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Booklet to help with unit 28 P4

Booklet to help with unit 28 P4. Compounds contain C=O (carbonyl group). The difference between aldehydes and ketones.

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Booklet to help with unit 28 P4

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  1. Booklet to help with unit 28 P4 Compounds contain C=O (carbonyl group)

  2. The difference between aldehydes and ketones The difference between an aldehyde and a ketone is the presence of a hydrogen atom attached to the carbon-oxygen double bond in the aldehyde. Ketones don't have that hydrogen.

  3. Aldehydes • The presence of the hydrogen atom attached to the carbon from the C=O bond makes aldehydes very easy to oxidise. Or, put another way, they are strong reducing agents

  4. What is formed when aldehydes are oxidised? It depends on whether the reaction is done under acidic or alkaline conditions. Under acidic conditions, the aldehyde is oxidised to a carboxylic acid. Under alkaline conditions, this couldn't form because it would react with the alkali. A salt is formed instead.

  5. Examples of reagents that oxidise aldehydes under acidic conditions • Potassium dichromate (VI) A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde. The Orange solution should then turn green as aldehyde is oxidised.

  6. Examples of reagents that oxidise aldehydes under alkaline conditions • Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+. This is made from silver(I) nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate. To carry out the test, you add a few drops of the aldehyde and warm gently in a hot water bath for a few minutes.

  7. Examples of reagents that oxidise aldehydes under alkaline conditions • Fehling's solution and Benedict's solution are variants of essentially the same thing. Both contain complexed copper(II) ions in an alkaline solution. Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minutes The blue solution produces a dark red precipitate of copper(I) oxide as the aldehyde becomes oxidised

  8. Ketones • Ketones are less reactive than aldehydes because there are two hydrocarbon chains which can block the C=O bond. This makes it harder for ketones to reactive than aldehydes. • Ketones do not undergo oxidation

  9. Chemicals that can reduce aldehydes and ketones The main chemicals that can reduce aldehydes and ketones are the compounds LiAlH4 and NaBH4

  10. Reduction of aldehyde • reduction of an aldehyde leads to a primary alcohol. • You get the same product if you use LiAlH4or NaBH4

  11. Reduction of ketone • Reduction of a ketone leads to a secondary alcohol

  12. Nucleophilic addition reactions of aldehydes and ketones • The C=O group can react with nucleophiles (molecules that are rich in electrons and are attracted to the slightly positively charged carbon atom from the C=O bond • An example of a nucleophile is HCN. See next slide for details on this reaction.

  13. Addition-elimination reactions of aldehydes and ketones • 2,4-dinitrophenylhydrazine (Brady's reagent) is used to test the presence of the C=O bond

  14. Addition-elimination reaction of aldehyde The Oxygen from the C=O bond is replaced by the Nitrogen side of the 2,4-dinitrophenylhydrazine molecule

  15. Addition-elimination reaction of ketone The Oxygen from the C=O bond is replaced by the Nitrogen side of the 2,4-dinitrophenylhydrazine molecule

  16. Other addition-elimination reactions • Reaction of Hydrazine with C=O bonds The above reaction is hydrazine with ketone. The reaction with aldehydes is the same but you would replace one of the R groups drawn on the ketone molecule with an H (for hydrogen atom) More examples of reactions with C=O bond are on the next slide

  17. Carboxylic acid • How carboxylic acid behaves as an acid

  18. Carboxylic acids react with alcohols to form esters Eg: when carboxylic acids react with ethanol

  19. Acetyl chlorides • Reaction with alcohols like ethanol

  20. Reactions of acetyl chlorides • Reaction with amine X can be a hydrocarbon group. For example in ethylamine, X would be C2H5 an ethyl group.

  21. Reactions of acetyl chloride • Reaction with ammonia

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