Organic chemistry is a fundamental branch of chemistry that deals with the structure, properties, and reactions of carbon-containing compounds. Understanding the general formulae for alkanes, alkenes, alcohols, and carboxylic acids is crucial for students preparing for the Cambridge IGCSE Chemistry (0620 - Core) exams. This knowledge not only forms the backbone of various organic reactions but also aids in the identification and classification of organic molecules.

Key Concepts

1. Alkanes

Alkanes are the simplest class of hydrocarbons, consisting solely of carbon and hydrogen atoms connected by single bonds. They are saturated hydrocarbons, meaning they contain the maximum number of hydrogen atoms possible for their carbon skeleton.

General Formula: The general formula for alkanes is given by:

$$C_nH_{2n+2}$$

Where n is the number of carbon atoms. This formula indicates that each additional carbon atom in an alkane adds two hydrogen atoms, plus two more.

Examples:

Methane (CH₄): The simplest alkane with one carbon atom.
Ethane (C₂H₆): Contains two carbon atoms.
Propane (C₃H₈): Contains three carbon atoms.

Physical Properties: Alkanes are generally non-polar, making them insoluble in water but soluble in organic solvents. They exhibit low boiling and melting points, which increase with molecular weight.

Chemical Properties: Alkanes undergo reactions such as combustion and substitution. Due to the single bonds, they are relatively unreactive compared to other hydrocarbons.

2. Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. This double bond results in a general formula that differs from alkanes.

General Formula: The general formula for alkenes is:

$$C_nH_{2n}$$

Each double bond reduces the number of hydrogen atoms by two compared to alkanes.

Examples:

Ethene (C₂H₄): The simplest alkene with one double bond.
Propene (C₃H₆): Contains three carbon atoms and one double bond.

Physical Properties: Alkenes are non-polar and exhibit properties similar to alkanes but often have slightly higher boiling points due to the presence of the double bond.

Chemical Properties: The presence of the double bond makes alkenes more reactive. They readily undergo addition reactions, such as hydrogenation, halogenation, and hydrohalogenation.

3. Alcohols

Alcohols are organic compounds characterized by the presence of one or more hydroxyl (-OH) groups attached to a carbon atom.

General Formula: The general formula for alcohols depends on their structure but can generally be represented as:

$$C_nH_{2n+2−m}O$$

Where m is the number of hydroxyl groups. For simple monohydric alcohols (one -OH group), the formula is often represented as:

$$C_nH_{2n+2}O$$

Examples:

Methanol (CH₃OH): The simplest alcohol with one carbon atom.
Ethanol (C₂H₅OH): Commonly used in beverages and as a solvent.

Physical Properties: Alcohols are polar molecules due to the hydroxyl group, making them soluble in water. They have higher boiling points compared to alkanes and alkenes of similar molecular weight.

Chemical Properties: Alcohols undergo various chemical reactions, including oxidation, esterification, and dehydration. Their reactivity is influenced by the presence and type of hydroxyl group.

4. Carboxylic Acids

Carboxylic acids are organic compounds containing the carboxyl group (-COOH), which consists of a carbonyl group bonded to a hydroxyl group.

General Formula: The general formula for carboxylic acids is:

$$C_nH_{2nO_2}$$

Alternatively, it can be expressed as:

$$R-COOH$$

Where R represents a hydrocarbon chain.

Examples:

Formic Acid (HCOOH): The simplest carboxylic acid.
Acetic Acid (CH₃COOH): Widely used in vinegar.

Physical Properties: Carboxylic acids are polar and can form hydrogen bonds, making them soluble in water. They typically have higher boiling points than alcohols of similar molecular weight due to dimer formation.

Chemical Properties: Carboxylic acids are acidic due to the ability to donate a proton from the hydroxyl group. They undergo reactions such as esterification, reduction, and decarboxylation.

Advanced Concepts

1. Structural Isomerism

Isomerism is a phenomenon where compounds have the same molecular formula but different structural arrangements. In alkanes and alkenes, structural isomerism is a key concept.

Alkanes: Alkanes can exhibit chain isomerism, where the carbon chain branches, leading to different structures with the same formula. For example, butane (C₄H₁₀) has two isomers: n-butane and isobutane.

Alkenes: Alkenes can show both chain isomerism and cis-trans (geometric) isomerism due to the rigidity of the carbon-carbon double bond. For instance, 2-butene exists as cis-2-butene and trans-2-butene.

2. Reaction Mechanisms

Understanding the mechanisms of organic reactions is essential for predicting products and intermediary steps.

Alkane Reactions: Alkanes primarily undergo substitution reactions, such as free radical halogenation. The mechanism involves initiation (formation of radicals), propagation (reaction of radicals with alkane), and termination (radical recombination).

$$ \begin{align*} &\text{Initiation: } Cl_2 \rightarrow 2 Cl^.\\ &\text{Propagation: } Cl^. + C_nH_{2n+2} \rightarrow C_nH_{2n+1}Cl + H^.\\ &\text{Propagation: } H^. + Cl_2 \rightarrow HCl + Cl^.\\ &\text{Termination: } Cl^. + Cl^. \rightarrow Cl_2\\ &\text{Termination: } C_nH_{2n+1}Cl + Cl^. \rightarrow C_nH_{2n+1}Cl_2\\ \end{align*} $$

Alkene Reactions: Alkenes undergo addition reactions due to the presence of the double bond. Common reactions include hydrogenation, halogenation, hydrohalogenation, and hydration.

$$ CH_2=CH_2 + H_2 \rightarrow CH_3-CH_3 $$

Alcohol Reactions: Alcohols can undergo dehydration to form alkenes, oxidation to form carbonyl compounds, and esterification to form esters.

Carboxylic Acid Reactions: Carboxylic acids undergo reactions such as esterification, reduction to primary alcohols or aldehydes, and decarboxylation to remove the carboxyl group.

3. Spectroscopic Identification

Identifying organic compounds often involves spectroscopic techniques such as Infrared (IR) spectroscopy and Nuclear Magnetic Resonance (NMR) spectroscopy.

IR Spectroscopy: Each functional group absorbs IR radiation at characteristic wavelengths. For example:

Alkanes: C–H stretching around 2850-2960 cm^-1.
Alkenes: C=C stretching around 1640-1680 cm^-1.
Alcohols: O–H stretching broad peak around 3200-3550 cm^-1.
Carboxylic Acids: Broad O–H stretch and C=O stretch around 1700 cm^-1.

NMR Spectroscopy: Provides information about the hydrogen environment in a molecule. Chemical shifts, coupling constants, and integration help in deducing the structure.

4. Interdisciplinary Connections

Organic chemistry principles are interconnected with various other scientific fields:

Biochemistry: Understanding alcohols and carboxylic acids is essential in studying metabolic pathways and enzyme functions.
Pharmaceutical Chemistry: Designing drugs requires knowledge of functional groups and their reactivity.
Environmental Science: Studying the impact of hydrocarbons and their derivatives on the environment involves organic chemistry concepts.
Material Science: Development of polymers and plastics utilizes knowledge of alkanes and alkenes.

5. Stereochemistry

Stereochemistry deals with the spatial arrangement of atoms in molecules. In alkenes, cis-trans isomerism is a fundamental concept.

Cis-Trans Isomerism: Occurs in alkenes when each carbon of the double bond has two different substituents. Cis isomers have similar groups on the same side, while trans isomers have them on opposite sides.

$$ \begin{align*} &\text{Cis-2-Butene:} \quad \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \quad (\text{CH}_3 \text{ groups on the same side})\\ &\text{Trans-2-Butene:} \quad \text{CH}_3-\text{CH}=\text{CH}-\text{CH}_3 \quad (\text{CH}_3 \text{ groups on opposite sides}) \end{align*} $$

This distinction affects the physical and chemical properties of the isomers, such as melting points and reactivity.

Comparison Table

Feature	Alkanes	Alkenes	Alcohols	Carboxylic Acids
General Formula	$C_nH_{2n+2}$	$C_nH_{2n}$	$C_nH_{2n+2}O$	$C_nH_{2nO_2}$
Bond Types	Single C–C bonds	At least one C=C double bond	Contains hydroxyl (-OH) group	Contains carboxyl (-COOH) group
Saturation	Saturated hydrocarbons	Unsaturated hydrocarbons	Saturated with -OH groups	Contains both saturated and functional groups
Physical State	Gases and liquids at room temperature	Gases and liquids at room temperature	Liquids or solids	Liquids or solids
Reactivity	Relatively unreactive; undergo substitution	More reactive; undergo addition reactions	Reactive; undergo oxidation, dehydration, esterification	Highly reactive; undergo acids-base reactions, esterification, decarboxylation

Summary and Key Takeaways

Alkanes: Saturated hydrocarbons with single bonds ($C_nH_{2n+2}$).
Alkenes: Unsaturated hydrocarbons with at least one double bond ($C_nH_{2n}$).
Alcohols: Contain hydroxyl groups, general formula $C_nH_{2n+2}O$.
Carboxylic Acids: Feature carboxyl groups, general formula $C_nH_{2nO_2}$.
Understanding functional groups is essential for predicting chemical behavior and reactivity.

Examiner Tip

Tips

To remember the general formulas, use the mnemonic "All Singles Have Two More" for alkanes ($C_nH_{2n+2}$) and "Eager Scientists Have Two" for alkenes ($C_nH_{2n}$). Practice drawing structural formulas to reinforce functional group recognition. Additionally, solving various reaction mechanism problems can enhance your understanding and retention for exam success.

Did You Know

Despite their simplicity, alkanes play a crucial role in everyday fuels like natural gas and gasoline. Alkenes are foundational in producing plastics such as polyethylene. Additionally, carboxylic acids are not only vital in organic synthesis but also contribute to the sour taste of fruits, thanks to acids like citric acid.

Common Mistakes

Students often confuse the general formulas of alkanes and alkenes, forgetting that alkenes have two fewer hydrogen atoms. Another common error is misidentifying functional groups, such as mistaking hydroxyl (-OH) for a carbonyl (C=O) group. Additionally, students might overlook the importance of the double bond in alkenes, impacting their understanding of reactivity.

FAQ

What is the general formula for alkenes?

The general formula for alkenes is $C_nH_{2n}$, indicating the presence of at least one carbon-carbon double bond.

How do you distinguish between primary, secondary, and tertiary alcohols?

Primary, secondary, and tertiary alcohols are distinguished by the number of carbon atoms bonded to the carbon bearing the hydroxyl group. Primary alcohols have one carbon attached, secondary have two, and tertiary have three.

Why are alkenes more reactive than alkanes?

Alkenes are more reactive than alkanes due to the presence of the carbon-carbon double bond, which is a region of high electron density, making them susceptible to addition reactions.

What are common uses of carboxylic acids?

Carboxylic acids are commonly used in the production of pharmaceuticals, food preservatives, and plastics. They are also important in the synthesis of esters used in fragrances and flavors.

How does the general formula of alkanes change with increasing carbon atoms?

As the number of carbon atoms in alkanes increases, the general formula $C_nH_{2n+2}$ ensures that each additional carbon adds two more hydrogens, maintaining the saturation of the molecule.

Can tertiary alcohols be oxidized?

Tertiary alcohols generally resist oxidation because there is no hydrogen atom attached to the carbon bearing the hydroxyl group, making oxidation difficult.

1. Acids, Bases, and Salts

1.1 Salt Preparation

1.1.1 Making soluble salts by reaction with excess metal

1.1.2 Making soluble salts by reaction with excess base

1.1.3 Making soluble salts by reaction with excess carbonate

1.1.4 Making soluble salts by titration

1.2 Solubility Rules

1.2.1 Solubility of sodium, potassium, ammonium salts

1.2.2 Solubility of nitrates, chlorides, sulfates, carbonates, and hydroxides

1.3 Hydrated and Anhydrous Salts

1.3.1 Definition of hydrated and anhydrous substances

1.4 Insoluble Salts