Reactions of carbonyl compounds L.O. To confidently identify different carbonyl compounds. To find out how carbonyl compounds react.

Uses of carbonyl compounds Propanone: Industrial solvent in paints, varnishes, nail-polish removers. Methanal: a. Preserving and embalming, as a germicide and insecticide b. Manufacturing plastic coatings such as Bakelite and melamine c. Manufacturing polymer adhesives (e.g. wood glues) Flavourings: Benzaldehyde (C6H5CHO) – fresh almonds Heptan-2-one (C5H11COCH3) – blue cheese!

Carbonyl compounds - bonding PLANAR WITH BOND ANGLES OF 120° The carbon is sp2 hybridised and three sigma (s) bonds are planar. The unhybridised 2p orbital of carbon is at 90° to these. This overlaps with a 2p orbital of oxygen to form a pi (p) bond as oxygen is more electronegative than carbon the bond is polar P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120° NEW ORBITAL

Physical properties of carbonyl compounds Polarity is LESS than that of hydroxyl group in alcohols. Therefore, aldehydes and ketones have weaker dipole-dipole interactions and lower b.p. than alcohols of comparable Mr (hence good in paints and varnishes as evaporate easily). Carbonyls show a limited/lack of hydrogen bonding between molecules, whereas the corresponding alcohol will show extensive intermolecular H bonding. Weaker polarity means aldehydes and ketones mix well with polar solvents such as water and will dissolve many organic compounds.

Identification reactions of carbonyl compounds 2,4-dinitrophenylhydrazine CONDENSATION REACTION C6H3(NO2)2NHNH2 -Carbonyl group is detected using Brady’s reagent (solution of 2,4-dinitrophenylhydrazine [2,4-DNPH] in dilute acid) -Simple test for aldehydes and ketones -Bright orange-yellow crystals (ppt) formed which identifies presence of carbonyl group (C=O). -Products are derivatives: 2,4-dinitrophenylhydrazones. These have sharp, well-defined melting points so can be used to characterise (identify) carbonyl compounds.

Identification reactions of carbonyl compounds Distinguishing aldehydes from ketones: - use a mild oxidising agent In each test the ALDEHYDE REDUCES the reagents used, producing a VISIBLE COLOUR CHANGE. 1. Tollen’s Reagent ammoniacal silver nitrate Mild oxidising agent which will oxidise aldehydes but not ketones Procedure: Warm aldehyde or ketone with Tollen’s reagent. Contains the diammine silver(I) ion - [Ag(NH3)2 ]+ The silver(I) ion is reduced to silver Ag+(aq) + e- Ag(s) The test is known as THE SILVER MIRROR TEST

Identification reactions of carbonyl compounds 2. Fehling’s Solution (or Benedict’s solution) Procedure: Warm aldehyde or ketone with Fehling’s solution. Contains a copper(II) complex (Cu2+ ions dissolved in aqueous alkali) ion giving a blue solution on warming. It will oxidise aliphatic (but not aromatic) aldehydes 2Cu2+ (aq) + 2e- + 2OH- (aq) Cu2O (s) + H2O (l) N.B: The silver mirror test is best as it works with ALL aldehydes Ketones do not react with Tollen’s Reagent or Fehling’s Solution

Identification reactions of carbonyl compounds 3. Acidified dichromate (VI) Ions Procedure: Warm aldehyde or ketone with acidified potassium dichromate (VI). Dilute H2SO4 used as source of H+ ions. K2Cr2O7 used as source of Cr2O72- Aldehydes will be oxidised BUT show no reaction with ketones Cr2O72- (aq) + 14H+ (aq) + 6e- 2Cr3+ (aq) + 7H2O (l) ORANGE (CrVI) GREEN (CrIII)

Chemical properties of carbonyl compounds Oxidation • Used to differentiate between aldehydes and ketones. • Mild oxidising agents work best (Tollen’s reagent, Fehling’s etc). • Aldehydes are easier to oxidise. • Powerful oxidising agents will oxidise ketones. Ketones oxidise to a mixture of carboxylic acids. ALDEHYDES: Easily oxidised to the corresponding carboxylic acid RCHO(l) + [O] RCOOH(l) CH3CHO(l) + [O] CH3COOH(l) KETONES: Oxidised under vigorous conditions to carboxylic acids with fewer carbon atoms (MCPBA [oxidises to ester], or conc KMn04 +reflux) C2H5COCH2CH3(l) + 3 [O] C2H5COOH(l) + CH3COOH(l)

Nucleophilic addition reactions Both aldehydes and ketones undergo Nu- addition reactions. Reaction involves addition to the C=O double bond. For carbonyls, attack is at the positive carbon centre. This is due to the difference in electronegativity between C and O. N.B: This is UNLIKE ALKENES, which are attacked by E+. This is because they are NON-POLAR Group Bond Polarity Attacking species Result ALKENES C=C NON-POLAR ELECTROPHILES ADDITION CARBONYLS C=O POLAR NUCLEOPHILES ADDITION

Nucleophilic addition reactions Reagent: Hydrogen cyanide - HCN (in the presence of KCN + small amount of dilute H2SO4). KCN is needed to help generate the Nu- (cyanide nucleophile CN-). Acid used as source of H+ ions. Conditions: Room temperature Product(s): Hydroxynitrile (cyanohydrin) Equation: CH3CHO + HCN CH3CH(OH)CN 2-hydroxypropanenitrile N.B: HCN is a weak acid and has difficulty dissociating into ions HCN H+ + CN¯ Therefore the reaction is aided by KCN and dilute acid which help produce more of the nucleophilic CN¯

Nucleophilic addition reactions Mechanism: Nucleophilic addition Step 1: CN- acts as nucleophile, attacking the slightly positive carbon. One of the C=O bonds breaks; a pair of electrons goes onto the O. Step 2: A pair of electrons is used to form a bond with H+. Overall, HCN has added to the carbonyl compound. STEP 1 STEP 2

Optical activity and Nucleophilic addition Mechanism: Nucleophilic addition Optical activity: The mechanism depends on both the concentration of the nucleophile and the concentration of the carbonyl (it is BIMOLECULAR). The product that results contains a chiral centre (carbonyl carbon) and so will show optical activity. N.B. See nucleophilic substitution reactions of halogenoalkanes for other examples of stereospecific mechanisms (resulting in retention or loss of optical isomerism) STEP 1 STEP 2

Alternative reagents for HCN reaction Reagent: Hydrogen cyanide - HCN (in the presence of trace amounts of base [NaOH]). NaOH helps generate CN- ions as HCN only very weakly acidic Conditions: Room temperature The reaction proceeds as previously described.

Reduction of carbonyl compounds Possible reagents: Sodium tetrahydridoborate (III) (sodium borohydride),NaBH4 (aq) Lithium tetrahydridoaluminate (III), LiAlH4 in ether, followed by water. N.B: The above 2 are the most common to be used with carbonyl compounds. 3. Sodium in ethanol (ethanol generates hydrogen) 4. Hydrogen under pressure with a nickel, platinum or palladium catalyst

Reduction of carbonyl compounds with NaBH4 Reagent: Sodium tetrahydridoborate (III) (sodium borohydride),NaBH4 Conditions: Aqueous solution at room temperature Mechanism: Nucleophilic addition (also reduction as it is addition of H- ) Nucleophile: H- (hydride ion) Product(s): Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s): CH3CHO + 2[H] CH3CH2OH CH3COCH3 + 2[H] CH3CHOHCH3 N.B: The water provides a proton to make the alcohol AND NaBH4 does not reduce C=C

Reduction of carbonyl compounds with LiAlH4 Reagent: Lithium tetrahydridoaluminate (lithium aluminium hydride) LiAlH4 Conditions: Dry ether followed by hydrolysis by water Mechanism: Nucleophilic addition (also reduction as it is addition of H- ) Nucleophile: AlH4- (dry conditions needed to prevent hydrolysis) Product(s): Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Can you draw the mechanism for this reaction?

Reduction of carbonyl compounds with LiAlH4

Reduction of carbonyl compounds with other functionalities Functional groups containing multiple bonds can be reduced C=C is reduced to CH-CH C=O is reduced to CH-OH CN is reduced to CH-NH2 Hydrogen: H2 H- (nucleophile) Reactions: Hydrogen reduces C=C and C=O bonds CH2 = CHCHO + 4[H] CH3CH2CH2OH Hydride ion H- reduces C=O bonds ONLY CH2 = CHCHO + 2[H] CH2=CHCH2OH

Reduction of carbonyl compounds with other functionalities EXPLANATION C=O is polar so is attacked by the nucleophilic H- C=C is non-polar so is not attacked by the nucleophilic H- Key point when choosing reducing agent – which group do you want reduced?

Reduction of carbonyl compounds with other functionalities What are the products when Compound X is reduced? COMPOUND X H2 NaBH4

Reduction of carbonyl compounds with other functionalities What are the products when Compound X is reduced? COMPOUND X H2 NaBH4 C=O is polar so is attacked by the nucleophilic H- C=C is non-polar so is not attacked by the nucleophilic H-

Iodoform reaction – identification test for carbonyl compounds containing a specific group Aldehydes and ketones containing the CH3C=O group will give a positive result with the triiodomethane reaction. Therefore, only aldehyde to give a positive result is ETHANAL Alcohols will also give a positive result if they can first oxidise to a ketone (secondary alcohols) containing the above group. Reaction: The organic compound added to a mixture of IODINE and dilute NaOH (or a mixture of KI and NaOCl [sodiun chlorate]) Result: A pale yellow precipitate of iodoform (CHI3)

Iodoform reaction – a short practical to see it in action….. Add 4cm3 of NaOH (aq) to each of 3 test tubes. To the first test tube add 6 drops of compound A. To the second test tube add 6 drops of compound B. To the third test tube ass 6 drops of compound C. Add I2 (aq) dropwise to each test tube until a faint brown colour remains. Allow the tubes to stand for a short time and then record your observations. Which of the compounds has produced the CHI3 ppt and so which contain(s) the CH3C=O group?