SolutionsFunctional Groups

1. Recognize the functional groups

Each colour marks a functional group:

• carboxylic acid
• ester
• aromatic ring
• phenol
• amide
• aldehyde
• ether
• alcohol
• amine
• alkene

2. Classify

a) State whether each alcohol is primary, secondary or tertiary:

1. CH₃CH₂OH — Primary: the carbon bearing the OH is bonded to only one carbon.
2. (CH₃)₂CHOH — Secondary: the carbon bearing the OH is bonded to two carbons.
3. (CH₃)₃COH — Tertiary: the carbon bearing the OH is bonded to three carbons.

b) State whether each amine is primary, secondary or tertiary:

1. CH₃CH₂NH₂ — Primary: the nitrogen is bonded to only one carbon.
2. CH₃CH₂NHCH₃ — Secondary: the nitrogen is bonded to two carbons.
3. CH₃CH₂N(CH₃)₂ — Tertiary: the nitrogen is bonded to three carbons.

Note that for alcohols we count the carbons bonded to the carbon bearing the OH, whereas for amines we count the carbons bonded directly to the nitrogen.

c) Which functional group does each compound in these closely related pairs have?

1. Acetic acid: carboxylic acid (–COOH)  /  methyl acetate: ester (–COO–). The ester lacks the acidic H of the OH.
2. Propanal: aldehyde (the C=O is at the end of the chain and bears an H)  /  propanone: ketone (the C=O is between two carbons).
3. Ethanamide: amide (the N is bonded to a C=O)  /  ethanamine: amine (the N is bonded to an sp³ carbon).

3. Which is the main function?

1. HOCH₂CH₂COOH — 3-Hydroxypropanoic acid. The carboxylic acid takes precedence over the alcohol: it gives the suffix “-oic acid” and the OH is named as “hydroxy-”.
2. CH₃COCH₂CH₂NH₂ — 4-Aminobutan-2-one. The ketone takes precedence over the amine: suffix “-one”; the NH₂ is named as “amino-”.
3. HOCH₂CH₂CHO — 3-Hydroxypropanal. The aldehyde takes precedence over the alcohol: suffix “-al”; prefix “hydroxy-”.
4. H₂NCH₂COOH — 2-Aminoethanoic acid (glycine). The acid takes precedence over the amine: suffix “-oic acid”; prefix “amino-”. It is the simplest amino acid.

4. From name to structure and back

a) Write the formula of these compounds:

1. ethyl ethanoate (ethyl acetate): CH₃COOCH₂CH₃
2. N-methylethanamide: CH₃CONHCH₃
3. but-3-en-2-ol: CH₂=CHCH(OH)CH₃
4. 4-hydroxybenzaldehyde: OHC–C₆H₄–OH (with the OH in position 4, para to the CHO)

b) Name these compounds:

1. CH₃CH₂CH₂COOCH₃ — Methyl butanoate (ester)
2. CH₃CH₂OCH₂CH₃ — Ethoxyethane or diethyl ether (ether)
3. (CH₃)₂CHCOOH — 2-Methylpropanoic acid (carboxylic acid)
4. CH₃CH₂CH(NH₂)CH₃ — Butan-2-amine (primary amine)

5. Same formula, different function

C₂H₆O

Ethanol, CH₃CH₂OH (alcohol), and dimethyl ether, CH₃OCH₃ (ether).

C₃H₆O

Propanal, CH₃CH₂CHO (aldehyde); propanone, CH₃COCH₃ (ketone); prop-2-en-1-ol, CH₂=CHCH₂OH (alcohol and alkene); methoxyethene, CH₂=CHOCH₃ (ether and alkene); cyclopropanol (alcohol), and cyclic ethers such as oxetane or methyloxirane.

C₃H₆O₂

Propanoic acid, CH₃CH₂COOH (acid); methyl ethanoate, CH₃COOCH₃, and ethyl methanoate, HCOOCH₂CH₃ (esters); 1-hydroxypropan-2-one, CH₃COCH₂OH (ketone and alcohol); 2-hydroxypropanal and 3-hydroxypropanal (aldehyde and alcohol), among others.

They are all functional isomers: same molecular formula, different functional group. You will study this in more detail in Isomerism.

6. Everyday chemistry