Alcohols

Alcohols burn readily in excess supply of oxygen to form carbon dioxide and water. Eg: C 2 H 5 OH (l) + 3O 2  (g) ---> 2CO 2  (g) + 3H 2 O (l) This reaction is exothermic that means it produces a large amount of heat energy. However, since alcohols are already partially oxidised (they contain an oxygen atom) they do not produce as much energy per unit mass as other fuels would when combusted.
 * Combustion of alcohols **

Combustion of alcohols is an oxidation process but when we talk about the oxidation of organic compounds we restrict it to the oxidation of the functional group and assume that the carbon chain remains intact. An alcohol (eg. Ethanol) can be oxidised by warming it with an aqueous solution of acidified potassium dichromate (K 2 Cr 2 O 7 ) (usually acidified using dilute sulphuric acid) which acts as an oxidising agent. As a result of the oxidation of ethanol, ethanal (an aldehyde) is produced. During this reaction the potassium dichromate (VI) changes colour from orange to green. The half equations for the reaction are: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">C 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 5 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">OH (aq) ---> CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">CHO (aq) + 2H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 2e - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Cr 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O 7 2- <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 14 H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 6e - <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> ---> 2Cr 3+ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 7H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (l) <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The overall reaction is: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">3C 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">H 5 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">OH (aq) + Cr 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O 7 2- <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 8H + <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">(aq) ---> 3CH 3 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">CHO (aq) + 2Cr 3+ <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> (aq) + 7H 2 <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">O (l)
 * Oxidation of ethanol **

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The simplified oxidation reaction can be shown as: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The reaction can go on further with ethanal being oxidized to ethanoic acid.



<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Ethanol and ethanoic acid both have hydrogen bonding between their molecules (due to H atom being bonded to the highly electronegative O atoms) thus they have a higher b.p than ethanal which dipole-dipole forces of attraction between its molecules. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Using this difference in b.p we can devise a way of getting a large yield of ethanal, before it gets oxidised to ethanoic acid from a reaction.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If we need a **high yield of the aldehyde** (ethanal) then the apparatus is arranged such that the lower boiling ethanal can be distilled from the mixture.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">If we need a high yield of the carboxylic acid (ethanoic acid) then we set up the apparatus for reflux, so that none of the aldehyde (ethanal) can escape, thus ensuring that most of it will remain and be oxidised to the ethanoic acid. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Alcohols can be primary, secondary or tertiary. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A // primary alcohol //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is one that contains __two hydrogen__ atoms and __one alkyl__ group bonded to the carbon atom that contains the alcohol group. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A // secondary alcohol //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is one that contains __one hydrogen__ atom and two __alkyl groups__ bonded to the carbon atom containing the alcohol group. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">A // tertiary alcohol //<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> is one that contains __three alkyl__ groups and __no hydrogen__ atoms bonded to the carbon atom containing the alcohol group. // (Remember that the R represents an alkyl group). // <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">All //primary alcohols// can be oxidised by warming with an aqueous solution of acidified potassium dichromate (VI) to first form the corresponding aldehyde and then further oxidized to form the carboxylic acid. (we have an example of oxidation of ethanol earlier where ethanol is oxidised to ethanal which is then oxidised to ethanoic acid).
 * OXIDATION OF PRIMARY, SECONDARY AND TERTIARY ALCOHOLS **
 * [[image:primary alcohol.png caption="Primary alcohol. "]] || [[image:Secondary alcohol.png caption="Secondary alcohol"]] || [[image:Tertiary alcohol.png caption="Tertiary alcohol"]] ||

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">//Secondary alcohols// can also be oxidised by an acidified aqueous solution of dichromate (VI) ions. However, secondary alcohols oxidise to form a **ketone**. And since the carbon atom bonded to the oxygen, in a ketone, contains no hydrogen atoms, the ketone can’t be oxidised further. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Eg:

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Tertiary alcohols can’t be oxidised without disrupting the carbon chain and they don’t react with an acidified aqueous solution of potassium dichromate (V).

By the end of this lesson you should be able to:
 * Use equtions to show the complete combustion of alcohols.
 * Use equations to show the oxidation reactions of alcohols.
 * Determine the products formed by the oxidation of primary and secondary alcohols.

__**<span style="color: #0000ff; font-family: 'Palatino Linotype','Book Antiqua',Palatino,serif;">Next up- Halogenoalkanes **__
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