Functional Groups – Chemistry (Medical Lab Technology)
1. Simple Explanation
In organic chemistry, a functional group is a specific arrangement of atoms that determines the characteristic chemical behavior of a molecule.
- Think of it as the “reactive part” of a molecule – the rest is just a scaffold.
- Functional groups are the basis for predicting reactivity, physical properties, and how a compound will behave in laboratory tests.
2. Key Points
| Category | Representative Groups | General Formula | Typical Properties | Common Lab Relevance |
|---|---|---|---|---|
| Alkyl‑substituted | Alkanes | R–C–C–C–R | Non‑polar, low reactivity | Baseline for comparison |
| Alkenes | - | R–C=C–R | Double bond, unsaturation | Olefin cross‑linking tests |
| Alkynes | - | R–C≡C–R | Triple bond | Reaction with HCl (hydrohalogenation) |
| Alcohols | - | R–OH | Donates H‑bond, high polarity | ( \text{AlkOH} ) – pH indicator, phenolphthalein test |
| Ethers | - | R–O–R | Ether‑oxygen, moderate polarity | Ether extraction, solvent choice |
| Aldehydes | - | R–CHO | Carbonyl + H | ( \text{AlkCHO} ) – Tollens/Fehling test |
| Ketones | - | R–CO–R | Carbonyl | Ketone test (Tollens negative) |
| Carboxylic Acids | - | R–COOH | Acidic, H‑bond donor | pH meter, titration |
| Esters | - | R–COO–R | Odor, esterification | Ester hydrolysis, smell test |
| Amides | - | R–CONH₂ | Polar, hydrogen bonding | Amide solubility, hydrolysis |
| Amines | - | R–NH₂ / R–NR₂ | Basic | Reaction with nitroprusside (Benedict’s) |
| Halides | - | R–X (X = F, Cl, Br, I) | Electrophilic | SN1/SN2 reactions |
| Phenols | - | Ar–OH | Acidic, ortho/para directing | FeCl₃ test (purple color) |
| Nitro | - | Ar–NO₂ | Electron‑withdrawing | Redox tests |
| Sulfur‑containing | - | R–S–R, R–SO₃H | Varied | S‑tests (e.g., Reagent 2,4‑dinitrophenylhydrazine) |
3. Functional Group Identification & Properties
| Functional Group | Common Tests | Reaction Type | Key Lab Observation |
|---|---|---|---|
| Alcohol | Phenolphthalein (pink) | Acid/base | Basic solutions → pink |
| Aldehyde | Tollens (silver mirror) | Redox | Reducing aldehyde → silver |
| Ketone | Tollens (no change) | Redox | No silver deposition |
| Carboxylic Acid | Acid–base titration | Neutralization | pH ~2–3, titrate with NaOH |
| Ester | NaOH hydrolysis | Saponification | Glycerol + free fatty acid |
| Amide | Hydrolysis | Acidic/Basic | Amide → carboxylic acid + amine |
| Amino | Benedict’s (blue to red) | Redox | Reducing amine → color change |
| Phenol | FeCl₃ (purple) | Coordination | Purple complex forms |
| Halide | AgNO₃ (precipitate) | Precipitation | White AgCl, AgBr, etc. |
4. Worked Example
Problem
Identify all functional groups present in 4‑chlorobenzyl alcohol and predict a suitable laboratory test for each.
Structure (simplified)
Cl
|
C6H4–CH2–OH
Step‑by‑Step
-
Check the main skeleton
- Aromatic ring (benzene) → no reactive functional groups by itself.
-
Look for heteroatoms
- One oxygen attached to a carbon → alcohol (–OH).
- One chlorine attached to a sp² carbon → alkyl halide.
-
Identify functional groups
- Benzylic alcohol (–CH2OH attached to aromatic ring).
- Chlorobenzene (–Cl on aromatic ring).
-
Predict tests
- Alcohol → Phenolphthalein (pink in basic solution).
- Chlorine → AgNO₃ test (no precipitate because Cl is covalently bonded; halide test is for alkyl halides, not aryl halides).
- Aromatic ring → Bromine water (no decolorization; aromatic rings are resistant).
- **Benzy