Isomerism & Reaction Map

Structural isomerism

Compounds with the same molecular formula but different connectivity of atoms. Example: C₄H₁₀ can be CH₃CH₂CH₂CH₃ (butane) or (CH₃)₂CHCH₃ (2-methylpropane).

Geometric (cis-trans) isomerism

Compounds with same formula and connectivity but different spatial arrangement around C=C double bond due to restricted rotation. Example: CH₃CH=CHCH₃; cis = substituents on same side, trans = opposite sides.

Cis isomer properties

Atom/group of atoms on same side of double bond; higher polarity and higher boiling point than trans isomer due to dipole moment.

Trans isomer properties

Substituents on opposite sides of double bond; lower polarity, lower boiling point, more stable due to less steric strain.

Optical isomerism (enantiomers)

Non-superimposable mirror images caused by a chiral carbon (four different groups attached). Rotate plane-polarised light in opposite directions.

Chiral carbon

Carbon bonded to four different atoms/groups; required for optical activity. Example: CH₃CH(OH)COOH (lactic acid).

Racemate

Equal mixture of two enantiomers; no net optical rotation.

Reaction map logic

A diagram connecting functional groups by reactions (e.g. oxidation, reduction, substitution, condensation, hydrolysis) to visualise pathways between compounds.

Reasoning unknowns

Use functional group tests and reaction map to deduce unknown compound; consider reagent, conditions, and observed product.

Alcohol → Aldehyde → Carboxylic acid pathway

Example: CH₃CH₂OH → CH₃CHO → CH₃COOH. Use distillation for aldehyde, reflux for acid.

Alkene → Alcohol → Ketone/Carboxylic acid pathway

Example: CH₂=CHCH₃ + H₂O/H₂SO₄ → CH₃CH(OH)CH₃ → CH₃COCH₃ (secondary alcohol oxidation).

Haloalkane → Alcohol → Aldehyde/Carboxylic acid pathway

Substitution then oxidation reactions; example: CH₃CH₂Br + NaOH → CH₃CH₂OH → CH₃CHO → CH₃COOH.

Amine → Amide → Carboxylic acid pathway

Example: CH₃NH₂ + CH₃COCl → CH₃CONHCH₃ → CH₃COOH after hydrolysis.

Ester → Alcohol + Carboxylic acid

Acidic hydrolysis: CH₃COOCH₃ + H₂O → CH₃OH + CH₃COOH.

Ester → Salt + Alcohol

Basic hydrolysis (saponification): CH₃COOCH₃ + NaOH → CH₃ONa + CH₃OH.

Polyester formation from diacid + diol

HOOC–CH₂–CH₂–COOH + HO–CH₂–CH₂–OH → –[OOC–CH₂–CH₂–COO–CH₂–CH₂–O]ₙ + H₂O. Shows condensation polymerisation.

Polyamide formation from diacid + diamine

HOOC–CH₂–CH₂–COOH + H₂N–CH₂–CH₂–NH₂ → –[OOC–CH₂–CH₂–CONH–CH₂–CH₂–NH]ₙ + H₂O. Shows condensation polymerisation.

Deducing unknowns

Combine test results (Tollens’, Benedict’s, bromine, litmus, smell) with reaction map pathways to identify functional groups and possible structures.

Markovnikov’s rule in addition

In HX addition to alkene, H adds to carbon with more H atoms; used to predict major product in hydrohalogenation.

Reflux vs distillation in pathways

Reflux: complete reaction, prevents loss of volatile compounds. Distillation: isolates intermediate (e.g., aldehyde before oxidation to acid).

Functional group interconversion logic

Know which reactions link functional groups (e.g., alcohol → aldehyde → carboxylic acid; acid → ester → amide) to navigate reaction maps.

Predicting optical activity

Compounds with chiral carbons: single enantiomer rotates plane-polarised light; racemate does not. Consider when deducing unknowns.