Chapter 8: Carbonyl Compounds (Aldehydes and Ketones) – Key Notes

Structure and Bonding

  • Carbonyl compounds contain the carbonyl group: > C = O (polar) and include aldehydes and ketones.
  • Carbonyl carbon is electrophilic; oxygen is nucleophilic.
  • Hybridisation: carbon in carbonyl group is sp2sp^2; bond angle ≈ 120<br/>".</li><li>Geometry:Trigonalplanararoundthecarbonylcarbon.</li><li>Thecarbonylbondresultsfromoverlapofthecarbon’sunhybridized2porbitalwithoxygen’s2porbitals;the120^<br /> ^". </li> <li>Geometry: Trigonal planar around the carbonyl carbon.</li> <li>The carbonyl bond results from overlap of the carbon’s unhybridized 2p orbital with oxygen’s 2p orbitals; theC=COπ−bondliesaboveandbelowtheplane.</li></ul><h3id="hybridisationandbonding">HybridisationandBonding</h3><ul><li>Hybridisationbasics:orbitalsmixtogiveneworbitalsofsimilarenergy;totalhybridorbitals=numberofintermixingorbitals.</li><li>Sigmavs.pibonds:<ul><li>π-bond lies above and below the plane.</li> </ul> <h3 id="hybridisationandbonding">Hybridisation and Bonding</h3> <ul> <li>Hybridisation basics: orbitals mix to give new orbitals of similar energy; total hybrid orbitals = number of intermixing orbitals.</li> <li>Sigma vs. pi bonds:<ul> <li>\sigmabonds:head−to−headoverlap;strong;freerotationpossible.</li><li>bonds: head-to-head overlap; strong; free rotation possible.</li> <li>\pibonds:lateraloverlapofp−orbitals;weaker;rotationisrestricted.</li></ul></li><li>Deltabetweenbonds: lateral overlap of p-orbitals; weaker; rotation is restricted.</li></ul></li> <li>Delta between\mathrm{C=C}andand\mathrm{C=O}:carbonylispolar;resonancepossibleinmanysystems,whereassimpleC=Cisnonpolarandhasdifferentresonancebehavior.</li></ul><h3id="nomenclatureofcarbonylcompounds">NomenclatureofCarbonylCompounds</h3><ul><li>Generalaldehydeformula:: carbonyl is polar; resonance possible in many systems, whereas simple C=C is nonpolar and has different resonance behavior.</li> </ul> <h3 id="nomenclatureofcarbonylcompounds">Nomenclature of Carbonyl Compounds</h3> <ul> <li>General aldehyde formula:R-CHO(Aldehydes/Alkanals).</li><li>Generalketoneformula:(Aldehydes/Alkanals).</li> <li>General ketone formula:R-CO-R'(Ketones/Alkanones).</li><li>Aldehydes:<ul><li>Commonnamesderivefromcorrespondingcarboxylicacidsbyreplacingtheacidsuffixwith“aldehyde”(e.g.,formaldehydefromformicacid).</li><li>IUPAC:replacetheendingofthealkanewith“−al”;longestchaincontainingthe–CHOgroupistheparent.</li></ul></li><li>Ketones:<ul><li>Commonnames:namebothsubstituentsthenadd“ketone”;simpleketonesusetheprefixindicatingsubstituents(e.g.,acetone,diethylketone).</li><li>IUPAC:replaceterminal‘e’with“−one”;longestchainisnumberedtogivecarbonylthelowestnumber.</li></ul></li><li>Examples:<ul><li>Aldehydes:(Ketones/Alkanones).</li> <li>Aldehydes:<ul> <li>Common names derive from corresponding carboxylic acids by replacing the acid suffix with “aldehyde” (e.g., formaldehyde from formic acid).</li> <li>IUPAC: replace the ending of the alkane with “-al”; longest chain containing the –CHO group is the parent.</li></ul></li> <li>Ketones:<ul> <li>Common names: name both substituents then add “ketone”; simple ketones use the prefix indicating substituents (e.g., acetone, diethyl ketone).</li> <li>IUPAC: replace terminal ‘e’ with “-one”; longest chain is numbered to give carbonyl the lowest number.</li></ul></li> <li>Examples:<ul> <li>Aldehydes: ext{HCHO}
    ightarrow ext{Methanal (Formaldehyde)},, ext{CH}3 ext{CHO} ightarrow ext{Ethanal (Acetaldehyde)},, ext{C}6 ext{H}_5 ext{CHO}
    ightarrow ext{Benzaldehyde}</li><li>Ketones:</li> <li>Ketones: ext{CH}3 ext{COCH}3
    ightarrow ext{Propanone (Acetone)},, ext{CH}3 ext{COCH}2 ext{CH}_3
    ightarrow ext{Butan-2-one}</li></ul></li></ul><h3id="aldehydesalkanals">Aldehydes(Alkanals)</h3><ul><li>Generalformula:</li></ul></li> </ul> <h3 id="aldehydesalkanals">Aldehydes (Alkanals)</h3> <ul> <li>General formula:R-CHO;atleastonehydrogenisattachedtothecarbonylcarbon.</li><li>Classification:<ul><li>Aliphaticaldehydes:alkylorHattachedto; at least one hydrogen is attached to the carbonyl carbon.</li> <li>Classification: <ul> <li>Aliphatic aldehydes: alkyl or H attached to-CHO(noaromaticring).</li><li>Aromaticaldehydes:arylgroupattachedto(no aromatic ring).</li> <li>Aromatic aldehydes: aryl group attached to-CHO(benzaldehydeisthesimplest).</li></ul></li><li>Commonaldehydes:<ul><li>Formaldehyde(Methanal):(benzaldehyde is the simplest).</li></ul></li> <li>Common aldehydes:<ul> <li>Formaldehyde (Methanal): ext{HCHO}</li><li>Acetaldehyde(Ethanal):</li> <li>Acetaldehyde (Ethanal): ext{CH}_3 ext{CHO}</li><li>Propionaldehyde(Propanal):</li> <li>Propionaldehyde (Propanal): ext{CH}3 ext{CH}2 ext{CHO}</li><li>Butyraldehyde(Butanal):</li> <li>Butyraldehyde (Butanal): ext{CH}3 ext{CH}2 ext{CH}_2 ext{CHO}</li><li>Benzaldehyde:</li> <li>Benzaldehyde: ext{C}6 ext{H}5 ext{CHO}
  • Aldehydes have the formyl group (>C=O attached to H).

    • Ketones (Alkanones)

    • General formula: R-CO-R';carbonylcarbonbondedtotwoalkyl/arylgroups.</li><li>Classification:<ul><li>Simpleketones(R=R′)</li><li>Mixedketones(R≠R′)</li><li>Symmetricalvs.asymmetrical</li></ul></li><li>Commonketones:<ul><li>Acetone:; carbonyl carbon bonded to two alkyl/aryl groups.</li> <li>Classification:<ul> <li>Simple ketones (R = R')</li> <li>Mixed ketones (R ≠ R')</li> <li>Symmetrical vs. asymmetrical</li></ul></li> <li>Common ketones:<ul> <li>Acetone: ext{CH}3 ext{COCH}3(IUPAC:Propan−2−one)</li><li>Butan−2−one,Pentan−3−one,Diethylketone,etc.</li></ul></li><li>Ketoneshavethecarbonylgroupbutdonothaveahydrogenonthecarbonylcarbon(unlikealdehydes).</li></ul><h3id="preparationandpropertiesofcarbonylcompounds">PreparationandPropertiesofCarbonylCompounds</h3><ul><li>Methodstopreparealdehydesandketones:<ul><li>Oxidationofprimaryalcohols→aldehydes;secondaryalcohols→ketones(withsuitableoxidants)</li><li>Dehydrogenationofalcohols</li><li>Hydrationofalkynes</li></ul></li><li>Importantspecificpreparations:<ul><li>Formaldehydefrommethanol(laboratory):methanol+airoverCu/Agat≈(IUPAC: Propan-2-one)</li> <li>Butan-2-one, Pentan-3-one, Diethyl ketone, etc.</li></ul></li> <li>Ketones have the carbonyl group but do not have a hydrogen on the carbonyl carbon (unlike aldehydes).</li> </ul> <h3 id="preparationandpropertiesofcarbonylcompounds">Preparation and Properties of Carbonyl Compounds</h3> <ul> <li>Methods to prepare aldehydes and ketones:<ul> <li>Oxidation of primary alcohols → aldehydes; secondary alcohols → ketones (with suitable oxidants) </li> <li>Dehydrogenation of alcohols</li> <li>Hydration of alkynes</li></ul></li> <li>Important specific preparations:<ul> <li>Formaldehyde from methanol (laboratory): methanol + air over Cu/Ag at ≈300^ {0}C→→ ext{HCHO} + ext{H}_2 ext{O}
    • Acetaldehyde from ethanol (oxidation with \mathrm{K2Cr2O7} or \mathrm{KMnO4})
    • Benzaldehyde from toluene via chromyl chloride in CCl₄, hydrolyzed to benzaldehyde
    • Acetone from isopropyl alcohol (oxidation with acidified \mathrm{K2Cr2O_7})
  • Important notes:
    • Tertiary alcohols do not undergo oxidation to aldehydes/ketones.

    • Physical Properties (Aldehydes & Ketones)

    • General properties:
      • Carbonyl group is polar; carbonyl carbon is electrophilic; oxygen bears partial negative charge.
      • Polarity leads to dipole moments; polarity influences solubility and boiling points.
      • Aldehydes and ketones are typically less reactive toward nucleophiles than carboxylic acids but react via polar carbonyl chemistry.
    • Ketones vs. Aldehydes: sols, bp, and solubility vary with aliphatic/aromatic nature; aromatic derivatives often less soluble in water.

    Reactions of Aldehydes and Ketones

    • Reduction to alcohols:
      • Aldehydes → primary alcohols with NaBH extsubscript{4} (or LiAlH extsubscript{4})
      • Ketones → secondary alcohols with NaBH extsubscript{4}
      • Equation example: ext{RCHO} + ext{NaBH}4 ightarrow ext{RCH}2 ext{OH}</li></ul></li><li>Oxidationtocarboxylicacids(strongoxidants):<ul><li>Aldehydes+oxidants(e.g.,</li></ul></li> <li>Oxidation to carboxylic acids (strong oxidants):<ul> <li>Aldehydes + oxidants (e.g., ext{K}2 ext{Cr}2 ext{O}7,, ext{KMnO}4)→) → ext{RCOOH}</li></ul></li><li>Tollen’sreagent(ammoniacalAgNO3):oxidizesaldehydestoacidswithformationofasilvermirror;ketonesdonotreact(weakoxidant).</li><li>Fehling’ssolution(Cu(II)inalkalinesolution):aldehydesgivebrick−redCu2Oprecipitate;ketonesdonotreact.</li><li>Schiff’sreagent:aldehydesreduceSchiff’sreagenttogiveapinkcolor;ketonesgivenocolorchange.</li><li>Cannizzaroreaction(noα−hydrogen):disproportionationofaldehydeslackingα−H(e.g.,formaldehyde,benzaldehyde)understrongbasetoyieldalcoholandcarboxylate.<ul><li>Mechanism(brief):nucleophilicattackbyOH−onaldehyde→hydridetransfertoanotheraldehyde→protontransfertoyieldalcoholandcarboxylate.</li></ul></li><li>AdditionwithHCN(cyanohydrinformation):aldehydesandketonesreactwithHCNtogivecyanohydrins;basecatalysisacceleratesthereaction.</li><li>Aldolcondensation:occursforaldehydes/ketoneswithα−hydrogen(e.g.,acetaldehyde);formaldehydedoesnotundergoaldolcondensation.</li><li>Generaldistinctiontests(aldehydesvsketones):<ul><li>Tollen’s:positiveforaldehydes;negativeforketones.</li><li>Fehling’s:positiveforaldehydes;negativeforketones.</li><li>Schiff’sreagent:positiveforaldehydes;negativeforketones.</li><li>ReductionwithNaBHextsubscript4:aldehydes→primaryalcohols;ketones→secondaryalcohols.</li></ul></li></ul><h3id="formaldehydemethanalhcho">Formaldehyde(Methanal,HCHO)</h3><ul><li>Specialfeatures:thecarbonylcarbonisattachedtoH(notanalkylgroup).</li><li>Preparation:laboratoryoxidationofmethanoloverCu/Agat≈</li></ul></li> <li>Tollen’s reagent (ammoniacal AgNO₃): oxidizes aldehydes to acids with formation of a silver mirror; ketones do not react (weak oxidant).</li> <li>Fehling’s solution (Cu(II) in alkaline solution): aldehydes give brick-red Cu₂O precipitate; ketones do not react.</li> <li>Schiff’s reagent: aldehydes reduce Schiff’s reagent to give a pink color; ketones give no color change.</li> <li>Cannizzaro reaction (no α-hydrogen): disproportionation of aldehydes lacking α-H (e.g., formaldehyde, benzaldehyde) under strong base to yield alcohol and carboxylate.<ul> <li>Mechanism (brief): nucleophilic attack by OH⁻ on aldehyde → hydride transfer to another aldehyde → proton transfer to yield alcohol and carboxylate.</li></ul></li> <li>Addition with HCN (cyanohydrin formation): aldehydes and ketones react with HCN to give cyanohydrins; base catalysis accelerates the reaction.</li> <li>Aldol condensation: occurs for aldehydes/ketones with α-hydrogen (e.g., acetaldehyde); formaldehyde does not undergo aldol condensation.</li> <li>General distinction tests (aldehydes vs ketones):<ul> <li>Tollen’s: positive for aldehydes; negative for ketones.</li> <li>Fehling’s: positive for aldehydes; negative for ketones.</li> <li>Schiff’s reagent: positive for aldehydes; negative for ketones.</li> <li>Reduction with NaBH extsubscript{4}: aldehydes → primary alcohols; ketones → secondary alcohols.</li></ul></li> </ul> <h3 id="formaldehydemethanalhcho">Formaldehyde (Methanal, HCHO)</h3> <ul> <li>Special features: the carbonyl carbon is attached to H (not an alkyl group).</li> <li>Preparation: laboratory oxidation of methanol over Cu/Ag at ≈300^ {0}C;reaction:; reaction: ext{CH}3 ext{OH} + frac{1}{2} ext{O}2
        ightarrow ext{HCHO} + ext{H}_2 ext{O}</li><li>Properties:colorlessgaswithpungentodor;highlysolubleinwater;soldas40<li>Tests:Schiff’sreagent,Tollen’sreagent(silvermirror),Fehling’ssolutionredppt.</li><li>Uses:disinfectant/antiseptic,formalin,silveringmirrors,Bakelite/resins.</li></ul><h3id="acetaldehydeethanalchextsubscript3cho">Acetaldehyde(Ethanal,CHextsubscript3CHO)</h3><ul><li>Preparation:oxidationofethanolwithoxidants(<br/>K<em>2Cr</em>2O<em>7orKMnO</em>4)toform</li> <li>Properties: colorless gas with pungent odor; highly soluble in water; sold as 40% formalin.</li> <li>Tests: Schiff’s reagent, Tollen’s reagent (silver mirror), Fehling’s solution red ppt.</li> <li>Uses: disinfectant/antiseptic, formalin, silvering mirrors, Bakelite/resins.</li> </ul> <h3 id="acetaldehydeethanalchextsubscript3cho">Acetaldehyde (Ethanal, CH extsubscript{3}CHO)</h3> <ul> <li>Preparation: oxidation of ethanol with oxidants (<br /> \mathrm{K<em>2Cr</em>2O<em>7} or \mathrm{KMnO</em>4}) to form ext{CH}_3 ext{CHO}
      • Properties: colorless, volatile liquid; miscible with water/ethanol
      • Tests: Schiff’s reagent pink, Fehling’s red ppt, Tollen’s silver mirror, iodoform test with I₂/NaOH, positive nitroprusside test
      • Uses: precursor for acetic acid, acetic anhydride, acetone, ethyl alcohol; dyes and resins; mirrors

      Benzaldehyde (C extsubscript{6}H extsubscript{5}CHO)

      • Structure: aryl group attached to aldehydic carbon
      • Preparation: oxidation of toluene with chromyl chloride in CCl₄, hydrolysis to benzaldehyde
      • Properties: colorless oily liquid; bitter almond odor; slightly heavier than water, insoluble in water
      • Tests: Schiff’s reagent (pink restoration), does not give red Fehling’s precipitate (unlike aliphatic aldehydes), Tollen’s gives silver mirror, NaOH gives brown resinous mass
      • Uses: dyes, flavors/perfumery, synthesis of cinnamaldehyde, cinnamic acid, benzoyl chloride

      Formaldehyde, Acetaldehyde, Benzaldehyde – Chemical Tests & Reactions

      • Aldehydes reduce Fehling’s and Tollen’s reagents and react with Schiff’s reagent; ketones do not.
      • Formaldehyde and acetaldehyde undergo Cannizzaro under strong base only if α-H is absent or not required (formaldehyde yes; acetaldehyde typically no).
      • Nucleophilic addition with HCN yields cyanohydrins (aldehydes and ketones).
      • Aldehydes can be oxidized to carboxylic acids (R-CHO → R-COOH) by strong oxidants.

      Acetone (Propan-2-one, CH₃COCH₃)

      • Properties: colorless liquid; miscible with water, alcohol, ether; good solvent.
      • Preparation: oxidation of isopropyl alcohol with acidified K₂Cr₂O₇.
      • Tests: iodine–NaOH gives a yellow ppt; sodium nitroprusside test (red color that shifts with pH).
      • Uses: nail polish remover, precursor for iodoform and chloroform, storage/transport of acetylene.
      • Chemistry:
        • Reduction to secondary alcohol (NaBH₄): ext{CH}3 ext{COCH}3
          ightarrow ext{CH}3 ext{CH(OH)CH}3</li><li>AdditionwithHCNformscyanohydrin(ketonescanreactsimilarlytoaldehydesforcyanohydrins).</li></ul></li><li>KetonesdonotreactwithTollens’orFehling’sreagents;theydonotgivepositivetestsforthesereagents.</li></ul><h3id="cannizzarosreactiondisproportionation">Cannizzaro’sReaction(Disproportionation)</h3><ul><li>Occursforaldehydeslackingα−hydrogen(e.g.,HCHO,Cextsubscript6Hextsubscript5CHO).</li><li>Understrongbase:onealdehydemoleculeisoxidizedtoacarboxylate,anotherisreducedtoaprimaryalcohol.</li><li>Example:</li> <li>Addition with HCN forms cyanohydrin (ketones can react similarly to aldehydes for cyanohydrins).</li></ul></li> <li>Ketones do not react with Tollens’ or Fehling’s reagents; they do not give positive tests for these reagents.</li> </ul> <h3 id="cannizzarosreactiondisproportionation">Cannizzaro’s Reaction (Disproportionation)</h3> <ul> <li>Occurs for aldehydes lacking α-hydrogen (e.g., HCHO, C extsubscript{6}H extsubscript{5}CHO).</li> <li>Under strong base: one aldehyde molecule is oxidized to a carboxylate, another is reduced to a primary alcohol.</li> <li>Example: ext{HCHO} + ext{HCHO}
          ightarrow ext{HCOO}^- + ext{CH}_3 ext{OH}$$
        • Mechanism (summary): nucleophilic attack by OH⁻, hydride transfer, proton transfer; rate-determining step involves hydride transfer.

        Aldol Condensation vs Cannizzaro

        • Formaldehyde does not undergo aldol condensation; acetaldehyde does undergo aldol condensation.
        • Cannizzaro reaction: aldehydes without α-H hydrogen undergo disproportionation; ketones do not.

        Distinction: Aldehydes vs Ketones (Distinction Tests)

        • Tollen’s reagent: positive for aldehydes; negative for ketones.
        • Fehling’s solution: positive for aldehydes; negative for ketones.
        • Schiff’s reagent: positive for aldehydes; negative for ketones.
        • Reduction with NaBH extsubscript{4}: aldehydes → primary alcohols; ketones → secondary alcohols.

        Health & Safety Aspects

        • Aldehydes (e.g., formaldehyde, acetaldehyde) are irritants; exposure can irritate eyes, skin, and respiratory tract; some are probable human carcinogens.
        • Prolonged exposure linked to various diseases (cancer, cardiovascular, neurodegenerative).
        • Ketones: excessive ketosis/ketoacidosis is a risk in medical contexts (diabetes-related).