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A2 Chemistry. Aldehydes and Ketones. Press “Esc” key to escape from any presentation and return to previous menu. H. H. H. H. C. C. C. C. O. O. O. O. ALDEHYDES and KETONES. Collectively known as CARBONYL compounds. Aldehydes. General functional group is. Abbreviated –CHO.

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  1. A2 Chemistry Aldehydes and Ketones Press “Esc” key to escape from any presentation and return to previous menu

  2. H H H H C C C C O O O O ALDEHYDES and KETONES Collectively known as CARBONYL compounds Aldehydes General functional group is Abbreviated –CHO CH3CH2CHO Propanal Named using (name of corresponding alkane….al) CH3CHO Ethanal H H H H H H H C C C C C C H H CH3CH2CH2CHO Butanal H H H H H H

  3. C C C C C C C C O O O O Ketones General functional group is C C C C Abbreviated C-CO-C H H H H H H H H H H C C CH3COCH3 Propanone H H H H H H Named using (name of corresponding alkane….one) H H H H CH3COCH2CH2CH3 Pentan-2-one C H H H H CH3COCH2CH3 Butanone

  4. PHYSICAL PROPERTIES 1. Boiling points HIGHER than similar Mr hydrocarbons because stronger dipole-dipole forces are present between aldehyde and ketone molecules. 2. Boiling points LOWER than similar Mr alcohols / acids because stronger hydrogen bonds are present between alcohol and acid molecules. 3. Provided Mr not too large, aldehydes and ketones are relatively soluble in water because the O atoms can hydrogen bond to H in water.

  5. PREPARING ALDEHYDES = first oxidation products of PRIMARY alcohols H H H R-C-O-H + [O]  R-C + H2O R-C + [O]  R-C O O H [O] provided by acidified dichromate(VI), Cr2O72-. Reaction mixture heated and IMMEDIATELY distilled to avoid further oxidation to the carboxylic acid. OH O eg Naming the organic products, write equations for oxidations of : (a) ethanol (b) 2-methylpropan-1-ol

  6. eg Naming the organic products, write equations for oxidations of : (a) ethanol (b) 2-methylpropan-1-ol (a) (b) H H H H H H CH3-C C-O-H + [O]  CH3-C C + H2O CH3-C C + [O]  CH3-C C CH3-C + [O]  CH3-C CH3-C-O-H + [O]  CH3-C + H2O Ethanal O O O O H H Ethanoic acid OH CH3 H H3C H CH3 H 2-Methylpropanal H OH O 2-Methylpropanoic acid O CH3

  7. PREPARING KETONES = oxidation products of SECONDARY alcohols H R1-C-O-H + [O]  R1-C-R2 + H2O R2 O [O] provided by acidified dichromate(VI), Cr2O72-. Reaction mixture heated under reflux since further oxidation does not occur.  ketones are not reductants since not oxidisable. Used in chemical tests to distinguish aldehydes from ketones, since aldehydes are reductants. eg Naming products, write equations for oxidations of : (a) propan-2-ol (b) 3-methylbutan-2-ol

  8. eg Naming products, write equations for the oxidations of : (a) propan-2-ol (b) 3-methylbutan-2-ol (a) (b) H H CH3-C-O-H + [O]  CH3-C-CH3 + H2O CH3-C-O-H + [O]  CH3-C-CH(CH3)CH3 + H2O CH3 CH(CH3)CH3 O O Propanone 3-Methylbutan-2-one

  9. DETECTING & IDENTIFYING ALDEHYDES & KETONES Unknown organic compound NO2 NO2 + 2,4-dintrophenylhydrazine dissolved in conc. H2SO4 (Brady’s reagent) R1 + O=C Orange ppt. if compound is an aldehyde or ketone R2 NO2 NO2 Ppt. is the HYDRAZONE derivative of the aldehyde or ketone NH-NH2 NH-N=C R1 R2 Isolate the hydrazone derivative, purify by recrystallisation from ethanol and measure its melting point + H2O Identify the particular aldehyde or ketone by matching melting point to databook values

  10. DISTINGUISHING ALDEHYDES FROM KETONES Heat with ORANGE acidified Cr2O72- Heat with BLUE alkaline Cu2+ ( + NaOH) Heat with COLOURLESS alkaline Ag+ ( + NH3) For Aldehyde : ORANGE  GREEN SOLN [Cr3+(aq)] BLUE SOLN  BRICK-RED PPT [Cu2O(s)] COLOURLESS SOLN  SILVER MIRROR [Ag(s)] ALL ARE REDUCTIONS OF METAL ION BY ALDEHYDE Cr(+6)  (+3) Cu(+2)  (+1) Ag(+1)  (0) ALDEHYDE IS OXIDISED TO THE CAROXYLIC ACID For Ketone : Remains COLOURLESS Remains ORANGE Remains BLUE

  11. REDUCTION OF ALDEHYDES & KETONES NB (2) preferred to reduce aldehydes & ketones – (1) and (3) also reduce acids etc = reverse of oxidation of alcohols Reducing agents = (1) H2 gas with Ni catalyst or (2) NaBH4 followed by water or (3)LiAlH4 in dry ether R1 R1 all represented by [H] C C=O R2 R2 Applicable to both aldehydes and ketones H + 2[H] OH Acid ALDEHYDE primary alcohol KETONE secondary alcohol

  12. Predict the reduction product(s) of : 1. Propanal2. Butan-2-one 3. Butan-2,3-dione 4. Hexanoic acid 5. Cyclohexanone CH3 CH2 CH3 C CH3 CH3 CH2 CH3 C C C H OH OH Butan-2-ol Propan-1-ol H H H H CH3CH2CH2CH2CH2CHO (hexanal) then CH3CH2CH2CH2CH2CH2OH (hexan-1-ol) OH OH Butan-2,3-diol H OH Cyclohexanol

  13. from NaBH4 :H- H H C C C H C O- :O- :O- OH H OH MECHANISM OF REDUCTION Stage 1 : Nucleophile (:H-) attacks +C of C=O + Stage 2 : Protonation of O- by water Primary or secondary alcohol formed + :OH- = NUCLEOPHILIC ADDITION

  14. NUCLEOPHILIC ADDITION OF HCN common to both aldehydes and ketones - :CN = the nucleophile, attacks +ve C of +C=O R1 R1 C=O C R2 R2 2-hydroxynitriles CN + HCN OH Note : HCN provided by mixing K+CN- with dil. H2SO4 because HCN is so weak that the concentration of :CN- would be very low.

  15. Predict the product(s) of HCN reacting with: • Methanal 2. Propanone • 3. Ethanal 4. Cyclohexanone CH3 H H C C C CH3 CH3 H 2-hydroxyethanenitrile 2-hydroxy-2-methylpropanenitrile CN CN CN OH OH OH CN 2-hydroxypropanenitrile 1-cyanocyclohexanol OH

  16. :CN- CN CN C C C CN C O- :O- :O- OH H OSO3H MECHANISM OF NUCLEOPHILIC ADDITION OF HCN Stage 1 : Nucleophile (:CN-) attacks +C of C=O + Stage 2 : Protonation of O- by H2SO4 Note: Exactly parallel to reduction + HSO4-

  17. CN R1 C R2 OH OPTICAL ACTIVITY OF 2-HYDROXYNITRILES If R1 and R2not the same, 2- hydroxynitrile will be optically active because it contains C bonded to 4 different groups = a CHIRALor ASYMMETRIC carbon atom  stereoisomers which are non-superimposable mirror images of eachother  rotate plane-polarised light EQUALLY but OPPOSITE Measured by polarimeter (see “isomerism” notes)

  18. mirror Optical isomers (“enantiomers”) of 2-hydroxynitriles HO OH C C NC CN R1 R1 R2 R2 A 50:50 mixture of such optical isomers is called a RACEMATE. This is OPTICALLY INACTIVE because rotational effects CANCEL.

  19. R1 R2 C R2 R2 O C C R1 CN OH R1 NC OH Why does aldehyde / ketone + HCN reaction produce a racemate? Aldehydes and ketones are PLANAR (flat) around the C=O carbon  CN- nucleophile may (50:50 chance) attack from “front” or “back” of this plane  50:50 mixture of 2 different optical isomers “Back” gives: “Front” gives: NB Racemisation provides evidence to support the proposed nucleophilic addition mechanism.

  20. THE IODOFORM REACTION Test for methyl aldehydes and ketones. Heat with I2 dissolved in aq. NaOH Also applies to alcohols of the type: OH O O R = H (aldehyde) or CnH2n+1 R R R CH3 CH3 -O C C CH + 3I2 + 4OH- i.e. ethanol or any secondary methyl alcohol These are oxidised to the corresponding methyl aldehyde or ketone by the alkaline I2 CHI3+ + 3I- + 3H2O CH3-CH(OH)-R + I2 + 2OH- CHI3= triiodomethane = “iodoform” = bright yellow precipitate CH3-CO-R + 2I- + 2H2O

  21. SAMPLE QUESTIONS • A. CH3CH2CHO B. (CH3)2CHCHO C. CH3CH2COCH2CH3 • Name compounds A – C • By selecting from A – C, select all the compounds which satisfy the following requirements. • Could be reduced to a secondary alcohol. • Could be oxidised to a carboxylic acid. • C be reduced to a primary alcohol. • (iii) Would produce a precipitate when treated Brady’s reagent. A. Propanal B. 2-Methylpropanal C. Pentan-3-one  ketone  C only  aldehyde  A and B only  aldehyde  A and B only  Carbonyl compound  A, B and C

  22. Name the compound formed when (a) propanal (b) methanal is treated with HCN. State the conditions for carrying out these reactions. Write an equation for each reaction. Are the products optically active? Explain. (a) 2-hydroxybutanenitrile (b) 2-hydroxyethanenitrile + KCN and dil. H2SO4 at room temperature H H C C CH3 CH2 H (a) CH3CH2CHO + HCN  CN CN (b) HCHO + HCN  OH OH Neither is optically active because: (a) racemisation will occur (b) the product does not contain a chiral C (R1 = R2 = H)

  23. Aldehydes and ketones can be reduced to alcohols by lithium tetrahydridoaluminate(III), LiAlH4. State the conditions under which this reagent is used. P. CH3CH(OH)COOH Q. CH3COCOOH P may be produced from Q by reduction, but this cannot be achieved with lithium tetrahydridoaluminate(III). What will be the product if this reagent is used? How would you obtain P from Q? In dry ether under reflux without heat C=O and COOH in Q will both be reduced • propan-1,2-diol (CH3CH(OH)CH2OH) Use a milder reducing agent  NaBH4 in water

  24. The End

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