Aldehydes and ketones

Aldehydes An aldehyde’s name ends in –al It always has the C=O bond on the first carbon of the chain so it does not need an extra number. It is by default number one on the chain Ethanal. The prefix oxo- should be used for compounds that contain a ketone group in addition to a carboxylic acid or aldehyde.Ketones end in -one When ketones have 5C’s or more in a chain then it needs a number to show the position of the double bond. E.g. pentan-2-one. If two ketone groups then di is put before –one and an an e is added to the stem. Carbonyls: Aldehydes and Ketones . Carbonyls are compounds with a C=O bond. They can be either aldehydes or ketones If the C=O is on the end of the chain with an H attached it is an aldehyde. The name will end in –al CH3CHO ethanal CH3COCH3 If the C=O is in the middle of the chain it is a ketone The name will end in -one propanone Solubility in water The smaller carbonyls are soluble in water because they can form hydrogen bonds with water. C CH3 CH3 O H O H Intermolecular forces in Carbonyls Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole forces. Reactions of carbonyls In comparison to the C=C bond in alkenes, the C=O is stronger and does not undergo addition reactions easily. The C=O bond is polarised because O is more electronegative than carbon. The positive carbon atom attracts nucleophiles. This is in contrast to the electrophiles that are attracted to the C=C .

Oxidation of Aldehydes RCHO + [O]  RCO2H C + [O]  O C H H C H H H H C C O H O H C H H H H Full Equation for oxidation 3CH3CHO + Cr2O7 2- + 8H+  3 CH3CO2H + 4H2O + 2Cr3+ Observation: the orange dichromate ion (Cr2O7 2-) reduces to the green Cr 3+ ion Aldehydes can also be oxidised using Fehling’s solution or Tollen’s Reagent. These are used as tests for the presence of aldehyde groups Tollen’s Reagent CH3CHO + 2Ag+ + H2O  CH3COOH + 2Ag + 2H+ Reagent: Fehling’s Solution containing blue Cu 2+ ions. Conditions: heat gently Reaction: aldehydes only are oxidised by Fehling’s Solution into a carboxylic acid and the copper ions are reduced to copper(I) oxide . . Observation: Aldehydes :Blue Cu 2+ ions in solution change to a red precipitate of Cu2O. Ketones do not react. Fehling’s solution CH3CHO + 2Cu2+ + 2H2O  CH3COOH + Cu2O + 4H+ Reagent: Tollen’s Reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(NH3 )2 ]+ . Conditions: heat gently Reaction: aldehydes only are oxidised by Tollen’s reagent into a carboxylic acid and the silver(I) ions are reduced to silver atoms Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no change. Reaction: aldehyde  carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: heat under reflux

Reduction of carbonyls Reducing agents such as NaBH4 (sodium tetrahydridoborate) or LiAlH4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols. Aldehydes will be reduced to primary alcohols Ketones will be reduced to secondary alcohols. propanone C C C O H H H H H H C H H C H C H H H O H C + 2[H]  H O C H H C H H H H + 2[H]  C O H H H C H H C H H H propanal Propan-1-ol Propan-2-ol Reagents: NaBH4 In aqueous ethanol Conditions: Room temperature and pressure NaBH4 contain a source of nucleophilic hydride ions (H-) which are attracted to the positive carbon in the C=O bond. Nucleophilic Addition Mechanism H+ from water or weak acid Catalytic Hydrogenation Carbonyls can also be reduced using catalytic hydrogenation Reagent: hydrogen and nickel catalyst Conditions: high pressure CH3CHO + H2  CH3CH2OH Example Equations CH3COCH3 + H2  CH3CH(OH)CH3

Addition of hydrogen cyanide to carbonyls to form hydroxynitriles Reaction: carbonyl  hydroxynitrile Reagent: sodium cyanide (NaCN) and dilute sulphuric acid. Conditions: Room temperature and pressure Mechanism: nucleophilic addition NC C R H OH hydroxynitrile The NaCN supplies the nucleophilic CNions. The H2SO4 acid supplies H+ ions needed in second step of the mechanism CH3COCH3+ HCN  CH3C(OH)(CN)CH3 CH3CHO + HCN  CH3CH(OH)CN When naming hydroxy nitriles the CN becomes part of the main chain 2-hydroxy-2-methylpropanenitrile 2-hydroxypropanenitrile Nucleophilic Addition Mechanism H+ from sulphuric acid NC C CH3 CH3 OH NC C CH3 H OH We could use HCN for this reaction but it is a toxic gas that is difficult to contain. The KCN/NaCN are still, however, toxic, because of the cyanide ion. H3C C CH3 O :H- δ + δ – : H+ C H H3C CH3 O H – H3C C CH3 O :CN- δ + δ – :- H+ C CN H3C CH3 O C CN H3C CH3 O H N Goalby chemrevise.org H3C C H O 3 :CN NC: C CH3 O H C NC CH3 OH H C H3C CN OH H There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen Nucleophilic addition of HCN to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the HCN attacking species: results in the formation of a racemate Mechanism for the reaction (drawn the same for both enantiomers) :CN- δ + δ – :-