Organic Compounds: Structure, Alkanes, and Functional Groups

Organic Compounds

What are Organic Compounds?

Organic compounds are chemical substances primarily composed of carbon (C) and hydrogen (H), often with additional elements such as oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P). These compounds form the basis of biomolecules essential for life, including proteins, carbohydrates, lipids, and DNA.

• Definition: Organic compounds contain mostly C and H atoms.

• Additional Elements: May include O, N, S, P.

• Biomolecules: Proteins, carbohydrates, lipids, DNA are examples.

Formulas:

• Molecular formula: Shows only the number of each atom (e.g., $C_3H_8$).

• Condensed structural formula: Shows all atoms, fewer bonds (e.g., $CH_3CH_2CH_3$).

• Lewis structure: Shows all atoms and bonds explicitly.

Example: Propane ($C_3H_8$) can be represented as:

• Molecular: $C_3H_8$

• Condensed: $CH_3CH_2CH_3$

• Lewis: All atoms and bonds shown

Types of Structural Formulas

Structural formulas are used to represent organic molecules in varying levels of detail.

• Condensed formula: Shows all atoms, not all bonds. Electron pairs are not shown.

• Skeletal structure: Shows only bonds between carbons as lines; hydrogens attached to carbons are implied.

Skeletal Structures

Skeletal ("bare-bones") structures are simplified representations used for organic molecules with three or more carbons.

• Only bonds between carbons are shown as lines.

• Hydrogens attached to carbons are not shown (implied).

• Heteroatoms (O, N, Cl, etc.) are shown explicitly.

Rules for Drawing Skeletal Structures

When drawing skeletal structures, follow these steps:

• Show only C–C bonds as lines.

• C–H bonds are not shown but are implied.

• Heteroatoms are shown with their symbols.

• Lone pairs are not shown.

Steps for Drawing Skeletal Structures

To draw a skeletal structure:

1. Count the number of carbons in the chain.

2. Draw the carbon skeleton in a zigzag pattern.

3. Add bonds to non-carbon atoms as needed.

Alkanes: The Simplest Organic Compounds

Definition of Alkanes

Alkanes are hydrocarbons containing only single bonds between carbon and hydrogen atoms. They are saturated compounds, meaning each carbon is bonded to the maximum number of hydrogens.

• Hydrocarbons: Made only of C and H.

• Saturated: Only single bonds; maximum hydrogens.

Straight-Chain Alkanes

Straight-chain alkanes have continuous, unbranched chains of carbon atoms. Their names depend on the number of carbons present.

• First four have historical names: methane, ethane, propane, butane.

• From five carbons onward, names use Greek prefixes + "-ane" ending.

• General formula: $C_nH_{2n+2}$

Table of the First 10 Alkanes

The following table lists the first 10 straight-chain alkanes, their formulas, and structures:

Number of Carbons	Prefix	Name	Molecular Formula	Condensed Structure	Skeletal Structure
1	Meth-	Methane	CH4	CH4	•
2	Eth-	Ethane	C2H6	CH3CH3	—
3	Prop-	Propane	C3H8	CH3CH2CH3	— —
4	But-	Butane	C4H10	CH3CH2CH2CH3	— — —
5	Pent-	Pentane	C5H12	CH3(CH2)3CH3	— — — —
6	Hex-	Hexane	C6H14	CH3(CH2)4CH3	— — — — —
7	Hept-	Heptane	C7H16	CH3(CH2)5CH3	— — — — — —
8	Oct-	Octane	C8H18	CH3(CH2)6CH3	— — — — — — —
9	Non-	Nonane	C9H20	CH3(CH2)7CH3	— — — — — — — —
10	Dec-	Decane	C10H22	CH3(CH2)8CH3	— — — — — — — — —

Cycloalkanes

Cycloalkanes are hydrocarbons with carbon atoms arranged in rings and only single bonds. They are named with the prefix "cyclo-" plus the corresponding alkane name.

• Smallest is cyclopropane (triangle, 3 C).

• Most common are cyclopentane (pentagon, 5 C) and cyclohexane (hexagon, 6 C).

Table of Cycloalkanes

Name	Molecular Formula	Skeletal Structure	Ball-and-Stick Model
Cyclopropane	C3H6	Triangle	3 C in triangle, each C with 2 H
Cyclobutane	C4H8	Square	4 C in square, each C with 2 H
Cyclopentane	C5H10	Pentagon	5 C in pentagon, each C with 2 H
Cyclohexane	C6H12	Hexagon	6 C in hexagon, each C with 2 H

Polarity of Alkanes

Alkanes are nonpolar molecules because C–H bonds share electrons almost equally. As a result, alkanes do not dissolve in water (a polar solvent).

• C–H bonds: Nonpolar

• Alkanes: Completely nonpolar

• Solubility: Do not mix with water

Alkanes as Fuels

Alkanes react with oxygen in combustion reactions to produce carbon dioxide and water. Cleaner and cheaper fuels are sought to replace fossil fuels.

• Combustion reaction: $\text{Alkane} + O_2 \rightarrow CO_2 + H_2O$

• Complete combustion produces only $CO_2$ and $H_2O$.

Common Uses of Alkanes

Alkanes are used as fuels and in various industrial applications.

• Methane ($CH_4$): Natural gas

• Propane ($C_3H_8$): Propane gas

• Isooctane: Gasoline

• Hexadecane: Diesel

• Hentriacontane: Wax/paraffin

Alkane Name	Formula	Where Commonly Found	Chemical Structure
Methane	CH4	Natural gas	•
Propane	C3H8	Propane gas	— —
Isooctane	C8H18	Gasoline	— — — — — — —
Hexadecane	C16H34	Diesel fuel	— — — — — — — — — — — — — — — —
Hentriacontane	C31H64	Candle wax (paraffin)	Long chain

Families of Organic Compounds — Functional Groups

Introduction to Functional Groups

Functional groups are specific groups of atoms within molecules that determine the chemical properties and reactivity of organic compounds. When an organic compound contains atoms other than C and H, these are called heteroatoms.

• Functional group: A set of atoms bonded in a specific way.

• Organic compounds are classified into families by their functional group.

• Each functional group has unique properties and reactivity.

Classification of Functional Groups

Functional groups can be classified as hydrocarbons (only C and H) or those containing heteroatoms (O, N, S).

• Hydrocarbons: Alkanes, alkenes, alkynes, aromatics.

• Most functional groups contain O, N, or S.

• Carbonyl group (C=O) is common in many families.

Functional Group as Reactive Part

The functional group is the reactive site of an organic molecule. The rest of the molecule is often represented by "R" to simplify structures and focus on the functional group.

• R: Represents the rest of the molecule, usually a carbon chain.

• Helps generalize structures and focus on the important part.

Use of R in Functional Groups

"R" is used generically in examples to represent any carbon group, allowing comparison of different functional groups regardless of chain size.

• All functional group examples use R to show the specific group.

Alkenes (Alquenos)

Alkenes are hydrocarbons containing at least one double bond (C=C) and are unsaturated. The double bond is shorter and stronger than a single bond, and alkenes are more reactive than alkanes.

• Double bond: $C=C$

• Unsaturated: Fewer hydrogens than alkanes

• Addition reactions: Double bond breaks, atoms are added

• Alkenes are more reactive than alkanes

Example: Ethene ($C_2H_4$) undergoes addition reactions to form ethane ($C_2H_6$).