Organic Compounds and Structures: Foundations of Organic Chemistry

Organic Compounds and Structures

What are Organic Compounds?

Organic compounds are substances in solid, liquid, or gaseous states that contain carbon in their molecules. The diversity of organic compounds arises from the unique bonding properties of carbon.

• General Classification: Organic compounds are classified as acyclic (straight or branched chains) or cyclic (homocyclic, aromatic, heterocyclic). They can also be saturated or unsaturated, and aromatic or non-aromatic.

Examples of Organic Compounds

• Acyclic Compounds: e.g., Propane (CH3CH2CH3), 3,4-dimethylhexane

• Cyclic Compounds: e.g., Cyclohexane, Tetrahydrofuran

• Aromatic Compounds: e.g., Benzene, Thiophene

Saturated hydrocarbons (alkanes) contain only single bonds, while unsaturated hydrocarbons (alkenes, alkynes) contain double or triple bonds, respectively.

Carbon: The Backbone of Life

Importance of Carbon

• Living organisms are primarily composed of carbon-based compounds.

• Carbon forms large, complex, and diverse molecules due to its ability to make four covalent bonds.

• Major classes of organic biomolecules: carbohydrates, proteins, nucleic acids, lipids.

Electron Configuration and Bonding of Carbon

Electron Configuration

• Carbon has four valence electrons and forms four covalent bonds.

• Electron dot diagram: :C:

• Hund’s rule: Electrons occupy orbitals singly before pairing.

Hybridization and Bond Formation

• sp3 hybridization: Four equivalent orbitals, tetrahedral geometry (bond angle ≈ 109.5°).

• sp2 hybridization: Three equivalent orbitals, trigonal planar geometry (bond angle ≈ 120°).

• sp hybridization: Two equivalent orbitals, linear geometry (bond angle ≈ 180°).

Bond Types:

• σ (sigma) bond: Head-on overlap of orbitals (single bonds).

• π (pi) bond: Sideways overlap (present in double and triple bonds).

The Formation of Bonds with Carbon

• Carbon forms four covalent bonds with a variety of atoms (H, O, N, etc.).

• In molecules with multiple carbons, each carbon bonded to four other atoms has a tetrahedral shape.

• Carbon can form single, double, and triple bonds, allowing for molecular diversity.

Hydrocarbons and Their Classification

Definition and Types

Hydrocarbons are compounds containing only carbon and hydrogen. They are classified as:

• Alkanes: Saturated hydrocarbons (single bonds only).

• Cycloalkanes: Saturated hydrocarbons with ring structures.

• Alkenes: Unsaturated hydrocarbons (at least one double bond).

• Cycloalkenes: Unsaturated ring structures (with double bonds).

• Alkynes: Unsaturated hydrocarbons (at least one triple bond).

General Physical Properties of Hydrocarbons

• Hydrocarbons are hydrophobic (not water soluble) due to non-polar C–H and C–C bonds.

• Density: Less than 1 g/mL (float on water).

• Boiling and melting points increase with increasing carbon chain length due to stronger London dispersion forces.

Name	Number of Carbons	Formula	Melting Point (°C)	Boiling Point (°C)
Methane	1	CH4	-183	-162
Ethane	2	CH3CH3	-172	-89
Propane	3	CH3CH2CH3	-188	-42
Butane	4	CH3(CH2)2CH3	-138	0
Pentane	5	CH3(CH2)3CH3	-130	36
Hexane	6	CH3(CH2)4CH3	-95	69
Heptane	7	CH3(CH2)5CH3	-91	98
Octane	8	CH3(CH2)6CH3	-57	126
Nonane	9	CH3(CH2)7CH3	-54	151
Decane	10	CH3(CH2)8CH3	-30	174

Alkanes (Saturated Hydrocarbons)

General Formula and Examples

• General formula: $C_nH_{2n+2}$

• Examples: CH4 (methane), C2H6 (ethane), C3H8 (propane), C4H10 (butane)

• Lewis structures show all bonds and atoms explicitly.

• Alkanes are nonpolar molecules.

Types of Carbons in Alkanes

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

• Secondary (2°) carbon: Connected to two other carbon atoms.

• Tertiary (3°) carbon: Connected to three other carbon atoms.

• Quaternary (4°) carbon: Connected to four other carbon atoms.

Example: In butane (C4H10), there are 2 primary and 2 secondary carbons. In isobutane, there are 3 primary and 1 tertiary carbon.

Isomerism

Constitutional (Structural) Isomers

• Compounds with the same molecular formula but different connectivity of atoms.

• Example: Butane and isobutane (C4H10) are constitutional isomers.

• Strategy: Draw all possible carbon skeletons for a given formula to find all isomers.

Name	Molecular Formula	Structural Formula	Number of Isomers
Methane	CH4	CH4	1
Ethane	C2H6	CH3CH3	1
Propane	C3H8	CH3CH2CH3	1
Butane	C4H10	CH3CH2CH2CH3, (CH3)3CH	2
Pentane	C5H12	CH3(CH2)3CH3, etc.	3
Hexane	C6H14	CH3(CH2)4CH3, etc.	5

Nomenclature of Alkanes

Common and IUPAC Systems

• Count the total number of carbon atoms and use the Latin root with the suffix “-ane”.

• For branched hydrocarbons, use prefixes (e.g., methyl-, ethyl-) and number the chain to give the lowest possible numbers to substituents.

• The name of every organic molecule has three parts: prefix (substituents), parent (longest chain), suffix (functional group).

Number of Carbons	Latin Root
1	meth
2	eth
3	prop
4	but
5	pent
6	hex
7	hept
8	oct
9	non
10	dec

Substituent Names (Alkyl Groups)

• One carbon: methyl (CH3–)

• Two carbons: ethyl (CH3CH2–)

• Three carbons: propyl (CH3CH2CH2–), isopropyl ((CH3)2CH–)

• Four carbons: butyl, isobutyl, sec-butyl, tert-butyl

Naming Steps (IUPAC):

1. Find the longest continuous carbon chain (parent chain).

2. Number the chain to give the first substituent the lowest possible number.

3. Name and number the substituents; use prefixes (di-, tri-, etc.) for multiples.

4. List substituents in alphabetical order (ignoring di-, tri-, etc.).

5. Combine substituent names and numbers with the parent and suffix into a single word.

Example: 3-ethyl-2,6-dimethylheptane

*Additional info: This summary covers the foundational concepts of organic compounds, hydrocarbon classification, physical properties, isomerism, and systematic nomenclature, as outlined in the provided lecture slides. The content is structured to serve as a comprehensive mini-textbook guide for introductory organic chemistry students.*