Carbonyl compounds
What is the steric reason for Aldehydes being generally more reactive than ketones in nucleophilic addition reactions?
(i) Steric reason: The presence of two relatively large substituents in ketones versus only one large substituent in aldehydes means that attacking nucleophiles are able to approach the carbonyl carbon in aldehydes with less steric hinderance than in ketones.
What is the electronic reason for Aldehydes being generally more reactive than ketones in nucleophilic addition reactions?
(ii) Electronic reason: Aldehydes have one electron donating alkyl group while ketones have two. Hence, an aldehyde- carbon has a higher partial positive charge compared to a ketone-carbon, making the former more susceptible towards nucleophilic attack.
Describe oxidation of carbonyl compounds.
The aldehyde group undergoes oxidation reactions with acidified potassium dichromate(VI), Tollens' reagent and Fehling's solution, while the ketone group does not. As such, these three tests are good distinguishing tests between aldehydes and ketones.
Why do Aldehydes and ketones have higher boiling points than corresponding hydrocarbons of similar number of electrons in the molecule?
The carbonyl group is polar, thus permanent dipole-permanent dipole interactions exist between molecules in aldehydes and ketones (in addition to dispersion forces) which require more energy to overcome compared to only dispersion forces present in non-polar alkanes. Hence, carbonyl compounds have higher boiling point than the corresponding alkanes.
What is the importance of cyanohydrins in synthesis?
The cyanohydrin product can be easily converted to other usable forms since the nitrile group (-CN) can be converted to other functional groups such as: ➢ carboxylic acids by acid hydrolysis using mineral acids such as dilute H2SO4 ➢ carboxylate salts by alkaline hydrolysis using alkalis such as dilute NaOH ➢ amines by reduction using lithium aluminium hydride (LiAlH4) in dry ether or hydrogen with nickel catalyst
Describe the stereochemistry of the carbonyl carbon in aldehydes and ketones in nucleophilic addition wth HCN.
The geometry about the carbonyl carbon atom in aldehydes and ketones is planar. Hence the nucleophile can attack the + carbonyl carbon atom in equal probability from the top or bottom of the plane, producing equal amounts of both enantiomers to give a racemic mixture, if the resulting sp3 carbon is chiral.
Why do aldehydes and ketones have lower boiling points than their corresponding alcohols and carboxylic acids?
Alcohols and carboxylic acids can form hydrogen bonds between molecules, which are stronger and require more energy to overcome than permanent dipole-permanent dipole interactions in the corresponding aldehydes and ketones. Hence, carbonyl compounds have lower boiling point than the corresponding alcohols and carboxylic acids.
What is the equation for oxidation of aldehyde with Tollen's reagent?
Aldehyde + 2[Ag(NH3)2]+ + 3OH- —> carboxylate ion + 2Ag + 4NH3 + 2H2O
Describe the oxidation of aldehyde with acidified potassium dichromate(VI) or acidified potassium manganate(VII).
Aldehyde has a hydrogen atom attached directly to the carbonyl carbon. This enables it to be oxidised to carboxylic acids.
What is the equation for the tri-iodomethane (iodoform) test of carbonyl compounds.
Ethanol or methyl ketones + 3I2(aq) + 4OH- —> carboxylate ion + CHI3 + 3I- + 3H2O
Describe the oxidation of aldehyde with Fehling's reagent.
Fehling's reagent is an alkaline solution of copper(II) tartrate. Aldehydes (except benzaldehyde and its derivatives) reduce the copper(II) in Fehling's reagent to the reddish-brown copper(I) oxide, which is precipitated.
How does adding trace amount of KCN as catalyst increase the rate of reaction of nucleophilic addition of aldehydes and ketones with HCN?
HCN is a weak acid, which only partially ionises in water to give CN−. HCN + H2O ⇌ H3O+ + CN- The reaction takes place very slowly if only HCN is used because the solution contains a very low concentration of CN- ions. The reaction can be speeded up by adding a trace amount of KCN, a salt which completely dissociates to provide free CN- ions. KCN (aq) → K+ (aq) + CN- (aq) CN- ions used in the addition reaction are regenerated at the end of the reaction as shown in the mechanism.
What are the reagents and conditions for nucleophilic addition of aldehydes and ketones with hydrogen cyanide?
HCN with trace KCN, cold KCN is used as a catalyst; CN- ion is regenerated at the end of the reaction.
How does adding a small amount of base increase the rate of reaction of nucleophilic addition of aldehydes and ketones with HCN?
HCN+H2O⇌H3O+ +CN− When a small amount of base is added, the H3O+ ions are neutralised, which by Le Chatelier's Principle, shifts the position of the above equilibrium to the right such that the concentration of the CN- increases. Hence the rate of reaction increases.
What are the reagents, condition and observations of oxidation of aldehyde with Fehling's reagent?
Reagents and conditions: Fehling's reagent, heat Observations: Reddish-brown (brick-red) precipitate formed Fehling's reagent deteriorates on keeping. and is usually prepared in two parts which are added together just before the test is carried out. It consists of: 1. Fehling's solution A (CuSO4 solution) and 2. Fehling's solution B (sodium potassium tartrate + excess NaOH) A deep blue solution is obtained on mixing the two solutions owing to the formation of copper(II) tartrate, the tartrate ions present prevent the precipitation of Cu(OH)2 by complexing with the copper(II) ions.
Compare the reducing agents LiAlH4 and NaBH4.
Lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4) are useful hydride reagents that can reduce aldehydes and ketones in excellent yields. LiAlH4 is a grayish powder that is soluble in ether. It reacts violently with water to form H2 gas and decomposes explosively when heated above 120 ºC. NaBH4 is a white, crystalline solid that can be handled in the open atmosphere and used in either water or alcohol solution. Due to the hazardous nature of LiAlH4 and ether (which forms highly explosive mixtures in air), the use of NaBH4 is usually preferred. Both LiAlH4 and NaBH4 can be considered as sources of the nucleophilic hydride anion, H-. They both have hydrogen atoms covalently bonded to aluminium and boron respectively and because hydrogen is more electronegative than aluminium and boron, the hydrogen atoms will bear the negative charge, making it a good nucleophile. As such, LiAlH4 and NaBH4 are able to react with aldehydes and ketones bearing the polar C=O group with an electron-deficient carbon. Aluminiumislesselectronegativethanboron,thusmoreofthenegativechargeintheAlH4- ion is borne by the hydrogen atoms. Therefore, LiAlH4 is a much stronger reducing agent, and is much less selective than NaBH4. Both LiAlH4 and NaBH4 do not reduce the non-polar electron-rich alkene group. The alkene functional group is reduced using H2(g) over a nickel catalyst.
What are the reagents and conditions and observations of oxidation of aldehyde with acidified potassium dichromate(VI) or acidified potassium manganate(VII)?
Reagents and conditions: K2Cr2O7 / dilute H2SO4 or KMnO4 / dilute H2SO4, heat under reflux Observations: Orange solution (Cr2O72-) turns green (Cr3+) or Purple solution (MnO4-) turns colourless (Mn2+)
What are the reagents and conditions of reduction of aldehydes and ketones?
Reagents and conditions: LiAlH4 in dry ether, room temperature When the aldehyde or ketone react with LiAlH4, the alkoxide ion is form. H2O needs to be added to protonate the alkoxide to form the alcohol. Other reducing agents like NaBH4 in methanol or, H2(g)/ Ni at high pressure may be used.
How can aldehydes be prepared?
Oxidation of primary alcohols
How can ketones be prepared?
Oxidation of secondary alcohols Oxidative cleavage of substituted alkenes
Describe aldehydes (RCHO).
R = H, alkyl or aryl For the 1st member of the aldehydes, R = H
Describe ketones (RCOR')
R/ R' = alkyl or aryl The 1st member of the ketones can only be propanone
What are the reagents and conditions and observations of Condensation reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) with carbonyl compounds?
Reagents and conditions: 2,4-dinitrophenylhydrazine (2,4-DNPH), room temperature Observations: Orange precipitate formed
What are the reagents, conditions and observations of tri-iodomethane (iodoform) test of carbonyl compounds.
Reagents and conditions: Aqueous I2 with NaOH(aq), warm Observations: Yellow crystals of CHI3 (iodoform) formed.
What are the reagents, conditions and observations of oxidation of aldehyde with Tollen's reagent?
Reagents and conditions: Tollens' reagent, heat Observations: Silver mirror formed [Ag(NH3)2]+ is unstable and should always be freshly prepared. It can be prepared as follows: 1. One drop of dilute NaOH(aq) is added to about 3 cm3 of AgNO3(aq) to produce a dark brown precipitate of Ag2O. 2. Dilute NH3(aq) is then added dropwise until the brown precipitate first formed just redissolves.
Describe isomerism of carbonyl compounds.
Saturated aliphatic aldehydes and ketones have the general formula CnH2nO where n equals to the number of the carbon atoms in the compound. The aliphatic aldehydes and ketones are constitutional isomers if they have the same number of carbon atoms, n, in the compound. For ketones, the ketone group may occur at different positions.
Describe the general mechanism of nucleophilic addition. (Draw)
Step 1: A nucleophile attacks the electron-deficient carbonyl carbon, breaking the pi bond of the C=O group, to form an alkoxide ion. Step 2: The tetrahedral intermediate is protonated by water or acid to give an alcohol as the final product.
Describe the tri-iodomethane (iodoform) test of carbonyl compounds.
This test specifically identifies aldehydes and ketones with the structure: O=C-CH3 | R "R" can be a hydrogen atom or a hydrocarbon group (alkyl or aryl). If "R" is hydrogen, the compound is ethanal, CH3CHO. Ethanal is the only aldehyde that gives positive tri-iodomethane test. This reaction breaks a C-C bond and removes a methyl (-CH3) group. It is therefore a useful method of shortening a carbon chain by a single carbon atom (step-down reaction).
Describe the oxidation of aldehyde with Tollen's reagent (silver mirror test).
Tollens' reagent (ammoniacal silver nitrate) contains diamminesilver(I) ions, [Ag(NH3)2]+. Aldehydes reduce silver(I) in Tollens' reagent to silver metal. Under carefully controlled conditions, silver metal will deposit on the walls of the reaction vessel as a mirror.
Describe what happens with benzaldehyde and Fehling's reagent?
benzaldehyde undergoes disproportionation instead due to the presence of NaOH in Fehling's reagent.
What can be done to increase the rate of reaction of nucleophilic addition of aldehydes and ketones with HCN?
1. Add trace amount of KCN as catalyst 2. Add a small amount of base (e.g. NaOH)
Describe the condensation reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH) to detect the presence of carbonyl compounds.
2,4-dinitrophenylhydrazine, also known as Brady's reagent, is a yellow solution. It reacts with aldehydes and ketones to form 2,4-dinitrophenylhydrazone, which is an orange precipitate. A water molecule is eliminated in the reaction. This is a characteristic test for the presence of aldehydes and ketones.
What is a carbonyl group?
A carbonyl group is composed of a carbon atom which is double bonded to an oxygen atom. Aldehydes and ketones.
Describe the solubility of aldehydes and ketones in water.
Aldehydes and ketones have both polar and non-polar regions. They can act as solvents to both polar and non-polar solutes, which gives them a wide range of uses as solvents. The smaller aliphatic aldehydes and ketones are soluble in water due largely to their ability to form hydrogen bonds with water molecules. Carbonyl compounds containing more than 5 carbons or aromatic rings, are virtually insoluble in water. As the number of carbon atoms in the alkyl chain increases, the strength of the dispersion forces between molecules of the carbonyl compound increases. Energy released from hydrogen bonding formed between the carbonyl group and water is less able to overcome the increasingly stronger dispersion forces, as well as the existing hydrogen bonding in water. In addition, the hydrogen bonding with water is disrupted by the larger non-polar alkyl chain.
Describe the boiling points of aldehydes and ketones with respect too their corresponding hydrocarbons, alcohols and Carboxylic acids.
Aldehydes and ketones have higher boiling points than corresponding hydrocarbons of similar number of electrons in the molecule, but have lower boiling points than their corresponding alcohols and carboxylic acids.
Describe nucleophilic addition mechanism of aldehydes and ketones with hydrogren cyanide, HCN.
Aldehydes and ketones undergo nucleophilic addition with hydrogen cyanide, HCN, under appropriate conditions to form cyanohydrins. Step 1: The carbonyl carbon is attacked by the CN- nucleophile ➢ The CN- ion acts as nucleophile and attacks the electron-deficient carbonyl carbon breaking the π-bond of the C=O. ➢ This leads to the formation of a stable tetrahedral intermediate anion (an alkoxide ion). ➢ This is the rate-determining step. Step 2: Protonation of the intermediate to give a cyanohydrin ➢ The tetrahedral alkoxide ion intermediate is protonated by attacking an undissociated HCN molecule to yield the cyanohydrin. ➢ The CN- nucleophile is regenerated. rate = k [carbonyl compound] [CN-] Nucleophilic addition with CN- ion is an important method of lengthening the carbon chain by 1 carbon (step-up reaction).
Describe the reduction and aldehydes and ketones.
Aldehydes and ketones undergo reduction to form primary and secondary alcohols respectively. Aldehyde + 2[H] —> primary alcohol Ketone + 2[H] —> secondary alcohol
Describe the relative reactivity of aldehydes and ketones in nucleophilic addition reactions.
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions.
What is the equation for oxidation of aldehyde with Fehling's reagent.
Aliphatic aldehyde + 2Cu2+ + 5OH- —> carboxylate ion + Cu2O + 3H2O
Which carbonyl compounds do not reduce Fehling's reagent?
Ketones, benzaldehyde and its derivatives do not reduce Fehling's reagent.
Describe the reactivity of aldehydes and ketones towards nucleophilic reagents and addition.
LiAlH4, NaBH4 and HCN are nucleophilic reagents. The δ+C=Oδ- bond is polar, so the aldehyde-carbon or the ketone-carbon has δ+ charge. δ+ carbon attracts nucleophiles. Hence, aldehydes and ketones react with the above nucleophilic reagents.
Describe the condensation reaction of carbonyl compounds.
Nucleophiles having two H atoms on a nitrogen atom readily add onto carbonyl compounds. But usually, the initially formed addition product is not isolated. The reaction can be viewed as an addition, followed by an elimination, hence it may be referred to as an addition-elimination reaction. Step 1: Addition of nucleophile to carbonyl compound Step 2: Elimination of a water molecule
