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

• Key Point 2: Inorganic compounds are composed mostly 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.

Hydrocarbons

Structure and Properties

Hydrocarbons are organic compounds consisting solely 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, making them insoluble in water but soluble in nonpolar solvents. Their physical properties include low density and low boiling points, which increase with molecular weight.

• Key Point 1: Hydrocarbons are non-polar and generally less dense than water.

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

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

• 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 a benzene ring.

• Example: Ethane (alkane), ethene (alkene), ethyne (alkyne), cyclohexane (cycloalkane), benzene (aromatic).

Alkanes

General Formula and Structure

Alkanes are saturated hydrocarbons containing only carbon and hydrogen atoms, 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 alkanes follows $C_nH_{2n+2}$.

• Example: For n = 2, ethane is $C_2H_6$.

Formulas Used in Organic Chemistry

Alkanes can be represented by molecular, structural, condensed, and line-angle formulas. Molecular formulas list the number and type of atoms, structural formulas show each atom and bond, condensed formulas group atoms, and line-angle formulas depict the carbon skeleton.

• Key Point 1: Molecular formula: C2H6, C3H8, C4H10, C5H12.

• Key Point 2: Structural formula shows all atoms and bonds; condensed formula groups atoms; line-angle formula simplifies the carbon skeleton.

• Example: Butane: Expanded structural formula, condensed formula (CH3CH2CH2CH3), line-angle formula.

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

Alkanes are non-polar, insoluble in water, and have low boiling and melting points. As the chain length increases, melting and boiling points, as well as density, increase.

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

• Key Point 2: Alkanes are less dense than water and are flammable.

• 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

This table compares the main properties of organic and inorganic compounds, including elements present, bonding, polarity, melting and boiling points, flammability, and solubility.

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 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

This table lists the molecular and condensed formulas, melting points, and boiling points for 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	-56.6	126.0
Nonane	C9H20	CH3CH2CH2CH2CH2CH2CH2CH2CH3	-53.6	150.7
Decane	C10H22	CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3	-29.5	174.0

Additional info:

These notes cover the foundational concepts of organic chemistry, focusing on hydrocarbons and their classification, structure, naming, and physical properties. The tables and diagrams included reinforce the distinctions between organic and inorganic compounds, as well as the structural diversity and properties of alkanes.