Chapter 11: Introduction to Organic Chemistry – Hydrocarbons

Organic Compounds

Definition and Characteristics

Organic compounds are molecules primarily composed of carbon atoms, often accompanied by hydrogen, and may also include oxygen, sulfur, nitrogen, and halogens (Cl, Br, I). Their formulas are written with carbon first, followed by hydrogen, then other elements. Organic compounds are prevalent in everyday products such as gasoline, medicines, shampoos, plastics, and perfumes. In contrast, inorganic compounds are composed of most metals and nonmetals, such as sodium chloride (NaCl).

• Key Point 1: Organic compounds always contain at least one carbon atom and usually many hydrogen atoms.

• Key Point 2: The bonding in organic compounds is mostly covalent, while inorganic compounds often have ionic or polar covalent bonds.

• Example: Propane (C3H8) is an organic compound used as a fuel; sodium chloride (NaCl) is an inorganic compound used as table salt.

Hydrocarbons

Structure and Properties

Hydrocarbons are a subset of organic compounds consisting only of carbon and hydrogen atoms. They can form chains or rings, with each carbon atom forming four covalent bonds. Hydrocarbons are non-polar, generally less dense than water, and have low boiling points. As the molecular weight increases, their melting and boiling points, as well as density, also increase.

• Key Point 1: Hydrocarbons are non-polar and not soluble in water, but soluble in nonpolar organic solvents.

• Key Point 2: Physical properties such as boiling point and density increase with molecular size.

• Example: Methane (CH4), ethane (C2H6), and propane (C3H8) are common hydrocarbons.

Classification of Hydrocarbons

Aliphatic and Aromatic Hydrocarbons

Hydrocarbons are classified into two main groups: aliphatic and aromatic. Aliphatic hydrocarbons include alkanes, alkenes, alkynes, and cycloalkanes. Aromatic hydrocarbons contain benzene rings and related structures.

• Key Point 1: Alkanes contain only single bonds; alkenes contain at least one double bond; alkynes contain at least one triple bond; cycloalkanes have carbon atoms bonded in rings.

• Key Point 2: Aromatic hydrocarbons are characterized by the presence of benzene rings.

• Example: Benzene (C6H6) is an aromatic hydrocarbon; ethane (C2H6) is an aliphatic alkane.

Alkanes

Structure and General Formula

Alkanes are saturated hydrocarbons containing only carbon and hydrogen atoms, with carbon-carbon and carbon-hydrogen single bonds. The general formula for a chain alkane is $C_nH_{2n+2}$, where n is the number of carbon atoms.

• Key Point 1: Alkanes are saturated, meaning all carbon-carbon bonds are single bonds.

• Key Point 2: The molecular formula for alkanes follows $C_nH_{2n+2}$.

• Example: For n = 10, the formula is C10H22.

Formulas Used in Organic Chemistry

Alkanes can be represented in several ways: molecular formula, structural formula, condensed formula, and line-angle formula. The molecular formula lists the number of each atom, the structural formula shows each atom and bond, the condensed formula groups atoms, and the line-angle formula represents the carbon skeleton.

• Key Point 1: Molecular formula: C2H6 (ethane), C3H8 (propane).

• Key Point 2: Structural formula visually shows the arrangement of atoms and bonds.

• Key Point 3: Condensed formula: CH3CH2CH3 (propane).

• Key Point 4: Line-angle formula: Each vertex or end represents a carbon atom.

Naming Alkanes

The names of alkanes are determined by the IUPAC system and end in "-ane." Alkanes with 1-4 carbons use prefixes: methane, ethane, propane, butane. Alkanes with 5-10 carbons use Greek prefixes: pentane, hexane, heptane, octane, nonane, decane.

• Key Point 1: Prefixes indicate the number of carbon atoms in the chain.

• Key Point 2: The molecular and condensed formulas correspond to the number of carbons and hydrogens.

• Example: Heptane: C7H16, condensed formula CH3(CH2)5CH3.

Physical Properties of Alkanes

As the length (molecular weight) of alkanes increases, their melting and boiling points, as well as density, increase. Alkanes are generally non-polar, not soluble in water, and have low boiling points compared to inorganic compounds.

• Key Point 1: Longer alkanes have higher melting and boiling points.

• Key Point 2: Alkanes are less dense than water and are non-polar.

• Example: Methane has a boiling point of -162°C, while decane has a boiling point of 174°C.

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 (F, Br, I)	C and H	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

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	-89.0
Propane	C3H8	CH3CH2CH3	-187.6	-42.1
Butane	C4H10	CH3CH2CH2CH3	-138.4	0.0
Pentane	C5H12	CH3CH2CH2CH2CH3	-129.7	36.1
Hexane	C6H14	CH3CH2CH2CH2CH2CH3	-95.2	68.7
Heptane	C7H16	CH3CH2CH2CH2CH2CH2CH3	-90.6	98.4
Octane	C8H18	CH3CH2CH2CH2CH2CH2CH2CH3	-56.8	126.0
Nonane	C9H20	CH3CH2CH2CH2CH2CH2CH2CH2CH3	-53.6	150.7
Decane	C10H22	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3	-29.5	174.0

Summary

Organic chemistry focuses on compounds containing carbon, especially hydrocarbons. Alkanes are saturated hydrocarbons with only single bonds, and their properties change with molecular size. Understanding the classification, structure, and naming conventions of hydrocarbons is fundamental to organic chemistry.