Introduction to Organic Chemistry: Hydrocarbons (Chapter 11 Study Notes)

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Introduction to Organic Chemistry: Hydrocarbons

11.1 Organic Compounds

Organic chemistry is the study of carbon compounds. Organic compounds are essential to life and are found in many everyday products.

Organic compounds are made primarily from carbon and hydrogen atoms.
They may also contain other nonmetals such as oxygen, sulfur, nitrogen, phosphorus, or halogens (F, Cl, Br, I).
Common examples include gasoline, medicines, shampoos, plastics, and perfumes.
The chemical formulas of organic compounds are written with carbon first, followed by hydrogen, then other elements.

Properties of Organic Compounds

Organic compounds typically have covalent bonds.
They have low melting and boiling points.
They are flammable and undergo combustion.
They are not soluble in water (nonpolar).

Example: Vegetable oil is a mixture of organic compounds and does not dissolve in water.

Comparison: Organic vs. Inorganic Compounds

Many inorganic compounds have high melting and boiling points.
Inorganic compounds that are ionic are usually soluble in water and most do not burn in air.
Example: Propane (C3H8) is an organic compound used as fuel; salt (NaCl) is an inorganic compound composed of Na+ and Cl- ions.

Hydrocarbons

Hydrocarbons are organic compounds that contain only carbon and hydrogen. In organic molecules, every carbon atom forms four bonds.

Saturated hydrocarbons contain only single bonds between carbon atoms.

Example: Methane (CH4)

Methane has a tetrahedral geometry with bond angles of 109°.
Carbon forms four covalent bonds to hydrogen.

Representations of methane include:

Space-filling model
Ball-and-stick model
Wedge-dash model
Expanded structural formula
Condensed structural formula

Example: Ethane (C2H6)

Each carbon forms three covalent bonds to hydrogen and one to the other carbon.
Ethane is also tetrahedral around each carbon atom.

11.2 Alkanes

Alkanes are hydrocarbons containing only single C–C and C–H bonds. The covalent bonds between carbon atoms are very strong, allowing for a large number of possible compounds.

Alkanes are named using the IUPAC system.
Names end in -ane.
Greek prefixes are used for chains with five or more carbons.

Naming Alkanes (IUPAC Rules)

Determine the type of hydrocarbon.
Find the longest continuous chain of carbon atoms.
Name the base chain using a prefix (number of carbons) and a suffix (-ane, -ene, -yne) to indicate the type.

IUPAC Names and Formulas of the First Ten Alkanes

Number of Carbon Atoms	IUPAC Name	Molecular Formula	Condensed Structural Formula	Line-Angle Formula
1	Methane	CH4	CH4	•
2	Ethane	C2H6	CH3–CH3	—
3	Propane	C3H8	CH3–CH2–CH3	— —
4	Butane	C4H10	CH3–CH2–CH2–CH3	— — —
5	Pentane	C5H12	CH3–(CH2)3–CH3	— — — —
6	Hexane	C6H14	CH3–(CH2)4–CH3	— — — — —
7	Heptane	C7H16	CH3–(CH2)5–CH3	— — — — — —
8	Octane	C8H18	CH3–(CH2)6–CH3	— — — — — — —
9	Nonane	C9H20	CH3–(CH2)7–CH3	— — — — — — — —
10	Decane	C10H22	CH3–(CH2)8–CH3	— — — — — — — — —

Hydrocarbon Prefixes

Meth-: 1 carbon
Eth-: 2 carbons
Prop-: 3 carbons
But-: 4 carbons
Pent-: 5 carbons
Hex-: 6 carbons
Hept-: 7 carbons
Oct-: 8 carbons
Non-: 9 carbons
Dec-: 10 carbons

Condensed Structural Formulas

Each carbon atom and its attached hydrogen atoms are written as a group.
A subscript indicates the number of hydrogen atoms bonded to each carbon atom.
Example: Butane: CH3–CH2–CH2–CH3

Expanded vs. Condensed Structural Formulas

Expanded: Shows each bond explicitly.
Condensed: Shows each carbon atom and its attached hydrogens as a group.

Writing Skeletal (Line-Angle) Formulas

Molecular formula: Shows total number of atoms (e.g., C4H10).
Line-angle formula: Shows the carbon skeleton; each vertex or end of a line represents a carbon atom.

Alkane Representations

Molecular formula
Ball-and-stick model
Expanded structural formula
Condensed structural formula
Line-angle formula

Structural Formulas for Butane (C4H10)

Expanded Structural Formula	Condensed Structural Formula	Line-Angle Formula
H–C–C–C–C–H	CH3–CH2–CH2–CH3	— — —

Cycloalkanes

Cycloalkanes are cyclic alkanes.
They have two fewer hydrogen atoms than the open-chain form.
Named by using the prefix cyclo- before the alkane name.
Example: Propane (C3H8) vs. Cyclopropane (C3H6).

Formulas of Common Cycloalkanes

Name	Ball-and-Stick Model	Condensed Structural Formula	Line-Angle Formula
Cyclopropane	3-carbon ring	CH2–CH2–CH2	△
Cyclobutane	4-carbon ring	CH2–CH2–CH2–CH2	□
Cyclopentane	5-carbon ring	CH2–CH2–CH2–CH2–CH2	⬠
Cyclohexane	6-carbon ring	CH2–CH2–CH2–CH2–CH2–CH2	⬡

11.3 Alkanes with Substituents

Alkanes with four or more carbon atoms can have side groups (branches or substituents) attached to the main chain.

Naming Hydrocarbons with Substituents (IUPAC)

Determine the type of hydrocarbon.
Find the longest continuous chain of carbon atoms.
Name the base chain using a prefix and suffix.
Number the chain from the end closest to the first branch (substituent).
If the first substituent is equally close to both ends, use the next substituent to determine numbering.
Indicate the position and name of each substituent; list substituents alphabetically.

Structural Isomers

Structural isomers have the same molecular formula but different arrangements of atoms.
Example: Butane (C4H10) has a straight-chain and a branched-chain isomer.

Alkane Substituents

Alkyl groups: Groups of carbon atoms attached to the main chain, named with an -yl ending (e.g., methyl, ethyl).
Halo substituents: Halogen atoms attached to the carbon chain, named as fluoro, chloro, bromo, or iodo.

Common Substituents Table

Formula	Name
CH3–	methyl
CH3–CH2–	ethyl
CH3–CH2–CH2–	propyl
CH3–CH2–CH2–CH2–	butyl
F–	fluoro
Cl–	chloro
Br–	bromo
I–	iodo

Naming Cycloalkanes with Substituents

For a single substituent, place its name in front of the cycloalkane name; no number is needed.
Example: Ethylcyclohexane

Drawing Formulas for Alkanes

The IUPAC name provides all necessary information to draw the condensed structural formula.
Example: 1-chloro-1,2-dimethylheptane: seven-carbon chain, methyl groups on carbons 1 and 2, chlorine on carbon 1.

Solubility and Density of Alkanes

Alkanes are nonpolar, insoluble in water, less dense than water, flammable in air, and found in crude oil.
In oil spills, alkanes float on water, forming a thin layer.

Combustion of Alkanes

Alkanes react with oxygen gas to produce carbon dioxide and water, releasing energy.

General equation for alkane combustion:

Example: Methane combustion

Example: Butane combustion

Alkenes and Alkynes

Alkenes contain at least one carbon-carbon double bond.
Alkynes contain at least one carbon-carbon triple bond.
They are called unsaturated hydrocarbons because they do not have the maximum number of hydrogen atoms.
They react with hydrogen gas to become alkanes.

Additional info: Alkenes have trigonal planar geometry (bond angles 120°); alkynes have linear geometry (bond angles 180°).