Alkanes and Their Properties

Structural Isomers

Structural isomers are compounds that share the same molecular formula but differ in their structural formulas and names. Key characteristics include:

Same molecular formula
Different structural formulas
Different names
Usually different physical properties

The number of structural isomers for alkanes increases rapidly with the addition of carbon atoms.

Substituents

In alkanes with four or more carbon atoms, side groups known as branches or substituents can be present:

Substituents are attached to the longest continuous carbon chain.
They replace at least one hydrogen atom in the alkane chain.

Alkyl groups are the substituents derived from carbon atoms, changing the suffix from -ane to -yl when a hydrogen atom is removed.

Examples of Alkyl Groups

Straight-Chain Alkanes	Alkyl Groups
Methane (CH₄)	Methyl (CH₃-)
Ethane (C₂H₆)	Ethyl (C₂H₅-)
Propane (C₃H₈)	Propyl (C₃H₇-)
Butane (C₄H₁₀)	Isopropyl (C₃H₇-)
Pentane (C₅H₁₂)	Various alkyl groups

Naming Alkanes (IUPAC Guidelines)

The naming of alkanes follows specific guidelines:

Name the longest continuous carbon chain, which is the parent chain.
Number the chain from the end closest to the first branch.
Count the alkyl groups and refer to the appropriate naming conventions.
Identify the lowest branch number by comparing from both ends.
List substituent names in alphabetical order, indicating their positions.

Examples of Isomers

For butane (C₄H₁₀), there are two isomers:

Butane
2-Methylpropane

Drawing Structures

To draw alkane structures:

Draw the main chain of carbon atoms with single bonds.
Number the carbon atoms in the chain.
Place substituents on the corresponding numbered carbon atoms.
Add the correct number of hydrogen atoms to ensure four bonds per carbon atom.

Sources of Alkanes

Alkanes can be sourced from:

Natural Gas: Composed of 80-90% methane, with smaller amounts of other alkanes.
Bottled Gas: Used in camping stoves and barbecues, typically containing propane and butane.

Petroleum Distillation

Crude oil is a significant source of hydrocarbons, and distillation separates it into fractions based on boiling points. Octane is a major component of gasoline.

Gasoline Characteristics

Gasoline consists of hydrocarbons ranging from C₅ to C₁₂. The octane number measures gasoline's resistance to knocking during combustion:

Isooctane: Octane number of 100 (burns smoothly)
Heptane: Octane number of 0 (more prone to knocking)

Additives like ethanol are used to improve combustion efficiency, while tetraethyllead has been phased out due to environmental concerns.

Reactions of Alkanes

Alkanes undergo several types of reactions:

Combustion Reactions: React with oxygen to produce carbon dioxide and water.
Substitution Reactions: Halogen atoms replace hydrogen atoms, typically involving free-radical mechanisms.
Dehydrogenation Reactions: Removal of hydrogen atoms to form unsaturated hydrocarbons, requiring catalysts and high temperatures.

Cycloalkanes

Cycloalkanes are cyclic alkanes named by adding "cyclo-" before the alkane name. Key features include:

Composed of carbon rings with all single C-C bonds.
Contain two fewer hydrogen atoms than their straight-chain counterparts.
Require at least three carbon atoms to form a ring.

Examples include cyclopentane and cyclohexane, which have stable structures and specific boiling points.

Examples of Cycloalkanes

Common cycloalkanes include:

Cyclopentane: C₅H₁₀
Cyclohexane: C₆H₁₂

Practice Naming Cycloalkanes

Examples for practice include naming cycloalkanes and drawing formulas for specific compounds like 1,3-dimethyl-2-propylcyclohexane.

Organic Chemistry Overview

Organic chemistry is the branch of chemistry focused on the study of carbon-containing compounds. These compounds are essential for biological life and can be found in both natural and synthetic forms.

Approximately 7 million organic compounds exist, representing about 90-96% of all known substances.
Each year, over 50,000 new organic compounds are synthesized.
Organic compounds can have systematic names (IUPAC) or common names.
Carbon atoms are unique in their ability to bond with each other, forming chains and rings.

Why Carbon?

Carbon is a versatile element due to its four valence electrons, which allows it to form stable covalent bonds with other atoms.

Carbon can achieve a stable octet through various bonding configurations:

Four single bonds (sp³ hybridization) - ALKANE
Two single bonds and one double bond (sp² hybridization) - ALKENE
One single bond and one triple bond (sp hybridization) - ALKYNE
Two double bonds (sp hybridization) - DIENE

Carbon can bond with itself to create:

Chains
Branches
Rings

Examples of Organic Compounds

Examples of organic compounds include:

Alkanes (e.g., Ethane)
Alkenes (e.g., Ethene)
Dienes (e.g., 1,3-Butadiene)
Alkynes (e.g., Ethyne)
Aromatic compounds (e.g., Benzene)

Organic Compounds

Organic compounds are characterized by the presence of carbon and typically hydrogen. Key points include:

A hydrocarbon contains only carbon and hydrogen.
A saturated hydrocarbon has only single bonds between carbon atoms.
Hydrocarbon derivatives contain other elements (e.g., O, N, S, P, halogens).

Classification of Hydrocarbons

Hydrocarbons can be classified into two main types:

Saturated hydrocarbons (only C-C single bonds)
Unsaturated hydrocarbons (at least one C=C double bond or C≡C triple bond)
Aromatic compounds (or arenes) are also included in the unsaturated category.

Examples of Hydrocarbons

Ethane (alkane)
Ethene (alkene)
1,3-Butadiene (diene)
Ethyne (alkyne)
Benzene (aromatic; arene)

Saturated Hydrocarbons

Saturated hydrocarbons consist solely of single bonds between carbon atoms, belonging to the alkane family. Characteristics include:

Carbon atoms can form four single covalent bonds.
Contain the maximum ratio of hydrogen to carbon atoms.
The molecular formula for an open-chain saturated n-alkane is C_nH_2n+2.

Naming Alkanes

The naming convention for organic compounds includes a prefix indicating the number of carbon atoms and a suffix indicating the family:

The prefix is determined by the number of carbon atoms in the longest continuous chain.
As the number of carbon atoms increases, so does the boiling point of the alkanes.
Common alkanes include:

Methane, ethane, propane, and butane (gases at room temperature)
Pentane, hexane, heptane, and octane (liquids at room temperature)

Prefixes for Alkanes

Prefixes for the base names of alkanes are essential for proper naming:

1 - Meth-
2 - Eth-
3 - Prop-
4 - But-
5 - Pent-
6 - Hex-
7 - Hept-
8 - Oct-

Structural Formulas

Alkanes can be represented in various structural forms:

Expanded structural formulas show each bond.
Condensed structural formulas represent carbon atoms and attached hydrogen atoms.
Line-bond formulas (zig-zag) omit hydrogen atoms, showing only the bonds between carbon atoms.

Conformations of Butane

In butane, the groups attached to a C-C single bond can rotate freely, resulting in different orientations known as conformations.

Classification of Alkanes

Alkanes can be further classified into:

n-Alkanes (linear or normal)
Isomers (structural isomers with different patterns of atom attachment)

Isomers

Isomers are different molecules that share the same molecular formula but differ in structure:

Structural isomers have different bonding patterns and names.
Conformers are not isomers; they are rotations around C-C bonds.

Examples of Alkyl Groups

Common alkyl groups include:

Methyl (–CH₃)
Ethyl (–C₂H₅)
Propyl (–C₃H₇)
Butyl (–C₄H₉)

Alkanes: Structures, Isomers, and Naming Conventions

Alkanes: Structures, Isomers, and Naming Conventions