Introduction
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 Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole bonding. 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 . nucleophile
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6.1.2 Carbonyl compounds
Not explicitly stated but required for subsequent topics.
Oxidation reactions PAG7
Oxidation Reactions Primary alcohol Tertiary alcohols Secondary alcohol aldehydes carboxylic acid ketones do not oxidise Potassium dichromate K2Cr2O7 is an oxidising agent that causes alcohols and aldehydes to oxidise. Key point: Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised. Reaction: aldehyde carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: heat under reflux 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 Tollen’s Reagent. This are used as a test for the presence of aldehyde groups Tollen’s Reagent CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+ 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.
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6.1.2 Carbonyl compounds
Reactions of carbonyl compounds (a) oxidation of aldehydes using Cr2O7 2–/H+ (i.e. K2Cr2O7/H2SO4) to form carboxylic acids In equations for organic redox reactions, [O] and [H] should be used.
Nucleophilic addition
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 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 H3C C CH3 O H- δ + δ – C H H3C CH3 – O H N Goalby chemrevise.org O H H +OHAddition 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 :CN- δ + δ – : – H+ C CN H3C CH3 O C CN
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6.1.2 Carbonyl compounds
(b) nucleophilic addition reactions of carbonyl compounds with: (i) NaBH4 to form alcohols (ii) HCN [i.e. NaCN(aq)/H+(aq)], to form hydroxynitriles (see also 6.2.4 b) (c) the mechanism for nucleophilic addition reactions of aldehydes and ketones with NaBH4 and HCN For NaBH4, the nucleophile can be considered as being the hydride ion, H–, with subsequent protonation of the organic intermediate from H2O. For HCN, initial nucleophilic attack is from CN– ions; subsequent protonation stage can be shown using H2O or H+. HSW1,2,8 Use of reaction mechanisms to explain organic reactions.
Characteristic tests PAG7
Reaction with 2,4-dinitro phenylhydrazine 2,4-DNP reacts with both aldehydes and ketones. The product is an orange precipitate, It can be used as a test for a carbonyl group in a compound. Use 2,4-DNP to identify if the compound is a carbonyl. Then to differentiate an aldehyde from a ketone use Tollen’s reagent. NO2 O2N H3C C H2N NH H O + NO2 O2N 3CH C NH NH H OH NO2 O2N 3CH C N NH H addition elimination of water .. 2,4-DNP orange precipitate You don’t need to learn these equations for the exam Tollen’s Reagent CH3CHO + 2Ag+ + H2O CH3COOH + 2Ag + 2H+ 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. The melting point of the crystal formed can be used to help identify which carbonyl was used. Take the melting point of orange crystals product from 2,4-DNP. Compare melting point with known values in database
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6.1.2 Carbonyl compounds
Characteristic tests for carbonyl compounds (d) use of 2,4-dinitrophenylhydrazine to: (i) detect the presence of a carbonyl group in an organic compound (ii) identify a carbonyl compound from the melting point of the derivative The equation for this reaction is not required. Structure of derivative not required. PAG7 (see also 6.3.1 c) HSW4 Qualitative analysis. (e) use of Tollens’ reagent (ammoniacal silver nitrate) to: (i) detect the presence of an aldehyde group (ii) distinguish between aldehydes and ketones, explained in terms of the oxidation of aldehydes to carboxylic acids with reduction of silver ions to silver. In equations involving Tollens’ reagent, [O] is acceptable. PAG7 (see also 6.3.1 c) HSW4 Qualitative analysis.
Credits: Neil Goalby