Organic Chemistry Basics

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1. Structural Representation of Organic Molecules Structural Formula: Shows all constituent atoms and covalent bonds as dashes. Example: Methane ($CH_4$) $$ \begin{array}{c} \text{H} \\ | \\ \text{H} - \text{C} - \text{H} \\ | \\ \text{H} \end{array} $$ Electron-dot (Lewis) Structures: Shows valence electrons as dots. Two dots between atoms represent one covalent bond. Example: Methane $$ \begin{array}{c} \text{H} \\ \text{H} : \text{C} : \text{H} \\ \text{H} \end{array} $$ Condensed Formula: Simplifies structural formula by hiding some or all covalent bonds and indicating identical groups by subscripts. Example: Ethane can be $CH_3 - CH_3$ or $CH_3CH_3$. Bond Line Formula (Zig-Zag Formula): Carbon and hydrogen atoms are not written. Lines represent C-C bonds in a zig-zag manner. Terminals and intersections denote carbon atoms with appropriate number of hydrogens. Heteroatoms and H atoms bonded to heteroatoms are shown. Example: Propane is $\text{CH}_3\text{CH}_2\text{CH}_3$ represented as $\wedge$. Ethanol ($C_2H_5OH$) is $\text{OH}$. 2. Drawing Molecules in Three Dimensions Wedge Formula: Solid wedge ($\blacktriangle$): Bond projecting up from the paper towards the reader. Dashed wedge ($\cdots\cdot$): Bond projecting backward, away from the reader. Normal line (—): Bond in the plane of the paper. Example: Methane (Fig. 14.1) Fischer Projection Formula (Cross Formula): Main carbon chain vertical. Each carbon is a cross. Horizontal lines: Bonds projecting up. Vertical lines: Bonds going below. Common in carbohydrate chemistry. (Fig. 14.2) Newman Projection Formula: Visualized by looking along a C-C bond. Front carbon: A point. Rear carbon: A circle. Bonds on front carbon: Radiate from point. Bonds on rear carbon: Radiate from circumference. Example: Ethane (Fig. 14.3) Sawhorse (Andiron/Perspective) Formula: C-C single bond: Long slanting line. Lower end: Front carbon. Upper end: Rear carbon. Remaining three bonds radiate from respective carbons. Example: Ethane (Fig. 14.4) Molecular Models: (Fig. 14.5) Framework model: Emphasizes skeletal pattern of bonds. Ball and stick model: Ball represents atom, stick represents bond. Space filling model: Emphasizes relative size of atoms, bonds not shown. 3. Classification of Organic Compounds 3.1 Based on Carbon Skeleton Organic compounds are classified into: Acyclic/Aliphatic (Open Chain): Molecules with chains of carbon atoms. Can be straight or branched. (Table 14.2) Example: Propane ($CH_3-CH_2-CH_3$), n-propyl alcohol ($CH_3-CH_2-CH_2-OH$), Isobutane ($\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}_3$) Cyclic (Closed Chain): Compounds forming ring-like structures. Homocyclic (Carbocyclic): Ring made only of carbon atoms. Alicyclic: Properties similar to aliphatic compounds (e.g., Cyclobutane, Cyclohexene). Aromatic: Special stability, often containing benzene ring. Benzenoid Aromatic: Contain benzene ring (e.g., Benzene, Naphthalene). Non-benzenoid Aromatic: Do not contain benzene ring (e.g., Tropone). Heterocyclic: Ring includes one or more heteroatoms (O, N, S). Heterocyclic Aromatic (e.g., Furan, Thiophene, Pyridine). Heterocyclic Non-Aromatic (e.g., Tetrahydrofuran, Piperidine). 3.2 Based on Functional Group Functional Group: Part of a molecule that undergoes change in a reaction (e.g., -OH in propanol). Homologous Series: Family of compounds with same functional group, differing by a constant $CH_2$ unit. Members (homologues) have similar chemical properties and gradually changing physical properties. (Table 14.4) Common Functional Groups: (Table 14.3) Sr. No. Name of Family/Functional Group Structure of Functional Group Example 1. Halide $-X$ $CH_3Br$ (methyl bromide) 2. Cyanide (or Nitrile) $-C \equiv N$ $CH_3CN$ (methyl cyanide) 5. Alcohol $-OH$ $CH_3OH$ (methyl alcohol) 10. Ether $-O-$ $CH_3-O-CH_3$ (dimethyl ether) 11. Aldehyde $\begin{array}{c} O \\ || \\ -C-H \end{array}$ $CH_3CHO$ (acetaldehyde) 12. Ketone $\begin{array}{c} O \\ || \\ -C- \end{array}$ $CH_3-CO-CH_3$ (acetone) 13. Carboxylic acid $\begin{array}{c} O \\ || \\ -C-OH \end{array}$ $CH_3COOH$ (acetic acid) 4. Nomenclature of Organic Compounds 4.1 Common/Trivial Names Older names, often with historical context or common usage. Example: Toluene (methylbenzene), Aniline (aminobenzene), Phenol (hydroxybenzene). (Table 14.5) 4.2 IUPAC Nomenclature Systematic method for naming organic compounds, giving a unique name. Based on parent hydrocarbon, branches, and functional groups. Straight Chain Alkanes: (Table 14.6) Name derived from number of carbon atoms. First four members (methane, ethane, propane, butane) have accepted trivial names as IUPAC names. Branched Saturated Hydrocarbons: Alkyl groups: Formed by removing one H from an alkane. 'ane' replaced by 'yl'. (Table 14.7) Example: Methane ($CH_4$) $\rightarrow$ methyl ($CH_3-$) Branched alkyl groups: (Table 14.8) Example: Isopropyl ($\text{CH}_3-\text{CH}(\text{CH}_3)- $), sec-Butyl ($\text{CH}_3-\text{CH}_2-\text{CH}(\text{CH}_3)- $). Rules for Branched Saturated Hydrocarbons: Select the longest continuous carbon chain (parent chain). Other carbons are side chains/branches/alkyl substituents. Number the parent chain to give the lowest locant number to substituents. Names of alkyl substituents are prefixes, listed alphabetically with locant numbers. Locant separated by hyphen. If numbering gives the same set of locants, choose the one giving smaller locant to the substituent with alphabetical priority. Use di, tri, tetra for identical substituents; locants listed together, separated by commas. Branched alkyl groups without accepted trivial names are numbered from the point of attachment to the parent chain. Unsaturated Hydrocarbons (Alkenes and Alkynes): Longest continuous chain must include C-C multiple bond. Numbering gives the lowest possible locant number to the multiple bond. 'ane' replaced by 'ene' (alkene) or 'yne' (alkyne). Position of multiple bond indicated by smaller locant number. If multiple bond is equidistant, number from end nearer to first branching. If two double/triple bonds, use 'diene'/'diyne'. 'a' of 'ane' retained. If both double and triple bonds, 'en' ending first, then 'yne'. Numbering gives preference to the multiple bond nearer to the end. If tie between double and triple bond, double bond gets lower number. Simple Monocyclic Hydrocarbons: Saturated: Prefix 'cyclo' to corresponding alkane name (e.g., Cyclopropane). Unsaturated: 'ene'/'yne' replace 'ane' (e.g., Cyclohexene). With side chains: Rules for branched hydrocarbons apply. Number ring starting from side chain. If alkyl group has more carbons than ring, compound named as derivative of alkane, ring as substituent (e.g., phenyl-substituted alkane). Compounds with Functional Groups: Functional groups act as prefixes or suffixes. (Table 14.9) Monofunctional compounds: Longest carbon chain containing functional group is parent chain. Numbering gives smallest locant to functional group. Parent name modified by suffix. Polyfunctional compounds: One functional group is principal (suffix), others are substituents (prefixes). Priority order: $COOH > SO_3H > COOR > COCl > CONH_2 > CN > CHO > C=O > OH > NH_2 > C=C > C \equiv C$. Substituted Benzene: Monosubstituted: Substituent name as prefix to "benzene" (e.g., Chlorobenzene). Trivial names often used (e.g., Toluene for methylbenzene). Disubstituted: Prefixes ortho (o-), meta (m-), para (p-) for 1,2-, 1,3-, 1,4- positions respectively. IUPAC uses numbering. Trisubstituted: Numbers used for relative positions, following alphabetical order and lowest locant rule. Common names of parent benzene derivatives can be used. 5. Isomerism Compounds with the same molecular formula but different structures. Structural Isomerism: Same molecular formula, different structural formulae. Chain Isomerism: Different carbon skeletons (e.g., Butane and 2-methylpropane). Position Isomerism: Same functional groups at different positions on the parent chain (e.g., But-1-ene and But-2-ene). Functional Group Isomerism: Same molecular formula, different functional groups (e.g., Dimethyl ether and Ethyl alcohol). Metamerism: Same molecular formula and functional group, but unequal distribution of carbon atoms on either side of the functional group (e.g., Ethoxyethane and Methoxypropane). Tautomerism: Rapid equilibrium between two structurally distinct molecules (e.g., Keto-enol tautomerism). Stereoisomerism: Same structural formula, different spatial arrangement of groups/atoms. Geometrical Isomerism Optical Isomerism 6. Theoretical Basis of Organic Reactions 6.1 Types of Cleavage of Covalent Bond Homolytic Cleavage: Each bonded atom takes one electron from the shared pair, forming free radicals. Represented by half-headed curved arrows ($\curvearrowright$). Favored by UV light, peroxides, high temperatures. Free radicals are highly reactive (e.g., $CH_3\cdot$ methyl free radical). Stability of alkyl free radicals: $3^\circ > 2^\circ > 1^\circ$. Free radicals are $sp^2$ hybridized, planar trigonal geometry. Heterolytic Cleavage: One atom takes both electrons from the shared pair, forming ions (carbocation and carbanion). Represented by full-headed curved arrows ($\curvearrowright$). Favored by polar solvents. Carbocation: Carbon atom with sextet of electrons and positive charge. $sp^2$ hybridized, trigonal planar geometry. Stability: $3^\circ > 2^\circ > 1^\circ$. Carbanion: Carbon atom with completed octet of electrons and negative charge. 6.2 Types of Reagent Electrophiles ($E^+$): Electron-loving species, accept electrons, electron deficient (e.g., $Br^+$, $CH_3^+$, $AlCl_3$). Nucleophiles ($Nu^-$): Nucleus-loving species, donate electrons, electron rich (e.g., $OH^-$, $H_2O$). 6.3 Electronic Effects in Organic Reactions (Permanent Effects) Inductive Effect: Permanent displacement of electron density along a sigma bond due to electronegativity difference. Represented by an arrow head in the center of the bond ($\rightarrow$). Decreases rapidly with distance. Electron Withdrawing (-I effect): Groups that pull electron density (e.g., $-Cl$, $-NO_2$, $-COOH$). Electron Donating (+I effect): Groups that push electron density (e.g., alkyl groups like $-CH_3$). Resonance Effect (Mesomeric Effect): Polarity developed in a molecule due to interaction between conjugated $\pi$ bonds or between $\pi$ bond and a p-orbital on an attached atom. Involves delocalization of $\pi$ electrons, represented by curved arrows. Produces resonance structures (contributing/canonical structures), linked by double-headed arrow ($\leftrightarrow$). The actual molecule is a resonance hybrid, more stable than any single resonance structure. Positive Resonance (+R or +M effect): Substituent donates lone pair electrons to the conjugated system, increasing electron density at ortho and para positions (e.g., $-OH$, $-NH_2$, halogens). Negative Resonance (-R or -M effect): Substituent withdraws electrons from the conjugated system, creating positive polarity at ortho and para positions (e.g., $-COOH$, $-NO_2$). Electromeric Effect: Temporary electronic effect exhibited by multiple-bonded groups in the excited state in the presence of a reagent. Electron pair completely shifts to one of the multiply bonded atoms, forming a charged separated structure. Hyperconjugation: Permanent electronic effect involving delocalization of $\sigma$ electrons of a C-H bond of an alkyl group directly attached to an unsaturated system or an empty p-orbital. Also called "no bond resonance". Explains stability of carbocations, free radicals, and alkenes. More $\alpha$-hydrogens lead to more hyperconjugative structures and greater stability. Stronger than inductive effect.