CDS Chemistry · Carbon and Its Compounds
Functional Groups and Isomerism
The functional groups that name an organic compound, which classes can be functional isomers of each other, the three ways of drawing a molecule in 3-D, and how Lassaigne's test finds nitrogen, sulphur and halogens.
Why this matters
Five CDS questions, four of them HARD. This is the part of CDS chemistry that reaches into Class 11 organic chemistry, so a little reading here goes further than anywhere else in the chapter.
Concept 1 of 3: Functional groups
Definition
The groups CDS uses:
- Alcohol –OH (ending -ol): ethanol C₂H₅OH, methanol CH₃OH.
- Aldehyde –CHO (-al): methanal HCHO. Ketone >C=O (-one): propanone CH₃COCH₃.
- Carboxylic acid –COOH (-oic acid): ethanoic acid CH₃COOH, propanoic acid C₂H₅COOH.
- Ester –COO– (-oate). Haloalkane –X (chloro-, bromo-).
- Methanol is poisonous: the liver oxidises it to methanal (formaldehyde), which coagulates protoplasm and can cause blindness. Ethanol is oxidised to ethanal and then to harmless acetate.
| Group | Formula | Example |
|---|---|---|
| Alcohol | –OH | Ethanol, C₂H₅OH |
| Aldehyde | –CHO | Methanal, HCHO |
| Ketone | >C=O | Propanone, CH₃COCH₃ |
| Carboxylic acid | –COOH | Propanoic acid, C₂H₅COOHQ |
| Haloalkane | –X (Cl, Br, I) | Chloromethane, CH₃Cl |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Carbon and Its Compounds · Functional Groups and Isomerism
Methanol's damage comes from methanal
Concept 2 of 3: Catenation, isomerism and 3-D projections
Definition
The ideas:
- Catenation: carbon links to itself in chains and rings. Tetravalency: carbon makes four bonds.
- Isomers: same molecular formula, different structure (chain, position or functional group).
- Functional isomer pairs: alcohol and ether; aldehyde and ketone; carboxylic acid and ester; cyanide and isocyanide. An alkyl halide formula fits no other class, so it shows no functional isomerism.
- Projections for drawing 3-D molecules: Fischer shows the eclipsed form, the least stable conformation; Newman can show eclipsed, staggered and skew forms; in a Sawhorse projection the bonds are drawn at 120° to each other.
| Class | Functional isomer |
|---|---|
| Alcohol (C₂H₆O: ethanol) | Ether (dimethyl ether) |
| Aldehyde (C₃H₆O: propanal) | Ketone (propanone) |
| Carboxylic acid | Ester |
| Cyanide (–CN) | Isocyanide (–NC) |
| Alkyl halide | None possibleQ CDS 2019 (I), HARD: alkyl halides cannot show functional isomerism. |
Practice this concept3 quick reps
The same idea in a real exam question:
Example 2 · Carbon and Its Compounds · Functional Groups and Isomerism
Fischer is the least stable view
Concept 3 of 3: Lassaigne's test for N, S and halogens
Definition
The test:
- The compound is fused with sodium; the extract holds NaCN, Na₂S and NaX.
- Halogens: acidify, add silver nitrate. AgCl is white, AgBr pale yellow, AgI yellow.
- Fluorine cannot be detected this way: silver fluoride is soluble, so no precipitate forms.
| Halogen | Silver halide | Seen as |
|---|---|---|
| Chlorine | AgCl | White precipitate |
| Bromine | AgBr | Pale yellow precipitate |
| Iodine | AgI | Yellow precipitate |
| Fluorine | AgF | No precipitate (soluble)Q |
Practice this concept3 quick reps
The same idea in a real exam question:
Example 3 · Carbon and Its Compounds · Functional Groups and Isomerism
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Functional groups5 rows
| Group | Formula | Example |
|---|---|---|
| Alcohol | –OH | Ethanol, C₂H₅OH |
| Aldehyde | –CHO | Methanal, HCHO |
| Ketone | >C=O | Propanone, CH₃COCH₃ |
| Carboxylic acid | –COOH | Propanoic acid, C₂H₅COOHQ |
| Haloalkane | –X (Cl, Br, I) | Chloromethane, CH₃Cl |
Catenation, isomerism and 3-D projections5 rows
| Class | Functional isomer |
|---|---|
| Alcohol (C₂H₆O: ethanol) | Ether (dimethyl ether) |
| Aldehyde (C₃H₆O: propanal) | Ketone (propanone) |
| Carboxylic acid | Ester |
| Cyanide (–CN) | Isocyanide (–NC) |
| Alkyl halide | None possibleQ CDS 2019 (I), HARD: alkyl halides cannot show functional isomerism. |
Lassaigne's test for N, S and halogens4 rows
| Halogen | Silver halide | Seen as |
|---|---|---|
| Chlorine | AgCl | White precipitate |
| Bromine | AgBr | Pale yellow precipitate |
| Iodine | AgI | Yellow precipitate |
| Fluorine | AgF | No precipitate (soluble)Q |
Watch out for (2)
- Methanol's damage comes from methanal→ Functional groups
- Fischer is the least stable view→ Catenation, isomerism and 3-D projections
Test yourself on Carbon and Its Compounds
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.