Aldehydes and Ketones Functional group formula? C=O F.G. name? The carbonyl group
Naming Aldehydes – how can you spot one? (C=O on the end of the chain i.e. C N o 1) end in ……al Ketones (C=O has C’s on both sides) end in ….one Position of C=O given as a number What is the largest chain where a number is not required? Butanone – 4 C’s Try 1,1,1-trifluoropentan-2-one 1-hydroxyhex-2-ene-4-one Watch out for benzaldehyde and phenylethanone
Reduction of Aldehydes and Ketones to Alcohols Reagents NaBH 4 (sodium borohydride) – a source of hydride (H - ions) then dilute sulphuric acid Conditions Room temperature Another source of H - ? LiAlH 4
Equation CH 3 CHO + 2[H] CH 3 CH 2 OH Draw a diagram showing how the reactants relate to the products Repeat the equation and diagram for propanone See questions page 17 of notes
How does it happen? The mechanism shows this What factors do you consider when trying to work out a mechanism? Polarity of bonds Areas of positive or negative charge Characteristics of particles present in the reactants – NaBH 4 is a source of H -.
Reduction mechanism – addition of hydrogen
How does it happen? Conditions – are free radicals likely to be formed? KCN contains which particles? K + and CN - (dot/cross diagram?) What do acids do? Try to create a mechanism (consider ethanal). Check names of products and note that in all cases a 2-hydroxy compound is formed See q’s page 18
Oxidation of Aldehydes Remember that ketones are not readily oxidised Extension note – ketones can be oxidised, but only under relatively severe conditions, where the carbon chain is broken. They are flammable ( burning= oxidation)
Oxidation of Aldehydes Reagents K 2 Cr 2 O 7 / sulphuric acid K 2 Cr 2 O 7 = potassium dichromate Conditions H&R When do you heat and distil?
Oxidation of Aldehydes Equation RCHO + [O] RCOOH Draw a diagram to illustrate this (not a mechanism) See Q,s page 20
Tests for Aldehydes/ketones 2,4-dinitrophenylhydrazine Test for? Carbonyl group C=O Observation? Orange precipitate