Introduction to Organic Chemistry: Hydrocarbons (Ch. 11) - Mini Study Guide

Organic Compounds

Definition and Characteristics

Organic compounds are molecules primarily composed of carbon atoms, often bonded with hydrogen, and may include oxygen, sulfur, nitrogen, and halogens (Cl, Br, I). The formulas of organic compounds are written with carbon first, followed by hydrogen, then other elements. These compounds are prevalent in everyday products such as gasoline, medicines, shampoos, plastics, and perfumes.

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

• Key Point 2: Inorganic compounds are composed of most metals and nonmetals (e.g., NaCl).

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

Properties of Organic vs. Inorganic Compounds

Comparative Table

Organic and inorganic compounds differ in their elemental composition, bonding, physical properties, and solubility.

Property	Organic	Example: C3H8	Inorganic	Example: NaCl
Elements Present	C and H, sometimes O, S, N, P, or Cl	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	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

Structure and Physical Properties

Hydrocarbons are organic compounds consisting only of carbon and hydrogen atoms. They can be arranged in 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, melting and boiling points, as well as density, increase.

• Key Point 1: Hydrocarbons are non-polar and not water soluble; they dissolve in nonpolar organic solvents.

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

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

Classification of Hydrocarbons

Types of Hydrocarbons

Hydrocarbons are classified into aliphatic and aromatic types. Aliphatic hydrocarbons include alkanes, cycloalkanes, alkenes, and alkynes, while aromatic hydrocarbons contain benzene rings.

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

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

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

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

• Aromatic: Contain benzene rings (e.g., benzene).

Alkanes

General Formula and Structure

Alkanes are saturated hydrocarbons containing only carbon and hydrogen, 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: An alkane with 10 carbon atoms: $C_{10}H_{22}$.

Formulas Used in Organic Chemistry

Molecular, Structural, Condensed, and Line-Angle Formulas

Organic compounds can be represented in several ways:

• Molecular formula: Lists the kind and number of each atom (e.g., C2H6).

• Structural formula: Shows each atom and bond in a molecule.

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

• Line-angle formula: Represents the carbon skeleton, with carbon atoms at line ends or corners.

Naming Alkanes

IUPAC System and Prefixes

The names of alkanes are determined by the IUPAC system and end in "-ane." Alkanes with 1-4 carbons use specific prefixes, while those with 5-10 carbons use Greek prefixes.

• Key Point 1: Methane (1C), Ethane (2C), Propane (3C), Butane (4C).

• Key Point 2: Pentane (5C), Hexane (6C), Heptane (7C), Octane (8C), Nonane (9C), Decane (10C).

• Example: CH3CH2CH2CH2CH2CH2CH3 is heptane (C7H16).

Name	# Carbons	Molecular Formula
Methane	1	CH4
Ethane	2	CH3CH3
Propane	3	CH3CH2CH3
Butane	4	CH3CH2CH2CH3
Pentane	5	CH3CH2CH2CH2CH3
Hexane	6	CH3CH2CH2CH2CH2CH3
Heptane	7	CH3CH2CH2CH2CH2CH2CH3
Octane	8	CH3CH2CH2CH2CH2CH2CH2CH3
Nonane	9	CH3CH2CH2CH2CH2CH2CH2CH2CH3
Decane	10	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3

Physical Properties of Alkanes

Trends in Melting and Boiling Points

As the length (molecular weight) of alkanes increases, their melting and boiling points, as well as density, increase. This is due to greater van der Waals forces between larger molecules.

• Key Point 1: Short-chain alkanes have lower boiling points than long-chain alkanes.

• Key Point 2: The boiling point order for C2H6, CH4, C6H14, C4H10 is: CH4 < C2H6 < C4H10 < C6H14.

• Example: Methane (CH4) has a boiling point of -162°C, while decane (C10H22) has a boiling point of 174°C.

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	CH3(CH2)3CH3	-129.7	36.1
Hexane	C6H14	CH3(CH2)4CH3	-95.2	68.7
Heptane	C7H16	CH3(CH2)5CH3	-90.6	98.4
Octane	C8H18	CH3(CH2)6CH3	-56.6	125.7
Nonane	C9H20	CH3(CH2)7CH3	-53.6	150.7
Decane	C10H22	CH3(CH2)8CH3	-29.5	174.0

Summary Table: Aliphatic vs. Aromatic Hydrocarbons

Structural Examples

Aliphatic hydrocarbons include straight-chain and cyclic compounds, while aromatic hydrocarbons contain benzene rings.

Type	Example	Structure
Aliphatic	Methane	CH4
Aliphatic	Propane	CH3CH2CH3
Aliphatic	Cyclohexane	C6H12
Aliphatic	Ethanol	CH3CH2OH
Aromatic	Benzene	C6H6
Aromatic	Ethylbenzene	C8H10
Aromatic	Naphthalene	C10H8

Practice Problems and Applications

Sample Questions

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

• Solution: Use the formula $C_nH_{2n+2}$: $C_{10}H_{22}$.

• Question: Rank the following alkanes from lowest to highest boiling point: C2H6, CH4, C6H14, C4H10.

• Solution: CH4 < C2H6 < C4H10 < C6H14.

Additional Info

Organic chemistry forms the basis for understanding biological molecules and industrial chemicals. Mastery of hydrocarbon classification, nomenclature, and physical properties is essential for further study in organic and biochemistry.