Organic Chemistry Basics
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Introduction: The Chemistry of Life
Organic chemistry is the chemistry of carbon and its compounds. For centuries, scientists thought "organic" compounds could only be made by living organisms—hence the name. But in 1828, Friedrich Wöhler synthesized urea (a compound found in urine) from inorganic starting materials, proving that the boundary between organic and inorganic chemistry is blurred. Today, organic chemistry encompasses not just life's molecules—proteins, carbohydrates, lipids, nucleic acids—but also plastics, dyes, pharmaceuticals, and millions of synthetic compounds.
In this chapter, you'll master the foundations of organic chemistry: understanding carbon's unique versatility, the nomenclature that lets chemists communicate complex structures, and the reactions that transform one organic compound into another.
Part 1: Carbon—The Versatile Element
Tetravalency of Carbon
Tetravalency: Carbon forms four covalent bonds (C has 4 valence electrons; needs 4 more to reach octet).
Consequences:
- Can bond with 4 hydrogen atoms: CH₄ (methane)
- Can bond with 3 hydrogen and 1 carbon: C₂H₆ (ethane, where C-C bond forms)
- Can form double bonds: C=C (alkenes) or C≡C (alkynes)
- Can bond with nonmetals: F, Cl, Br, I, O, N, S, P
Catenation
Catenation: Carbon's unique ability to bond with other carbon atoms, forming long chains and rings.
Why catenation?
- Carbon-carbon bonds are strong and stable
- Carbon is small enough to form multiple bonds efficiently
- This allows millions of different carbon chains
Examples:
- Linear chain: CH₃-CH₂-CH₂-CH₃ (butane)
- Branched chain: CH₃-CH(CH₃)-CH₃ (isobutane)
- Ring structures: Benzene (C₆H₆), cyclohexane (C₆H₁₂)
Allotropes of Carbon
Allotropes: Different forms of the same element with different properties.
Diamond (C)
- Structure: Each carbon bonded to 4 other carbons in tetrahedral geometry; forms a 3D network
- Hardness: Hardest known natural substance (used for drill bits, cutting tools)
- Melting point: 3550°C (extremely high)
- Conductivity: Poor (electrons are fixed in C-C bonds)
- Density: 3.51 g/cm³
Graphite (C)
- Structure: Layers of carbon atoms; each carbon bonded to 3 others in planar sheets. Layers are held by weak van der Waals forces
- Hardness: Soft; layers slide past each other easily
- Melting point: 3727°C (sublimes; high)
- Conductivity: Excellent (delocalized electrons within layers)
- Density: 2.22 g/cm³
- Uses: Pencil lead (not actually lead!), lubricant, electrodes, crucibles
Buckminsterfullerene (C₆₀)
- Structure: 60 carbon atoms arranged in a sphere (soccer ball shape)
- Discovery: 1985 (Nobel Prize in 1996)
- Uses: Emerging applications in nanotechnology
- Nickname: "Buckyball"
Amorphous Carbon
- Structure: Disordered; contains microcrystalline diamond and graphite
- Examples: Charcoal, carbon black
- Uses: Charcoal for water purification; carbon black in inks and tires
[Memory Hook] Diamond = hardest, transparent. Graphite = soft, conducts electricity. C₆₀ = soccer ball shape.
Part 2: Hydrocarbons
Hydrocarbons: Organic compounds containing only carbon and hydrogen.
Alkanes (CₙH₂ₙ₊₂)
Definition: Saturated hydrocarbons (all single C-C bonds; no double/triple bonds).
General formula: CₙH₂ₙ₊₂ (for straight-chain alkanes)
Examples:
- CH₄ (methane, n=1)
- C₂H₆ (ethane, n=2)
- C₃H₈ (propane, n=3)
- C₄H₁₀ (butane, n=4)
- C₅H₁₂ (pentane, n=5)
IUPAC Nomenclature (prefixes for carbon number):
- 1 carbon: meth-
- 2 carbons: eth-
- 3 carbons: prop-
- 4 carbons: but-
- 5 carbons: pent-
- 6 carbons: hex-
- 7 carbons: hept-
- 8 carbons: oct-
- 9 carbons: non-
- 10 carbons: dec-
Suffix: -ane (for alkanes)
Examples:
- Methane: CH₄
- Ethane: CH₃-CH₃
- Propane: CH₃-CH₂-CH₃
- Butane: CH₃-CH₂-CH₂-CH₃
Physical Properties:
- Nonpolar; hydrophobic (don't dissolve in water)
- Soluble in nonpolar solvents (oil, gasoline)
- Low melting/boiling points (for short chains)
- Density increases with carbon number
Chemical Properties:
- Relatively inert at room temperature (strong C-C and C-H bonds)
- Combustible: CₙH₂ₙ₊₂ + O₂ → CO₂ + H₂O (exothermic)
- Substitution reactions: Cl₂ replaces H atoms under UV light
[Memory Hook] Alkanes: CₙH₂ₙ₊₂; saturated (single bonds); inert; burn completely to CO₂ + H₂O
Alkenes (CₙH₂ₙ)
Definition: Unsaturated hydrocarbons with one C=C double bond.
General formula: CₙH₂ₙ (one degree of unsaturation)
Examples:
- C₂H₄ (ethene, or ethylene): CH₂=CH₂
- C₃H₆ (propene): CH₃-CH=CH₂
- C₄H₈ (butene): CH₃-CH₂-CH=CH₂
Naming suffix: -ene
Physical Properties:
- Slightly polar; more polar than alkanes
- Low boiling points (but higher than corresponding alkanes)
- Insoluble in water
Chemical Properties:
Addition reactions: Readily add molecules across the double bond
- CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br (decolorizes bromine; test for unsaturation)
- CH₂=CH₂ + H₂O → CH₃-CH₂OH (hydration; ethene → ethanol)
Polymerization: Alkenes undergo addition polymerization
- n(CH₂=CH₂) → (-CH₂-CH₂-)ₙ (polyethylene)
Real examples:
- Ethene (ethylene): Plant hormone; ripens fruits; used in plastics
- Propene: Used to make polypropylene (PP plastic)
- Butene: Gasoline component
[Memory Hook] Alkenes: CₙH₂ₙ; unsaturated (double bond); addition reactions; decolorize Br₂
Alkynes (CₙH₂ₙ₋₂)
Definition: Unsaturated hydrocarbons with one C≡C triple bond.
General formula: CₙH₂ₙ₋₂ (two degrees of unsaturation)
Examples:
- C₂H₂ (ethyne, or acetylene): HC≡CH
- C₃H₄ (propyne): CH₃-C≡CH
Naming suffix: -yne
Physical Properties:
- Polar; polarity increases with triple bond
- High reactivity due to weak π bonds (easier to break than σ bonds)
Chemical Properties:
Addition reactions: Very reactive; add 2 molecules per triple bond
- HC≡CH + Br₂ → CHBr=CHBr + Br₂ → CHBr₂-CHBr₂
Uses:
- Acetylene (ethyne): Used in oxy-acetylene welding torches (produces extremely hot flame)
- Industrial synthesis of chemicals
[Memory Hook] Alkynes: CₙH₂ₙ₋₂; triple bond; very reactive; acetylene used in welding
Part 3: Functional Groups
A functional group is a characteristic arrangement of atoms that determines the chemical properties of an organic compound.
Alcohol (-OH)
Structure: A hydroxyl group (-OH) attached to a carbon atom
General formula: ROH (where R is an alkyl group)
Examples:
- CH₃OH (methanol, or wood alcohol): Toxic; used as fuel
- CH₃CH₂OH (ethanol, or grain alcohol): Beverage, disinfectant, fuel
- CH₃CH₂CH₂OH (propanol): Disinfectant
Properties:
- Polar (due to O-H bond)
- Soluble in water (hydrogen bonding)
- React with acids to form esters
- Oxidize to aldehydes (for primary alcohols) or ketones (for secondary alcohols)
Aldehyde (C=O)
Structure: A carbonyl group (C=O) at the end of a carbon chain
General formula: RCHO (where R is hydrogen or an alkyl group)
Examples:
- HCHO (formaldehyde): Preservative for biological specimens
- CH₃CHO (acetaldehyde): Intermediate in ethanol synthesis
Properties:
- Polar (due to C=O bond)
- Readily oxidized to carboxylic acids
- Easily reduced to alcohols
- Undergo addition reactions
Ketone (C=O)
Structure: A carbonyl group (C=O) in the middle of a carbon chain
General formula: RCOR' (where R and R' are alkyl groups)
Examples:
- CH₃COCH₃ (acetone): Solvent for nail polish
- CH₃COCH₂CH₃ (butanone): Industrial solvent
Properties:
- Similar to aldehydes but more stable
- Cannot be easily oxidized (C=O is already in middle of chain)
- More common in natural products (many plants contain ketones)
Carboxylic Acid (-COOH)
Structure: A carbonyl group bonded to a hydroxyl group: -COOH
General formula: RCOOH
Examples:
- HCOOH (formic acid): Found in ant venom
- CH₃COOH (acetic acid): Vinegar (5% aqueous solution)
- CH₃CH₂COOH (propionic acid): Preservative in bread
Properties:
- Weakly acidic (donates H⁺)
- Form dimers (two molecules hydrogen bonded)
- React with bases to form salts (neutralization)
- Esterify with alcohols
Ester (-COO-)
Structure: Derived from carboxylic acid + alcohol via condensation
General formula: RCOOR'
Formation: Acid + Alcohol → Ester + Water
- CH₃COOH + CH₃OH → CH₃COOCH₃ + H₂O
Examples:
- CH₃COOC₂H₅ (ethyl acetate): Solvent, nail polish remover
- Fats and oils: Esters of glycerol and fatty acids
Properties:
- Neutral; pleasant fruity smell
- Insoluble in water
- Easily hydrolyzed by acids or bases (saponification)
Amine (-NH₂)
Structure: Nitrogen bonded to carbon(s) and hydrogens
General formula: RNH₂ (primary), R₂NH (secondary), R₃N (tertiary)
Examples:
- CH₃NH₂ (methylamine): Pungent fishy smell
- C₆H₅NH₂ (aniline): Used in dyes and pharmaceuticals
Properties:
- Weakly basic (nitrogen accepts H⁺)
- Polar (due to N-H bonds)
- Hydrogen bonding with water
Ether (-O-)
Structure: Oxygen bonded to two carbon groups
General formula: ROR'
Examples:
- CH₃OCH₃ (dimethyl ether): Gas at room temp
- C₂H₅OC₂H₅ (diethyl ether): Volatile liquid; once used as anesthetic
Properties:
- Polar (due to bent C-O-C)
- Poor hydrogen bonding donors (but can accept)
- Volatile; many are flammable
[Memory Hook] -OH = alcohol. C=O = aldehyde (at end) or ketone (in middle). -COOH = carboxylic acid. -COO- = ester. -NH₂ = amine. -O- = ether.
Part 4: Isomerism
Isomers: Compounds with the same molecular formula but different structural arrangements.
Structural Isomerism
Chain Isomerism
- Different carbon skeleton
- Example: Butane (C₄H₁₀)
- Butane: CH₃-CH₂-CH₂-CH₃ (straight chain)
- Isobutane: CH₃-CH(CH₃)-CH₃ (branched)
Position Isomerism
- Functional group at different positions
- Example: Propanol (C₃H₈O)
- 1-Propanol: CH₃-CH₂-CH₂-OH
- 2-Propanol: CH₃-CH(OH)-CH₃
Functional Group Isomerism
- Different functional groups
- Example: Propanal and Propanol (both C₃H₆O)
- Propanal: CH₃-CH₂-CHO (aldehyde)
- Propanol: CH₃-CH(OH)-CH₃ (alcohol, specifically 2-propanol)
Stereoisomerism
Cis-Trans Isomerism (Geometric)
- Alkene carbons cannot rotate (double bond is rigid)
- Groups on same side = cis; opposite sides = trans
Example: 2-Butene (C₄H₈)
Cis-2-butene: Trans-2-butene:
CH₃ CH₃ CH₃ H
C=C C=C
H H H CH₃
(CH₃ groups on same side) (CH₃ groups on opposite sides)
Different physical properties (different boiling points, colors in some cases)
Part 5: IUPAC Nomenclature Rules
Steps to name an organic compound:
- Identify the longest carbon chain (parent chain)
- Identify and name functional groups (suffix)
- Number the chain to give functional groups lowest numbers
- Add prefixes for substituents (alkyl groups, halogens, etc.)
- Combine prefix + parent + suffix
Example: CH₃-CHBr-CH₂-CH(CH₃)-CH₃
- Longest chain: 5 carbons (pentane)
- Functional groups: Alkyl groups (methyl, bromo) on a pentane chain
- Number:
CH₃-CHBr-CH₂-CH(CH₃)-CH₃ 1 2 3 4 5 - Substituents: Bromo at position 2; methyl at position 4
- Name: 4-Methyl-2-bromopentane
Example: CH₃-CO-CH₂-CH₃ (Butanone)
- Longest chain: 4 carbons
- Functional group: Ketone (-one suffix)
- Number:
(Ketone at position 2; lowest number)CH₃-CO-CH₂-CH₃ 1 2 3 4 - Name: 2-Butanone (or Methyl ethyl ketone, MEK)
[Memory Hook] IUPAC naming: Find longest chain, identify functional groups, number to give lowest numbers, add prefixes.
Part 6: Organic Reactions
Addition Reactions
Definition: Atoms or groups add across a double or triple bond.
Example: Hydration of ethene CH₂=CH₂ + H₂O → CH₃-CH₂OH (ethanol)
Substitution Reactions
Definition: One atom/group is replaced by another.
Example: Chlorination of methane CH₄ + Cl₂ → CH₃Cl + HCl (under UV light)
Oxidation Reactions
Definition: Loss of electrons (or gain of oxygen, or loss of hydrogen).
Example: Oxidation of ethanol to acetaldehyde CH₃CH₂OH + [O] → CH₃CHO + H₂O (oxidizing agent needed)
Conclusion
Organic chemistry is the chemistry of carbon's extraordinary versatility. From simple methane to complex proteins, carbon's ability to form multiple bonds and catenate creates the diversity of organic compounds. Mastering hydrocarbons, functional groups, and nomenclature opens the door to understanding both the molecules of life and the plastics and pharmaceuticals of modern society.
23 MCQ Questions
Q1: Which property makes carbon unique in its ability to form diverse compounds?
- A) Its small size
- B) Its tetravalency and ability to catenate
- C) Its high electronegativity
- D) Its high atomic mass
Q2: Catenation refers to:
- A) Carbon bonding with hydrogen
- B) Carbon bonding with other carbon atoms
- C) Carbon bonding with oxygen
- D) Carbon bonding with metals
Q3: Which allotrope of carbon is the hardest?
- A) Graphite
- B) Diamond
- C) Buckminsterfullerene (C₆₀)
- D) Amorphous carbon
Q4: Graphite conducts electricity while diamond does not because:
- A) Graphite has more electrons
- B) Graphite has delocalized electrons in layers
- C) Diamond is harder
- D) Graphite is denser
Q5: The general formula for alkanes is:
- A) CₙH₂ₙ
- B) CₙH₂ₙ₊₂
- C) CₙH₂ₙ₋₂
- D) CₙHₙ
Q6: Which alkene addition reaction can be used to test for unsaturation?
- A) Addition of hydrogen
- B) Addition of water
- C) Addition of bromine (which decolorizes)
- D) Addition of chlorine
Q7: In IUPAC nomenclature, the suffix "-ene" indicates:
- A) A single bond
- B) A double bond
- C) A triple bond
- D) An aromatic ring
Q8: Alkynes have the general formula:
- A) CₙH₂ₙ₊₂
- B) CₙH₂ₙ
- C) CₙH₂ₙ₋₂
- D) CₙHₙ
Q9: Acetylene (ethyne) is used in:
- A) Making plastics
- B) Welding torches (produces high heat)
- C) Refrigeration
- D) Disinfection
Q10: Which functional group is found in alcohols?
- A) -CHO
- B) -COOH
- C) -OH
- D) -NH₂
Q11: The difference between an aldehyde and a ketone is:
- A) Aldehydes are more reactive
- B) Ketones have C=O at the end of the chain; aldehydes have C=O in middle
- C) Aldehydes have C=O at the end; ketones have C=O in the middle
- D) They have different molecular formulas
Q12: Acetic acid is the main component of:
- A) Vinegar
- B) Olive oil
- C) Gasoline
- D) Alcohol
Q13: An ester is formed from a carboxylic acid and an:
- A) Aldehyde
- B) Ketone
- C) Alcohol
- D) Amine
Q14: Which functional group is responsible for the basic properties of amines?
- A) The nitrogen atom
- B) The carbon-nitrogen bond
- C) The lone pair on nitrogen (can accept protons)
- D) The hydrogens attached to nitrogen
Q15: Chain isomerism occurs in:
- A) CH₄ (methane)
- B) C₂H₆ (ethane)
- C) C₃H₈ (propane)
- D) C₄H₁₀ (butane)
Q16: Position isomerism of propanol refers to:
- A) Different straight-chain arrangements
- B) Different positions of the -OH group
- C) Different molecular formulas
- D) Different functional groups
Q17: Cis-trans isomerism is possible in:
- A) Alkanes
- B) Alkenes (C=C double bond present)
- C) Aromatic compounds
- D) Carboxylic acids
Q18: The IUPAC name of CH₃-CHBr-CH₂-CH₃ is:
- A) 2-Brombutane
- B) 3-Brombutane
- C) 1-Brombutane
- D) 2-Bromo-2-methylpropane
Q19: The correct IUPAC name for CH₃-CO-CH₂-CH₃ is:
- A) Butanone
- B) 2-Butanone
- C) Methyl ethyl ketone
- D) Both B and C
Q20: Addition polymerization of ethene produces:
- A) Ethanol
- B) Polyethylene (plastic)
- C) Acetaldehyde
- D) Vinyl chloride
Q21: In the name "3-Methylpentane", the "3" refers to:
- A) The longest chain has 3 carbons
- B) The functional group position
- C) The position of the methyl substituent on the longest chain
- D) The number of double bonds
Q22: Formaldehyde (HCHO) is commonly used for:
- A) Fuel
- B) Preserving biological specimens
- C) Making plastics
- D) Disinfection
Q23: Which functional group makes an organic compound acidic?
- A) -OH (hydroxyl)
- B) -COOH (carboxylic acid)
- C) -CHO (aldehyde)
- D) -NH₂ (amine)
Answer Key: 1-B, 2-B, 3-B, 4-B, 5-B, 6-C, 7-B, 8-C, 9-B, 10-C, 11-C, 12-A, 13-C, 14-C, 15-D, 16-B, 17-B, 18-A, 19-D, 20-B, 21-C, 22-B, 23-B