F324: Module 1: Carbonyl compounds, Carboxylic acids and esters

Question 1

C) What is the aldehyde functional group similar to but must not be confused with?

Answer 1

The hydroxyl functional group in alcohols which is written as COH whereas the aldehyde functional group is written as CHO.

Question 2

C) Why are the lower members of the homologous series of aldehydes and ketones completely miscible with water?

Answer 2

The C=O is significantly polar, it has a permanent dipole, so enables hydrogen bonds to be formed. However, this is not so with longer chains because the long chain gets in the way and interferes with/ breaks hydrogen bonds with the water.

Question 3

C) How are aldehydes and ketones prepared?

What is the colour change?

Answer 3

Heat a primary alcohol with acidified dichromate and a sulphuric acid catalyst (distil otherwise the aldehyde is easily oxidised into a carboxylic acid) for an aldehyde and the same but with a secondary alcohol for a ketone. (tertiary alcohols do not oxidise)

The mixtures go from orange to green as the dichromate ion which is orange changes to the chromium ion which is green

Question 4

C) How are aldehydes and ketones reduced to their respective alcohols?

Answer 4

NaBH4 (sodium borohydride) is used as a reducing agent. The aldehyde or ketone is warmed with the reducing agent in a solvent of water or ethanol. The reducing agent is represented by [H].

Question 5

C) What are the two steps of nucleophilic addition when a carbonyl group is reduced?

Answer 5

1) Nucelophilic attack by the hydride ion where the H- donates a pair of electrons to the double bond and the double bond breaks and a pair of electrons is donated to the delta negative oxygen (the H adds on to the carbon)
2) Protonation of the intermediate by water where the pair of electrons on the now negatively charged oxygen are donated to a delta positive hydrogen in a molecule of water and the pair of electrons in the bond between the oxygen and the hydrogen are donated to the oxygen, forming OH- and the H adds on to form an OH alcohol group on the original molecule.

Question 6

C) How do you test for the presence of a carbonyl

Answer 6

Add 2,4-DNP (Brady’s reagent) and if a carbonyl is present, an orange ppt should form and the melting point for each derrivative is unique to the carbonyl so can be used to identify the compound.

Question 7

C) How do you distinguish between an aldehyde and a ketone?

Answer 7

Add tollens reagent and a silver mirror will form with an aldehyde only.

Question 8

C) What is an aryl functional group?

Answer 8

Any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, be it phenyl, naphthyl, thienyl, indolyl, etc.

Question 9

C) What is the general formula for aliphatic carboxylic acids?

Answer 9

CnH2n+1COOH

Question 10

C) Why are carboxylic acids weak acids?

Answer 10

In water, soluble carboxylic acids will ionise as the OH bond breaks however this only occurs in a small number of molecules (partial dissociation) so its a weak acid.

Question 11

C) Describe the intermolecular forces present in a carboxylic acid.

Answer 11

• Van der Waals forces due to the hydrocarbon chain
• Dipole dipole interactions due to the polar C=O bond
• Hydrogen bonding forces due to the OH group

Question 12

C) Why can carboxylic acids ionise?

Answer 12

Due to the delocalisation of the negative charge over the carbon and two oxygen atoms so when the H dissociates, the negative charge is spread which increases the energetic stability of the anion.

Question 13

C) What are the products when a carboxylic acid reacts with

• a metal
• a base
• a metal carbonate

Answer 13

• salt plus hydrogen gas
• salt plus water
• salt plus water plus carbon dioxide

Question 14

C) How are esters formed?

Answer 14

Reflux carboxylic acid and alcohol with an acid catalyst, producing an ester plus water

Question 15

C) What is glacial ethanoic acid?

Answer 15

Ethanoic acid that is pure and free of all water- it freezes in the bottle at 16.7 degrees.

Question 16

C) What is the general formula of esterification using an acid anhydride instead of a carboxylic acid and why are acid anhydrides often used?

Answer 16

Acid anhydride + alcohol –> ester +carboxylic acid

-They are more reactive, no catalyst is needed and the reaction is irreversible

Question 17

C) How can esters be hydrolysed? What are the products?

Answer 17

Using water or through acid or alkali hydrolysis.
Water- original reactants formed
Acid-Original reactants formed
Alkali-Salt of the acid formed

Question 18

C) Why is benzoic acid, a product of hydrolysis of a benzoate ester, only sparingly soluble in water?

Answer 18

Because of its non polar benzene ring.

Question 19

C) What is a monoglyceride, diglyceride and triglyceride?

Answer 19

• One of the alcohol groups on the glycerol (propane-1,2,3-triol) has been esterified
• Two of the alcohol groups have been esterified
• Three of the alcohol groups have been esterified

Question 20

C) How is a triglyceride formed?

Answer 20

Esterification of a fatty acid (carboxylic acids obtained from oils or fats) with glycerol where the carboxylic acid group forms an ester bond with the alcohol group of the glycerol.

Question 21

C) What is a trans fat?

Why do they have higher melting points than other fats?

Answer 21

An unsaturated fatty acid where the double bond(s) cause cis and trans isomers to be formed. It is the trans isomer that is usually a straight chain, rather than bent, which means that the molecules can pack closer together and therefore have a higher melting point and are worse for your health.

Question 22

C) Why are trans fats bad for you?

Answer 22

They increase the amount of low density lipoproteins (“bad cholesterol”) and lower the amount of high density lipoproteins. This results in an increased risk of high blood pressure, heart attacks and strokes because the diameter of the arteries are narrowed.

Question 23

C) Describe how esters and fatty acids are useful in the production of biofuels.

Answer 23

• Triglycerides are extracted from plants
• They are processed by heating with a strong alkali which hydrolyses the triglycerides into fatty acids and glycerol
• Methanol is added which reacts with the carboxylic acids which makes the methyl esters that we use as biodiesel

Question 24

(1 mark)

What kind of reaction takes place when NaBH4 is used to reduce an aldehyde or ketone to an alcohol?

Answer 24

Nucleophilic addition