|Lecture Notes: 4 June||
Lewis Definition: In this definition:
Note in the following reaction that although the dimethyl ether is both a Brønsted (it could accept a proton) and Lewis base, boron trifluoride is only an acid in the Lewis sense:
Alkanes are also known as aliphatic compounds because of their fat-like properties (oily, water repellent). They are saturated hydrocarbons. That is they are compounds of just hydrogen and carbon with the maximum number of hydrogens/carbon.
You should know (memorize) the first 12 straight chain (normal) alkanes: methane (C1), ethane (C2), propane (C3), butane (C4), pentane (C5), hexane (C6), heptane (C7), octane (C8), nonane (C9), decane (C10), undecane (C11), and dodecane (C12). Note that they all have the formula CnH2n+2.
Note that they may be drawn as structural formulae (bonds shown as lines), condensed structural formulae in which the bonds are understood, or mixtures of the two. Be sure you can draw both types and be able to translate between them. Note also that a "straight chain" does not have to be, nor is it always preferably, drawn straight!
Alkanes may also be branched, to form isomers: compounds having the same formula (same numbers of atoms), but different structures (linked in different ways). The smallest alkane with isomers is then Butane. It can be linear, or there can be a one carbon branch in the center:
Isomers like these, where the atoms are connected in different order, are known as constitutional isomers. We will see other, more subtle types of isomer later. Notice that the number of isomers grows very rapidly with the number of carbons. As seen in table on pg. 59 of your text there are over 4 billion triacontanes (30 C alkane isomers). You will get a chance to explore isomerism in the lab this week and next week. These lab materials will not be collected, but they will be on exams!
Alkyl groups: When saturated hydrocarbon chains occur as side chains on larger molecules we refer to them as alkyl groups. Thus methane becomes methyl, ethane becomes ethyl, propane becomes propyl, etc. But now propane can give rise to two propyl groups, as shown below:
Similarly we see four butyl groups, two from butane and two from isobutane:
Note the use of sec- and tert- denoting secondary and tertiary carbons at the attachment points. These terms refer to the number of carbons bonded to a "central" carbon: a primary (1°) carbon has a single carbon bonded to it; a secondary (2°) carbon has two carbons bonded to it; a tertiary (3°) carbon has three carbons bonded to it; and a quaternary (4°) carbon has four carbons bonded to it. Hydrogens bonded to these carbons are given the same designations (1°-3° hydrogens).
In order to name the amazing complexity of organic molecules the International Union of Pure and Applied Chemistry (IUPAC) has created a rational systematic nomenclature for them. In the IUPAC system each chemical name has three parts:
For now we will focus on alkanes, so we only need one suffix: -ane
To name an alkane then we can follow the four steps below (or the more detailed outline in your text on pp. 60-62):
Note that nomenclature is also largely based on Functional groups. As a general rule the most highly oxidized carbon will take precedence in naming. Thus, as we shall see later, a compound with both an acid group and an alcohol group will be named as an acid, etc. Of course with as complex a system as organic chemistry there will be violations to this general rule.
When naming a compound the name is com[posed of three parts: the prefix, the infix, and the suffix:
Of course things will get a bit more complex when there is more than one functional group as we shall see later.
Alkane chains with three or more carbons can also exist in ring forms, where the two terminal carbons have bonded together, displacing two hydrogens. Thus they have the general formula CnH2n. We can represent them most readily as polygons using what is known as skeletal drawings.
In these drawings only C-C bonds are shown with each intersection of two lines and each line end representing a C. Hydrogens are not shown - the viewer adds a sufficient number to give 4 bonds to each carbon. All other elements are shown.
Last modified 26 February 2006