Basic Principles of Organic Chemistry — MHT-CET Formulas and Traps

IUPAC Nomenclature, Functional Groups and Homologous Series

Homologous Series: One CH₂ at a Time

Homologous step

M(\text{C}_{n+1}\text{H}_{2n+4}) - M(\text{C}_n\text{H}_{2n+2}) = 14\ \text{g mol}^{-1}

Naming From a Structure, and Reading Bond-Line Formulas

Naming order

\text{locants: principal group} \to \text{C=C / C≡C} \to \text{substituents (lowest set)};\quad \text{prefixes alphabetical}

Heterocyclic Compounds by Heteroatom

Compound	Ring size	Heteroatom	Aromatic?
Pyrrole	5	N	Yes
Furan	5	O	Yes
Thiophene	5	S	Yes The only sulphur ring in the list.
Pyridine	6	N	Yes
Piperidine	6	N	No — saturated Piperidine has N, not S; it is the reduced form of pyridine.
Pyran	6	O	No Pyran contains NO nitrogen.

Pyr- names carry nitrogen except pyran; fur- is oxygen; thio- is sulphur.

Priority Order of Functional Groups

Rank	Group	Suffix
1	−COOH	-oic acid
2	−SO₃H	-sulphonic acid Above esters and acid chlorides — the 2025 paper keys it over −COCl.
3	−COOR	-oate
4	−COCl	-oyl chloride
5	−CONH₂	-amide
6	−CN	-nitrile
7	−CHO	-al
8	>C=O	-one Lowest of the carbonyl family.
9	−OH	-ol
10	−NH₂	-amine
11	C=C, C≡C	-ene, -yne Never principal over a heteroatom group.

Acid first, then its derivatives in the order ester, chloride, amide, then nitrile, aldehyde, ketone, alcohol, amine.

Common traps

Taking the difference as 12 or 2

A CH₂ unit is one carbon AND two hydrogens: 12 + 2 = 14. Both 12 and 2 are offered.

Reading 'pyran' as a nitrogen ring

Pyrrole, pyridine and piperidine all carry N, so the prefix feels like nitrogen — but pyran is the OXYGEN six-ring. It is the planted answer in every 'no nitrogen' question.

Putting the acid chloride above the ester

The derivative order is ester, THEN acid chloride, then amide: −COOR > −COCl > −CONH₂. The reversed pair is offered as an option.

Numbering from the substituent-rich end

The double bond outranks bromo and methyl for the lowest locant; only when the C=C locant ties do the substituents decide. 4-bromo-3-methylhex-3-ene is the planted misnumbering.

Isomerism and Stereochemistry

Learn this subtopic in the notes

Structural Isomerism: Chain, Position, Functional, Metamerism

Kinds of structural isomerism

\text{chain} \;|\; \text{position} \;|\; \text{functional group} \;|\; \text{metamerism (alkyls about the group)}

Chiral Carbons and Optical Activity

Chirality test

\text{C}abcd \text{ with } a \neq b \neq c \neq d \Rightarrow \text{chiral centre}

Enantiomers and the Fischer Projection

Fischer convention

\text{vertical bonds away from the viewer;}\quad \text{horizontal bonds towards the viewer}

Common traps

Calling ether-versus-alcohol pairs metamers

Metamers keep the SAME functional group. Dimethyl ether and ethanol have different groups — functional isomers. Metamerism is two ethers (or two amines) with the alkyls redistributed.

Checking only the carbon that carries the halogen

In 2-bromo-3-methylbutane the halogen carbon IS the chiral one, but in general the stereocentre can be elsewhere. Check C2 for two identical groups, then walk the chain.

Marking 'same chemical properties' as the false statement

Enantiomers DO react alike with ordinary reagents. The false claim is 'superimposable mirror images' — a superimposable mirror image is the same molecule, and the pair would not exist.

Electronic Effects, Hybridisation, Intermediates and General Reactions

Learn this subtopic in the notes

Hybridisation of Carbon and Counting From a Formula

Hybridisation from bonding

4\ \sigma \Rightarrow sp^3;\qquad 3\ \sigma + 1\ \pi \Rightarrow sp^2;\qquad 2\ \sigma + 2\ \pi \Rightarrow sp

Inductive and Resonance Effects; Carbocation Stability

Carbocation order

3^\circ > 2^\circ > 1^\circ > \text{CH}_3^+ \quad (\text{+I and hyperconjugation})

Grignard Reagent, Dry Ice and the Clemmensen Reduction

Grignard with dry ice

\text{R-MgX} \xrightarrow{\ \text{(i) CO}_2\text{, dry ether; (ii) H}_3\text{O}^+\ } \text{R-COOH}

Common traps

Hearing 'isopentane' as an alkene

The iso- prefix marks a branch, not a double bond. Isopentane is 2-methylbutane, fully saturated: every carbon sp³.

Calling −OH a withdrawer because oxygen is electronegative

Through the σ bond it is (−I), but attached to a π-system the lone pair donates by resonance and +R wins: −OH and −OR are net DONORS. −COOH, with no lone pair to give and a C=O to pull, withdraws by both routes.

Forgetting the extra carbon from CO₂

Ethyl-MgBr does not give ethanoic acid. The CO₂ carbon joins the chain: ethyl becomes propanoic acid. Count R plus one.

Basic Principles of Organic Chemistry formulas

IUPAC Nomenclature, Functional Groups and Homologous Series

Homologous Series: One CH₂ at a Time

Naming From a Structure, and Reading Bond-Line Formulas

Heterocyclic Compounds by Heteroatom

Priority Order of Functional Groups

Common traps

Isomerism and Stereochemistry

Structural Isomerism: Chain, Position, Functional, Metamerism

Chiral Carbons and Optical Activity

Enantiomers and the Fischer Projection

Common traps

Electronic Effects, Hybridisation, Intermediates and General Reactions

Hybridisation of Carbon and Counting From a Formula

Inductive and Resonance Effects; Carbocation Stability

Grignard Reagent, Dry Ice and the Clemmensen Reduction

Common traps

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