Lecture Notes #3

The Carbon Backbone

Learning Goals

• Understand how to draw and interpret line-bond structures in organic chemistry.

• Identify and name basic hydrocarbon structures and functional groups.

Hydrocarbons

Definition and Types

Hydrocarbons are organic molecules consisting solely of carbon (C) and hydrogen (H) atoms. Other atoms present in organic molecules are called heteroatoms (e.g., N, O, F).

• In stable molecules, carbon forms four bonds.

• Hydrocarbons are classified by the types of bonds between carbon atoms: single (alkanes), double (alkenes), and triple (alkynes).

Hybridization and Geometry

• sp3: Tetrahedral geometry, 4 σ bonds. Example: methane ($CH_4$).

• sp2: Trigonal planar geometry, 3 σ bonds, 1 π bond. Example: ethene ($C_2H_4$).

• sp: Linear geometry, 2 σ bonds, 2 π bonds. Example: ethyne ($C_2H_2$).

Alkanes

Structure and Representation

Alkanes are saturated hydrocarbons containing only single C–C and C–H bonds. Their carbon atoms are sp3 hybridized, resulting in a tetrahedral geometry.

• Line-bond structures are used for simplicity: each line represents a bond, and each vertex or line end represents a carbon atom.

• Wedge-dash notation is used to show three-dimensional geometry:

  • Wedge: bond coming out of the page.

  • Dash: bond going into the page.

Rules for Line-Bond Structures

1. Every line represents a bond.

2. Every end or vertex of a line represents a carbon atom (unless another atom is drawn).

3. Hydrogen atoms are usually omitted; it is assumed each carbon has enough hydrogens to make four bonds.

Naming Alkanes

Alkanes are named according to the number of carbon atoms in the chain:

Formula	Name	Lewis Structure	Condensed Formula	Line-Bond
CH4	Methane	H–C–H	CH4	(single point)
C2H6	Ethane	H3C–CH3	CH3CH3	—
C3H8	Propane	H3C–CH2–CH3	CH3CH2CH3	—/—
C4H10	Butane	H3C–CH2–CH2–CH3	CH3(CH2)2CH3	—/—/—
C5H12	Pentane	H3C–(CH2)3–CH3	CH3(CH2)3CH3	—/—/—/—
C6H14	Hexane	H3C–(CH2)4–CH3	CH3(CH2)4CH3	—/—/—/—/—
C7H16	Heptane	H3C–(CH2)5–CH3	CH3(CH2)5CH3	—/—/—/—/—/—
C8H18	Octane	H3C–(CH2)6–CH3	CH3(CH2)6CH3	—/—/—/—/—/—/—

Alkane Classification

Types of Carbon Atoms

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

• Secondary (2°) carbon: bonded to two other carbons.

• Tertiary (3°) carbon: bonded to three other carbons.

• Quaternary (4°) carbon: bonded to four other carbons.

These terms apply only to sp3 hybridized carbons in alkanes.

Groups & Abbreviations

Common Alkyl Groups

Name	Abbreviation	Structure
Methyl	Me	–CH3
Ethyl	Et	–CH2CH3
Propyl	Pr	–CH2CH2CH3
Iso-propyl	i-Pr	–CH(CH3)2
n-butyl	n-Bu	–CH2CH2CH2CH3
tert-butyl	t-Bu	–C(CH3)3

sp2 Hydrocarbons

Alkenes

• Alkenes contain at least one C=C double bond.

• Each carbon in the double bond is sp2 hybridized.

• Example: Ethene ($C_2H_4$).

Aromatic Rings

• Aromatic rings (e.g., benzene) have alternating single and double bonds in a cyclic structure.

• Common error: aromatic rings are not alkenes; they react differently.

Alkene Classification

Substitution Patterns

• Mono-substituted: one group attached to the double bond.

• Di-substituted: two groups attached (can be cis/trans).

• Tri-substituted: three groups attached.

• Tetra-substituted: four groups attached.

E vs. Z Alkenes

Stereochemistry of Alkenes

• Trans (E): highest priority groups are on opposite sides of the double bond.

• Cis (Z): highest priority groups are on the same side.

• Priority is determined by atomic number (Cahn-Ingold-Prelog rules).

sp Hydrocarbons

Alkynes

• Alkynes contain at least one C≡C triple bond; carbons are sp hybridized.

• Classified as internal or terminal depending on the position of the triple bond.

• Example: 1-butyne (terminal), 2-butyne (internal).

Groups & Abbreviations (Aromatic and Functional)

Name	Abbreviation	Structure
R (residue)	R	Any group not relevant to discussion
Allyl	n/a	–CH2CH=CH2
Phenyl	Ph	–C6H5
Benzyl	Bn	–CH2C6H5
Aromatic/aryl	Ar	Any aromatic ring
Acetyl	Ac	–C(O)CH3

Applications: Petroleum and Alkanes

Petroleum Composition

• Petroleum is a complex mixture; about 30% is alkanes.

• Alkanes in petroleum are used for fuels:

  • Gasoline: pentanes to octanes

  • Diesel, kerosene, jet fuel: nonanes to hexadecanes

• Burning petroleum produces CO2, contributing to greenhouse gases.

Practice Questions

Example 1: Secondary Carbons

How many secondary carbons are in the given molecule?

• Answer: Two

Example 2: Trans Alkenes

Which of the following are trans alkenes?

• Answer: Structure B

Summary

• Hydrocarbons are classified by bond type and substitution.

• Line-bond structures simplify organic molecule representation.

• Alkanes, alkenes, and alkynes have distinct geometries and naming conventions.

• Functional groups and abbreviations are essential for organic nomenclature.

Additional info: This guide covers foundational concepts from Organic Chemistry Chapter 1 (Structure and Bonding) and Chapter 3 (Structure and Stereochemistry of Alkanes), with context from Chapter 2 (Functional Groups).