Chemistry (0620) | Formulae, functional groups and terminology

Welcome to Organic Chemistry! (Section 11.1: Formulae, Functional Groups and Terminology)

Hello future Chemists! Organic Chemistry is the study of carbon compounds—basically, the chemistry of life! Don't worry if it seems overwhelming at first. This section is all about learning the language: how to draw these molecules, how to name their families, and what makes them tick chemically. Mastering this vocabulary is the key to unlocking the rest of the Organic Chemistry section. Let’s dive in!

Part 1: Describing Organic Molecules (The Different Formulae)

In organic chemistry, one formula is often not enough to describe a molecule because atoms can be arranged in different ways. We use three main types of formulae:

1. Displayed Formula (Core Requirement 1)

This is the most detailed formula. It shows you exactly how every single atom is connected and shows all the bonds between them.

• What it is: A full diagram showing every atom and every covalent bond.
• Use: Helps you visualise the 3D shape and confirm the bonding arrangement.

Example: Ethanol ($C_2H_6O$)
If you drew the displayed formula for ethanol, you would clearly see the C–C bond, the C–H bonds, and the crucial C–O and O–H bond (the alcohol functional group).

2. Structural Formula (Supplement Requirement 7)

The structural formula gives an unambiguous description of how the atoms are arranged in the molecule without needing to draw all the bonds explicitly. This is a condensed way of writing the structure.

• What it is: Groups of atoms attached to each carbon atom are written together.
• Use: Quickly describes the structure while saving space.

Example Comparison:

Molecular Formula: \(C_2H_6O\)
Structural Formula: \(CH_3CH_2OH\) (This tells us that the first carbon has 3 H atoms, and the second carbon has 2 H atoms and an OH group attached.)

Structural Formula Examples you need to know:

• Ethene: \(CH_2=CH_2\)
• Ethanol: \(CH_3CH_2OH\)
• Ethyl ethanoate (an ester): \(CH_3COOCH_3\)

3. General Formula (Core Requirement 2)

The general formula is a single expression that represents any member of a homologous series (a family of compounds).

• What it is: A formula using 'n' (the number of carbon atoms) to find the number of hydrogen or other atoms.

General Formulae you must know:

• Alkanes: \(C_nH_{2n+2}\) (e.g., if n=2, \(C_2H_6\))
• Alkenes: \(C_nH_{2n}\) (e.g., if n=3, \(C_3H_6\))
• Alcohols: \(C_nH_{2n+1}OH\) (e.g., if n=1, \(CH_3OH\))
• Carboxylic Acids: \(C_nH_{2n+1}COOH\) (e.g., if n=1, \(CH_3COOH\))

Part 2: Structure, Bonding, and Reactivity

Saturated vs. Unsaturated (Core Requirements 5 & 6)

This classification tells us about the type of carbon-carbon bonding present.

Saturated Compounds (Alkanes)

• Molecules where all carbon-carbon bonds are single bonds (C–C).
• They are "saturated" because they hold the maximum number of hydrogen atoms possible.
• They are generally less reactive (only undergoing substitution reactions).
• Analogy: A saturated sponge can’t hold any more water.

Unsaturated Compounds (Alkenes)

• Molecules that have one or more carbon-carbon bonds that are not single bonds (i.e., double bonds C=C or triple bonds).
• They have fewer hydrogen atoms than their saturated counterparts.
• The double bond is a point of high reactivity, allowing them to easily undergo addition reactions.
• Analogy: An unsaturated sponge has room to absorb more water (or atoms!).

Functional Groups (Core Requirement 3)

A functional group is an atom or group of atoms within a molecule that determines the chemical properties of the compound.

Think of it like the engine in a car. All cars (molecules) have wheels and a chassis (the carbon backbone), but the engine (functional group) determines how it drives (how it reacts chemically).

• Alkanes: Have no functional group (just C–C and C–H single bonds).
• Alkenes: The functional group is the carbon-carbon double bond (\(C=C\)).
• Alcohols: The functional group is the hydroxyl group (\(–OH\)).
• Carboxylic Acids: The functional group is the carboxyl group (\(–COOH\)).

Part 3: Homologous Series (The Family of Molecules)

Definition and Characteristics (Core & Supplement Requirements 4 & 9)

A homologous series is a family of organic compounds that share similar features due to the presence of the same functional group.

General Characteristics of a Homologous Series:

1. Same Functional Group: All members have the same functional group (e.g., all alcohols have the –OH group). This is why they have similar chemical properties.
2. Same General Formula: All members can be represented by the same general formula (e.g., all alkanes fit \(C_nH_{2n+2}\)).
3. Difference by a –CH₂– Unit: Each successive member differs from the previous one by a simple \(–CH_2–\) unit (called the methylene group).

   Example: Methane (\(CH_4\)) -> Ethane (\(C_2H_6\)) -> Propane (\(C_3H_8\)).

4. Trend in Physical Properties: As the chain length increases (more \(–CH_2–\) units), the physical properties show a gradual change (e.g., boiling point and density increase).
5. Similar Chemical Properties: Because they share the same functional group, their reactions are generally the same (e.g., all alkenes react with bromine water).

Part 4: Structural Isomers (Same Ingredients, Different Recipe)

What Are Structural Isomers? (Supplement Requirement 8)

Don't worry if this sounds complicated—it's just a matter of juggling the atoms!

Structural Isomers are compounds that have the same molecular formula but different structural formulae. They have the same total number of atoms of each element, but those atoms are connected differently.

Because the connectivity (structure) is different, isomers often have different physical properties (like melting and boiling points) and slightly different chemical properties.

Key Example: Butane (\(C_4H_{10}\))

The molecular formula \(C_4H_{10}\) can exist in two ways:

1. Butane (Straight chain): The four carbon atoms are linked in a single row.
   Structural Formula: \(CH_3CH_2CH_2CH_3\)
2. Methylpropane (Branched chain): Three carbons are in the main chain, and the fourth carbon atom is branched off the middle one.
   Structural Formula: \(CH_3CH(CH_3)CH_3\)

Both compounds have exactly 4 Carbons and 10 Hydrogens, but their structures (and names!) are different.

Example: Butene (\(C_4H_8\))

Alkenes, like butene, can also have isomers depending on where the double bond is placed:

1. But-1-ene: The double bond is between the first and second carbon.
   Structural Formula: \(CH_2=CHCH_2CH_3\)
2. But-2-ene: The double bond is between the second and third carbon.
   Structural Formula: \(CH_3CH=CHCH_3\)