KETONES William, Nanda, Hafiz & Mio

Introduction  Alcohols are found in two forms:  Primary: -OH group attached at the end of the chain.  Secondary: -OH group attached to a carbon atom not located at the end of the chain.

Formation of ketones  When a secondary alcohol is heated with the oxidising agent potassium dichromate (VI), it oxidises into a ketone. CH 3 CH(OH)CH 3 + [O]  CH 3 COCH 3 + H 2 O propan-2-ol potassium dichromate propanone water  Ketones cannot be oxidised any further.

So what is a ketone?  A ketone is a carbonyl compound where one of the carbon atoms along the chain (not at the ends) is double-bonded to an oxygen atom.

Naming convention Carbon atomsName 3propanone 4butanone 5pentanone 6hexanone 7heptanone 8octanone 9nonanone 10decanone

Applications  Polymer precursor  Pharmaceuticals  Solvents  e.g.:  acetone  methylethyl ketone  cyclohexanone

Reduction of ketones  When a ketone is warmed with the reducing agent NaBH 4 (sodium tetrahydridoborate), it reverts into a secondary alcohol. CH 3 COCH 3 + 2[H]  CH 3 CH(OH)CH 3 propanone reducing agent propan-2-ol  We can think of it as a nucleophilic addition, where H - ions from the reducing agent act as the nucleophile.

Testing for ketones  Testing for ketones and aldehydes in a solution may be done with 2,4-dinitrophenylhydrazine (2,4- DNPH).  If the solution contains either, an orange precipitate will be formed.  This is an example of condensation reaction between 2,4-DNPH and the ketone.

Testing with Tollens’ reagent  Both aldehydes and ketones form an orange precipitate with 2,4-DNPH.  To differentiate, we may react the solution with Tollens’ reagent – a solution of silver nitrate in excess ammonia.  When it is warmed with an aldehyde, a silver precipitate builds up.  Ketones, however, will remain colourless.

Fehling’s solution  is an alkaline solution containing copper (II) ions.  It oxidises and turns from clear blue to an opaque red/orange when heated with an aldehyde.  Ketones cannot be oxidised any further, so the solution remains blue.

Nucleophilic addition with HCN  The carbonyl group C=O is highly polarised, as the oxygen atom has more electron pairs (and therefore more negative).  The carbon atom, being more positive, is open to addition reactions* from nucleophilic ions (i.e. negative ones). *”nucleophilic attack” is a proper term and sounds cooler. O O C C

Mechanism  Addition, as you’ll recall, involves the breakdown of a double bond into a single bond.  An example of a nucleophile is the CN - ion in HCN, hydrogen cyanide. O O C C CN O-O- O-O- C C H H H+H+ H+H+

O O C C H H CN - - -

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