Alkanes | Organic Chemistry 1

Reactions of alkanes are studied in this chapter: important concepts of saturated hydrocarbons (equivalent atoms, primary, secondary, tertiary, quaternary atoms in chemistry, stability of radicals), 2 types of reactivity in organic chemistry (heterolytic vs. homolytic), radical chain reactions, halogenation of alkanes (mechanism, products and selectivity)

IUPAC Nomenclature

Prefix - Parent - Suffix

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

 

Parent name:

Number of C atoms:

1
2
3
4
5
6
7
8
9
10

Parent name:

meth
eth
prop
but
pent
hex
hept
oct
non
dec

Some other common substituents:

 

Substituents name:

Carbon substituents bonded to a carbon chain are called alkyl groups. To name them, the -ane ending of the corresponding alkane is replace by -yl

CH- = methyl
CH3CH- = ethyl

Nomenclature of Alkanes

How to name an alkane:
 

  1. Identify the parent chain
    - choose the longest continuous chain of carbon atoms
    - in case of 2 chains of equal length, choose the chain with more 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 lower number
    - if the first substituent is the same distance from both ends, number the chain to give the second substituent the lower number
    - if the numbering the parent chain gives the same numbers from either end of the chain after taking account all substituents, assign the lower number alphabetically to the first substituent
     
  4. Write the name of the alkane as a single word
    - use hyphens to separate different prefixes and commas to separate numbers
    - arrange the substituents alphabetically (di, tri, tetra are not counted)

 

1. Longest chain: 7 carbon atoms ⇒ heptane
2. 2 methyl substituents + 1 ethyl substituent
3. The right end is closest to a substituent: number the C atoms from right to left
4. 4-ethyl-3,3-dimethylheptane

Properties of Alkanes

Alkanes:

Organic compounds having only C-C and C-H single bonds. Acyclic alkanes have the molecular formula CnH2n+2. They are also called saturated hydrocarbons because they have the maximum number of hydrogens per carbon
 

General properties:

- Geometry: alkanes are composed of sp3 hybridized carbon atoms ⇒ tetrahedral geometry
- Reactivity: alkanes have nonpolar C-C and C-H bonds and no functional group ⇒ alkanes are not very reactive

Chemically Equivalent Atoms

2 atoms are chemically equivalent when they have an identical environment. For example, the 6 hydrogen atoms in ethane are considered to be chemically equivalent since they are bonded to a carbon atom bonded to 2 other hydrogens and to a methyl group. They have the same environment

 

Ethane: all H are equivalent

1,1,4,4-tetramethylcyclohexane: 2 groups of equivalent H

 

This concept is very important in spectroscopy (NMR) and in reactions (e.g. redical halogenation of alkanes)

Primary, Secondary, Tertiary, Quaternary in Chemistry

For carbon atoms:

Carbon atoms are classified by the number of other carbons directly bonded to them

- Primary carbon: a carbon attached to only ONE other carbon atom
- Secondary carbon: a carbon attached to TWO other carbon atoms
- Tertiary carbon: a carbon attached to THREE other carbon atoms
- Quaternary carbon: a carbon attached to FOUR other carbon atoms

 

For hydrogen atoms, chemical functions or radicals:

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

- 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 attached
 

Radicals

Radicals are formed by homolytic cleavage. Their stability increases along the series from primary to secondary to tertiary (hyperconjugation); therefore, the energy required to create them decreases
 

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Radical Chain Mechanism

Radical chain reactions proceed in 3 stages: initiation, propagation and termination
 

Chlorination of Methane:

Initiation (At the beginning of the reaction)
Homolytic cleavage of the Cl-Cl bond:


Propagation (Steps 1 and 2 several times in a row)
Step 1 - Abstraction of an H atom by Cl:


Step 2 - Abstraction of a Cl atom by R:


Chain Termination (At the end of the reaction)
Radical-radical combination:

Products of an Alkane Monohalogenation

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

 

Give all the possible products resulting from the mono-chlorination of propane:
 

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Selectivity

Tertiary radicals are more stable and form faster 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 statistics
 

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