Organic chemistry and macromolecules
Organic Chemistry and Functional Groups
This independent foundation chapter develops nomenclature, functional group recognition, structural, geometric and optical isomerism, reaction mechanisms and types, and the test-tube reactions that distinguish aldehydes, ketones, alkenes and alcohols.
Nomenclature and isomerism
- Choose the longest chain containing the principal functional group, number to give the lowest locants, and apply the priority order: carboxylic acid > ester > amide > aldehyde > ketone > alcohol > amine > alkene > alkane.
- Structural isomers differ in connectivity; positional isomers differ in where a group sits; for E/Z isomerism of an acyclic alkene, each alkene carbon needs two different substituents. A tetrahedral carbon with four different groups is a common source of chirality, but axial and other forms mean it is not universally required.
Reaction types
- Alkanes undergo free-radical substitution with Cl2 or Br2 under UV light.
- Alkenes undergo electrophilic addition; Markovnikov's rule places H on the carbon with more hydrogens; bromine water is decolourised.
- Primary alcohols oxidise to aldehydes and then carboxylic acids with acidified dichromate; secondary alcohols give ketones; tertiary alcohols resist oxidation.
- Haloalkanes undergo substitution (aqueous KOH) or elimination (hot ethanolic KOH).
- Carbonyls: Appropriate aldehydes and ketones can form coloured 2,4-DNP derivatives under the specified test conditions. Tollens’ and Fehling’s are reducing tests rather than infallible labels: some other reducing species respond, and many aromatic aldehydes do not give the standard Fehling result. Interpret the complete supplied evidence and conditions.
- Carboxylic acids with alcohols form esters under acid catalysis; esters hydrolyse back.
Worked example
A compound of formula C3H6O gives an orange precipitate with 2,4-DNP but no silver mirror with Tollens' reagent. Degree of unsaturation = (2×3 + 2 − 6)/2 = 1, consistent with one C=O. Positive 2,4-DNP confirms a carbonyl; the negative Tollens' test rules out an aldehyde, so the compound is the ketone propanone.
Traps that cost marks
- Using Tollens' and Fehling's interchangeably, or expecting them to react with ketones.
- Thinking tertiary alcohols oxidise to ketones.
- Applying Markovnikov's rule backwards.
- Calling any carbon with four bonds chiral; the four groups must differ.
- Missing the C=C requirement that each carbon carries two different substituents for geometric isomerism.
Proteins, lipids and polymer structure
An organic functional group can appear in a small molecule or repeatedly along a macromolecule. Always identify the actual linkage and the model given. An amino acid can contain both amino and carboxyl functions. In the elementary peptide model, reaction of a carboxyl group with an amino group forms an amide linkage and eliminates water. Hydrolysis cleaves that linkage with water. A linear peptide containing n amino-acid residues has n minus one peptide links when its ends are not joined into a ring. This counting statement concerns the supplied linear structure, not every cyclic or branched biological assembly.
Protein primary structure describes amino-acid sequence. Secondary and tertiary organization depend on interactions and conformational constraints; denaturation can disturb higher-level structure without necessarily hydrolyzing the peptide backbone. Do not equate loss of enzyme activity with a change in DNA sequence or assume that all denaturation is reversible. If an exercise supplies an isoelectric point, a pH above that point commonly corresponds to a more negative average net charge, while a lower pH favors a more positive charge. Use the stated model rather than predicting a complete titration curve from one number.
A triacylglycerol contains glycerol joined to three fatty-acid residues through ester links. In an idealized saponification balance, base-promoted ester hydrolysis gives glycerol and carboxylate salts. It does not directly produce three neutral free acids while excess strong base remains. Unsaturation changes possible addition chemistry and chain packing. Compare otherwise similar structures: a cis double bond introduces a bend that can hinder close packing, but melting behavior also depends on chain length, composition and crystal structure.
For an alkene addition-polymerization model, the carbon-carbon double bond opens to join repeat units without eliminating water for every addition. Step-growth polymer formation may instead connect complementary functional groups; some such reactions eliminate a small molecule, often water or hydrogen chloride, while others do not. A diol and a dicarboxylic acid can form ester links in a simplified polyester model. Functionality, conversion, branching and crosslinking affect the resulting architecture, so step growth is not simply a synonym for one universal byproduct.
Thermoplastics can soften and flow under suitable heating because their chains are not permanently connected into one dense covalent network. A strongly crosslinked thermoset generally does not remelt into the original processing fluid; excessive heating can cause decomposition. Crosslinking restricts large-scale chain motion and may produce swelling rather than complete dissolution in an otherwise compatible solvent. These tendencies do not substitute for a particular material’s supplied data.
Distinguish mechanical size reduction from cleavage of polymer bonds. Fragmenting a plastic into smaller particles does not demonstrate chemical mineralization. Bio-based feedstock does not automatically imply biodegradability, and a compostability claim applies to specified conditions and tests. In a repeat-unit molar-mass calculation, state whether end groups, branching and the distribution of chain lengths are neglected before dividing a polymer molar mass by a repeat-unit mass.