Chapter 5: Carbon Compounds (Organic Chemistry)

Overview

Organic chemistry is the study of carbon-containing compounds, which form the basis of all living organisms and countless synthetic materials. This vast field encompasses millions of compounds with diverse structures and properties. From the fuels that power our vehicles to the medicines that heal our bodies and the plastics that shape modern life, organic compounds are essential to our existence. This chapter explores the fundamental concepts of organic chemistry, including hydrocarbons, functional groups, homologous series, and the specific reactions and properties of important organic families.

Learning Objectives

After studying this chapter, you should be able to:

• Distinguish between organic and inorganic carbon compounds
• Understand the classification of carbon compounds into hydrocarbons and non-hydrocarbons
• Define homologous series and their characteristics
• Identify and name different functional groups
• Describe the properties and reactions of alkanes, alkenes, alcohols, and carboxylic acids
• Understand isomerism and its significance
• Apply IUPAC nomenclature rules to organic compounds
• Analyze practical applications of organic compounds in daily life

5.1 Types of Carbon Compounds

What are Carbon Compounds?

Carbon compounds are compounds that contain the element carbon. They are divided into two main groups based on their structure and properties.

Organic vs Inorganic Carbon Compounds

Hydrocarbons

Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms.

Saturated Hydrocarbons

• Only contain single carbon-carbon bonds (C-C)
• Examples: Alkanes (methane, ethane, propane)

Unsaturated Hydrocarbons

• Contain at least one double (C=C) or triple (C≡C) bond between carbon atoms
• Examples: Alkenes (ethene), alkynes (ethyne)

Non-Hydrocarbons

Non-hydrocarbon organic compounds contain atoms other than carbon and hydrogen, such as oxygen, nitrogen, sulfur, etc.

Major Classes

• Alcohols: Contain hydroxyl group (-OH)
• Carboxylic acids: Contain carboxyl group (-COOH)
• Esters: Contain ester group (-COO-)
• Amides: Contain amide group (-CONH-)

---

Did You Know?

Carbon's unique ability to form four covalent bonds allows it to create long chains, branched structures, and rings. This versatility is why there are over 10 million known carbon compounds, making organic chemistry the largest branch of chemistry!

5.2 Homologous Series

What is a Homologous Series?

Homologous series is a family of organic compounds with the same functional group and similar chemical properties. Members differ by a -C$H_2$- unit.

Characteristics of Homologous Series

1. Same functional group
2. Similar chemical properties
3. Can be represented by a general formula
4. Successive members differ by -C$H_2$- unit
5. Physical properties change gradually as molecular size increases

Major Homologous Series

IUPAC Nomenclature

IUPAC naming follows systematic rules:

• Prefix: Indicates number of carbon atoms (meth-, eth-, prop-, but-, pent-, hex-)
• Suffix: Indicates homologous series (-ane, -ene, -ol, -oic acid)

---

SPM Exam Tips

• Remember the general formulas:
  • Alkanes: Cₙ$H_2$ₙ₊₂
  • Alkenes: Cₙ$H_2$ₙ
  • Alcohols: Cₙ$H_2$ₙ₊₁OH
  • Carboxylic acids: Cₙ$H_2$ₙ₊₁COOH
• Always count carbon atoms correctly when naming
• Functional groups determine chemical properties
• Homologous series members show gradual property changes

5.3 Alkanes

What are Alkanes?

Alkanes are saturated hydrocarbons with general formula Cₙ$H_2$ₙ₊₂. They are relatively unreactive.

Sources and Production

Natural sources: Petroleum (crude oil) and natural gas Industrial production: Separated by fractional distillation

Physical Properties

Trends with increasing carbon number:

• Boiling point increases: Due to stronger van der Waals forces between larger molecules
• Melting point increases: Larger molecules require more energy to melt
• Density increases: Larger molecules are more dense
• Solubility: Insoluble in water but soluble in organic solvents

Chemical Properties

1. Combustion

Complete combustion (excess oxygen): produces $CO_2$ and $H_2O$

$$CH_4(g) + 2O_2(g) → CO_2(g) + 2H_2O(l)$$

Incomplete combustion (limited oxygen): produces CO and/or carbon (soot) and $H_2$O

$$2CH_4(g) + 3O_2(g) → 2CO(g) + 4H_2O(l)$$

2. Substitution Reactions

Reacts with halogens (C$l_2$, B$r_2$) in the presence of UV light

CH_4(g) + C_l_2(g) --(UV)→ C_H_3Cl(g) + HCl(g) (chloromethane)

3. Cracking

High temperature decomposition of large hydrocarbons to smaller ones

$$C_5H_12(g) → C_5H_10(g) + C_5H_10(g)$$

Key Terms

• Fractional distillation: Process of separating liquids with different boiling points
• Substitution reaction: Reaction where one atom/group is replaced by another
• Cracking: Breaking down large hydrocarbon molecules into smaller ones

---

Safety Reminder

When working with organic compounds:

• Use proper eye protection and lab coats
• Work in a well-ventilated area (many organic compounds are flammable)
• Be careful with open flames (flammability risk)
• Follow proper waste disposal procedures
• Never mix unknown chemicals

5.4 Alkenes

What are Alkenes?

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond (C=C) and general formula Cₙ$H_2$ₙ.

Laboratory Preparation

Dehydration of alcohols:

$$C_2H_5OH(l) → C_2H_4(g) + H_2O(l)$$

Ethanol heated with porcelain chips or excess aluminum oxide

Physical Properties

Similar to alkanes, but:

• Generally lower boiling points than corresponding alkanes
• Insoluble in water but soluble in organic solvents

Chemical Properties (Addition Reactions)

The double bond is weak and easily broken, making alkenes more reactive than alkanes.

1. Combustion

$$C_2H_4(g) + 3O_2(g) → 2CO_2(g) + 2H_2O(l)$$

Burns with more smoky flame due to higher carbon percentage

2. Hydrogenation

$$C_2H_4(g) + H_2(g) --(Ni, 180°C)→ C_2H_6(g)$$

Adds hydrogen to convert alkene to alkane (used in margarine production)

3. Halogenation

$$C_2H_4(g) + Br_2(aq) → C_2H_4Br_2(aq) (1,2-dibromoethane)$$

Removes brown color from bromine water (test for unsaturation)

4. Hydration

$$C_2H_4(g) + H_2O(g) --(H_3PO_4, 300°C)→ C_2H_5OH(l)$$

Adds water to produce alcohol

5. Addition of Hydrogen Halides

$$C_2H_4(g) + HBr(g) → C_2H_5Br(l)$$

Adds HBr, HCl, or HI

6. Oxidation with Strong Oxidizing Agents

$$C_2H_4 → CH_2OH-CH_2OH (ethane-1,2-diol)$$

Passes through acidified potassium manganate(VII) Purple color disappears (test for unsaturation)

7. Addition Polymerization

$$n(C_2H_4) → [-CH_2-CH_2-]_n (polyethene)$$

Alkene molecules (monomers) join to form long chains (polymers)

Key Terms

• Addition reaction: Reaction where atoms are added to double/triple bonds
• Test for unsaturation: Reaction with bromine water or acidified KMn$O_4$
• Polymerization: Process where small molecules join to form large molecules

---

Did You Know?

Polyethene (polyethylene) is the world's most widely used plastic, with annual production exceeding 100 million tons. It's used for everything from plastic bags to bottles to medical devices!

5.5 Isomerism

What is Isomerism?

Isomers are molecules with the same molecular formula but different structural formulas.

Chain Isomerism

Occurs when isomers have different carbon chain structures

Example: Butane, $C_4H_10$

1. n-Butane: Straight chain C_H_3-C_H_2-C_H_2-C_H_3

2. 2-Methylpropane: Branched chain C_H_3-CH(C_H_3)-C_H_3

Properties of Isomers

Similar chemical properties (if same functional group) Different physical properties (boiling point, density, etc.) Branch-chain isomers usually have lower boiling points due to smaller surface area and weaker van der Waals forces

Key Terms

• Isomer: Compound with same molecular formula but different structure
• Structural formula: Shows how atoms are bonded in a molecule
• Chain isomerism: Isomers with different carbon chain arrangements

5.6 Alcohols

What are Alcohols?

Alcohols are a homologous series with functional group hydroxyl (-OH) and general formula Cₙ$H_2$ₙ₊₁OH.

Naming

Suffix: '-a' from alkane changed to '-ol' Example: methane → methanol, ethane → ethanol

Ethanol Production

1. Fermentation

$$C_6H_12O_6(aq) --(Yeast)→ 2C_2H_5OH(aq) + 2CO_2(g)$$

Glucose or starch broken down by yeast in anaerobic conditions Produces ~15% ethanol solution

2. Hydration of Ethene

$$C_2H_4(g) + H_2O(g) --(H_3PO_4, 300°C)→ C_2H_5OH(l)$$

Industrial method

Physical Properties

Short-chain alcohols are completely miscible with water due to -OH group forming hydrogen bonds with water molecules. Higher boiling points than corresponding alkanes due to hydrogen bonding between alcohol molecules.

Chemical Properties

1. Combustion

$$C_2H_5OH(l) + 3O_2(g) → 2CO_2(g) + 3H_2O(l)$$

Burns with blue flame without soot

2. Oxidation

C_2H_5OH + 2[O] → C_H_3COOH + H_2O

By strong oxidizing agents like acidified $KMnO_4$ or $K_2Cr_2O_7$ Purple $KMnO_4$ decolorized; orange $K_2Cr_2O_7$ turns green

3. Dehydration

$$C_2H_5OH(l) --(H_2SO_4, heat)→ C_2H_4(g) + H_2O(l)$$

Removes water to form alkene

Uses of Alcohols

Ethanol: Beverages, solvents, fuel, antiseptics Methanol: Industrial solvents, fuel (highly toxic)

Key Terms

• Fermentation: Biochemical process where sugar is converted to ethanol and C$O_2$
• Dehydration: Reaction involving removal of water
• Oxidation: Loss of electrons (in organic chemistry, often addition of oxygen)

5.7 Carboxylic Acids

What are Carboxylic Acids?

Carboxylic acids are a homologous series with functional group carboxyl (-COOH) and general formula Cₙ$H_2$ₙ₊₁COOH (n≥0). They are weak acids.

Naming

Suffix: '-a' from alkane changed to '-oic acid' + 'Acid' Example: methane → methanoic acid, ethane → ethanoic acid

Ethanoic Acid Preparation

Oxidation of ethanol using strong oxidizing agents and heating:

$$C_2H_5OH + 2[O] → CH_3COOH + H_2O$$

Physical Properties

Short-chain members are soluble in water due to -COOH group forming hydrogen bonds with water. Higher boiling points than corresponding alcohols due to dimer formation through two hydrogen bonds.

Chemical Properties (Weak Acid Behavior)

Partial ionization in water:

C_H_3COOH(aq) ⇌ C_H_3COO⁻(aq) + H⁺(aq)

Shows all typical acid reactions:

1. With Bases → Salt + Water

$$CH_3COOH + NaOH → CH_3COONa + H_2O (sodium ethanoate)$$

2. With Carbonates → Salt + Water + C$O_2$

2CH_3COOH + CaC$O_3$ → (CH_3COO)₂Ca + H_2O + C$O_2$

3. With Reactive Metals → Salt + Hydrogen

$$2CH_3COOH + Mg → (CH_3COO)₂Mg + H_2$$

4. Reaction with Alcohols (Esterification)

$$CH_3COOH(l) + C_2H_5OH(l) --(conc. H_2SO_4, heat)⇌ CH_3COOC_2H_5(l) + H_2O(l)$$

Carboxylic acid reacts with alcohol to produce ester and water

Key Terms

• Weak acid: Acid that partially ionizes in water
• Esterification: Reaction between carboxylic acid and alcohol to produce ester
• Dimer: Two molecules joined together

5.8 Esters

What are Esters?

Esters are a homologous series with functional group carboxylate (-COO-) and general formula RCOOR'. They are known for their fruity odors.

Formula and Naming

Formula: RCOOR' where R comes from carboxylic acid, R' comes from alcohol Naming: Alcohol name + '-il' + Acid name + '-oate' Example: Ethanol + Propanoic acid → Ethyl propanoate

Preparation

Esterification (see carboxylic acids section):

$$CH_3COOH(l) + C_2H_5OH(l) ⇌ CH_3COOC_2H_5(l) + H_2O(l)$$

Physical Properties

Colorless liquids, insoluble in water (less dense, float on water), volatile with pleasant odors.

Natural Sources and Odors

Uses

Food flavorings, fragrances in cosmetics and soaps, solvents.

Key Terms

• Ester: Organic compound formed from esterification reaction
• Fragrant odor: Characteristic smell of many esters

5.9 Fats

What are Fats?

Fats are natural esters formed from reaction between one glycerol molecule and three fatty acid molecules (triglycerides).

Structure

Glycerol: Alcohol with three -OH groups (propane-1,2,3-triol) Fatty acids: Long-chain carboxylic acids (12-24 carbon atoms)

Saturated vs Unsaturated Fats

Types of Unsaturated Fats

• Monounsaturated: One C=C bond
• Polyunsaturated: More than one C=C bond

Conversion of Oil to Solid Fat

Hydrogenation (addition reaction):

Oil (unsaturated) + $H_2$ --(Ni, heat)→ Solid fat (saturated)

Used in margarine production

Health Considerations

Saturated fats: Linked to increased cholesterol levels Unsaturated fats: Generally healthier, may reduce cholesterol levels

Key Terms

• Glycerol: Alcohol with three -OH groups
• Fatty acid: Long-chain carboxylic acid
• Triglyceride: Ester formed from glycerol and three fatty acids
• Hydrogenation: Addition of hydrogen to unsaturated fats

---

SPM Exam Tips

• For esterification reactions, remember the conditions: concentrated $H_2$S$O_4$ and heat
• Always show reversible arrow (⇌) for esterification
• Know the odor associations for common esters
• Remember that fats are triglycerides (glycerol + 3 fatty acids)
• Understand the health implications of saturated vs unsaturated fats

Summary

Key Concepts

1. Organic chemistry studies carbon-containing compounds
2. Hydrocarbons contain only carbon and hydrogen
3. Homologous series have same functional group and gradual property changes
4. Alkanes are saturated, relatively unreactive, undergo combustion and substitution
5. Alkenes are unsaturated, more reactive, undergo addition reactions
6. Alcohols contain -OH group, undergo combustion, oxidation, and dehydration
7. Carboxylic acids contain -COOH group, are weak acids, undergo esterification
8. Esters have pleasant odors, used as flavorings and fragrances
9. Fats are natural triglycerides, can be saturated or unsaturated

Nomenclature Rules

• Count carbon atoms systematically
• Use correct prefixes (meth-, eth-, prop-, etc.)
• Use correct suffixes (-ane, -ene, -ol, -oic acid)
• For esters: alcohol name + '-il' + acid name + '-oate'

Reaction Types

• Combustion: Reaction with oxygen
• Substitution: Replacement of one atom/group
• Addition: Addition to double/triple bonds
• Oxidation: Addition of oxygen or loss of electrons
• Esterification: Acid + alcohol → ester + water
• Fermentation: Sugar → ethanol + C$O_2$

Practical Applications

• Fuels: Alkanes and alcohols
• Polymers: Addition polymerization of alkenes
• Flavorings: Esters in food industry
• Solvents: Various organic compounds
• Medicines: Complex organic molecules

Practice Questions

1. Explain the difference between saturated and unsaturated hydrocarbons with examples.

2. Write the structural formulas and names of isomers for pentane ($C_5H_{12}$).

3. Describe the reactions that can occur when ethene reacts with: a) Bromine water b) Hydrogen with nickel catalyst c) Steam with phosphoric acid catalyst

4. Explain why esters have pleasant odors and describe their uses in everyday life.

---

Related Topics:

• Chapter 1: Chemical Equilibrium
• Chapter 2: Rate of Reaction
• Chapter 3: Thermochemistry
• Chapter 4: Electrochemistry
• Chapter 6: Oxidation and Reduction
• Chapter 7: Manufactured Substances