Introduction to Organic Chemistry: Hydrocarbons (GOB Chemistry Study Notes)

Organic Compounds and Their Properties

Definition and Characteristics of Organic Compounds

Organic chemistry is the study of compounds containing carbon, typically bonded to hydrogen and sometimes other elements such as oxygen, nitrogen, sulfur, phosphorus, or halogens. Organic compounds are commonly found in everyday products like gasoline, medicines, shampoos, plastics, and perfumes. The unique properties of organic compounds arise from their covalent bonding and the versatility of carbon atoms.

• Contains carbon and hydrogen: All organic compounds have at least these two elements.

• May include other nonmetals: Oxygen, sulfur, nitrogen, phosphorus, or halogens.

• Functional groups: Groups of atoms bonded in specific ways, giving compounds similar properties.

• Common products: Gasoline, medicines, shampoos, plastics, perfumes.

Properties of Organic vs. Inorganic Compounds

Organic compounds differ from inorganic compounds in several key physical and chemical properties. Understanding these differences is essential for classifying substances and predicting their behavior.

• Covalent bonds: Organic compounds are mostly covalent.

• Low melting and boiling points: Organic compounds typically melt and boil at lower temperatures.

• Flammability: Organic compounds are flammable and undergo combustion.

• Solubility: Organic compounds are generally not soluble in water.

• Inorganic compounds: Often ionic, have high melting/boiling points, usually soluble in water, and do not burn in air.

Property	Organic	Inorganic
Elements Present	C and H, sometimes O, S, N, P, or Cl	Most metals and nonmetals
Bonding	Mostly covalent	Mostly ionic
Melting Point	Usually low	Usually high
Flammability	High	Low
Solubility in Water	Not soluble	Most are soluble

Hydrocarbons and Their Structures

Hydrocarbons: Definition and Types

Hydrocarbons are organic compounds composed solely of carbon and hydrogen. They are classified based on the types of bonds between carbon atoms:

• Saturated hydrocarbons (alkanes): Only single bonds between carbon atoms.

• Unsaturated hydrocarbons: Contain double or triple bonds (not covered in detail here).

Each carbon atom in organic molecules forms four bonds, achieving a stable octet. Methane (CH4) is the simplest hydrocarbon, with a tetrahedral shape and bond angles of 109°.

Structural Representations of Hydrocarbons

Organic compounds can be represented in several ways:

• Expanded structural formula: Shows all atoms and bonds.

• Condensed structural formula: Groups each carbon with its attached hydrogens.

• Line-angle structural formula: Uses zigzag lines; ends and corners represent carbon atoms.

Three-Dimensional Models

Three-dimensional models help visualize the spatial arrangement of atoms in molecules. Ethane (C2H6) and propane (C3H8) are examples of simple hydrocarbons with tetrahedral geometry.

Alkanes: Structure, Naming, and Isomerism

Alkanes: Definition and Properties

Alkanes are saturated hydrocarbons containing only single bonds. They can exist as straight chains or rings (cycloalkanes) and are named using the IUPAC system, with names ending in "-ane." The prefixes meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, and dec- indicate the number of carbon atoms.

Number of Carbons	Name	Molecular Formula	Condensed Formula	Line-Angle Formula
1	Methane	CH4	CH4	—
2	Ethane	C2H6	CH3CH3	—
3	Propane	C3H8	CH3CH2CH3	—
4	Butane	C4H10	CH3CH2CH2CH3	—
5	Pentane	C5H12	CH3CH2CH2CH2CH3	—

Drawing Structural Formulas for Alkanes

To draw structural formulas for alkanes, follow these steps:

• Step 1: Draw the carbon chain.

• Step 2: Add hydrogen atoms to each carbon using single bonds (expanded formula).

• Step 3: Combine H atoms with each C atom (condensed formula).

• Step 4: Draw the line-angle formula as a zigzag line (ends and corners represent C atoms).

Conformations of Alkanes

Alkanes can rotate about their single C–C bonds, resulting in different spatial arrangements called conformations. This flexibility allows groups attached to each carbon to move freely.

Structural Isomerism

Structural isomers are compounds with the same molecular formula but different arrangements of atoms. For example, butane (C4H10) has two isomers: straight-chain and branched-chain. Pentane (C5H12) has three possible structural isomers.

Cycloalkanes

Definition and Properties of Cycloalkanes

Cycloalkanes are cyclic alkanes with two fewer hydrogen atoms than their open-chain counterparts. They are named by adding the prefix "cyclo-" to the alkane name corresponding to the number of carbon atoms in the ring.

Name	Ball-and-Stick Model	Condensed Formula	Line-Angle Formula
Cyclopropane	Triangle	C3H6	△
Cyclobutane	Square	C4H8	□
Cyclopentane	Pentagon	C5H10	⬟
Cyclohexane	Hexagon	C6H12	⬢

Alkanes with Substituents

Substituents and Alkyl Groups

Branched-chain alkanes have side groups called substituents attached to the main carbon chain. Substituents include alkyl groups (carbon branches) and halo groups (halogen atoms).

• Alkyl groups: Named with an "-yl" ending (e.g., methyl, ethyl).

• Halo groups: Named as fluoro, chloro, bromo, or iodo.

Formula	Name
CH3-	methyl
CH3CH2-	ethyl
CH3CH2CH2-	propyl
Cl-	chloro
Br-	bromo
F-	fluoro
I-	iodo

Naming Alkanes with Substituents (IUPAC System)

To name alkanes with substituents:

• Name the longest carbon chain as the parent.

• Identify and name each substituent.

• Number the chain from the end nearest the first substituent.

• List substituents in alphabetical order, using numbers to indicate their positions.

• Separate numbers with commas and numbers/names with dashes.

Example: 2-chloro-3-methylpentane

For cycloalkanes with substituents, the substituent name is placed in front of the cycloalkane name. No number is needed for a single substituent.

Naming Haloalkanes

Haloalkanes are alkanes in which a halogen atom replaces a hydrogen atom. The halogen is named and numbered according to its position on the carbon chain.

Formula	IUPAC Name	Common Name
CH3Cl	Chloromethane	Methyl chloride
CH3CH2Br	Bromoethane	Ethyl bromide
CH3CH2F	2-Fluoropropane	Isopropyl fluoride

Summary Table: Structural Isomers of Pentane

Condensed Formula	Line-Angle Formula
CH3CH2CH2CH2CH3	— — — — —
CH3CH(CH3)CH2CH3	— — — —
CH3C(CH3)2CH3	— — —

Additional info: These notes cover the foundational concepts of organic chemistry, focusing on hydrocarbons, their properties, structural representations, naming conventions, and isomerism, as relevant to GOB Chemistry.