Revision: Std. XI >> Basic Principles of Organic Chemistry MAH-MHT CET (PCM/PCB) Basic Principles of Organic Chemistry

The branch of chemistry that deals with compounds originally derived from living organisms, such as sugar, starch, proteins, and acetic acid.

or

Organic compounds are the hydrides of carbon (hydrocarbons) and their derivatives. The branch of chemistry dealing with these compounds is called organic chemistry.

The branch of chemistry that deals with compounds obtained from non-living sources or minerals, such as common salt, blue vitriol, and nitrates.

Nomenclature is the system of assignment of names to organic compounds.

Compounds having the same molecular formula but different structural formula are known as Isomers and the phenomenon is known as Isomerism. (iso = same, meros = parts).

The stabilising interaction that involves delocalisation of σ-electrons of C–H bond of an alkyl group linked directly to an atom of unsaturated system or to an atom having unshared p-orbital, is called hyperconjugation.

e.g.

2D Representations

Type	Description	Example
Dash (Structural) Formula	Shows all atoms and all covalent bonds by dashes	\[\begin{array}{cc} \phantom{.....}\ce{H}\phantom{.....}\ce{H}\phantom{....}\ce{H}\phantom{.....}\\ \phantom{.....}|\phantom{.....}|\phantom{.....}|\phantom{......}\\ \phantom{.}\ce{H-C-C-C-H}\phantom{..}\\ \phantom{.....}|\phantom{.....}|\phantom{.....}|\phantom{......}\\ \phantom{.....}\ce{H}\phantom{....}\ce{H}\phantom{....}\ce{H}\phantom{.....} \end{array}\]
Condensed Formula	Groups atoms together without showing all bonds	CH₃CH₂CH₃ or CH₃–CH₂–CH₃
Bond Line / Zig-Zag Formula	Skeletal structure — lines represent C–C bonds; carbon atoms at line intersections and ends; H atoms implied

3D Representations

(i) Based on Functional Group:

(ii) Based on Carbon Skeleton:

• The IUPAC system provides a unique, systematic way to name carbon compounds based on structure, replacing confusing common names.
• An IUPAC name has three parts: prefix, parent, and suffix, reflecting the carbon chain and functional group.
• The parent name is based on the longest carbon chain, and its ending changes to –ane, –ene, or –yne depending on the number of bonds.
• Functional groups are shown as prefixes or suffixes, and the chain is numbered to give them the lowest possible number.
• If the suffix begins with a vowel, the final ‘e’ in the parent alkane name is dropped (e.g., propane → propanone).

1. Longest chain rule — The longest continuous C–C linkage is the parent chain
2. Lowest locant rule — Number the parent chain from the end that gives substituents the lowest possible locant numbers
3. Alphabetical order rule — When multiple substituents are present, name them in alphabetical order (ignoring multiplying prefixes like di, tri)
4. First point of difference rule — If functional groups are at equivalent positions, number so that the first different locant is the lowest
5. Complex group rule — For branched side chains, the carbon through which the complex group is attached to the main chain is always given locant number 1
6. Double and triple bond priority — If both are at equal distance from ends, double bond is preferred; if asymmetrical, use lowest locant sum rule then first point of difference rule
7. Multiple functional groups — Priority order: –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > C=O > –OH > –NH₂ > C=C > –C≡C–

1. Structural Isomerism

Same molecular formula but different connectivity (bonding) of atoms.

Types:

1. Chain isomerism: Different carbon skeleton (straight/branched)
2. Position isomerism: Functional group at different positions
3. Functional isomerism: Different functional groups (e.g., alcohol vs ether)
4. Metamerism: Different alkyl groups on either side of same functional group
5. Tautomerism: Dynamic equilibrium between two forms (keto ↔ enol)

2. Stereoisomerism

Same molecular formula and bonding but different spatial arrangement.

Types:

I. Geometrical isomerism:

• Due to restricted rotation (double bond)
• Forms cis (same side) and trans (opposite side)

II. Optical isomerism:

• Mirror image isomers (enantiomers)
• Show optical activity (rotate plane polarized light)

1. Electrophiles (E⁺) — Electron-loving

Attack regions of high electron density.

2. Nucleophiles (Nu⁻) — Nucleus-loving

Attack regions of low electron density (electron-deficient centres).

• Permanent effect due to shift of σ-electrons
• Electrons move towards more electronegative atom
• Effect decreases with distance (negligible after 3 carbons)

e.g-

• Resonance occurs when a single Lewis structure cannot accurately describe a molecule.
• Multiple structures (canonical structures) differing only in the arrangement of electrons (not atoms) are drawn.
• The actual molecule is a resonance hybrid — a weighted average of all canonical structures.
• Resonance energy = Actual bond energy − Energy of most stable resonating structure.

Conditions for Writing Resonance Structures:

• Same atomic positions in all structures.
• Same number of unpaired electrons.
• Nearly the same energy.
• Negative charge on the more electronegative atom; positive charge on the electropositive atom.
• Like charges should not reside on adjacent atoms.

Examples:

Resonance:

• When a single Lewis structure cannot explain all properties of a compound, two or more structures (canonical forms) are written
• The real molecule does not exist as separate forms, it exists as a resonance hybrid
• Resonance hybrid is a weighted average of all canonical forms
• Due to resonance, electrons are delocalised over the molecule
• Resonance hybrid is more stable (lower energy) than any single structure

Resonance Effect (Mesomeric Effect):

A temporary effect involving the complete transfer of shared π-electrons to one of the atoms in a multiple bond, in the presence of an attacking reagent.

Type	Direction of π-electron transfer	Example
+E Effect	π electrons transferred to the atom to which the reagent gets attached
–E Effect	π electrons transferred to the atom to which the reagent does not attach

Also called the Baker-Nathan effect or no-bond resonance.

Conditions for hyperconjugation:

• The compound must have at least one sp²-hybridised carbon (alkene, alkyl carbocation, or alkyl free radical)
• The α-carbon must have at least one H–C bond attached to the unsaturated system
• More the number of H–C bonds at α-carbon → greater the stabilisation → more stable the alkene/carbocation

Resonance vs. Hyperconjugation: