Alkanes | Organic Chemistry 1

The IUPAC nomenclature of a chemical compound is in the following form: Prefix - Parent - Suffix

• Prefix: identity, location, and number of substituents
• Parent name: the longest continuous chain of carbon atoms
• Suffix: priority functional group present in the molecule

1. Identify the parent chain
   - choose the longest continuous chain of carbon atoms
   - in case of 2 chains of the same length, choose the chain with the most substituents
2. Identify and name the substituents as alkyl groups
3. Number the atoms in the parent chain
   - number the parent chain to give the first substituent the lowest number
   - if the first substituent is the same distance from both ends, number the chain to give the second substituent the lower number
   - if numbering the parent chain gives the same numbers from each end of the chain after considering all substituents, assign the lowest number to the first substituent in alphabetical order
4. Write the name of the alkane as a single word
   - use hyphens to separate the different prefixes, and commas to separate numbers
   - list the substituents in alphabetical order (di, tri, tetra are not counted)

Alkanes with unbranched carbon chains are simply named by the number of carbons in the chain: methane (CH₄), ethane (CH₃CH₃), propane (CH₃CH₂CH₃), butane (CH₃CH₂CH₂CH₃), propane, hexane, heptane, octane, nonane, decane, etc.

An alkane is an organic compound with only C-C and C-H single bonds. Acyclic alkanes have the molecular formula C_nH_2n+2. They are also called saturated hydrocarbons because they have the maximum number of hydrogens per carbon

• Alkanes have low boiling and melting points compared to more polar compounds of comparable size. Their boiling and melting points increase with the number of carbons (increase in surface area)
• Alkanes are less dense than water ⇒ alkanes float above water
• Geometry: alkanes are composed of sp³ hybridized carbon atoms ⇒ tetrahedral geometry
• Reactivity: alkanes have nonpolar C-C and C-H bonds and have no functional groups ⇒ alkanes are not very reactive

2 atoms are chemically equivalent when they have an identical environment. This notion of equivalence is very important in spectroscopy (NMR) and in reactions (for example, the radical halogenation of alkanes)

To determine if 2 atoms are chemically equivalent, the most common method is to determine how these atoms are related by molecular motion (rotation of σ bonds, ring flip, axial and equatorial positions) or by symmetry (plane of symmetry, axis of rotation, center of inversion)

Carbon atoms are classified according to the number of other carbons directly attached to them

• Primary carbon: a carbon bonded to ONE other carbon atom
• Secondary carbon: a carbon bonded to TWO other carbon atoms
• Tertiary carbon: a carbon bonded to THREE other carbon atoms
• Quaternary carbon: a carbon bonded to FOUR other carbon atoms

Hydrogen atoms, chemical functions or radicals are classified according to the type of carbon atom to which they are attached

• Primary: hydrogen / chemical function / radical on a primary carbon
• Secondary: hydrogen / chemical function / radical on a secondary carbon
• Tertiary: hydrogen / chemical function / radical on a tertiary carbon

A radical is a reactive intermediate with a single unpaired electron, formed by the homolytic cleavage of a covalent bond

The stability of radicals increases along the series from primary to secondary to tertiary due to the electronic effects of attached alkyl groups (hyperconjugation); therefore, the energy required to create them decreases in that order

A radical initiator is a compound containing a particularly weak bond that serves as a source of radicals. Common examples of radical initiators are halogens (Cl₂, Br₂), azo compounds (R-N=N-R^') and organic peroxides (R-O-O-R^')

Halogenation of an alkane produces a hydrocarbon derivative in which one or more halogen atoms have been substituted for hydrogen atoms

The reaction between an alkane and a halogen in the presence of UV light (hv) or heat leads to the formation of an alkyl halide (or haloalkane)

The mechanism of halogenation of alkanes occurs in 3 steps:

1. Initiation step: homolytic cleavage of the Cl-Cl bond
2. Propagation step: abstraction of a hydrogen atom by the Cl radical followed by an abstraction of a chlorine atom by the alkyl radical
3. Chain termination step: radical-radical combination

1. Identify all equivalent hydrogen groups in the starting alkane
2. Replace one hydrogen atom of an equivalent hydrogen group with a halogen atom

The total number of products is equal to the number of equivalent hydrogen groups (ignoring stereoisomers)

Tertiary radicals are more stable and form more rapidly than secondary and primary radicals. Therefore, their relative ratios are greater than the statistical ratios. On the contrary, the relative ratios of primary radicals are lower than the statistical ratios