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

Organic Compounds

Definition and Characteristics

Organic compounds are molecules primarily composed of carbon and hydrogen, often with other elements such as oxygen, sulfur, nitrogen, and halogens. Their formulas are written with carbon first, followed by hydrogen, then other elements. Organic compounds are prevalent in everyday products like fuels, medicines, plastics, and perfumes.

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

• Key Point 2: Inorganic compounds are composed of metals and nonmetals, such as sodium chloride (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	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	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 form chains or rings, with each carbon atom forming four covalent bonds. Hydrocarbons are classified as non-polar molecules, generally insoluble in water but soluble in nonpolar solvents. Their density and boiling points are typically lower than water, but increase with molecular weight.

• Key Point 1: Hydrocarbons are non-polar and not water soluble.

• Key Point 2: As the length of the carbon chain increases, melting and boiling points, as well as density, increase.

• Example: Propane (C3H8) and methane (CH4) are common hydrocarbons.

Classification of Hydrocarbons

Aliphatic and Aromatic Hydrocarbons

Hydrocarbons are divided into two main categories: aliphatic and aromatic. Aliphatic hydrocarbons include alkanes, alkenes, alkynes, and cycloalkanes, 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 single bonds between carbon atoms. 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 an alkane with n carbons is $C_nH_{2n+2}$.

• Example: An alkane with 10 carbons: $C_{10}H_{22}$.

Formulas Used in Organic Chemistry

Alkanes can be represented by molecular, structural, condensed, and line-angle formulas. Each provides different levels of detail about the molecule's structure.

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

• Structural formula: Shows each atom and bond (e.g., expanded structure).

• Condensed formula: Groups atoms by carbon (e.g., CH3CH2CH3).

• Line-angle formula: Represents the carbon skeleton with lines and 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 (1 carbon), ethane (2), propane (3), butane (4).

• Key Point 2: Pentane (5), hexane (6), heptane (7), octane (8), nonane (9), decane (10).

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

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 with Molecular Weight

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 (e.g., methane, ethane) have very low boiling points.

• Key Point 2: Long-chain alkanes (e.g., octane, decane) have higher boiling points and densities.

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

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

Melting and Boiling Points

The table below summarizes the molecular and condensed formulas, melting points, and boiling points for the first ten straight-chain alkanes.

Alkane	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	-56.6	125.7
Nonane	C9H20	CH3CH2CH2CH2CH2CH2CH2CH2CH3	-53.6	150.7
Decane	C10H22	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3	-29.5	174.0

Summary: Key Concepts in Hydrocarbon Chemistry

• Organic compounds are carbon-based and often nonpolar, with low melting and boiling points.

• Hydrocarbons are classified as aliphatic (alkanes, alkenes, alkynes, cycloalkanes) or aromatic (benzene rings).

• Alkanes follow the general formula $C_nH_{2n+2}$ and are named according to IUPAC rules.

• Physical properties of alkanes increase with molecular weight.