Module 4 revision guide: Compounds with C=O group

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1 opyright N Goalby Bancroft's School Module 4 revision guide: ompounds with = group arbonyls: Aldehydes and Ketones arbonyls are compounds with a = bond, they can be either aldehydes or ketones. 3 ethanal If the = is on the end of the chain with an attached it is an aldehyde. The name will end in al 3 3 propanone If the = is in the middle of the chain it is a ketone The name will end in -one Solubility in water The smaller carbonyls are soluble in water because they can form hydrogen bonds with water. 3 3 Pure carbonyls cannot hydrogen bond, but bond instead by permanent dipole bonding. Reactions of carbonyls The = bond is polarised because is more electronegative than carbon. The positive carbon atom attracts nucleophiles. In comparison to the = bond in alkenes, the = is stronger and does not undergo addition reactions easily. This is in contrast to the electrophiles that are attracted to the =. xidation Reactions Potassium dichromate K 2 r 2 7 is an oxidising agent that causes alcohols and aldehydes to oxidise. xidation of Aldehydes Primary alcohol aldehydes carboxylic acid Secondary alcohol Tertiary alcohols Reaction: aldehyde carboxylic acid Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. onditions: heat under reflux ketones do not oxidise + [] Key point: Aldehydes can be oxidised to carboxylic acids but ketones cannot be oxidised. Full Equation for oxidation r r 3+ bservation: the orange dichromate ion (r ) reduces to the green r 3+ ion R + [] R 2 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 Reagent: Tollen s Reagent formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of [Ag(N 3 ) 2 ] +. onditions: heat gently Reaction: aldehydes only are oxidised by Tollen s reagent into a carboxylic acid and the silver(i) ions are reduced to a silver atoms bservation: with aldehydes, a silver mirror forms coating the inside of the test tube Ketones result in no change 3 + 2Ag Ag Fehling s solution Reagent: Fehling s Solution containing blue u 2+ ions. onditions: 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.. bservation: Aldehydes :Blue u 2+ ions in solution change to to red precipitate of u 2. Ketones do not react 3 + 2u u

2 opyright N Goalby Bancroft's School Reduction of carbonyls Reagents: NaB 4 In aqueous ethanol onditions: Room temperature and pressure Reducing agents such as NaB 4 (sodium tetrahydridoborate) or LiAl 4 (lithium tetrahydridoaluminate) will reduce carbonyls to alcohols. Aldehydes will be reduced to primary alcohols + 2[] propanal Propan-1-ol Ketones will be reduced to secondary alcohols. + 2[] propanone Propan-2-ol Nucleophilic Addition Mechanism + from water or weak acid NaB 4 contain a source of nucleophilic hydride ions ( - ) which are attracted to the positive carbon in the = bond. atalytic ydrogenation arbonyls can also be reduced using catalytic hydrogenation Reagent: hydrogen and nickel catalyst onditions: high pressure Example Equations () 3 Addition of hydrogen cyanide to carbonyls to form hydroxynitriles Reaction: carbonyl hyroxynitrile Reagent: sodium cyanide (NaN) and dilute sulphuric acid. onditions: Room temperature and pressure Mechanism: nucleophilic addition The NaN supplies the nucleophilic N - ions. The 2 S 4 acid supplies + ions needed in second step of the mechanism N R hydroxy nitrile N 3 ()(N) 3 3 N 2-hydroxy-2-methylpropanenitrile 3 When naming hydroxy nitriles the N becomes part of the main chain 3 + N 3 ()N 2-hydroxypropanenitrile N 3 We could use N for this reaction but it is a toxic gas that is difficult to contain. The KN/NaN are still, however, toxic, because of the cyanide ion Nucleophilic Addition Mechanism + from sulphuric acid 2

3 opyright N Goalby Bancroft's School Nucleophilic addition of N to aldehydes and ketones (unsymmetrical) when the trigonal planar carbonyl is approached from both sides by the N attacking species: results in the formation of a racemate N: 3 :N Mechanism for the reaction (drawn the same for both enantiomers) N 3 There is an equal chance of either enantiomer forming so a racemate forms. No optical activity is seen 3 N arboxylic Acids Acidity The carboxylic acid are only weak acids in water and only partially dissociate, but they are strong enough to displace carbon dioxide from carbonates. 3 2 (aq) 3 2- (aq)+ + (aq) Solubility in Water δ + The smaller carboxylic (up to 4) δ acids dissolve in water in all δ + δ proportions but after this the solubility δ + rapidly reduces. They dissolve 3 δ + δ because they can hydrogen bond to δ the water molecules. δ δ + Delocalisation The carboxylic acid salts are stabilised by delocalisation, which makes the dissociation more likely. 3 delocalised 3 The delocalised ion has equal - bond lengths. If delocalisation did not occur, the = bond would be shorter than the - bond. Salt formation reactions of carboxylic acids arboxylic acids can form salts with metals, alkalis and carbonates. acid + metal (Na) salt + hydrogen Na 3 2- Na acid + alkali (Na) salt + water Na 3 2- Na acid + carbonate (Na 2 3 ) salt + water Na Na The pi charge cloud has delocalised and spread out. The delocalisation makes the ion 3 more stable and therefore more likely to form. The effervescence caused by production of 2 with carboxylic acids with solid Na 2 3 or aqueous Na 3 can be used as a functional group test for carboxylic acids xidation of methanoic acid arboxylic acids cannot be oxidised by using oxidising agents but methanoic acid is an exception as its structure has effectively an aldehyde group + [] It forms carbonic acid ( 2 3 ) which can decompose to give 2 3

4 opyright N Goalby Bancroft's School Esterification arboxylic acids react with alcohols,in the presence of a strong acid catalyst, to form esters and water. Esters have two parts to their names, eg methyl propanoate. arboxylic Acid + Alcohol + Ester + water The bit ending in anoate comes from the carboxylic acid and includes the in the = bond. The bit ending in yl comes from the alcohol that has formed it and is next to the single bonded oxygen Ethanoic acid Ethanol Ethyl Ethanoate + 2 The reaction is reversible. The reaction is quite slow and needs heating under reflux, (often for several hours or days). Low yields (50% ish) are achieved. An acid catalyst ( 2 S 4 ) is needed. Uses of Esters Esters are sweet smelling compounds that can be used in perfumes and flavourings. Esters can have pleasant smells For use in perfumes they need to be non toxic, soluble in solvent such as ethanol, volatile (turns into gas easily), and not react with water. Esters can be used as solvents for polar organic substances Ethyl ethanoate is used as a solvent in glues and printing inks Esters can be used as plasticisers for polymers Although polar, they do not form hydrogen bonds (reason: there is no hydrogen bonded to a highly electronegative atom) thus, they have much lower b.p. than the hydrogen-bonded carboxylic acids they came from. They are also almost insoluble in water ften pure polymers have limited flexibility because the polymer chains cannot move over each other. Incorporating some plasticiser into the polymer allows the chains to move more easily and the polymer can become more flexible. ydrolysis of esters Esters can be hydrolysed and split up by either heating with acid or with sodium hydroxide. i) with acid reagents: dilute acid (l) conditions: heat under reflux This reaction is the reverse reaction of ester formation. When an ester is hydrolysed a carboxylic acid and an alcohol are formed ethyl propanoate This reaction is reversible and does not give a good yield of the products. ii) with sodium hydroxide reagents: dilute sodium hydroxide conditions: heat under reflux This reaction goes to completion Na Na methyl propanoate sodium propanoate methanol The carboxylic acid salt product is the anion of the carboxylic acid. The anion is resistant to attack by weak nucleophiles such as alcohols, so the reaction is not reversible. 4

5 opyright N Goalby Bancroft's School Fats and soaps Fats and oils are ESTERS of glycerol and long chain carboxylic acids (fatty acids) Vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic (fatty) acids 2 2 ( 2 ) 16 3 ( 2 ) 16 3 ( 2 ) K 2 2 glycerol ( 2 ) 16 2 K + long chain carboxylic (fatty) acids Glycerol forms hydrogen bonds very easily and is readily soluble in water. It is used in cosmetics, food and in glues Soap Long chain carboxylic (fatty) acids act as soaps. The polar - 2 end is hydrophilic and mixes with water. The long non-polar hydrocarbon chain is hydrophobic and mixes with grease. So this allows the grease and water to mix and be washed away Biodiesel biodiesel is a mixture of methyl esters of long chain carboxylic acids Vegetable oils can be converted into biodiesel by reaction with methanol in the presence of a (strong alkali) catalyst 2 2 ( 2 ) 14 3 ( 2 ) 14 3 ( 2 ) ( 2 ) It can be argued that biodiesel produced from this method is classed as carbon neutral as any carbon dioxide given off when the biofuel is burnt would have been extracted from the air by photosynthesis when the plant grew. This does not take into account any energy needed to irrigate plants, extract the oil, heat the reaction with methanol mixture or process the fuel. If the energy for this process comes from fossil fuels then the biofuel produced is not carbon neutral It also does not take into account the effect on land available for food production 5

6 arboxylic acid derivatives: acyl chlorides and acid anhydrides opyright N Goalby Bancroft's School Acyl hlorides 3 l ethanoyl chloride Acyl chlorides are much more reactive than carboxylic acids The l and 3 groups are classed as good leaving groups (to do with less effective delocalisation.) This makes acyl chlorides and acid anhydrides much more reactive than carboxylic acids and esters Acid Anhydrides 3 3 ethanoic anhydride. Acid anhydrides have a similar reactivity to acyl chlorides and therefore bring about the same changes in functional groups. The main difference is the byproducts. Acyl chlorides mostly give off l. Acid anhydrides give off R Reaction with water hange in functional group: acyl chloride carboxylic acid Reagent: water onditions: room temp. hange in functional group: acid anhydride carboxylic acid Reagent: water onditions: room temp. Rl (l) + 2 R 2 + l (g) l (g) l bservation: Steamy white fumes of l are given off 3 3 (R) R Nucleophilic Addition Elimination Mechanism Reaction with alcohol 3 hange in functional group: acyl chloride ester Reagent: alcohol onditions: room temp. Rl (l) R l (g) l l bservation: Steamy white fumes of l are given off hange in functional group: acid anhydride ester Reagent: alcohol onditions: room temp. 3 3 (R) R R This reaction for making esters is much better than using carboxylic acids as the reaction is much quicker and it is not a reversible reaction Nucleophilic Addition Elimination Mechanism 6

7 opyright N Goalby Bancroft's School Reaction with ammonia hange in functional group: acyl chloride primary amide Reagent: ammonia onditions: room temp. hange in functional group: acid anhydride primary amide Reagent: ammonia onditions: room temp. Rl (l) +2N 3 RN 2 + N 4 l (s) (R) 2 + 2N 3 RN 2 + R 2- N N 3 + N 4 l (s) 3 3 l + 2N N 3 2- N N 2 bservation: white smoke of N 4 l is given off Nucleophilic Addition Elimination Mechanism Reaction with primary amines hange in functional group: acyl chloride secondary amide Reagent: primary amine onditions: room temp. hange in functional group: acid anhydride secondary amide Reagent: primary amine onditions: room temp. 3 Rl +2 3 N 2 RN N 3+ l - (R) N 2 RN 3 + R 2-3 N N 3+ l N 2 3 N 3 + l +2 3 N N N-methylethanamide 3 N 3 3 N-methylethanamide Nucleophilic Addition Elimination Mechanism Making Aspirin Aspirin Aspirin is made from 2- hydroxybenzoic acid which contains a phenol group. In the reaction the phenol group is turned into an ester by reacting it with the reactive ethanoic anhydride Ethanoic anhydride is used instead of acid chlorides because it is cheaper, less corrosive, less vulnerable to hydrolysis, and less dangerous to use. 7

H H O C C O H Carboxylic Acids and Derivatives C CH 2 C. N Goalby chemrevise.org. Strength of carboxylic acids.

H H O C C O H Carboxylic Acids and Derivatives C CH 2 C. N Goalby chemrevise.org. Strength of carboxylic acids. 19 arboxylic Acids and Derivatives Naming arboxylic acids These have the ending -oic acid but no number is necessary for the acid group as it must always be at the end of the chain. The numbering always