Carbonyls, acids and esters

Question 1

What is the carbonyl group?

Answer 1

C=O

Question 2

How is the double bond in the carbonyl group formed?

Answer 2

• The double bond consists of 2 types of bonds between the C and O, the σ bond and the π bond.
• The σ bond is formed by the front-on overlap of orbitals in the C and O atoms, each contributing 1 electron to the bonding pair of electrons.
• The π bond is formed by the side-on overlap of p orbitals in the C and O atoms, each contributing 1 electron the the bonding pair of electrons, forming 2 bonding regions; one above and one below the σ bond.
• The front-on overlap is a better overlap so the σ bond is stronger than the π bond.

Question 3

What is the difference between the C=C bond and the C=O bond?

Answer 3

O is much more electronegative than C and attract the bonding pairs of electrons more, resulting in the C=O bond being polar whereas the C=C bond is not.

Question 4

How are carbonyls named?

Answer 4

• Aldehydes are named with -al as the suffix and the numbering of the carbon the carbonyl is attached to ensures the smallest number possible.
• Ketones are named with -one as the suffix and the numbering of the carbon the carbonyl is attached to also ensures the smallest number possible.

Question 5

How are aldehydes made?

Answer 5

• Oxidation of primary alcohols with a suitable oxidising agent; for example, acidified dichromate ions (K₂Cr₂O₇ + H₂SO₄) will convert the primary alcohol into an aldehyde, then into a carboxylic acid.
• The aldehyde needs to be distilled as it’s being made in order to prevent it from becoming a carboxylic acid.
• As the reaction occurs, the colour of the dichromate solution changes from orange to green.

Question 6

How are ketones made?

Answer 6

• Oxidation of secondary alcohols with a suitable oxidising agent, for example, acidified dichromate ions (K₂Cr₂O₇ + H₂SO₄) will convert the secondary alcohol into a ketone.
• Because the ketone is the only and final product in this reaction, the reaction may be carried out under reflux to ensure that all the alcohol has been converted into ketones.
• As the reaction occurs, the colour of the dichromate solution changes from orange to green.

Question 7

How are carbonyls reduced?

Answer 7

Carbonyls can be reduced by heating them with a suitable reducing agent (e.g. NaBH₄, Sodium borohydryde). Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols. NaBH₄ is a relatively weak reducing agent so will only attack the carbonyl groups on a multi-functional group compound.

Question 8

What is the mechanism for the reduction of carbonyls?

Answer 8

Nucleophilic addition

Question 9

What are the steps involved in nucleophilic addition?

Answer 9

1. NaBH₄ is a source for H^- ions, which acts as the nucleophiles.
2. The δ+ C atom in the C=O bond attracts the H^- ion, which attacks the C atom to form a new C-H bond with its lone pair of electrons.
3. The π bond breaks and a negatively charged intermediate is formed on the O atom in the C=O bond.
4. The O^- intermediate donates a pair of electrons to an H atom in H₂O, forming a dative O-H bond. An O-H bond breaks in H₂O, with the electron pair going to the oxygen atom to form an OH^- ion.
5. An alcohol and a hydroxide ion is formed.

Question 10

What is the general formula for the reduction of an aldehyde?

Answer 10

R-CHO + 2 [H] → R-CH₂OH

Question 11

What is the general formula for the reduction of a ketone?

Answer 11

R-CO-R’ + 2 [H] → R-CH(OH)-R’

Question 12

What is the test for a carbonyl?

Answer 12

1. 2,4-DNPH (2,4-Dinitrophenylhydrazine) is added to a mixture of methanol and sulfuric acid to make Brady’s reagent.
2. If an orange precipitate is formed after adding Brady’s reagent to the compound in question, then the compound is a carbonyl.

Question 13

How is an aldehyde distinguished from a ketone?

Answer 13

1. Determine that the compound is a carbonyl via the 2,4-DNPH test.
2. Tollen’s reagent is made by making Ag₂O from NaOH and AgNO₃ and then redissolving in aqueous ammonia.
3. Heat the compound in question with a few drops of Tollen’s reagent in a test tube. If a ‘silver mirror’ is formed on the sides of the test-tube, the compound is an aldehyde. If not, the compound is a ketone.
4. Ag₂O is a weak oxidising agent and will oxidise an aldehyde to a carboxylic acid to form silver solid, but will not oxidise a ketone.

Question 14

How is the exact identity of a carbonyl identified after performing the 2,4-DNPH and silver mirror test?

Answer 14

1. The orange 2,4-DNPH derivative of the carbonyl from the first test is purified with recrystallisation and then filtered under reduced pressure.
2. The purified orange/yellow crystal then undergoes melting point analysis and the results are compared to a data table in order to determine the carbonyl compound derives from.

Question 15

What is the functional group present in carboxylic acids?

Answer 15

Carboxyl group (-COOH).

Question 16

What is the solubility of carboxylic acids in water?

Answer 16

Carboxylic acids are very soluble in water due to the fact that they contain a polar hydroxyl (-OH) group which is capable of making hydrogen bonds with water molecules. However, the solubility decreases as the size of the molecule increases becasue the size of the non-polar part of the molecule incapable of hydrogen bonding with water also increases, making it harder for the molecule to dissolve.

Question 17

Why are hydroxyl groups on carboxylic acids more acidic than those on alcohols?

Answer 17

A lone pair of electrons on the O in the carboxyl -OH groups can partially delocalise across the 2 Os (C-O^- and C=O) in the carboxyl group when -OH dissociates to -O^-. This means that -O^- only partially exists in carboxylic acids, whereas -O^-exists fully in dissociated alcohols. The high -ve charge on the alcohol -O^- quickly reattracts any dissociated H⁺ ions, so alcohol -OH do not dissociate easily, whereas the partial -ve charge on the -O^- has a weaker attraction towards dissociated H⁺ ions so carboxyl -OH dissociate more easily than alcohol -OH.

Question 18

What is the name given to salts of carboxylic acids formed from typical acid reactions?

Answer 18

Carboxylate salts.

Question 19

What is the test for a carboxylic acid?

Answer 19

React with carbonate ions to produce effervescence or use indicator to test pH.

Question 20

How is an ester made?

Answer 20

Heating an alcohol with a carboxylic acid in the presence of a concentrated acid catalyst (e.g. sulfuric acid). The reaction is:

R-OH + R’-COOH ⇔ R-COO-R’ + H₂O

The equilibrium lies to the right since the concentrated acid also acts as a dehydrating agent and removes water formed from this reaction, shifting p.o.e to the right.

Question 21

What is a more efficient method for forming an ester?

Answer 21

Gently heating an acid anhydride with an alcohol to form an ester and a carboxylic acid. The reaction is non-reversible therefore the yield is higher than that of a carboxylic acid-alcohol reaction. The equation is:

R-COOCO-R + R’-OH → R-COO-R’ + R-COOH

Question 22

What is hydrolysis?

Answer 22

A reaction with water or hydroxide ions that that breaks a compound into two compounds.

Question 23

How can an ester be hydrolysed by an acid?

Answer 23

When making an ester with a carboxylic acid and an alcohol with acid catalyst, the reaction is reversible, with the reverse reaction being the hydrolysis reaction. The catalyst increases rate of forward and reverse reaction equally, but p.o.e shifted to the right as concentrated acid is a dehydrating agent and removes water. During acid hydrolysis reaction, an ester is refluxed with a dilute acid catalyst to eliminate the dehydrating properties of the catalyst and move p.o.e bach toward the left. The equation is:

R-COO-R’ + H₂O ⇔ R-COOH + R’-OH

Question 24

How can an ester be hydrolysed by an alkali?

Answer 24

Esters can also be hydrolysed by alkali. This reaction is non-reversible therefore produces a higher yield than the acid hydrolysis. The equation is:

R-COO-R’ + OH^- → R-COO^- + R’-OH

Instead of the acid, the carboxylate salt is produced since any acid produced by the reaction immediately reacts with the alkali to form a salt.

Question 25

What are some of the uses for esters?

Answer 25

Esters are responsible for many natural flavours and aromas found in plants.

• Oil of wintergreen is the ester that can be obtained by steam distilling organic plant matter. It can be used as an ointment for mild pain relief.
• Benzyl ethanoate is an important ester in the jasmine flower aroma and is used in many perfums and fragrences, as wells as to give the apple/pear taste in some drinks.

Question 26

What is the difference between a fat and an oil?

Answer 26

Fats have melting points above room temperature and oils have melting points above room temperature. This means that fats are solid and oils are liquids at room temperature.

Question 27

What are fatty acids?

Answer 27

Carboxylic acids made from long lengths of hydrocarbon varying in structure containing a number of single and double bonds. Most natural fatty acids have an even number of carbon atoms.

Question 28

What is the systematic name for glycerol?

Answer 28

Propane-1,2,3-triol

Question 29

What is a triglyceride?

Answer 29

Most commonly consumed plant and animal fats are triglycerides. They consist of a glycerol molecule bonded to 3 fatty acid molecules via ester bonds as a result of 3 separate condensation reactions.

Question 30

What is the difference between an unsarturated and a saturated fats?

Answer 30

A saturated fat contains fatty acids with hydrocarbon chains containing no double bonds.

A monounsaturated fat contains one double bond and a polyunsaturalted fat contains more than 1 double bond.

Question 31

What is a shorthand notation for representing fatty acids?

Answer 31

NUMBER OF CARBONS : NUMBER OF DOUBLE BONDS (POSITION OF DOUBLE BONDS, COUNTING CARBOXYL GROUP CARBON AS CARBON 1)

Example: Octadec-9-enoic acid is 18 : 1 (9)

Question 32

How are triglycerides formed?

Answer 32

Simple esterification reactions between glycerol and a mixture of fatty acids (or the same fatty acids to obtain simple triglycerides).

Question 33

What are LDLs?

Answer 33

Low density lipoproteins, responsible for carrying cholesterol from the liver to tissues but can be deposited on artery walls. Associated with raising blood cholesterol.

Question 34

What are HDLs?

Answer 34

High Density Lipoproteins, responsible for carrying cholesterol from tissue back to liver for storage/disposal. Associated with lowering blood cholesterol.

Question 35

Why are trans-fats thought to be bad for health whereas cis-fats are not?

Answer 35

Cis-fats have a bent structure so cannot be packed together tightly and contain weaker intermolecular forces, resulting in lower melting points. They increase HDL levels and decrease LDL levels.

Trans-fats have linear structures similar to that of saturated fats and therefore can be packed together tightly. They contain stronger intermolecular forces, resulting in higher melting points. They behave similarly to saturated fats and increase LDL levels and decrease HDL levels.

Question 36

How is biodiesel made?

Answer 36

Transesterification process between an alcohol and a triglyceride in the presence of a hydroxide catalyst (NaOH or KOH) forming methyl esters (biodiesel) and glycerol.

Question 37

What are the advantages of biodiesel?

Answer 37

• Renewable source of energy.
• Can replace fossil fuels.
• A potential method for recycling used vegetable oil.
• Cheaper than fossil fuels.

Question 38

What are the disadvantages of biodiesel?

Answer 38

• Field that can be used to grow food are used to grow oil plants instead for biofuels.
• Energy and methanol required for manufacture process still comes from fossil fuels.