Introduction to Organic Chemistry: Alkanes – Structured Study Notes

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Chapter 12: Introduction to Organic Chemistry – Alkanes

12.1 The Nature of Organic Molecules

Organic chemistry is the study of carbon compounds, which are characterized by the tetravalent nature of carbon and the diversity of covalent bonding patterns. Organic molecules often contain hydrogen, oxygen, and nitrogen in addition to carbon.

Carbon is tetravalent: It forms four covalent bonds, allowing for a variety of molecular shapes and structures.
Covalent bonds are the primary type of bonding in organic molecules.
Carbon can form single, double, or triple bonds with other atoms, leading to different molecular geometries:
- 4 groups attached: Tetrahedral
- 3 groups attached: Trigonal planar
- 2 groups attached: Linear
When carbon bonds to more electronegative elements (e.g., O, N, halogens), polar covalent bonds result.
Organic molecules have specific three-dimensional shapes, which influence their chemical behavior.
Most organic compounds are insoluble in water and do not conduct electricity; only small polar molecules or those with many polar groups are water-soluble.

Ethane structure and model Ethylene and acetylene structures and models Chloromethane structure and polarity Methane tetrahedral structure Common organic bonds (C-N, C-O, etc.)

12.2 Families of Organic Molecules: Functional Groups

Functional groups are specific atoms or groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. Organic compounds are classified into families based on their functional groups.

Functional group: An atom or group of atoms with characteristic chemical behavior.
Functional groups dictate the chemistry of organic molecules more than molecular size or complexity.
Hydrocarbons (contain only C and H):
- Alkanes: Only single bonds, no functional group.
- Alkenes: Contain C=C double bonds.
- Alkynes: Contain C≡C triple bonds.
- Aromatic compounds: Six-membered rings with alternating double bonds.
Other families (contain heteroatoms):
- Alkyl halides: C–halogen bond
- Alcohols: C–OH bond
- Ethers: C–O–C linkage
- Amines: C–N bond
- Carbonyl compounds (C=O): Aldehydes, ketones, carboxylic acids, anhydrides, esters, amides
- Sulfur-containing: Thiols, sulfides, disulfides

Worked example: identifying functional groups Alkene structure example Alcohol structure example Aromatic hydrocarbon example Ketone structure example Amine and amide functional groups Aromatic and disulfide functional groups

Worked Example: Proposing Structures from Formulas

(a) Amines (C2H7N): Contains a C–NH2 group.
(b) Alkynes (C3H4): Contains a C≡C triple bond.
(c) Ethers (C4H10O): Contains a C–O–C linkage.

Amine structure example Alkyne structure example Ether structure example

12.3 The Structure of Organic Molecules: Alkanes and Their Isomers

Alkanes are hydrocarbons with only single bonds. Isomers are compounds with the same molecular formula but different structures.

Alkane general formula: CnH2n+2
Isomers arise as the number of carbons increases.
Straight-chain alkanes: All carbons in a row.
Branched-chain alkanes: At least one carbon is attached to more than two other carbons.
Constitutional (structural) isomers: Same formula, different connectivity.
Functional group isomers: Isomers that differ in both connectivity and functional group family.

Taxol structure for functional group identification Methane formation Propane formation Straight-chain and branched-chain butane Ethyl alcohol and dimethyl ether as functional group isomers Isomers of hexane (C6H14)

12.4 Drawing Organic Structures

Organic structures can be represented in several ways: structural formulas, condensed formulas, and line (line-angle) structures.

Condensed structure: Bonds are implied; groups are written together (e.g., CH3CH2CH3).
Line structure: Each vertex or line end represents a carbon; hydrogens on carbons are not shown.
Non-carbon/hydrogen atoms must be shown explicitly.
Parentheses and subscripts can indicate repeating groups (e.g., CH3(CH2)4CH3).

Line structure example Condensed to line structure conversion Line structure with numbered carbons Line structure with methyl branches Line structure with OH and Cl groups Line structure to condensed structure conversion Line structure with carbons labeled Adding hydrogens to line structure Final condensed structure Line structure with noncarbon atoms Adding hydrogens to noncarbon atoms Final condensed structure with branches

12.5 The Shapes of Organic Molecules

Alkanes have tetrahedral geometry around each carbon. Single bonds allow for free rotation, resulting in different conformations (conformers).

Conformers: Different spatial arrangements due to rotation around single bonds.
Most molecules adopt the lowest-energy, least crowded conformation.
Conformers are interconvertible and have the same connectivity.

Conformations of butane Isomers and conformers of C7H16 Comparing isomers and conformers Conformers with methyl group on same carbon Isomers with methyl group on different carbons

12.6 Naming Alkanes

The IUPAC system provides a systematic way to name alkanes based on the number of carbons and the presence of substituents.

A chemical name has three parts: prefix (substituent location), parent (longest chain), suffix (family).
Straight-chain alkanes: Named by the number of carbons + "-ane" (e.g., methane, ethane, propane).
Branched-chain alkanes: Identify and number the main chain, then name and number substituents.
Alkyl groups: Derived from alkanes by removing one hydrogen (e.g., methyl, ethyl).
Carbons are classified as primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) based on the number of other carbons attached.

IUPAC naming structure Names of straight-chain alkanes Alkyl group formation Common alkyl groups Primary, secondary, tertiary, quaternary carbons Common abbreviations in organic chemistry Longest chain identification Numbering the main chain

12.7 Properties of Alkanes

Alkanes are nonpolar molecules with weak intermolecular forces (London dispersion forces). Their physical properties vary with molecular size.

First four alkanes (methane, ethane, propane, butane) are gases at room temperature.
Alkanes with 5–15 carbons are liquids; 16+ carbons are waxy solids.
Alkanes are odorless or have mild odor, colorless, tasteless, nontoxic, insoluble in water, and flammable.

12.8 Reactions of Alkanes

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

Combustion: Reaction with oxygen to produce CO2, H2O, and heat.
Halogenation: Replacement of a hydrogen atom by a halogen (Cl or Br) via free radical mechanism, often producing a mixture of products.
In halogenation, only one hydrogen is replaced at a time; prolonged reaction can replace all hydrogens.

12.9 Cycloalkanes

Cycloalkanes are alkanes with carbon atoms arranged in a ring. Their general formula is CnH2n.

Forming a ring requires an extra C–C bond and loss of two hydrogens.
Small rings (cyclopropane, cyclobutane) have bond angles less than the ideal tetrahedral angle, making them less stable.
Cyclopentane and cyclohexane are more stable and common in nature.
Cycloalkanes are nonpolar, insoluble in water, flammable, and more rigid than open-chain alkanes due to restricted rotation.

12.10 Drawing and Naming Cycloalkanes

Cycloalkanes are typically drawn using polygons to represent the ring. Naming follows the same rules as for open-chain alkanes, with the ring as the parent structure.

If only one substituent is present, numbering is unnecessary.
With multiple substituents, numbering starts at the group with alphabetical priority and proceeds to give the lowest possible numbers to substituents.

Summary Table: Names of Straight-Chain Alkanes

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