Organic Compounds and Structures: Foundations of Organic Chemistry

Organic Compounds and Structures

What are Organic Compounds?

Organic compounds are chemical substances that contain carbon atoms bonded to other elements such as hydrogen, oxygen, or nitrogen. They exist in solid, liquid, or gaseous states and are the basis of all known life forms.

• Definition: Compounds containing carbon in their molecular structure.

• Classification: Organic compounds are classified as acyclic (open-chain) or cyclic (ring-containing), with further subdivisions into straight chain, branched chain, homocyclic, aromatic, heterocyclic, saturated, and unsaturated types.

Classification Diagram:

• Acyclic (Straight Chain, Branched Chain)

• Cyclic (Homocyclic: Alicyclic, Aromatic; Heterocyclic)

• Saturated vs. Unsaturated; Aromatic vs. Non-aromatic

Examples of Organic Compounds

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

• Cyclic Compounds: Cyclohexane (homocyclic), Tetrahydrofuran (heterocyclic)

• Aromatic Compounds: Benzene, Thiophene

Saturated hydrocarbons: Alkanes (single bonds only) Unsaturated hydrocarbons: Alkenes (double bonds), Alkynes (triple bonds)

Carbon: The Backbone of Life

Carbon's unique ability to form four covalent bonds allows it to create large, complex, and diverse molecules essential for life. Major classes of biological organic compounds include:

• Carbohydrates (sugars)

• Proteins (amino acids)

• Nucleic acids (DNA, RNA)

• Lipids (fatty acids, steroids, waxes)

Atomic Structure and Bonding in Carbon

Electron Configuration of Carbon

• Carbon has four valence electrons, allowing it to form four covalent bonds.

• Electron dot diagram: :C:

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

Electron configuration: 1s2 2s2 2p2

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 of p orbitals (in double and triple bonds)

The Formation of Bonds with Carbon

• Carbon forms four covalent bonds with a variety of atoms.

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

• Carbon can form single, double, and triple bonds.

• Common bonding partners: hydrogen, oxygen, nitrogen.

Hydrocarbons and Their Classification

Hydrocarbons

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

• Alkanes: Saturated hydrocarbons (single bonds only)

• Cycloalkanes: Saturated 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

• Hydrophobic (not water soluble) due to non-polar C–H and C–C bonds

• Low density (less than 1 g/mL)

• 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: Structure and Properties

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

Polarity: 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 in Organic Compounds

Constitutional (Structural) Isomers

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

• To identify constitutional isomers, compare the number and arrangement of atoms in each molecule.

Example: Butane and isobutane (C4H10) are constitutional 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
Heptane	C7H16	CH3(CH2)5CH3, etc.	9
Octane	C8H18	CH3(CH2)6CH3, etc.	18
Nonane	C9H20	CH3(CH2)7CH3, etc.	35
Decane	C10H22	CH3(CH2)8CH3, etc.	75

Nomenclature of Alkanes

Common System

• Count the total number of carbon atoms in the molecule.

• Use the Latin root for the number of carbons, followed by the suffix “-ane”.

• Unbranched hydrocarbons use the prefix “normal” or “n-”.

• Branched hydrocarbons use specific prefixes (e.g., iso-, sec-, tert-).

IUPAC System

• Find the longest continuous chain of carbon atoms (parent chain).

• Use the appropriate Latin root and add the “-ane” suffix.

• Name and number substituents (alkyl groups) attached to the parent chain.

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

• List substituents in alphabetical order, using prefixes (di-, tri-, etc.) for multiples.

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

Summary of Alkane Nomenclature Steps

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

2. Number the chain to give substituents the lowest possible numbers.

3. Name and number all substituents.

4. Combine substituent names and numbers with the parent name and suffix.

Example: 3-ethyl-2,6-dimethylheptane

*Additional info: This summary covers the foundational concepts of organic compounds, their classification, bonding, hydrocarbon types, isomerism, and systematic nomenclature, as outlined in the provided lecture slides. These topics correspond to Chapters 1, 2, and 4 of a standard Organic Chemistry curriculum.*