Diversity of Organic Compounds (VCE SSCE Chemistry): Revision Notes

Diversity of Organic Compounds

What are organic compounds?

Carbon compounds form the foundation of all life on Earth. In fact, more than 90% of all known chemical compounds contain carbon. Because carbon compounds were first discovered in living organisms, the study of carbon-based substances became known as organic chemistry, and these substances are called organic molecules.

Most organic molecules have a hydrocarbon skeleton, meaning they contain carbon and hydrogen as their main components. Common additional elements found in organic molecules include oxygen, nitrogen, sulfur, and chlorine.

Why carbon is special

Electronic structure and bonding capacity

Carbon's unique properties stem from its electronic structure. A carbon atom has 6 electrons in total:

• 2 electrons in the first shell
• 4 electrons in the second (outer) shell

We can write this electronic configuration as 2,4 or as $1s^2 2s^2 2p^2$.

Carbon's bonding versatility

Carbon's four valence electrons give it remarkable versatility in forming compounds:

1. Chain and ring formation: Each carbon atom can bond with up to four other carbon atoms, and each of those can bond with four more carbons. This allows carbon to form long chains, branched structures, and ring-shaped molecules.
2. Multiple bond types: Carbon can form single, double, or triple bonds between carbon atoms:
   • Saturated molecules contain only single carbon-carbon bonds
   • Unsaturated molecules contain one or more double or triple carbon-carbon bonds
3. Diverse bonding arrangements: Carbon's four bonds can be arranged in different ways:
   • Four single bonds with four different atoms
   • One double bond and two single bonds with three atoms
   • Two double bonds with two atoms
   • One triple bond and one single bond with two atoms

Bond strength and stability

The strength of chemical bonds is measured by bond energy, which is the amount of energy required to break 1 mole of covalent bonds in the gaseous state. Higher bond energy indicates a stronger, more stable bond.

Carbon compared to silicon

Silicon and carbon are similar elements—both are in group 14 of the periodic table and have four valence electrons. However, their bonding characteristics differ significantly:

Covalent bonds with carbon	Bond energy (kJ mol⁻¹)	Covalent bonds with silicon	Bond energy (kJ mol⁻¹)
$C≡C$	839	$Si≡Si$	do not form
$C=C$	614	$Si=Si$	do not form readily
$C-C$	348	$Si-Si$	226
$C-H$	413	$Si-H$	323
$C-O$	360	$Si-O$	466

Key observations from this comparison:

• Silicon cannot readily form multiple bonds with itself
• Carbon-carbon single bonds (348 kJ mol⁻¹) are much stronger than silicon-silicon bonds (226 kJ mol⁻¹)
• The one exception is the $Si-O$ bond (466 kJ mol⁻¹), which explains why silicon dioxide (quartz) is such a stable and abundant mineral

Strength of carbon-carbon bonds

The carbon-carbon single bond is exceptionally strong compared to bonds between other atoms of the same type:

Covalent bonds between atoms of the same type	Bond energy (kJ mol⁻¹)
$C-C$	348
$N-N$	163
$O-O$	146
$F-F$	155
$S-S$	226

Summary of carbon's diversity

Carbon forms such a wide variety of compounds because:

• It can form strong, stable bonds with many other elements
• It can form stable single, double, and triple bonds with itself
• The strength of carbon-carbon bonds enables the formation of chain-like and ring-like structures
• Its four valence electrons allow for diverse molecular architectures

Representing organic molecules

Chemists use several different notations to represent organic molecules, each with its own advantages. Understanding these different representations is essential for studying organic chemistry.

Molecular formulas

Molecular formulas show the number and type of each element present in a molecule. For example, $C_2H_6O$ and $C_4H_8O$ are molecular formulas.

Structural formulas

Structural formulas show the exact arrangement of atoms in a molecule, including the location and type of all covalent bonds.

When carbon forms four single bonds, the electron pairs in each bond repel each other (according to VSEPR theory). This causes the bonds to arrange themselves in three dimensions at angles of approximately 109.5° to each other, creating a tetrahedral shape.

Semi-structural formulas

A semi-structural formula (also called a condensed formula) shows the connections between atoms without displaying the full three-dimensional arrangement. This notation is more compact than structural formulas.

In semi-structural formulas:

• The carbon chain is written on one line
• Single bonds are typically not shown
• Double and triple bonds are often shown
• Hydrogen atoms are written next to the carbon they're bonded to
• Branching groups are shown in brackets after the carbon they're attached to
• Repeated $-CH_2-$ groups can be written as $(CH_2)_n$

Skeletal structures

Skeletal structures are a shorthand notation used for complex organic molecules. They show only the carbon-carbon bonds and functional groups, making the structure clearer and easier to draw.

What skeletal structures leave out:

• Carbon atoms (implied at each junction and end of lines)
• Hydrogen atoms bonded to carbon atoms
• Bonds between carbon and hydrogen

What skeletal structures show:

• Carbon-carbon single, double, and triple bonds
• Functional groups (groups of atoms that determine the molecule's properties)
• Bonds to and within functional groups

The table above compares all four representation methods for three different molecules, showing how each notation system presents the same chemical information in different ways.

Isomers: same formula, different structure

Isomers are molecules that contain the same number and type of atoms (same molecular formula) but have these atoms arranged in different ways. The existence of isomers is a major reason for the enormous diversity of carbon compounds.

Although isomers have the same molecular formula, they often have different physical and chemical properties because their atoms are connected differently.

Structural isomers

Structural isomers form when atoms with the same molecular formula bond together in different arrangements. You can create structural isomers by:

1. Changing the length of the main carbon chain (creating branches)
2. Changing the position of functional groups
3. Combining both changes

Isomers formed by changing chain length

Carbon chains can branch, creating molecules with the same number of carbon and hydrogen atoms but different chain lengths and structures.

Isomers formed by changing functional group position

When molecules contain functional groups, isomers can form by placing the functional group at different positions along the carbon chain.

Alcohols are organic molecules containing a hydroxyl functional group ($-OH$). For the molecular formula $C_4H_{10}O$, several alcohol isomers are possible:

These two isomers show the hydroxyl group attached to different carbon atoms:

• Butan-1-ol: $-OH$ group on the first carbon
• Butan-2-ol: $-OH$ group on the second carbon

Alkenes contain a carbon-carbon double bond, which acts as a functional group. The position of this double bond can also create isomers:

These two butene isomers ($C_4H_8$) have the double bond in different locations:

• But-1-ene: double bond between the first and second carbons
• But-2-ene: double bond between the second and third carbons

Important considerations about isomers

When isomers are possible: Isomers can only exist when:

• The molecule contains a functional group, AND
• The carbon chain is long enough that the functional group can occupy different positions

Isomers with multiple changes

It's possible to create isomers by changing both the chain length and the functional group position. Here are two more alcohol isomers with the formula $C_4H_{10}O$:

These structures differ from the earlier butanol isomers in both their carbon chain arrangement and the position of the hydroxyl group.

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