Chapter 11: Introduction to Organic Chemistry – Hydrocarbons

Introduction to Organic Chemistry

Overview of Organic Compounds

Organic chemistry is the study of compounds primarily composed of carbon and hydrogen, often with oxygen, nitrogen, sulfur, or halogens. These compounds are foundational to life and are found in fuels, medicines, plastics, and more. In contrast, inorganic compounds typically consist of metals and nonmetals and do not always contain carbon.

• Organic compounds contain at least one carbon atom, usually bonded to hydrogen, and may include O, S, N, or halogens.

• Inorganic compounds are composed of metals and nonmetals, such as sodium chloride (NaCl).

• Formulas for organic compounds are written with carbon first, then hydrogen, followed by other elements.

• Examples: Propane (C3H8) is an organic fuel; sodium chloride (NaCl) is an inorganic salt.

Table Purpose: This table compares the properties of organic and inorganic compounds, including elements present, bonding, melting/boiling points, and solubility.

Hydrocarbons

Definition and Structure

Hydrocarbons are the simplest organic compounds, consisting only of carbon and hydrogen. They can form chains or rings, with each carbon atom forming four covalent bonds. Hydrocarbons are classified as either aliphatic (chains or non-aromatic rings) or aromatic (containing benzene-like rings).

• Aliphatic hydrocarbons include alkanes, alkenes, alkynes, and cycloalkanes.

• Aromatic hydrocarbons contain benzene rings and related structures.

Table Purpose: This diagram classifies hydrocarbons into aliphatic and aromatic types, with examples of each subclass.

Physical Properties of Hydrocarbons

• Hydrocarbons are non-polar molecules, making them insoluble in water but soluble in nonpolar solvents.

• They are generally less dense than water.

• Hydrocarbons have low boiling points and low melting points compared to inorganic compounds.

• As the molecular weight (chain length) increases, melting and boiling points, as well as density, increase.

Alkanes

Structure and General Formula

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. The general formula for a straight-chain alkane is:

$\mathrm{C}_n\mathrm{H}_{2n+2}$

• n = number of carbon atoms

• Each carbon forms four single covalent bonds (either with hydrogen or other carbons).

• Examples: Methane (CH4), Ethane (C2H6), Propane (C3H8), Butane (C4H10).

Formulas Used in Organic Chemistry

• Molecular formula: Shows the number and type of atoms (e.g., C2H6).

• Structural formula: Shows each atom and bond explicitly.

• Condensed formula: Groups atoms to show connectivity (e.g., CH3CH2CH3).

• Line-angle formula: Represents the carbon skeleton with lines; each vertex or end represents a carbon atom.

Naming Alkanes (IUPAC System)

The names of alkanes are determined by the International Union of Pure and Applied Chemistry (IUPAC) system. All alkane names end in "-ane." The prefixes indicate the number of carbon atoms in the chain.

Name	# Carbons	Molecular Formula
Methane	1	CH4
Ethane	2	CH3CH3
Propane	3	CH3CH2CH3
Butane	4	CH3CH2CH2CH3
Pentane	5	CH3CH2CH2CH2CH3
Hexane	6	CH3CH2CH2CH2CH2CH3
Heptane	7	CH3(CH2)5CH3
Octane	8	CH3(CH2)6CH3
Nonane	9	CH3(CH2)7CH3
Decane	10	CH3(CH2)8CH3

Table Purpose: This table lists the names, formulas, and melting/boiling points of the first ten straight-chain alkanes.

Physical Properties and Trends in Alkanes

• Alkanes are nonpolar and insoluble in water.

• They have low melting and boiling points, which increase with molecular weight (chain length).

• Alkanes are less dense than water and are flammable.

• Example: Methane (CH4) boils at -162°C, while decane (C10H22) boils at 174°C.

Examples and Applications

• Propane (C3H8): Used as a fuel for heating and cooking.

• Octane (C8H18): A major component of gasoline.

• Butane (C4H10): Used in lighters and portable stoves.

Summary Table: Properties of Organic vs. Inorganic Compounds

Property	Organic	Example: C3H8	Inorganic	Example: NaCl
Elements Present	C and H, sometimes O, S, N, P, or Cl	C3H8	Most metals and nonmetals	Na and Cl
Particles	Molecules	C3H8	Mostly ions	Na+ and Cl-
Bonding	Mostly covalent	Covalent	Many are ionic, some covalent	Ionic
Polarity of Bonds	Nonpolar, unless a strongly electronegative atom is present	Nonpolar	Most are ionic or polar covalent	Ionic
Melting Point	Usually low	-188°C	Usually high	801°C
Boiling Point	Usually low	-42°C	Usually high	1413°C
Flammability	High	Burns in air	Low	Does not burn
Solubility in Water	Not soluble unless a polar group is present	No	Most are soluble unless nonpolar	Yes

Visualizing Hydrocarbons

Figure Purpose: This figure shows the structures of common aliphatic (methane, propane, cyclohexane, ethanol) and aromatic (benzene, ethylbenzene, naphthalene) hydrocarbons, illustrating the diversity of organic molecules.

Practice Problem Example

• Question: An alkane used as a motor lubricant contains 10 carbon atoms. How many hydrogen atoms are present?

• Solution: Use the general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$ with n = 10:

$\mathrm{C}_{10}\mathrm{H}_{(2 \times 10) + 2} = \mathrm{C}_{10}\mathrm{H}_{22}$

• Answer: 22 hydrogen atoms

Key Takeaways

• Organic compounds are carbon-based and have distinct properties compared to inorganic compounds.

• Hydrocarbons are the simplest organic compounds and are classified as alkanes, alkenes, alkynes, cycloalkanes, or aromatics.

• Alkanes are saturated hydrocarbons with the general formula $\mathrm{C}_n\mathrm{H}_{2n+2}$.

• Physical properties such as melting and boiling points increase with molecular size in alkanes.