Unit 13: Fundamental Principles of Organic Chemistry

1. IUPAC Nomenclature of Organic Compounds (up to 6 carbon atoms)

IUPAC (International Union of Pure and Applied Chemistry) nomenclature provides a systematic method for naming organic compounds, ensuring that each unique structure has a unique name. This section focuses on naming compounds containing up to six carbon atoms.

Steps for Naming Organic Compounds:

Step 1: Select the Longest Carbon Chain (Parent Chain)
Identify the longest continuous chain of carbon atoms. This chain forms the basis of the compound's name. If there are multiple chains of the same length, choose the one with the most substituents.

Example: In CH₃-CH(CH₃)-CH₂-CH₃, the longest chain is four carbons (butane), not three (propane) if you start from the methyl group as part of the main chain.
Step 2: Number from the End Nearest to the Substituent
Number the carbon atoms in the parent chain starting from the end that gives the lowest possible numbers to the substituents. If there are multiple substituents, give the lowest numbers to the first point of difference. If a multiple bond (double or triple) is present, it takes precedence over alkyl substituents in numbering.

Example: For CH₃-CH(CH₃)-CH₂-CH₃, numbering from left gives 2-methylbutane, while from right gives 3-methylbutane. 2-methylbutane is correct as 2 is lower than 3.
Step 3: Name and Locate Substituents
Identify all groups attached to the parent chain that are not hydrogen atoms. Name these substituents (e.g., -CH₃ is methyl, -CH₂CH₃ is ethyl). Precede the substituent name with the number of the carbon atom to which it is attached.

Example: In 2-methylpropane, the -CH₃ group is a methyl substituent located on the second carbon.
Step 4: Name the Parent Chain with the Appropriate Suffix
The parent chain's name depends on the number of carbon atoms and the type of carbon-carbon bonds present:
- Alkanes: Suffix -ane (all single bonds).
- Alkenes: Suffix -ene (at least one double bond). The position of the double bond is indicated by the lowest possible number.
- Alkynes: Suffix -yne (at least one triple bond). The position of the triple bond is indicated by the lowest possible number.
Example: A four-carbon chain with all single bonds is butane. A four-carbon chain with a double bond at the first carbon is 1-butene.

Rules for Naming:

Alphabetical Order of Substituents: If there are two or more different substituents, list them in alphabetical order. Prefixes like di-, tri-, tetra- are ignored when determining alphabetical order.
Di/Tri/Tetra Prefixes for Multiple Identical Groups: If the same substituent appears multiple times, use prefixes di- (for two), tri- (for three), tetra- (for four), etc., before its name. Each substituent's position must be indicated by a number, separated by commas.

Examples of IUPAC Nomenclature:

CH₄: Methane (1 carbon, alkane)
CH₃-CH₃: Ethane (2 carbons, alkane)
CH₃-CH₂-CH₃: Propane (3 carbons, alkane)
CH₃-CH₂-CH₂-CH₃: Butane (4 carbons, alkane)
CH₃-CH₂-CH₂-CH₂-CH₃: Pentane (5 carbons, alkane)
CH₃-CH₂-CH₂-CH₂-CH₂-CH₃: Hexane (6 carbons, alkane)
CH₃-CH(CH₃)-CH₃: 2-methylpropane (Longest chain is 3 carbons, methyl group at C2)
CH₃-CH(CH₃)-CH(CH₃)-CH₃: 2,3-dimethylbutane (Longest chain is 4 carbons, two methyl groups at C2 and C3)
CH₂=CH-CH₂-CH₃: 1-butene (4 carbons, double bond at C1)
CH₃-C≡C-CH₂-CH₃: 2-pentyne (5 carbons, triple bond at C2)

2. Qualitative Analysis of Organic Compounds

Qualitative analysis involves determining the elements present in an organic compound. Common elements like carbon and hydrogen are almost always present. Nitrogen, sulphur, and halogens (Cl, Br, I) are detected using specific tests, most notably Lassaigne's Test.

Lassaigne's Test (Sodium Fusion Test):

Organic compounds are generally covalent and do not readily ionize. To detect elements like N, S, and halogens, they are converted into their ionic forms by fusing the organic compound with sodium metal. The resulting sodium salts are then extracted with distilled water to form a sodium fusion extract (SFE), which is used for subsequent tests.

Organic Compound + Na &xrightarrow{\text{heat}} Ionic Sodium Salts

Detection of Nitrogen:

Principle: Nitrogen in the organic compound reacts with sodium and carbon (from the organic compound itself) to form sodium cyanide (NaCN). Na + C + N &xrightarrow{\Delta} NaCN
Procedure: The SFE is boiled with ferrous sulphate (FeSO₄) solution and then acidified with concentrated sulphuric acid (H₂SO₄) or hydrochloric acid (HCl).
Reaction & Observation:
1. FeSO₄ + 2NaCN → Fe(CN)₂ + Na₂SO₄
2. Fe(CN)₂ + 4NaCN → Na₄[Fe(CN)₆] (Sodium ferrocyanide)
3. In the presence of Fe³⁺ ions (formed by oxidation of Fe²⁺ during heating) and acid, Na₄[Fe(CN)₆] reacts to form Prussian blue precipitate. 3Na₄[Fe(CN)₆] + 4Fe³⁺ → Fe₄[Fe(CN)₆]₃ + 12Na⁺
  Observation: Formation of a deep blue or green precipitate (Prussian blue) confirms the presence of nitrogen.

Detection of Sulphur:

Principle: Sulphur in the organic compound reacts with sodium to form sodium sulphide (Na₂S). 2Na + S &xrightarrow{\Delta} Na₂S
Procedure: A portion of the SFE is treated with lead acetate solution.
Reaction & Observation: Na₂S + (CH₃COO)₂Pb → PbS↓ + 2CH₃COONa
Observation: Formation of a black precipitate of lead sulphide (PbS) confirms the presence of sulphur.
Alternative Test: SFE can also be treated with sodium nitroprusside solution. A violet colour indicates the presence of sulphur. Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS] (Violet colour)

Detection of Halogens (Cl, Br, I):

Principle: Halogens (X) in the organic compound react with sodium to form sodium halide (NaX). Na + X &xrightarrow{\Delta} NaX
Procedure: A portion of the SFE is acidified with dilute nitric acid (HNO₃) and then treated with silver nitrate (AgNO₃) solution. The acidification is crucial to decompose any NaCN or Na₂S that might interfere with the test.
Reaction & Observation: NaX + AgNO₃ → AgX↓ + NaNO₃
- Chlorine (Cl): Forms AgCl, a white precipitate, which is soluble in ammonium hydroxide (NH₄OH).
- Bromine (Br): Forms AgBr, a pale yellow precipitate, which is sparingly soluble in ammonium hydroxide.
- Iodine (I): Forms AgI, a yellow precipitate, which is insoluble in ammonium hydroxide.

3. Isomerism in Organic Compounds

Isomerism is the phenomenon where two or more compounds have the same molecular formula but different structural or spatial arrangements of atoms. These compounds are called isomers.

A. Structural Isomerism:

Structural isomers (or constitutional isomers) have the same molecular formula but differ in the connectivity of their atoms, leading to different structural formulas.

Chain Isomerism (or Skeletal Isomerism):
Compounds have the same molecular formula but differ in the arrangement of the carbon skeleton (parent chain).

Example: C₄H₁₀
- CH₃-CH₂-CH₂-CH₃ (n-Butane - straight chain)
- CH₃-CH(CH₃)-CH₃ (Isobutane or 2-methylpropane - branched chain)
Position Isomerism:
Compounds have the same molecular formula and the same carbon skeleton, but differ in the position of the functional group or substituent on the carbon chain.

Example: C₃H₈O (Propanol)
- CH₃-CH₂-CH₂-OH (1-Propanol - -OH group on C1)
- CH₃-CH(OH)-CH₃ (2-Propanol - -OH group on C2)
Functional Isomerism:
Compounds have the same molecular formula but contain different functional groups.

Example: C₂H₆O
- CH₃-CH₂-OH (Ethanol - an alcohol)
- CH₃-O-CH₃ (Dimethyl ether - an ether)
Metamerism:
A specific type of structural isomerism where compounds have the same molecular formula and the same functional group, but differ in the nature of the alkyl groups attached to the functional group (typically polyvalent functional groups like ethers, ketones, or secondary amines).

Example: C₄H₁₀O (Ethers)
- CH₃-CH₂-O-CH₂-CH₃ (Diethyl ether - two ethyl groups around oxygen)
- CH₃-O-CH₂-CH₂-CH₃ (Methyl propyl ether - methyl and propyl groups around oxygen)
Tautomerism:
A dynamic equilibrium between two structural isomers that differ in the position of a proton and a double bond. The isomers are called tautomers.

Example: Keto-enol tautomerism

R-CH₂-C(=O)-R' ⇌ R-CH=C(OH)-R'

Here, the keto form (containing a carbonyl group) and the enol form (containing a hydroxyl group attached to a carbon-carbon double bond) interconvert.

B. Stereoisomerism:

Stereoisomers have the same molecular formula and the same connectivity of atoms, but differ in the spatial arrangement of their atoms.

Geometrical Isomerism (cis-trans Isomerism):
Arises due to restricted rotation around a carbon-carbon double bond or in cyclic structures. Substituents can be on the same side (cis) or opposite sides (trans) of the double bond or ring plane.

Condition: Each carbon atom of the double bond must be attached to two different groups.

Example: C₄H₈ (2-Butene)
- cis-2-butene: Both methyl groups are on the same side of the C=C double bond.
- trans-2-butene: Both methyl groups are on opposite sides of the C=C double bond.
Optical Isomerism (d and l forms / Enantiomerism):
Occurs in molecules that are non-superimposable mirror images of each other. These isomers are called enantiomers and possess chirality.

Condition: Presence of a chiral carbon (also called an asymmetric carbon), which is a carbon atom bonded to four different groups.

Properties: Optical isomers rotate the plane of plane-polarized light in opposite directions (dextrorotatory 'd' or '+' rotates to the right; levorotatory 'l' or '-' rotates to the left) but have identical physical and chemical properties in a non-chiral environment.

Example: Lactic acid (CH₃-CH(OH)-COOH) has a chiral carbon (the carbon bonded to -CH₃, -OH, -H, and -COOH).
- One enantiomer rotates light clockwise (d-lactic acid).
- The other enantiomer rotates light counter-clockwise (l-lactic acid).

4. Reaction Mechanism

A reaction mechanism describes the step-by-step sequence of elementary reactions by which overall chemical change occurs. It involves understanding how bonds break and form, and the nature of reactive intermediates.

Bond Fission:

Homolytic Fission:
The breaking of a covalent bond in such a way that each atom retains one of the bonding electrons. This process leads to the formation of highly reactive species called free radicals.

A—B → A^. + B^. (Each dot represents an unpaired electron)

Example: The breaking of a chlorine molecule under UV light: Cl—Cl &xrightarrow{UV light} Cl^. + Cl^.
Heterolytic Fission:
The breaking of a covalent bond in such a way that one atom retains both bonding electrons, while the other atom gets none. This leads to the formation of ions: a positively charged species (carbocation or cation) and a negatively charged species (carbanion or anion).

A—B → A⁺ + B^- (If B is more electronegative)

A—B → A^- + B⁺ (If A is more electronegative)

Example: CH₃—Cl → CH₃⁺ + Cl^- (Formation of a carbocation and a chloride ion)

Reactive Intermediates:

Electrophiles (Electron-loving species):
Species that are electron-deficient and seek electron-rich centers (nucleophiles) to form a new bond. They are typically positively charged or have an incomplete octet.

Examples: H⁺, Cl⁺, NO₂⁺ (nitronium ion), BF₃ (boron trifluoride), AlCl₃ (aluminum chloride).
Nucleophiles (Nucleus-loving species):
Species that are electron-rich and seek positively charged or electron-deficient centers (electrophiles) to donate an electron pair and form a new bond. They are typically negatively charged or have lone pairs of electrons.

Examples: OH^- (hydroxide ion), CN^- (cyanide ion), NH₃ (ammonia), H₂O (water), R^- (carbanion).
Free Radicals:
Neutral species that have an unpaired electron. They are highly reactive and are formed by homolytic fission.

Examples: Cl^. (chlorine radical), CH₃^. (methyl radical).

5. Electronic Effects

Electronic effects describe how the presence of certain atoms or groups influences the electron distribution within a molecule, thereby affecting its stability, reactivity, and physical properties. These effects are crucial for understanding reaction mechanisms.

A. Inductive Effect (+I and -I):

The inductive effect is a permanent displacement of electron density along a sigma (single) bond chain due to the difference in electronegativity between atoms or groups. It weakens rapidly with distance.

+I Effect (Positive Inductive Effect / Electron-Donating Inductive Effect):
Groups that donate or push electron density towards the rest of the carbon chain. They are typically alkyl groups.

Order of +I effect: (CH₃)₃C- > (CH₃)₂CH- > CH₃CH₂- > CH₃- (Tertiary > Secondary > Primary > Methyl)

Examples: Alkyl groups (-CH₃, -C₂H₅, -CH(CH₃)₂).

Impact: Increases electron density, stabilizes carbocations, destabilizes carbanions, decreases acidity.
-I Effect (Negative Inductive Effect / Electron-Withdrawing Inductive Effect):
Groups that withdraw or pull electron density away from the rest of the carbon chain. These are typically electronegative atoms or groups with highly electronegative atoms.

Order of -I effect (some common groups): -NO₂ > -CN > -COOH > -F > -Cl > -Br > -I > -OH > -OR > -C₆H₅

Examples: -NO₂ (nitro), -COOH (carboxyl), -CN (cyano), -F (fluoro), -Cl (chloro).

Impact: Decreases electron density, stabilizes carbanions, destabilizes carbocations, increases acidity.

B. Resonance Effect (+R and -R) / Mesomeric Effect:

The resonance effect involves the delocalization of pi (π) electrons or lone pairs of electrons within a conjugated system (alternating single and double bonds). This effect is permanent and operates through pi bonds, influencing the electron density at various positions in the molecule.

+R Effect (Positive Resonance Effect / Electron-Donating Resonance Effect):
Groups that donate electrons into a conjugated system, typically by sharing a lone pair of electrons with the pi system. This increases electron density, especially at ortho and para positions in a benzene ring.

Examples: -OH (hydroxyl), -OCH₃ (methoxy), -NH₂ (amino), -OR (alkoxy), -X (halogens, though they also have a strong -I effect).

Impact: Activates aromatic rings towards electrophilic substitution, stabilizes carbocations, increases basicity.
-R Effect (Negative Resonance Effect / Electron-Withdrawing Resonance Effect):
Groups that withdraw electrons from a conjugated system, typically through pi bonds. This decreases electron density, especially at ortho and para positions in a benzene ring.

Examples: -NO₂ (nitro), -COOH (carboxyl), -CHO (aldehyde), -CN (cyano), -COR (ketone), -SO₃H (sulfonic acid).

Impact: Deactivates aromatic rings towards electrophilic substitution, stabilizes carbanions, increases acidity.