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The Structure of Alkanes




What is Alkanes ?

** Alkanes are saturated hydrocarbons; that is, they contain only carbon-carbon single bonds.

** In this context, saturated means that each carbon has the maximum number of hydrogens bonded to it.

** We often refer to alkanes as aliphatic hydrocarbons because the physical properties of the higher members of this class resemble those of the long carbon-chain molecules we find in animal fats and plant oils (Greek: aleiphar, fat or oil).

Classification of Hydrocarbon

** A hydrocarbon that contains one or more carbon-carbon double bond, triple bond, or benzene ring is classified as an unsaturated hydrocarbon.

** the figure shows the Classification of Hydrocarbon


The Structure of Alkanes

** Methane (CH4) and ethane (C2H6) are the first two members of the alkane family.

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



** The shape of methane is tetrahedral and all H-C-H bond angles are 109.5°. Each carbon atom in ethane is also tetrahedral, and all bond angles are approximately 109.5°.

** Although the three-dimensional shapes of larger alkanes are more complex than those of methane and ethane, the four bonds about each carbon are still arranged in a tetrahedral manner, and all bond angles are approximately 109.5°.

**The next three alkanes are propane, butane, and pentane.

** In the following representations, these hydrocarbons are drawn first as condensed structural formulas that show all carbons and hydrogens. They are also drawn in an even more abbreviated form called a line-angle formula.

** 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

** We can write structural formulas for alkanes in still another abbreviated form.

** The structural formula of pentane, for example, contains three CH2 (methylene) groups in the middle of the chain. We can collect them and write the structural formula of pentane as CH3(CH2)3CH3.

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



** Note that the names of all these alkanes end in -ane. We will have more to say about naming alkanes in the following lessons.

General molecular formula of Alkanes

** Alkanes have the general molecular formula CnH2n+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

** Constitutional isomers are compounds that have the same molecular formula but different structural formulas.

** By “different structural formulas,” we mean that constitutional isomers differ in the kinds of bonds they have (single, double, or triple) and/or in the connectivity of their atoms.

** For the molecular formulas CH4, C2H6, and C3H8, only one connectivity is possible.

** 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.



** Butane and 2-methylpropane are constitutional isomers; they are different compounds and have different physical and chemical properties. Their boiling points, for example, differ by approximately 11°C.

** To determine whether two or more structural formulas represent constitutional isomers (that is, different compounds with the same molecular formula), write the molecular formula of each and then compare them.

**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



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