The Structure of Alkanes

What is Alkanes ?

Classification of Hydrocarbon

** the figure shows the Classification of Hydrocarbon

The Structure of Alkanes

** Figure shows Lewis structures and molecular models for Methane and Ethane.

** In a line-angle formula, each vertex and line ending represents a carbon atom. Although we do not show hydrogen atoms in line-angle formulas, we assume that they are there in sufficient numbers to give each carbon four bonds.

Structural formulas for Alkanes

** Table gives the names and molecular formulas of the first 20 alkanes.

General molecular formula of Alkanes

** Alkanes have the general molecular formula C_nH_2n+2.

**Thus, given the number of carbon atoms in an alkane, we can determine the number of hydrogens in the molecule and its molecular formula. For example, decane, with 10 carbon atoms, must have ( 2 × 10) + 2 = 22 hydrogen atoms and a molecular formula of C₁₀H₂₂.

Constitutional isomers of alkanes

** For the molecular formula C₄H₁₀, two connectivities are possible. In one of these, named butane, the four carbons are bonded in a chain; in the other, named 2-methylpropane, three carbons are bonded in a chain with the fourth carbon as a branch on the chain.

**All compounds that have the same molecular formula, but different structural formulas (different connectivities of their atoms), are constitutional isomers.

Problems

(1) Do the condensed formulas in each pair represent the same compound or constitutional isomers.

Solution

(a) The molecules are drawn here as both condensed structural formulas and line angle formulas. Each formula has an unbranched chain of six carbons; the two are identical and represent the same compound.

(b) Each formula has a chain of five carbons with two -CH₃ branches. Although the branches are identical, they are at different locations on the chains; these formulas represent constitutional isomers

(2) Write line-angle formulas for the five constitutional isomers with the molecular formula C₆H₁₄ 

Solution

In solving problems of this type, you should devise a strategy and then follow it. Here is one such strategy: 

First, draw a line-angle formula for the constitutional isomer with all six carbons in an unbranched chain.

Then, draw line-angle formulas for all constitutional isomers with five carbons in a chain and one carbon as a branch on the chain.

Finally, draw line-angle formulas for all constitutional isomers with four carbons in a chain and two carbons as branches.

No constitutional isomers with only three carbons in the longest chain are possible for C₆H₁₄.

Number of constitutional isomers in Alkanes

** The ability of carbon atoms to form strong bonds with other carbon atoms results in a staggering number of constitutional isomers.

** As the table shows, there are 3 constitutional isomers with the molecular formula C₅H₁₂, 75 constitutional isomers with the molecular formula C₁₀H₂₂, and almost 37 million with the molecular formula C₂₅H₅₂. 

** for even a small number of carbon and hydrogen atoms, a very large number of constitutional isomers is possible. Because constitutional isomers have different chemical properties, a rich diversity of chemistry is possible within these sets.

Reference: Organic chemistry / William H. Brown , Christopher S. Foote , Brent L. Iverson , Eric V. Anslyn , Bruce M. Novak . ( sixth edition) . United States