1092662sefw64-F324-Carbonyls (1)

OCR F324 Carbonyl compounds

Carbonyl Chemistry

The carbonyl functional group (>C=O) is found in aldehydes and ketones. It also forms part of the carboxylic acid (-COOH) and ester (-COO-) functional group.

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Reactions of carbonyl compounds

• describe the oxidation of alcohols using Cr2O72–/H+ (ie K2Cr2O7/H2SO4), including:

(i) the oxidation of primary alcohols to form aldehydes and carboxylic acids, including the control of the oxidation product using different reaction conditions(ii) the oxidation of secondary alcohols to form ketones

• describe the oxidation of aldehydes using Cr2O72–/H+ to form carboxylic acids

• describe the reduction of carbonyl compounds using NaBH4 to form alcohols

• outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides, such as NaBH4. The nucleophile can be considered as being the hydride ion, H–, with subsequent protonation of the organic intermediate from H2O. In equations for organic redox reactions, [O] and [H] should be used.

Characteristic tests for carbonyl compounds

• describe the use of 2,4-dinitrophenylhydrazine to:(i) detect the presence of a carbonyl group in an organic compound (the structure of the derivative and the equation for this reaction is not required) (ii) identify a carbonyl compound from the melting point of the derivative

• describe the 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.

Properties of carboxylic acids• explain the water solubility of carboxylic acids in terms of hydrogen bonding and dipole–dipole interaction

• describe the reactions of carboxylic acids with metals, carbonates and bases.

Esters, triglycerides, unsaturated and saturated fats

• describe esterification of carboxylic acids with alcohols, in the presence of an acid catalyst

• describe reaction of acid anhydrides with alcohols to form esters• describe the hydrolysis of esters:

(i) in hot aqueous acid to form carboxylic acids and alcohols,(ii) in hot aqueous alkali to form carboxylate, salts and alcohols;

• state the uses of esters in perfumes and flavourings

• describe a triglyceride as a triester of glycerol (propane-1,2,3-triol) and fatty acids

• compare the structures of saturated fats, unsaturated fats and fatty acids, including cis and trans isomers, from systematic names and shorthand formulae e.g. octadecanoic acid, 18,0; octadec-9-enoic acid, 18,1(9); octadec-9,12-enoic acid, 18,2(9,12)

• compare the link between trans fatty acids, the possible increase in ‘bad’ cholesterol and the resultant increased risk of coronary heart disease and strokes. Consider the link between unsaturated and saturated fats and current concerns about heart disease and obesity• describe and explain the increased use of esters of fatty acids as biodiesel, and the subsequent use of biodiesel to increase the contribution to energy requirements from renewable fuels.

δ+ δ -

C = O


Aldehydes and ketones - Naming

- longest chain is 5 carbons, so "-pentan-" stem- ketones have "-one" ending- C=O is located on 3rd carbon in chain- chain also has a methyl side-group- methyl group on 2nd carbon (numbering from the end

which produces the smallest numbers in the name, i.e. from the right as drawn)

2-methylpentan-3-one

- longest chain is 5 carbons so "-pentan-" stem- aldehydes have an "-al" ending- numbering starts from the end with the aldehyde- methyl group is therefore on 4th carbon

4-methylpentanal

The simplest aromatic aldehyde and ketone are:benzaldehyde- colourless liquid (at RT) with almond-like aroma- almond essence, gives flavour to marzipan

phenylethanone- used to create fragrances which resemble cherry,

strawberry, honeysuckle or jasmine

How the carbonyl group reactsFirstly we need to understand a little more about the carbonyl group:1) Like a C=C double bond, it is comprised of a sigma bond and a pi bond formed

by the overlap of p-orbitals on the C and O atoms

2) Unlike a C=C double bond, the C=O bond has a dipole. The electrons in the sigma and pi bonds are more attracted to the O than the C. This is because oxygen is much more electronegative than carbon. As a result the C is δ+ and the O is δ-.

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CH3 CH2 C CH

O

CH3

CH3

CH3 CH CH2 CH2 C

O

HCH3

COH

CH3 C

O


Because of this the reactions of C=O are different to C=C; it is attacked by different reagents.

Chemical tests to distinguish carbonyl compounds1: Detecting an aldehyde or ketoneAldehydes and ketones react with Brady's Reagent to form an orange or yellow precipitate. Brady's Reagent is a solution of 2,4-dinitrophenylhydrazine (usually shortened to 2,4-DNP or 2,4-DNPH) in methanol and sulphuric acid. No precipitate is formed with other carbonyl compounds such as carboxylic acids or esters.

A few drops of the carbonyl compound are put in a test tube with about 5cm3 of Brady's reagent. The precipitate formed, referred to as a 2,4-dinitrophenylhydrazone derivative, can be used to help identify the specific aldehyde or ketone by measuring its melting point. This works well because the different derivatives have melting points which are many degrees apart.e.g. heptan-2-one b.p. = 151°C m.p of 2,4-DNP derivative = 90°Ccyclohexanone b.p. = 156°C = 162°Coctan-2-one b.p. = 173°C = 58°C

Identifying an aldehyde/ketone from the 2,4-DNPH derivative: The orange/yellow solid substance is purified by recrystallisation Melting point is determined Melting point is compared to a database of published melting points for these

derivativesYou will not be asked to write an equation for the formation of a 2,4-DNP derivative or to draw the structure of a 2,4-dinitrophenylhydrazone derivative.

2: Telling an aldehyde from a ketoneA further test is necessary to distinguish an aldehyde from a ketone. Aldehydes can be further oxidised to carboxylic acids, but ketones cannot.

Tollens' reagent is a weak oxidising agent containing silver nitrate in ammonia. The oxidising agent is the aqueous silver (I) ion, Ag+

(aq). When warmed with Tollens reagent, an aldehyde is oxidised to a carboxylic acid, and the silver ions in solution are reduced to silver metal. A "silver mirror" is formed on the walls of the test tube (or sometimes just a silver-grey solid is formed).

Oxidation of the aldehyde:R-CHO + [O] R-COOH

Reduction of the silver ions:Ag+

(aq) + e- Ag(s)

Note: Notice how in the structural formula above, the aldehyde is –CHO not COH. This is to prevent if being confused with an alchohol functional group.

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Oxidation Reactions involving carbonyl compoundsRecall that alcohols can be oxidized to form aldehydes, ketones and carboxyl acids. The product depends on:

- which alcohol you use (primary or secondary – tertiary alcohols don't react)- the reaction conditions

Reagent:The oxidizing agent is a solution containing acidified dichromate ions (H+/Cr2O7

2-)This is usually made using dilute sulphuric acid in which potassium dichromate is dissolved. The dichromate ions are an oxidising agent (so they themselves are reduced during the reaction):

Cr2O72- Cr3+

oxidation number of Cr changes from +6 to +3colour changes from orange to green

Make sure you learn this colour change which indicates that the dichromate ions have oxidised something !

In balanced equations we are allowed to represent the oxidising agent as [O].

The r eactions: Consider three of the four isomers of butanol – these examples give us primary, secondary and tertiary alcohols.

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Writing the equations: it is OK to use [O] for the oxidising agent remember there may be water formed as a product as well

Primary alcohols can react to form aldehydes:e.g. CH3CH2OH + [O] CH3CHO + H2O

Primary alcohols can also react to form carboxylic acids:e.g. CH3CH2OH + 2[O] CH3COOH + H2O

Secondary alcohols can only react to form ketones:e.g. CH3CH(OH)CH3 + [O] CH3COCH3 + H2O

Aldehydes react to form carboxylic acids:e.g. CH3CHO + [O] CH3COOH

Reduction reactions of carbonyl compoundsWe can reverse these reactions, turning aldehydes and ketones into alcohols, by reacting them with a suitable reducing agent.

Reagent:A suitably gentle reducing agent is NaBH4, sodium borohydride (sodium tetrahydridoborate III). Sodium borohydride is a source of hydride ions, H-, which are the actual reducing agent. We can represent these in equations by [H]. Sodium borohydride is not sufficiently good a reducing agent to reduce carboxylic acids.

Conditions:Carried out by warming the carbonyl compound with the reducing agent. Water is often used as the solvent.

Equations:Aldehydes are reduced to primary alcohols, e.g.

CH3CH2CHO + 2[H] CH3CH2CH2OH

Ketones are reduced to secondary alcohols, e.g.

CH3COCH3 + 2[H] CH3CH2(OH)CH3

Mechanism – NUCLEOPHILIC ADDITION: The H- ion is a hydrogen atom with an extra electron in its shell, meaning it has a lone pair, and is negatively charged. It can donate this lone pair to form a bond, so it is a nucleophile. Nucleophiles attack δ+ centres: in this case the δ+ carbon of the C=O.

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In the first stage of the reaction, the hydride ion attacks the δ+ carbon, forming a new bond, and at the same time the pi-bond between C and O atoms breaks, leaving an intermediate with a C-O single bond and a negative charge on the O atom.

In the second stage of the reaction, the oxygen in the intermediate donates a lone pair to take a hydrogen atom from a water molecule, forming a dative bond and releasing a hydroxide ion.

e.g. reduction of propanal

Carboxylic Acids and EstersCarboxylic acids contain the –COOH functional group, attached to an alkyl stem. They are widely found in nature, from the methanoic acid in ants, the ethanoic acid in vinegar to the citric acid in citrus fruits.

Esters contain the –COO- functional group, with an alkyl group attached to either side.Esters are synthesised by the reaction of carboxylic acids with alcohols, and are widely used in perfumes and flavourings since they are responsible for the flavours of many foods and the scents of flowers. Many esters are found in essential oils, obtained by steam distillation of plant matter e.g. benzyl ethanoate CH3COOCH2C6H5 is found in many flowers and is used to give apple/pear flavours in drinks and food; and in perfumes, shampoos, fabric softeners, soap, hairsprays and deodorants.

Naming For carboxylic acids, name the alkyl group, then replace the final 'e' with 'oic acid'. Remember that the carbon in the -COOH group is part of the alkyl chain for naming purposes, so:

HCOOH is methanoic acid CH3COOH is ethanoic acid etc.

The -COOH group can only go on the end of a chain, so no numbering is required to identify its position. Like in an aldehyde, the carbon in the –COOH group becomes number 1 in the chain.

For esters, the first part of the name is the name of the alkyl group attached to the oxygen atom in the –COO- group. This is named as usual (methyl, ethyl etc.). The second part of the name comes from the other alkyl group – the one containing the carbon of the COO

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Diagram:

C

O

O H OH

H

H

OH

:

: δ+

δ+ δ-

δ-


group. This is named like a carboxylic acid, by removing the 'e' and substituting 'oate'. Hence:

CH3COOC3H7 is propyl ethanoateCH3CH2COOCH3 is methyl propanoateCH3COOCH2(C6H5) is benzyl ethanoate

Solubility of carboxylic acidsShort chain carboxylic acid (1 – 4 carbons) are readily soluble in water. They can dissolve in water because the highly polar C=O and O-H groups in the carboxylic acid functional group can form hydrogen bonds to the water molecules. The alkyl group is non-polar and does not interact with the water molecules.

As the size of the alkyl group increases the solubility decreases. In order to fit the carboxylic acid molecule in between the water molecules, pre-existing hydrogen bonds in the water have to be broken. If the carboxylic acid molecules are small, the hydrogen bonds made with the carboxylic acid balance those which had to be broken, and the carboxylic acid dissolves. If the carboxylic acid molecules are large, many more hydrogen bonds in water would have to be broken than could be formed to the carboxylic acid, so the molecule is much more insoluble.

Reactions of carboxylic acids Carboxylic acids take part in typical acid reactions – reacting with metals, carbonates and bases to form salts. These salts are IONIC; the metal providing the positive ion and the carboxylic acid forming a negative carboxylate ion.

They are weak acids (partially dissociated in aqueous solution) unlike mineral acids such as HCl, H2SO4 or HNO3 which are fully dissociated. This means that carboxylic acids react more slowly.

1) with reactive metalscarboxylic acid + metal metal carboxylate (salt) + hydrogen

e.g. CH3COOH + Na CH3COO-Na+ + ½ H2

ethanoic acid sodium ethanoate

Observations: effervescence, the metal dissolves. Evaporation of the resulting solution will give a white crystals of sodium ethanoate

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CH3

O

O

CH3

O

O

CH3

O H

HO

CH3

O

+ H2O


2) with aqueous bases (e.g. metal hydroxides)carboxylic acid + metal hydroxide metal carboxylate (salt) + water

e.g. CH3COOH + NaOH CH3COO-Na+ + H2O

Observations: no fizzing etc ! This is a neutralisation reaction. An indicator would be needed to see when the solution had become neutral. Crystals of the salt can be obtained by evaporation.

3) with metal carbonatescarboxylic acid + metal carbonate metal carboxylate + water + carbon dioxide

e.g. 2 CH3COOH + Na2CO3 2 CH3COO-Na+ + CO2 + H2O

Observations: fizzing. Metal carbonate dissolves. Crystals of the salt can be obtained by evaporation.

Practice:1) Write balanced equations to show the reactions of butanoic acid with:

i) lithium, ii) calcium hydroxide, iii) magnesium carbonate

4) Carboxylic acids also react with alcohols to form esters. This is called an ESTERIFICATION.

conc. sulphuric acid catalyste.g. CH3CH2COOH + CH3OH CH3CH2COOCH3 + H2O

propanoic acid methanol methyl propanoate

Reactions of Esters1) Esters can also be formed from the reaction between an ACID ANHYDRIDE and an alcohol:

Acid anhydrides are molecules formed from two carboxylic acid molecules, by the removal of H2O:

When an acid anhydride is gently heated with an alcohol, the ester is formed and the yield is much better than we get by reacting carboxylic acids with alcohols.

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Definition: Esterification is the reaction of a carboxylic acid with an alcohol to produce an ester and water.


Practice: 2) Write an equation using displayed formulae or skeletal formulae for the reaction between an acid anhydride and an alcohol which would allow you to make the ester methyl propanoate.

2) Esters can be hydrolysed to form a carboxylic acid (or its salt) and an alcohol. i) Acid hydrolysis

Conditions: heat under refluxdilute sulphuric acid or hydrochloric acid catalyst

HCl(aq)

Reaction: CH3COOCH2CH2CH3 + H2O ⇌ CH3COOH + HOCH2CH2CH3

propyl ethanoate + water ethanoic acid + propan-1-ol

ii) Alkaline hydrolysisConditions: heat under reflux

aqueous sodium or potassium hydroxide

The reaction in this case is not reversible, and leads to the formation of the sodium salt of the carboxylic acid.

CH3COOCH2CH2CH3 + NaOH CH3COO-Na+ + HOCH2CH2CH3

propyl ethanoate + sodium hydroxide sodium ethanoate + propan-1-ol

This reaction is done in soap-making to turn fats into soaps – called saponification.

Practice: 3) Butyl hexanoate can be hydrolysed with aqueous acid or with aqueous alkali.

i) draw the structure of butyl hexanoateii) write equations for the acid and alkaline hydrolysis

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Definition: Hydrolysis is a reaction with water or hydroxide ions that breaks a chemical compound into two compounds


Fats and Oils (where we find carboxylic acid and ester functional groups in nature)N.B. Fats and oils differ only in their melting point. Fats are solid at room temperature and oils are liquid

The building-blocks of animal and vegetable fats and oils are long-chain carboxylic acids called fatty acids.

The long hydrocarbon chains can be saturated, monounsaturated, or polyunsaturated – most naturally-occurring fatty acids have an even number of carbon atoms because of the way they are made in nature.

Monounsaturated fats have one double bond, and this is commonly found between the 9th

and 10th carbon (remember numbering starts from the C in the COOH).

Shorthand notation:number of carbon atoms in chaincolonnumber of double bonds(position)

e.g.hexadecanoic acid (aka palmitic acid) CH32(CH2)14COOH 16:0

- palm oil, also found in butter, cheese, milk, meat

octadec-9-enoic acid (aka oleic acid) CH3(CH2)7CH=CH(CH2)7COOH 18:1(9)- in olive oil

octadec-9,12-dienoic acid (aka linoleic acid)CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH 18:2(9,12)

- in evening primrose oil

TriglyceridesOils and fats are esters formed from the fatty acids. Triglycerides are found naturally in animal and vegetable fats. They are triesters of propane-1,2,3-triol (glycerol), formed with three fatty acid molecules.

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O

OH

O

OH

O

OH

glycerol + hexadecanoic acid a triglyceride ester

H

C

C

C

H

OHH

H OH

H OH

+ 3 CH3(CH2)14COOH

H C

H

O

CH O

CH O

H

C

O

C

O

C

O

(CH2)14CH3

(CH2)14CH3

(CH2)14CH3

+ 3 H2O

O

OHcis-octadec-9-enoic acid


The general structure of a triglyceride is:

A simple triglyceride is formed from three molecules of the same fatty acid, forming the triester. The reaction is exactly the same as the alcohol/carboxylic acid esterifications you have met before.

Note:Natural triglycerides are mostly mixed, derived from two or three fatty acids.

Context 1 – Food and healthWe need to eat some fat as part of a balanced diet since fats have important functions in the body:

- protecting organs- providing insulation- as a long-term energy store

Unsaturated fatty acids show cis- trans- stereoisomerism (because there is no rotation around the C=C double bond, and because each of the carbons also has an H atom on it).

In nature, fatty acids usually exist in the cis- form. These fatty acids cannot pack closely together, and since intermolecular forces are short-range forces cis-fatty acids exist as liquids at room temperature.

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O

OHtrans-octadec-9-enoic acid


The equivalent trans-fatty acid molecules can pack much more closely, and so the intermolecular forces are stronger and they have higher melting points than their cis- equivalents.

Unsaturated fats have historically been regarded as healthier than saturated fats.Foods containing cis-fats present little danger to our health, but trans-fats are now thought to increase the risk of coronary heart disease.

The food industry uses many unsaturated fats, but in order to make them more solid often some of the double bonds are removed by hydrogenation (e.g. to make margarine). One of the side-effects of hydrogenation is that many of the remaining double bonds can end up being converted to the less-healthy trans- form.

So HDLs are "good lipoproteins". LDLs can deposit lipids onto artery walls building up fatty deposits that restrict blood flow. Trans-fatty acids, like saturated fats in the body, raise LDL levels and lower HDL levels increasing the risk of heart disease.

Context 2 – BiodieselWe know from GCSE that we can make biodiesel from plant oils e.g. waste cooking oils, or rapeseed oil in the UK; and the reasons for doing so (non-renewable nature of fossil fuels, carbon neutrality of biofuels). Chemically, these biofuels are the methyl or ethyl esters of fatty acids.

The triglygerides forming the oil are reacted with methanol or ethanol, in the presence of sodium or potassium hydroxide (catalyst). The ester links in the triglyceride break and methyl or ethyl esters are formed, plus glycerol. This process is transesterification. The atom economy of the process is improved by selling the glycerol to pharmaceutical or cosmetics industries.

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Definitions: High-density lipoproteins (HDLs) can remove cholesterol from the arteries and transport it back to the liver for excretion or re-utilisation.

Low-density lipoproteins (LDLs) responsible for carry cholesterol and triglycerides from the liver to the tissues

O

OO

O

O

O

OHCH3OH+ +

O

O


Answers to practice questions:1) Write balanced equations to show the reactions of butanoic acid with:

i) lithium, Li + CH3CH2CH2COOH CH3CH2CH2COO-Li+ + ½H2

or 2Li + 2CH3CH2CH2COOH 2CH3CH2CH2COO-Li+ + H2

ii) calcium hydroxide, Ca(OH)2 + 2CH3CH2CH2COOH (CH3CH2CH2COO-)2Ca2+ + 2H2O

iii) magnesium carbonateMgCO3 + 2CH3CH2CH2COOH (CH3CH2CH2COO-)2Mg2+ + H2O + CO2

2) Write an equation using displayed formulae or skeletal formulae for the reaction between an acid anhydride and an alcohol which would allow you to make the ester methyl propanoate.

propanoic anhydride + methanol methyl propanoate + propanoic acid

3) Butyl hexanoate can be hydrolysed with aqueous acid or with aqueous alkali.- draw the structure of butyl hexanoate

- write equations for the acid and alkaline hydrolysis

HCl(aq)

Acidic: C5H11COOC4H9 + H2O ⇌ C5H11COOH + HOC4H9

Alkaline: C5H11COOC4H9 + NaOH C5H11COO-Na+ + HOC4H9

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1092662sefw64-F324-Carbonyls (1)