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 additional elements such as oxygen, nitrogen, sulfur, and halogens. These compounds are foundational to life and are found in a wide range of everyday products, including fuels, medicines, plastics, and personal care items.

• Organic compounds always contain at least one carbon atom and many hydrogen atoms.

• Other elements that may be present include O (oxygen), S (sulfur), N (nitrogen), and halogens (Cl, Br, I).

• The chemical formula for organic compounds is typically written with carbon first, followed by hydrogen, then other elements.

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

Inorganic compounds are generally composed of metals and nonmetals and do not follow the same structural rules as organic compounds.

Comparison of Organic and Inorganic Compounds

Organic and inorganic compounds differ in their elemental composition, bonding, physical properties, and solubility. The table below summarizes these differences:

Property	Organic	Example: C3H8	Inorganic	Example: NaCl
Elements Present	C and H, sometimes O, S, N, P, or Cl (F, Br, I)	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, a few are nonpolar 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

Hydrocarbons

Definition and Structure

Hydrocarbons are a class of organic compounds consisting entirely of carbon and hydrogen atoms. They can form chains or rings, with each carbon atom forming four covalent bonds to satisfy its valency.

• Hydrocarbons are non-polar molecules and are generally insoluble in water but soluble in nonpolar solvents.

• They are less dense than water and have low boiling points, which increase with molecular weight.

• Hydrocarbons are classified as aliphatic (straight or branched chains, or rings) or aromatic (containing benzene-like rings).

Physical Properties of Hydrocarbons

• Non-polar and not water soluble

• Soluble in nonpolar organic solvents

• Lower density than water

• Low boiling and melting points, which increase with molecular size

Example: Propane (C3H8) is a hydrocarbon used as a fuel.

Alkanes

Definition and General Formula

Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. Their general formula is:

$C_nH_{2n+2}$

where n is the number of carbon atoms in the molecule.

• All bonds are single covalent bonds (sigma bonds).

• Alkanes are the simplest type of hydrocarbon.

Structural Representation of Alkanes

Alkanes can be represented in several ways:

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

• Structural formula: Shows each atom and bond explicitly.

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

• Line-angle formula: Each vertex or end of a line represents a carbon atom; hydrogens are implied.

Naming Alkanes (IUPAC System)

The International Union of Pure and Applied Chemistry (IUPAC) system is used to name alkanes. The names of straight-chain alkanes end in "-ane" and use prefixes to indicate the number of carbon atoms:

Name	# Carbons	Molecular Formula	Condensed Formula
Methane	1	CH4	CH4
Ethane	2	C2H6	CH3CH3
Propane	3	C3H8	CH3CH2CH3
Butane	4	C4H10	CH3CH2CH2CH3
Pentane	5	C5H12	CH3(CH2)3CH3
Hexane	6	C6H14	CH3(CH2)4CH3
Heptane	7	C7H16	CH3(CH2)5CH3
Octane	8	C8H18	CH3(CH2)6CH3
Nonane	9	C9H20	CH3(CH2)7CH3
Decane	10	C10H22	CH3(CH2)8CH3

Physical Properties and Trends in Alkanes

As the length (and molecular weight) of an alkane increases, its melting point, boiling point, and density also increase. This is due to the greater surface area and stronger London dispersion forces between larger molecules.

• Boiling point order: CH4 < C2H6 < C4H10 < C6H14

• Alkanes are less dense than water and are nonpolar.

Classification of Hydrocarbons

Types of Hydrocarbons

Hydrocarbons are divided into two main categories: aliphatic and aromatic.

• Aliphatic hydrocarbons include:

  • Alkanes: Only single bonds (e.g., ethane, CH3CH3).

  • Cycloalkanes: Alkanes with carbon atoms bonded in rings (e.g., cyclohexane).

  • Alkenes: At least one double bond (e.g., ethene, CH2=CH2).

  • Alkynes: At least one triple bond (e.g., ethyne, HC≡CH).

• Aromatic hydrocarbons contain benzene-like rings (e.g., benzene, C6H6).

Summary Table: Names and Formulas of the First Ten Straight-Chain Alkanes

Name	Molecular Formula	Condensed Formula	Melting Point (°C)	Boiling Point (°C)
Methane	CH4	CH4	−182.5	−162.2
Ethane	C2H6	CH3CH3	−183.0	−88.6
Propane	C3H8	CH3CH2CH3	−187.7	−42.1
Butane	C4H10	CH3(CH2)2CH3	−138.3	−0.5
Pentane	C5H12	CH3(CH2)3CH3	−129.7	36.1
Hexane	C6H14	CH3(CH2)4CH3	−95.0	68.7
Heptane	C7H16	CH3(CH2)5CH3	−90.6	98.4
Octane	C8H18	CH3(CH2)6CH3	−56.8	125.7
Nonane	C9H20	CH3(CH2)7CH3	−53.6	150.8
Decane	C10H22	CH3(CH2)8CH3	−29.7	174.0

Practice Example

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

Solution: Use the general formula for alkanes: $C_nH_{2n+2}$ For n = 10: $C_{10}H_{(2 \times 10) + 2} = C_{10}H_{22}$

Answer: 22 hydrogen atoms.

Summary

• Organic compounds are carbon-based and differ from inorganic compounds in structure and properties.

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

• Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$.

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