning about alcohols
This tutorial will help you as you study alcohols.
Functional group and types of alcohols
Alcohols contain an OH functional group which is not directly attached to an arene (aromatic) ring. They have the general formula ROH.
There are three types of alcohols depending on how many hydrogen atoms are also attached to the carbon to which the OH group is attached.
Primary alcohols have two hydrogens attached to this carbon i.e. -CH2-OH. For instance, CH3CH2CH2OH is the primary alcohol propanol.
Secondary alcohols have one hydrogen attached to this carbon, that is >CH-OH. For example, (CH3) 2CHOH is the secondary alcohol propan-2-ol.
Tertiary alcohols have no hydrogens attached to this carbon. That is, =C-OH, e.g. (CH3) 3COH is the tertiary alcohol 2-methyl propan-2-ol
Alcohols can contain more than one OH group. These are known as polyhydric alcohols.
Alcohols and ethers are structurally isomeric, e.g. ethanol CH3CH2OH and methoxymethane CH3OCH3
Relevant FAQ on React:
Write the structural formulae for the linear alcohols that are derived from butane.
Nomenclature
Alcohols are usually identified by adding the suffix ol to the name of the molecule but it is sometimes preferably to use the prefix hydroxy- to the name.
When naming an alcohol molecule, start by picking the longest carbon chain to which the OH group is attached. The number of carbon atoms in this chain gives the basic name of the molecule ( e.g. a four-carbon chain would be butan-). Then number this chain, starting from the end which gives the lowest possible number to the carbon atom to which the OH group is attached. Add the number of this carbon, plus ol, to the basic name. For instance, CH3CH2CH(OH)CH3 is butan-2-ol not butan-3-ol. Finally, add the name(s) of any side-chains to the main chain indicating the carbon atom to which they are attached, e.g. CH3CH(CH3)CH(OH)CH3 is 3-methyl butan-2-ol.
This question may help here:
I have noticed in the textbook that diols and triols seem to keep the E in their alkane stem:
e.g. ethaNE-1,2-diol and propaNE-1,2,3-triol.
whereas "normal" alcohols are just propaN-2-ol.
Should diols and triols keep the NE in the alkane stem, and single -ols lose the E?
Physical properties
The OH group enables alcohol molecules to undergo hydrogen bonding, both with themselves and with other molecules which can hydrogen bond. As a result, alcohols have higher melting and boiling points than other molecules of similar rmm which cannot hydrogen bond. Examples are the ethers with which they are isomeric.
For the same reason, alcohols are also soluble in polar solvents such as water. However, the extent of this solubility gets less as the size of the alkyl R- group gets larger.
Relevant FAQs
When a solution contains (by mass) 10% ethanol, 25% butanol, 40% propanol and 25% methanol. The solvent is:
a. ethanol
b. methanol
c. butanol
d. propanol
e. none of these
Preparation of alcohols
There are two principal ways of preparing alcohols in the laboratory:
Addition of water to alkenes (an electrophilic addition reaction), e.g.
C3H6 + H2O -> C3H7OH
Hydrolysis of halogenoalkanes (a nucleophilic substitution reaction) e.g.
C4H9X + H2O -> C4H9OH + HX
Ethanol has been obtained by fermentation for thousands of years. This reaction involves anaerobic respiration by the yeast organism of sugar in plant materials. Examples are grape juice when making wine, or malted barley when making beer. This produces ethanol plus CO2.
Other alcohols can also be made from various plant materials. See for instance the link to 'Chemicals from maize' in this web guide and follow the navigation: Products>Sorbitol.
Propane-1,2,3-triol (glycerol or glycerine) is a trihydric alcohol. This is formed as a by-product of the manufacture of soap from fats and vegetable oils.
Look at these questions:
I have an exam question which says:
when 3 bromo-2-methylpentane (CH3)2CHCHBrCH2CH3 reacts with aqueous potassium hydroxide, an alcohol is formed
Name the type of reaction taking place and the role of the reagent. I have stated tha the reaction is an elimination reaction but dont know what the role of the reagent. I suspect K is the reagent ... Help please!
I have read in "Chemistry In Context, second edition, page 542, that to produce an alcohol from an amine, use HCl and sodium nitrite (NaNO2) above 10 degrees. However, I can't seem to find any information on this. Could you please clarify?
Oxidation of alcohols
Primary alcohols can be oxidised first to aldehydes and then to carboxylic acids. Sodium dichromate(VI) solution in concentrated sulphuric acid is the oxidising reagent normally used. Gentle warming produces the aldehyde. Further heating, usually under reflux, produces the carboxylic acid. The reaction mixture changes from orange to green as the reaction proceeds. Potassium manganate(VII) solution acidified with sulphuric acid can also be used. In this case the reaction mixture goes colourless (or a brown precipitate may be formed) as the reaction proceeds. Here are two examples.
C3H7OH + [O] -> C2H5CHO + H2O
C2H5CHO + [O] -> C2H5COOH
Secondary alcohols can only be oxidised to ketones using either of these oxidising reagents, like this:
(CH3)2CHOH + [O] -> (CH3)2CO + H2O
Tertiary alcohols are not oxidised by these oxidising reagents.
Oxidation of ethanol by acidified sodium dichromate was the reaction used in the original breathalysers.
Here are some useful questions.
I really do not understand what it is I need to know for the dehydration and oxydation of alcohols. I'm really confused. I don't understand what's happening.
In the partial oxidation of an alcohol (e.g. butan-1-ol -> butanal), which is the oxidising agent and which is the other reagent? I know that sodium dichromate is used (oxidising agent?) but I am not sure what is meant by the term reagent, is it the sulphuric acid?
Silver nitrate acts as a catalyst in the reaction between ethanol and dichromate ions. If the volume of silver nitrate solution changed would there be any effect on the rate of the reaction?
What do chemists mean by the term 'reflux' when descibing the conditions for an organic reaction such as the complete oxidation of a primary alcohol?
What is the product of the reaction between a primary alcohol or aldehyde and MnO4/OH ?
Dehydration of alcohols
Passing an alcohol in the vapour state over a catalyst of heated solid aluminium oxide (or other similar materials) causes a dehydration reaction to occur to form an alkene, e.g.
(CH3)2CHOH -> CH3CH=CH2 + H2O
Relevant FAQs
I really do not understand what it is I need to know for the dehydration and oxydation of alcohols. I'm really confused. I don't understand what's happening.
During the experiment in Topic 2, converting cyclohexanol into cyclohexene, why do you add sodium chloride solution to the distillate that is formed by heating the reaction mixture? Is it to separate the layers? Afterwards, anhydrous calcium chloride is added. Is that to remove the excess water and impurities? Then you re-distill the distillate. Why?
What are the conditions for converting cyclohexanol to cyclohexene?
When an alcohol is dehydrated to produce an alkene, what are the reasons for the actual yield of the alkene being so much lower than the theoretical yield?
How is dehydration used in chemistry?
How would you go about synthesising an alkane from an alcohol? All I can think of doing in dehydrating the alcohol to an alkene, but I cant think of what to do next. (Nuffield Chemistry Students' book, Study task p 482)
Other reactions of alcohols
Like water, alcohols react with the alkali metals to produce hydrogen although the reaction rate is slower. Here is an example.
2CH3CH2CH2OH + 2Na -> 2CH3CH2CH2ONa + H2
The other product in this example is sodium propoxide. This is ionic, containing the Na+ and the CH3CH2CH2- ions.
Alcohols react with carboxylic acids to give esters (a nucleophilic substitution reaction). For instance
CH3COOH + CH3CH2OH <-> CH3COOCH2CH3 + H2O
The product in this case is ethyl ethanoate. Note that the reaction is reversible.
Alcohols react with PCl5 to give the corresponding chloroalkanes, e.g.
CH3CH2CH2OH + PCl5 -> CH3CH2CH2Cl + POCl3 + HCl
Like most organic compounds, alcohols are flammable. They produce CO2 and H2O on combustion. Oxygen in the air will also oxidise ethanol to ethanoic acid at room temperature. This is the process by which vinegars are formed.
Relevant FAQs
How do I write the structural formula for sodium propoxide?
Why can sodium metal can be used for drying diethyl ether and benzene but not ethanol?
What are the steps in the nucleophilic substitution reaction with a carboxylic acid and an alcohol (diagrams if possible)?
When an alcohol and a carboxylic acid form an ester, my teacher says you lose the OH group from the carboxylic acid and the hydrogen from the OH group from the alcohol. The text book says that the reaction attacks the C to O double bond in the carboxylic acid. Which one is it please?
What is the connection between wine and vinegar?
Tests for alcohols
There is no specific test for alcohols, but they are sometimes identified by the fact that they give hydrogen when reacted with sodium.
When PCl5 is added to an alcohol, steamy fumes of HCl are evolved. An example is when PCl5 is added to other compounds containing an OH group like water or carboxylic acids.
Ethanol and secondary alcohols with a methyl group attached to the same carbon as the OH group will also give a positive iodoform test.
Whether an alcohol is primary, secondary or tertiary can be determined by
whether or not it is oxidised by acidified sodium dichromate solution and, if it is,
whether the product is an alkanal (aldehyde) or alkanone (ketone).
These questions may help.
For a practical I have to do I am given a organic compound and I have to find whether it's an alcohol, alkene or alkane.
What does phosphorus(V) chloride test for? What is the chemical test for an alcohol?
Does PCl5 react with both a carboxylic acid and an alcohol? How would I know which is which if they do react? What is a good test for alcohol and a carboxylic acid?
How can you test between an ester and a tertiary alcohol?
Is there a way using iodoform test that will help us distinguish between the tertiary and methyl secondary alcohol? I think tertiary alcohol will respond negatively the iodoform test.
I'm confused about a set of tests in organic chemistry. I came across a question about primary, secondary and tertiary alcohols and how to differentiate between them. I know that when you oxidise them with potassium dichromate the tertiary alcohol does not react so it eliminates that one but then primary and secondary both turn green and I know that primary results in an aldehyde and secondary results in a ketone, but how do you tell between a ketone and an aldehyde?
How do you distinguish between the different alcohols?
For our A2 coursework, we have to identify an organic compound. I have sorted out most of the tests but I am unsure how to distinguish between a primary and tertiary alcohol. Is there a 'wet test' which would enable me to do this?
If an alcohol was warmed with Benedict's solution, would a precipitate be formed?
Investigations
You may have decided to undertake an investigation related to alcohols (or substances containing alcohols). The following questions may be helpful.
I am currently analysing my results for a practical assignment to find out how the molecular structure of an alcohol affects the enthalpy change of combustion. However, I have encountered a problem; I cannot explain the very slight difference in theoretical and practical enthalpy changes between alcohols with the same chain length but different position of OH or hydroxyl groups. e.g. propan-1-ol and propan-2-ol. Is it something to do with electronegativity? Please could you help me out?
I am determining the alcohol content of wine. I have completed the distillation procedure and am now completing a titration in which you oxidise the wine sample with an excess of potassium dichromate(VI) followed by a back titration with iron(II) irons using ammonium iron(II) sulphate. An article by Derek Denby says you need to standardise the ammonium iron(II) sulphate by titrating it directly with the potassium dichromate(VI) - what does this mean and how to you determine an end point for this titration?
I am doing an investigation on the analysis of wine. I have looked at an article from Chemistry Review (Jan 98) by Derek Denby and in it he talks about how to find out what the alcohol content of wine is. One method is to oxidise ethanol using an excess of potassium dichromate and then doing a back titration using ammonium iron(II) sulphate. I really need an indicator for this experiment or to know when the end point is for the indicator sodium diphenylamine sulphonate. I haven't had much luck with the internet so I thought you could help!
I am investigating wines for my individual investigation, but am a bit unsure of the theory behind part of it. When analysing the ethanol content of the wine using the reaction of ethanol with dichromate ions, and then a back titration with iron(II) ions, I was wondering what the complex ion formed is. The indicator I will use is sodium diphenylamine sulphonate and phosphoric acid is used to provide hydrogen ions. The indicator is needed because the chromium ions reduced form a complex with the water I presume and the phosphate ions from the acid in a ligand exchange reaction. I was wondering if you knew precisely what this ion is likely to be, or whether it actually matters.
End of alcohols tutorial
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updated: 12 January 2007
