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Chemistry · Unit 13

Fundamental Principles of Organic Chemistry: MDCAT Notes and MCQs

Definitions

  • Organic chemistry: chemistry of carbon compounds (hydrocarbons and their derivatives) — excluding CO, CO2, carbonates, cyanides, carbides.
  • Vital force theory (Berzelius) was disproved by Wöhler (1828), who synthesized urea from ammonium cyanate.
  • Carbon's special features: tetravalency, catenation, multiple bonds, isomerism.

Structural classification

  • Open-chain (acyclic / aliphatic): straight or branched — alkanes, alkenes, alkynes.
  • Closed-chain (cyclic):
    • Homocyclic / carbocyclic (ring of only C): alicyclic (cyclohexane) and aromatic (benzene, naphthalene).
    • Heterocyclic (ring contains N, O or S): pyridine, furan, thiophene, pyrrole.

Functional groups

An atom or group of atoms that determines the characteristic properties of a compound.

ClassGroupClassGroup
Alcohol–OHCarboxylic acid–COOH
EtherR–O–REster–COOR
Aldehyde–CHOAmine–NH2
Ketone>C=OAmide–CONH2
Alkyl halide–XNitrile–C≡N

Isomerism

Structural isomers differ in connectivity (chain, position, functional group, metamerism, tautomerism). Stereoisomers have the same connectivity but different spatial arrangement:

  • Geometric (cis–trans) isomerism: needs restricted rotation (C=C or ring) and two different groups on each double-bonded carbon. cis-2-butene vs trans-2-butene; maleic (cis) vs fumaric (trans) acid. Cis isomers usually polar with higher b.p.; trans more stable, higher m.p. 1-butene shows no geometric isomerism.
  • Optical isomerism: needs a chiral (asymmetric) carbon with four different groups. Non-superimposable mirror images = enantiomers; rotate plane-polarized light equally in opposite directions (d/+ and l/−). Equimolar mixture = racemic mixture (optically inactive). Examples: lactic acid, 2-butanol, glyceraldehyde, amino acids (except glycine).
PMDC learning outcomes for this unit
  • Organic compounds: 13.1 Define organic chemistry and organic compound.
  • Organic compounds: 13.2 Classify organic compounds on structural basis.
  • Organic compounds: 13.3 Define functional group.
  • Organic compounds: 13.4 Explain stereoisomerism and its types.

Fundamental Principles of Organic Chemistry MCQs with answers

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Q1
Vital force theory was rejected after Wöhler synthesized:
AMethane
BGlucose
CUrea
DAcetic acid
Show answer
Answer: C (Urea). Wöhler (1828) made urea from ammonium cyanate.
Q2
Pyridine is an example of:
AHeterocyclic compound
BAromatic homocyclic compound
COpen-chain compound
DAlicyclic compound
Show answer
Answer: A (Heterocyclic compound). Pyridine's ring contains a nitrogen atom.
Q3
Which compound shows geometric (cis–trans) isomerism?
A1-Butene
B2-Butene
CPropene
DEthene
Show answer
Answer: B (2-Butene). Each double-bonded carbon of 2-butene carries two different groups (H and CH3).
Q4
A carbon atom attached to four different groups is called:
APrimary carbon
BChiral carbon
CQuaternary carbon
Dsp² carbon
Show answer
Answer: B (Chiral carbon). A chiral (asymmetric) carbon makes optical isomerism possible.
Q5
Which compound shows optical isomerism?
APropanoic acid
BEthanol
CAcetone
DLactic acid
Show answer
Answer: D (Lactic acid). Lactic acid (CH3–CH(OH)–COOH) has a chiral carbon.
Q6
An equimolar mixture of two enantiomers is called:
ARacemic mixture
BDiastereomer
CMeso compound
DTautomer
Show answer
Answer: A (Racemic mixture). A racemic mixture is optically inactive due to external compensation.
Q7
The functional group of ketones is:
A–COOH
B–CHO
C–OH
D>C=O
Show answer
Answer: D (>C=O). Ketones have a carbonyl group bonded to two carbon atoms.
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