Free Radical Reactions in Chemistry

Alkanes and Cycloalkanes

• Alkanes are the simplest organic compounds, composed of only carbon and hydrogen atoms
• Alkanes have only carbon-to-carbon and carbon-to-hydrogen single bonds
• Alkane naming follows a simple counting system (e.g., methane, ethane, propane, butane) corresponding to 1, 2, 3, and 4 carbons respectively.
• The general formula for alkanes is CnH2n+2, where 'n' is the number of carbon atoms in the chain.
• Pent- means 5, Hex- means 6, Hept- means 7, Oct- means 8, Non- means 9, and Dec- means 10 carbon atoms
• Isomers are compounds with the same molecular formula but different structural formulas, leading to different properties. Butane (C4H10) exists as two isomers: butane and iso-butane.

Intended Learning Outcomes (ILOs)

• Students will gain an understanding of the physical properties of alkanes.
• Students will have a good idea about the nomenclature of alkanes.
• Students will learn methods to prepare alkanes.
• Students will become familiar with alkane reactions.

Contents

• Alkane definition, formula, and structure
• Physical characteristics of alkanes
• Nomenclature of alkanes
• Cycloalkanes properties and examples
• Preparation of alkanes
• Reactions of alkanes

Hydrocarbons

• Hydrocarbons are the simplest organic compounds, composed of carbon and hydrogen atoms
• Alkanes are a type of hydrocarbon with only carbon-to-carbon and carbon-to-hydrogen single bonds.
• Alkane naming follows a systematic pattern

Physical Characteristics of Alkanes

• The electronegativity difference between carbon and hydrogen is small (0.4), resulting in nonpolar C—H bonds.
• The primary intermolecular forces are van der Waals dispersion forces.
• The strength of these forces increases with the size of the molecule.
• Melting and boiling points increase with molecule size due to stronger intermolecular forces. For straight-chain alkanes, the boiling point increases with increasing chain length/size.
• Branching of the alkane chain decreases the surface area and therefore the strength of the van der Waals dispersion forces resulting in lower melting and boiling points.

Van der Waals Dispersion Forces

• This is the weakest type of intermolecular force
• It occurs due to temporary fluctuations in electron distribution within a molecule resulting in temporary dipoles.
• This temporary dipole affects nearby molecules, inducing further temporary dipoles
• The strength of dispersion forces is directly related to the size and shape of the molecule

Cycloalkanes

• Cycloalkanes have similar properties to alkanes, but they have higher melting and boiling points than straight-chain alkanes due to increased surface contact.
• Cycloalkanes are also insoluble in water, as they are nonpolar.
• The solubility of cycloalkanes is in organic solvents.

IUPAC Names for Alkanes

• IUPAC (International Union of Pure and Applied Chemistry) system systematically names alkanes.
• Alkane names have four parts: a Locant, Prefix, Parent, and Suffix.
  - Locant indicates the position of substituents - Prefix designates substituent groups - Parent indicates the main carbon chain length - Suffix denotes the presence of only single bonds.

Alkyl Groups

• Alkyl groups are created by removing a hydrogen atom from an alkane.
• Alkyl groups have the general formula CnH2n+1.
• They are named by replacing the -ane ending with -yl.

IUPAC Nomenclature of Simple Alkanes

• Step 1: Identify the parent hydrocarbon (longest carbon chain)
• Step 2: Number the atoms in the longest chain, prioritizing the branch point closets to the end of the chain
• Step 3: Identify and number the substituents (attaching groups) locating their attachment points.
• Step 4: Write the name as a single word, using hyphens and commas for numbers as necessary
• For identical substituents, use prefixes (di, tri, etc.)

IUPAC Nomenclature of Branched Alkanes

• Name the branched substituent as if it were a parent compound, numbered from the point of attachment to the main chain
• The branched substituent is named and placed within parentheses
• Use prefixes to indicate the number of repeated substituents

Reactions of Alkanes

• Alkanes are relatively unreactive, their carbon-carbon and carbon-hydrogen bonds are strong.
• Alkanes can undergo oxidation/combustion reactions.
• Combustion: When alkanes are burned with sufficient oxygen, they produce carbon dioxide and water, releasing energy.
• Pyrolysis/cracking: Thermal decomposition of higher alkanes into smaller alkanes in the absence of oxygen.
• Halogenation: free radical substitution of a hydrogen atom in the alkane with a halogen atom (e.g., Cl, Br).
• Photochemical reaction: The reaction between methane and chlorine in the presence of UV light, yielding chloromethane through a free radical chain reaction.
  • Chain initiation step: UV light breaks apart chlorine to form chlorine free radicals
  • Chain propagation step: The free radical abstracts a hydrogen from methane and forms a new radical
  • Chain termination step: Two free radicals combine to form a stable compound.

Nomenclature of Alkanes 2- Common Names

• For alkanes beyond propane, straight-chain alkanes are sometimes indicated by the prefix n (for normal), to distinguish them from branched-chain alkanes.
• The prefix iso is used to indicate that the chain has a branched structure, where all carbons do not form a straight chain, except one carbon.
• The prefix neo is used with a branched structure where the carbon with the most substitutions is second to last carbon in the chain

Common Names

• Common names for alkyl groups such as isopropyl, sec-butyl, isobutyl, and tert-butyl, are also useful.

Cycloalkanes

• When carbon atoms join to form rings, these become cycloalkanes.
• Cyclopropane, cyclobutane, cyclopentane, and cyclohexane are cycloalkanes that have rings of 3, 4, 5, and 6 carbon atoms respectively.
• The general formula for cycloalkanes is CnH2n.

Preparation of Alkanes

• Catalytic reduction/hydrogenation: Alkynes and alkenes can be converted to alkanes by adding hydrogen in the presence of a transition metal catalyst.
• Clemmensen reduction: Reduce carbonyl groups (aldehydes and ketones) into methylene groups using zinc amalgam and hydrochloric acid
• Grignard's reagent: Hydrolyze an alkyl halide with magnesium and water to yield an alkane.
• Wurtz reaction: Heating an alkyl halide with sodium metal to create alkyl-alkyl bonds and produce an alkane.
• Soda lime reaction: Decarboxylation of carboxylic acids or their salts by heating with soda lime, producing an alkane

Test your knowledge on free radical chain reactions, focusing on initiation, propagation, and termination steps. This quiz covers key concepts such as the interaction between chlorine radicals and methane. Assess your understanding of how free radicals behave during chemical reactions.