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Chapter 12: Alkanes and the Nature of Organic Molecules

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Alkanes and Hydrocarbons

Introduction to Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of hydrogen and carbon atoms. They are the simplest type of organic molecules and serve as the foundation for more complex organic chemistry.

  • Alkanes: Hydrocarbons with only single bonds between carbon atoms (saturated hydrocarbons).

  • Alkenes: Hydrocarbons containing at least one double bond between carbon atoms.

  • Alkynes: Hydrocarbons containing at least one triple bond between carbon atoms.

  • Aromatic Hydrocarbons: Hydrocarbons with ring structures and alternating double bonds (e.g., benzene).

  • General Formula for Alkanes:

  • Example: Methane (), Ethane (), Propane ()

The Nature of Organic Molecules

Bonding and Structure

Organic molecules are primarily composed of nonmetals and feature covalent bonds. Carbon is tetravalent, always forming four bonds, which allows for a variety of molecular shapes and structures.

  • Tetravalency of Carbon: Carbon forms four covalent bonds, leading to diverse structures.

  • Bond Types:

    • Single bond: Tetrahedral geometry (109.5° bond angles)

    • Double bond: Trigonal planar geometry (120° bond angles)

    • Triple bond: Linear geometry (180° bond angles)

  • Polar Covalent Bonds: When carbon bonds to more electronegative elements (e.g., O, N), polar covalent bonds result.

  • Physical Properties: Most organic molecules are insoluble in water, do not conduct electricity, and have specific 3D shapes.

Families of Organic Molecules

Functional Groups

Functional groups are specific atoms or groups of atoms within a molecule that determine its chemical behavior and reactivity. Molecules with the same functional group tend to undergo similar reactions.

  • Hydrocarbons: Alkanes, alkenes, alkynes, aromatics

  • Halides: Alkyl halides (F, Cl, Br, I)

  • Alcohols: Contains -OH group

  • Ethers: Contains -O- group

  • Amines: Contains -NH2 group

  • Aldehydes: Contains -CHO group

  • Ketones: Contains -CO- group

  • Carboxylic Acids: Contains -COOH group

  • Esters: Contains -COO- group

  • Amides: Contains -CONH2 group

  • Sulfur Compounds: Thiols (-SH), disulfides (-S-S-), sulfides (-S-)

Family

Structure

Suffix

Alkane

C-C

-ane

Alkene

C=C

-ene

Alkyne

C≡C

-yne

Aromatic

Ring

-none

Alcohol

-OH

-ol

Ether

-O-

-none

Aldehyde

-CHO

-al

Ketone

-CO-

-one

Carboxylic Acid

-COOH

-ic acid

Amine

-NH2

-amine

Halide

-X (F, Cl, Br, I)

-none

Thiols

-SH

-none

Structure of Organic Molecules: Alkanes and Isomers

Alkanes and Their Isomers

Alkanes are hydrocarbons with only single bonds. As the number of carbon atoms increases, the possibility of isomers arises. Isomers are compounds with the same molecular formula but different structural arrangements.

  • Straight-chain alkanes: Continuous chain of carbon atoms.

  • Branched-chain alkanes: Carbon chains with branches.

  • Constitutional (structural) isomers: Same formula, different connectivity.

  • Functional group isomers: Same formula, different functional groups.

  • Example: Butane () has both straight-chain and branched isomers.

Drawing Organic Structures

Condensed and Line Structures

Organic molecules can be represented in several ways to simplify their visualization and understanding.

  • Condensed Structures: Bonds between C-C and C-H are implied, not shown explicitly.

  • Line Structures: Each vertex represents a carbon atom; lines represent bonds. Hydrogens attached to carbon are not shown.

  • Guidelines:

    • Each line represents a C-C bond.

    • Each vertex is a carbon atom.

    • Atoms other than C or H must be shown explicitly.

  • Example: Butane can be drawn as a straight line with four vertices (carbons).

Shapes of Organic Molecules

Conformations and Conformers

The 3D arrangement of atoms in a molecule is called its conformation. Alkanes can rotate around C-C single bonds, resulting in different spatial arrangements called conformers.

  • Conformation: 3D arrangement achieved through bond rotation.

  • Conformers: Structures with identical atom connections but different spatial arrangements.

  • Example: Butane has several conformations due to rotation around C-C bonds.

Naming Alkanes

IUPAC System and Substituents

The International Union of Pure and Applied Chemistry (IUPAC) system provides rules for naming alkanes based on the number of carbons, the presence of branches (substituents), and their positions.

  • Prefix: Indicates substituents.

  • Parent: Indicates the longest carbon chain.

  • Suffix: Indicates the family (-ane for alkanes).

  • Substituent: Atom or group attached to the parent chain.

  • Alkyl Groups: Formed by removing a hydrogen from an alkane (e.g., methyl, ethyl, propyl).

  • Substitution Patterns:

    • Primary (1°): Carbon attached to one other carbon.

    • Secondary (2°): Carbon attached to two other carbons.

    • Tertiary (3°): Carbon attached to three other carbons.

    • Quaternary (4°): Carbon attached to four other carbons.

  • Naming Steps:

    1. Name the longest chain (parent).

    2. Number the chain from the end nearest a branch.

    3. Identify and number substituents.

    4. Write the name as a single word, using hyphens to separate numbers.

    5. List substituents alphabetically; use prefixes (di-, tri-, tetra-) for multiples.

Number of Carbons

Name

1

Methane

2

Ethane

3

Propane

4

Butane

5

Pentane

6

Hexane

7

Heptane

8

Octane

9

Nonane

10

Decane

Properties of Alkanes

Physical and Chemical Properties

Alkanes are characterized by their mild odor, nonpolarity, and low reactivity. They are insoluble in water but soluble in nonpolar organic solvents.

  • Odor: Mild, often tasteless.

  • Color: Colorless.

  • Solubility: Insoluble in water, soluble in nonpolar solvents.

  • Toxicity: Generally non-toxic.

  • Density: Less dense than water.

  • Flammability: Highly flammable.

  • Reactivity: Not very reactive except under specific conditions.

Reactions of Alkanes

Combustion and Halogenation

Alkanes undergo two main types of reactions: combustion and halogenation.

  • Combustion: Reaction with oxygen to produce carbon dioxide and water.

    • General Equation:

  • Halogenation: Replacement of a hydrogen atom by a halogen (Cl or Br), initiated by heat or light. Occurs stepwise, one hydrogen at a time.

  • Example: Methane reacts with chlorine to form chloromethane, dichloromethane, etc.

Cycloalkanes

Structure and Properties

Cycloalkanes are alkanes that form ring structures. They require an additional C-C bond and lose two hydrogen atoms compared to their straight-chain counterparts.

  • General Formula:

  • Examples: Cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane

  • Properties: Similar to alkanes but with ring strain in smaller rings.

Drawing and Naming Cycloalkanes

Guidelines for Cycloalkane Nomenclature

Cycloalkanes are named using the 'cyclo-' prefix and the parent alkane name. Substituents are numbered starting at the group with alphabetical priority.

  • Parent Compound: Use the cycloalkane name (e.g., cyclohexane).

  • Substituents: Name and number substituents; start numbering at the group with alphabetical priority.

  • Example: 4-ethyl-3-methylcyclohexane

  • Do not use: Names like 'cyclohexylmethane'; always use the cycloalkane as the parent.

Additional info: Academic context was added to clarify functional groups, isomer types, and naming conventions. Tables were reconstructed for functional groups and alkane names. Examples and formulas were expanded for completeness.

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