Organic Introduction to Alkanes AICE Chemistry Organic Introduction to Alkanes

What is Organic Chemistry? This is the area of Chemistry that focuses on compounds consisting of primarily carbon and hydrogen. The are three primary areas of study: Structure Properties Reactions

Alkanes These are the organic compounds that have the generic formula CnH(2n+2) It is this ratio of Carbon to Hydrogen that places an organic compound into the Alkanes category. Examples include the following: CH4 C3H8 C8H18

Where do Alkanes come from? Alkanes, like most of the hydrocarbons, are obtained from the fossil fuels – crude oil production. The student should realize that crude oil (raw material pumped from the ground (or from under the sea bed) is a mixture of very large hydrocarbons. (lots of carbon atoms) The simpler alkanes are obtained by a process called “cracking”.

Cracking ?? This is an industrial process that uses heat to break carbon-carbon bonds in crude oils. The result is a set of “smaller” hydrocarbons that may be separated by the differences in their boiling points. The AICE student may be asked about “cracking” and may also be asked to supply a product in an equation representing a hypothetical cracking reaction. Remember that the Law of Conservation of Mass still applies in organic chemistry.

A sample question regarding cracking. A crude oil has the chemical formula C26H54 . This crude material is “cracked” and it is observed that one of the products has the formula C14H28. Write a balanced equation for the reaction, clearly indicating the identity of the other product. Solution: this is nothing more than subtracting. There is one reactant and there are two products. Simply apply Conservation of Mass. Note – this is your 1st organic reaction !!!

Let’s move on to structure… In alkanes, all of the carbon atoms will be in sp3 hybrids and will form 4 sigma bonds. That means that all of the bond angles will be 109.5 degrees. The carbon atoms will form a “backbone” for the molecule and all of the hydrogen atoms will be on the “outside” of the molecule.

What about names and formulas? The key to this part of the discussion is to realize that Chemistry has assigned prefixes to represent specific numbers of carbon atoms in the molecule. You will simply have to memorize these prefixes and the number of carbon atoms being defined.

So what are these prefixes? C1 = meth C2 = eth C3 = prop C4 = but C5 = pent C6 = hex C7 = hept C8 = oct C9 = non C10 = dec Note that we do not typically write a “1” for a subscript – that is only there to establish the numerical pattern.

Think about the carbon-hydrogen ratio in alkanes to answer the following questions. What is the formula of an alkane that has 5 carbon atoms? How many hydrogen atoms are there in a molecule of octane? (notice the naming – the –ane suffix tells you that it is an alkane) An alkane has 22 hydrogen atoms. What is the name of this alkane? A cracking process starts with C16H34. One of the products has the formula C10H20. What is the name of the other product?

Representing alkanes: There are three different methods that may be used to represent a given alkane. The first is the regular chemical formula method (the Stock system) in which you simply use symbols and subscripts. For example, butane has 4 carbons and 10 hydrogens and would be represented as C4H10

The second method: This is called a structural formula. It is sort of like a regular chemical formula in that it does have symbols and subscripts, but the formula also gives the reader an insight as to how the molecule is assembled. In the case of butane, the structural formula would be: CH3CH2CH2CH3

And finally… We have a “displayed” formula. This method of representing alkanes will have you actually drawing the molecule to illustrate the bonding. In the case of alkanes, it is not typically necessary to show the actual bond angles.

A quick check: Write the chemical formula for a molecule of hexane. Write the structural formula for a molecule of pentane. Write the displayed formula for a molecule of heptane. What is the name of the compound illustrated below? H H H H H H H H C C C C C C C H H H H H H H H

What if the organic molecule has a “branched-chain” structure? The naming here gets a bit more complicated because you have to account for the actual structure of the branch itself and indicate the location of that new addition to the molecule. Now go to one of the computers, copy the following web address into a browser, and read through the page presented. Then work through the interactive exercise on naming branched chain alkanes. www.sciencegeek.net/APchemistry/organic/ochem.shtml