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Chapter 34 Carbonyl Compounds 34.1 34.2 34.3 34.4 34.5 34.6 1 Introduction Nomenclature of Carbonyl Compounds Physical Properties of Carbonyl Compounds Preparation of Carbonyl Compounds Reactions of Carbonyl Compounds Uses of Carbonyl Compounds New Way Chemistry for Hong Kong A-Level Book 3B 34.1 Introduction (SB p.2) Aldehydes and ketones : carbonyl compounds, contain group General formula of aldehydes: Examples: 2 New Way Chemistry for Hong Kong A-Level Book 3B 34.1 Introduction (SB p.2) General formula of ketones: Examples: 3 New Way Chemistry for Hong Kong A-Level Book 3B 34.1 Introduction (SB p.3) • Carbonyl carbon is sp2-hybridized • The bond is formed by the head-on overlap of an sp2 hybrid orbital of C and one p prbital of O • The bond is formed by the side-way overlap of p orbitals of C and O 4 New Way Chemistry for Hong Kong A-Level Book 3B 34.1 Introduction (SB p.3) 5 • The three atoms that are bonded to the carbonyl carbon forms a trigonal planar structure • The bond angles between three attached atoms are 120 New Way Chemistry for Hong Kong A-Level Book 3B 34.1 Introduction (SB p.3) • Oxygen is more electronegative • The carbonyl oxygen bears a partial negative charge and the carbonyl carbon bears partial positive charge 6 New Way Chemistry for Hong Kong A-Level Book 3B 34.2 Nomenclature of Carbonyl Compounds (SB p.3) Aldehydes Aldehydes are named by replacing the final “-e” of the name of the corresponding alkane with “-al” Examples: 7 New Way Chemistry for Hong Kong A-Level Book 3B 34.2 Nomenclature of Carbonyl Compounds (SB p.4) Ketones • Ketones are named by replacing the final “-e” of the name of the corresponding alkane with “-one”. • The parent chain is then numbered in the way that gives the carbonyl carbon atom the lowest possible number, and this number is used to indicate its position. Examples: 8 New Way Chemistry for Hong Kong A-Level Book 3B 34.2 Nomenclature of Carbonyl Compounds (SB p.4) Check Point 34-1 (a) Draw the structural formulae of all carbonyl compounds having the molecular formula C4H8O. Give their IUPAC names. Answer 9 New Way Chemistry for Hong Kong A-Level Book 3B 34.2 Nomenclature of Carbonyl Compounds (SB p.4) Check Point 34-1 (b) Draw the structural formulae of all straight-chain carbonyl compounds having the molecular formula C5H10O. Answer 10 New Way Chemistry for Hong Kong A-Level Book 3B 34.2 Nomenclature of Carbonyl Compounds (SB p.4) Check Point 34-1 (c) Explain why there is no such a compound called “ethanone”. Answer (c) Ketones are compounds with the group situated between two carbon chains. Therefore, the simplest ketone is the one with three carbon atoms. “Ethanone”, however, suggests that there are two carbon atoms in it and it does not exist. 11 New Way Chemistry for Hong Kong A-Level Book 3B 34.3 Physical Properties of Carbonyl Compounds (SB p.5) • Simple aldehydes and ketones are gases or liquids at room temperature • Aliphatic aldehydes have unpleasant and pungent smell • Ketones and benzaldehyde have a pleasant and sweet odour Carbonyl compound Aldehydes: Methanal Ethanal Propanal Butanal Methylpropanal Benzaldehyde 12 Formula Boiling point (°C) HCHO CH3CHO CH3CH2CHO CH3(CH2)2CHO (CH3)2CHCHO C6H5CHO –21 20.8 48.8 75.7 64.2 179 Melting Density at point (°C) 20°C (g cm–3) –92 –124 –81 –99 –65.9 –26 New Way Chemistry for Hong Kong A-Level Book 3B — 0.783 0.807 0.817 0.790 1.046 34.3 Physical Properties of Carbonyl Compounds (SB p.5) Carbonyl compound Formula Ketones: Propanone Butanone Pentan-3-one Pentan-2-one 3-Methylbutan-2-one Hexan-2-one Phenylethanone 13 CH3COCH3 CH3COCH2CH3 CH3CH2COCH2CH3 CH3CO(CH2)2CH3 CH3COCH(CH3)2 CH3CO(CH2)3CH3 C6H5COCH3 Boiling Melting Density at point (°C) point (°C) 20°C (g cm–3) 56.2 79.6 102 102 95 127 202 New Way Chemistry for Hong Kong A-Level Book 3B –95.4 –86.9 –39.9 –77.8 –92 –56.9 19.6 0.791 0.806 0.814 0.811 0.803 0.812 1.028 34.3 Physical Properties of Carbonyl Compounds (SB p.5) Boiling Point and Melting Point 14 • Carbonyl compounds have higher b.p. and m.p. than hydrocarbons of similar relative molecular masses ∵ the presence of dipole-dipole interactions • Carbonyl compounds have lower b.p. and m.p. than the corresponding alcohols ∵ dipole-dipole interactions are weaker than intermolecular hydrogen bonds New Way Chemistry for Hong Kong A-Level Book 3B 34.3 Physical Properties of Carbonyl Compounds (SB p.6) Density • The densities of aliphatic carbonyl compounds are lower than that of water at 20°C • Aromatic carbonyl compounds are slightly denser than water 20°C • Densities increase with increasing relative molecular masses 15 New Way Chemistry for Hong Kong A-Level Book 3B 34.3 Physical Properties of Carbonyl Compounds (SB p.6) Solubility • Aldehydes and ketones of low molecular masses show appreciable solubilities in water ∵ carbonyl oxygen can form strong hydrogen bonds with water molecules • e.g. propanone and ethanal are soluble in water in all proportions 16 New Way Chemistry for Hong Kong A-Level Book 3B 34.3 Physical Properties of Carbonyl Compounds (SB p.7) Example 34-1 (a) In each pair of compounds below, select the one you would expect to have a higher boiling point. (i) A: CH3CH2CHO B: CH3CH2CH2OH (ii) C: D: (iii)E: CH3CH2CH2CHO F: CH3CH2CH2CH3 (iv) G: Solution:H: (a) (i) (ii) (iii) (iv) 17 B D E H New Way Chemistry for Hong Kong A-Level Book 3B Answer 34.3 Physical Properties of Carbonyl Compounds (SB p.7) Example 34-1 (b) Propanone, CH3COCH3, is completely soluble in water, but octan-4-one, CH3CH2CH2COCH2CH2CH2CH3, is almost insoluble in soluble in water. Explain their difference in solubility. Answer Solution: (b) This is because the solubility in water decreases as the hydrophobic hydrocarbon portion lengthens 18 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.7) Dehydrogenation of Alcohols Industrially, lower members of aldehydes and ketones are prepared by passing alcohol vapour over hot silver catalyst 19 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.8) Oxidation of Alcohols Example of oxidizing agents: acidified K2Cr2O7 • Aldehydes are prepared by oxidation of 1° alcohols • Ketones are prepared by oxidation of 2 ° alcohols 20 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.8) Oxidative Cleavage of Alkenes (Ozonolysis) • Ozone reacts with alkenes vigorously to from ozonides • Ozonides are reduced by Zn and H2O to give aldehydes and/or ketones 21 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.8) Decarboxylation of Acid Salts Aldehydes can be prepared by heating a mixture of calcium methanoate and calcium carboxylate e.g. 22 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.9) Ketones can be prepared by heating calcium carboxylate e.g. 23 New Way Chemistry for Hong Kong A-Level Book 3B 34.4 Preparation of Carbonyl Compounds (SB p.9) Reduction of Acyl Chlorides • Aldehydes can be prepared by reducing acyl chlorides by treatment with H2 in the presence of Pd / BaSO4 catalyst and S • The purpose of adding sulphur is to poison the catalyst, so that the reduction does not proceed to produce alcohols 24 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.9) Nucleophilic Addition Reactions • Carbonyl group is susceptible to nucleophilic attack ∵ carbonyl carbon bears a partial positive charge • Nucleophiles use its lone pair electrons to form a bond with carbonyl carbon • One pair of bonding electrons of the carbon-oxygen bond shift out to the carbonyl oxygen 25 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.10) • The electron-rich oxygen transfers its electron pair to a proton addition of Nu – H to the carbonyl group • The carbonyl carbon changes from a trigonal planar geometry (i.e. sp2-hybridized) to a tetrahedral geometry (i.e. sp3-hybridized) 26 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.10) • Aldehydes are more reactive than ketones ∵ inductive effect and steric effect 1. The inductive effect The carbonyl carbon in ketones is less electrondeficient because two alkyl groups release electrons whereas only one present in aldehydes 27 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.10) 2. The steric effect • Aldehyde molecules are relatively open to the attack of nucleophiles ∵ one group being attached to the carbonyl carbon is a small hydrogen atom • In ketones, the two alkyl or aryl substituents cause a greater steric hindrance to the nucleophiles 28 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.11) • Due to the above 2 factors, the general order of reactivity of carbonyl compounds: • The delocalization of electrons from the benzene ring reduce the electron deficiency of the carbonyl carbon atom and makes aromatic carbonyl compounds even less reactive than aliphatic ketones 29 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.11) Addition of Hydrogen Cyanide Addition of hydrogen cyanide to the carbonyl group to form 2-hydroxyalkanenitriles (also known as cyanohydrins) 30 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.11) Examples: 31 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.11) Mechanism for the nucleophilic addition of hydrogen cyanide to the carbonyl group: 32 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.11) • HCN is a poor nucleophile while CN– is much stronger the reaction can be catalyzed by a base stronger than CN–, as the base can increase the concentration of CN– HCN + OH– CN– + H2O • As HCN is very toxic and volatile, it is safer to generate it in the reaction mixture • Mixing KCN or NaCN with dilute H2SO4 at 10 – 20°C gives HCN: 2KCN + H2SO4 2HCN + K2SO4 2NaCN + H2SO4 2HCN + Na2SO4 33 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.12) • 2-Hydroxyalkanenitriles are useful intermediates in organic synthesis • On acid hydrolysis, 2-hydroxyalkanenitriles are converted to 2-hydroxycarboxylic acids or 2-alkenoic acids e.g. 34 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.12) • With the use of reducing agents (e.g. LiAlH4), 2-hydroxyalkanenitriles are reduced to amines e.g. 35 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.13) Addition of Sodium Hydrogensulphate(IV) Carbonyl compounds react reversibly with excess 40% aqueous hydrogensulphate(IV) solutions at room temperature 36 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.13) Examples: 37 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.13) The reaction mechanism: • The reaction is initiated by the attack of nucleophile, HSO3– 38 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.13) • This reaction is very sensitive to steric hindrance and is limited to aliphatic aldehydes and sterically unhindered ketones • This reaction can be used for the separation and purification of the aldehydes and ketones, as they can be regenerated by treating the bisulphite addition product with aqueous alkalis or dilute acids. 39 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.14) Example 34-2 Outline how you are going to separate a mixture of butanone (b.p. 79.6°C) and 1-chlorobutane (b.p. 78.5°C) in diethyl ether. Answer 40 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.14) Solution: The mixture of butanone and 1-chlorobutane cannot be separated by distillation as their boiling points are too close. However, they can be separated through the nucleophilic addition reaction of sodium hydrogensulphate(IV). With the addition of sodium hydrogensulphate(IV) to the mixture, only butanone reacts to give the bisulphite addition product which is soluble in water. Then the organic later (containing 1chlorobutane) and the aqueous layer (containing the bisulphite addition product of butanone) are separated using a separating funnel. 1Chlorobutane can be obtained by distilling off the ether. On the other hand, with the addition of a dilute acid, the bisulphite addition product is converted to the carbonyl compound (i.e. butanone) which dissolves in diethyl ether. The organic layer (containing butanone) is separated from the aqueous layer by means of a separating funnel. Butanone is obtained after distilling off the ether. 41 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.14) Addition – Elimination (Condensation) Reactions 42 • Addition – elimination reactions involve the first addition of two molecules to form an unstable intermediate followed by the spontaneous elimination of the elements of water • e.g. reaction of aldehydes or ketones with the derivatives of ammonia New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.14) Reaction with Hydroxylamine • Carbonyl compounds react with hydroxylamine (NH2OH) to form oximes 43 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.15) Examples: 44 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.15) Reaction with 2,4-dinitrophenylhydrazine • Carbonyl compounds react with 2,4-dinitrophenylhydrazine to form 2,4-dinitrophenylhydrazones 45 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.15) Examples: 46 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.16) 47 • The oximes and 2,4-dinitrophenylhydrazones are used to identify unknown aldehydes and ketones • They are insoluble solids and have sharp characteristic melting points • The products are purified by recrystallization from ethanol and then filtered and washed under suction • Their melting points are determined and compared with that in data book to identify the original aldehyde or ketone New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.16) Carbonyl compound Aldehydes: Methanal Ethanal Propanal Butanal Benzaldehyde Ketones: Propanone Butanone Pentan-2-one Pentan-3-one Hexan-2-one Phenylethanone 48 Formula Melting point of 2,4-dinitrophenylhydrazone (°C) HCHO CH3CHO CH3CH2CHO CH3CH2CH2CHO C6H5CHO 167 146, 164 (2 forms) 156 123 237 CH3COCH3 CH3CH2COCH3 CH3CH2CH2COCH3 CH3CH2COCH2CH3 CH3CH2CH2CH2COCH3 C6H5COCH3 128 115 141 156 107 250 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.17) Check Point 34-2 (a) Carbonyl compoundsreactions always undergo nucleophilic addition (a) Compare the addition of carbonyl compounds andreactions. alkenes.As the carbon atom in the carbonyl group bears a partial positive charge, the carbonyl group is susceptible to nucleophilic Answer attack. In the case of alkenes, they always undergo electrophilic addition reactions. As the bonding electrons of the carbon-carbon double bond are only loosely held by the carbon atoms and are exposed, the carboncarbon double bond is susceptible to electron-loving reagents (i.e. electrophiles) 49 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.17) Check Point 34-2 (b) Describe briefly how you can distinguish between two carbonyl compounds having similar boiling points. Answer (b) The two compounds can be distinguished by determining the melting points of their 2,4-dinitrophenylhydrazone derivatives. 50 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.17) Oxidations • Aldehydes can be oxidized to carboxylic acids by strong oxidizing agents such as KMnO4 and K2Cr2O7, and also by mild oxidizing agents such as Tollen’s reagent and Fehling’s reagent 51 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.18) Reaction with Potassium Manganate(VII) and Potassium Dichromate(VI) • Aldehydes are oxidized readily by common oxidizing agents such as KMnO4/H+ and K2Cr2O7/H+ 52 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.18) • Generally, ketones do not undergo oxidation as their oxidation involves the cleavage of the strong carboncarbon bond • More severe conditions are required to bring about the oxidation 53 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.18) Reaction with Tollens’ Reagent (Silver Mirror Test) • Tollens’ reagent contains Ag(NH3)2+ • Ag(NH3)2+ oxidizes aldehydes to carboxylic acids while it is reduced to metallic silver which deposits on the wall of the reaction vessel as silver mirror 54 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.19) • Aldehydes are mixed with Tollens’ reagent in a clean test tube and placed in water bath kept at 60°C • If the wall of the reaction vessel is not clean enough, a silver mirror cannot be formed and a black precipitate is deposited instead • All ketones give a negative result of the silver mirror test • This reaction can be used to distinguish aldehydes from ketones 55 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.18) Reaction with Fehling’s Solution (Fehling’s Test) • Fehling’s solution is an alkaline solution of copper(II) tartrate. It is a blue solution • Aliphatic aldehydes reduce the Cu2+ ion in Fehling’s solution to form a brick-red precipitate of Cu2O 56 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.18) Addition of an aliphatic aldehyde Fehling’s solution • Ketones and aromatic aldehydes give a negative result of Fehling’s test • This reaction can be used to distinguish aliphatic aldehydes from ketones and aromatic aldehydes 57 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.20) Reductions • Aldehydes and ketones undergo reduction reactions forming 1° and 2° alcohols respectively 58 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.20) Reaction with Lithium Tetrahydridoaluminate • Lithium tetrahydridoaluminate (also called lithium aluminium hydride, LiAlH4) is a powerful reducing agent • It reduces aldehydes to 1° alcohols and ketones to 2° alcohols 59 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.20) • LiAlH4 is able to reduce carboxylic acid and esters to give alcohols • LiAlH4 reacts violently with water, therefore the reaction must be carried out in anhydrous solutions, usually in dry ether 60 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.21) Reaction with Sodium Tetrahydridoborate In practice, the reduction of aldehydes and ketones to alcohols is carried out by sodium tetrahydridoborate (also called sodium borohydride, NaBH4) 61 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.21) • NaBH4 is a less powerful reducing agent than LiAlH4 • NaBH4 reduces only aldehydes and ketones • The reduction by NaBH4 can be carried out in water or alcohol solutions 62 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.21) Triiodomethane Formation (Iodoform Reaction) Aldehydes or ketones having the group react with iodine in aqueous sodium hydroxide solution to give a bright yellow precipitate of iodoform (CHI3) 63 New Way Chemistry for Hong Kong A-Level Book 3B 34.5 Reactions of Carbonyl Compounds (SB p.21) • Ethanol and secondary alcohols with the group also give a positive result of iodoform test ∵ the group is first oxidized to group and further oxidized to give the carboxylate and the iodoform • 64 Iodoform test is test for the presence of New Way Chemistry for Hong Kong A-Level Book 3B or 34.5 Reactions of Carbonyl Compounds (SB p.22) Check Point 34-3 Draw the structural formulae of the major organic products A to H in the following reactions: (a) A: (a) (b) B: KCN/H2SO4 conc. HCl CH3CH2CHO A B 20°C 2,4-dinitrophenylhydrazine CH3CH2CHO C (b) C: (c) 1. LiAlH4 / dry ether CH3CH2CHO D + 2. H3O (d) (c) D: CH3CH2CH2OH NaBH4 CH3CH = CHCH2CHO E HO (d) E: CH3CH = CHCH22 CH2OH 65 New Way Chemistry for Hong Kong A-Level Book 3B Answer 34.5 Reactions of Carbonyl Compounds (SB p.22) Check Point 34-3 Draw the structural formulae of the major organic products A to H in the following reactions: (e) (e) (f) F: (f) G: Answer H: CHI3 66 New Way Chemistry for Hong Kong A-Level Book 3B 34.6 Uses of Carbonyl Compounds (SB p.22) As Raw Materials for Making Plastics Urea-methanal Urea-methanal is produced by condensation polymerization of urea and methanal under heat and pressure with the elimination of a water molecule 67 New Way Chemistry for Hong Kong A-Level Book 3B 34.6 Uses of Carbonyl Compounds (SB p.23) In the presence of excess methanal, cross linkages will be formed 68 New Way Chemistry for Hong Kong A-Level Book 3B 34.6 Uses of Carbonyl Compounds (SB p.24) Urea-methanal • thermosetting polymer (cannot be softened and insoluble in any solvents) • excellent electrical insulator • resistant to chemical attack • used for moulding electrical sockets 69 New Way Chemistry for Hong Kong A-Level Book 3B 34.6 Uses of Carbonyl Compounds (SB p.24) Perspex Propanone is converted to methyl 2-methylpropenoate, which is the monomer for the production of perspex 70 • Perspex is a dense, transparent solid • Used to make safety goggles, advertising signs and carside light protectors New Way Chemistry for Hong Kong A-Level Book 3B 34.6 Uses of Carbonyl Compounds (SB p.24) As Solvents Propanone • Liquid with a boiling point of 56.2°C • Can dissolve a variety of organic compounds • Important solvent used in industry and in the laboratory 71 New Way Chemistry for Hong Kong A-Level Book 3B The END 72 New Way Chemistry for Hong Kong A-Level Book 3B