Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328

Brief Organic Chemistry

Summer 2004

Lecture Notes: 8 June

© R. Paselk 2004
 
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Alkanes, cont.

Chemical Properties of Alkanes, cont.

Chemistry: As we noted last time, the alkanes are largely inert (they are sometimes referred to as paraffins because of their lack of affinity or reactivity - polyethylene is a very long chain alkane). Their most important reaction is with with oxygen at high temperatures.

CH3CH3 Æ CH2=CH2 + H2

Note that in each case a source of energy is required to initiate the reaction - we must break the C-C or C-H bonds to allow the reactions to take place.

Sources of Alkanes. Most alkanes are derived from fossil sources: natural gas (a mixture of 90-95% methane, 5-10% ethane, and a variety of other low boiling gases such as propane, butane and isobutane), petroleum or "crude oil" (a liquid mixture of thousands of compounds, mostly hydrocabons, resulting from the breakdown of organisms in sedimentary deposits), and coal (a mix of solid carbon, oils and mineral components, mostly the remians of vast amounts of vegetation deposited in ancient swamps and forests).

The most interesting of these three sources to us is petroleum, the major source of organic fuels for tramsportation (gasoline, aviation fuel, diesel), lubricating oils, and the raw organic materials for most of the synthetic materials of modern society such as plastics, fibers, drugs, dyes, etc.

In order to make these products we first need to separate the various fractions of the petroleum using fractional distillation (see figure in text)

 

Stereochemistry

Stereochemistry is the study of the geometry or three-dimensional structure of molecules and the consequences of this geometry. We've looked at cis-trans isomerism and constitutional isomerism.

Stereoisomerism (geometrical isomerism). Stereoisomers are isomers which have the same formulae and order of connection, but differ in 3-D structure - cis-trans isomers are an example. We now want to look at a very important type of isomers in organic chemistry and particularly in biological systems - mirror image or chiral stereoisomers. These are molecules which have no planes of symmetry, but which look like their mirror images, just as your right hand has no plane of symmetry, but is the mirror image of your left hand. The two members of a chiral pair are called enantiomers.

In chemistry chiral molecules occur most frequently (though not exclusively) as a result of carbon atoms having four different groups attached to them (models). Thus any carbon with four different groups will not have a plane of symmetry through it - it will a local center of chirality (chiral center), a stereogenic center. [examples of chiral molecules: aa's, fats, sugars, cholesterol (3-OH, 5,6-ene,10-methyl, 19-methyl, 17-R]

Notice that the chirality of a molecule resides in the entire molecule, which is caused by having one or more chiral centers. Also, as we'll see later, molecules can have chiral centers without being chiral (a Barbie Doll has chiral hands and feet, but is not itself chiral - its the same as its mirror image!

Fischer Projections: representations of chiral centers as perpendicular sets of lines on a flat surface {see p 412-3 in McMurry}. In this convention of drawing chiral centers it is understood that each center is represented as perpendicular lines, with the vertical pair going back into the plane of the "paper" above and below the chiral center, while the horizontal pair comes out to the plane:

Models

 

 

R,S System

So how do we designate the configuration of a chiral center? Two conventions, the R,S sustem favored by organic chemists, and the older D,L system widely used by biologists and biochemists.

R,S system is based on priority rules:

  1. Each atom bonded to the stereocenter is assigned a priority based on atomic number - the higher the number the higher the prioity
  2. If two atoms have same atomic number go to second atom etc.
  3. multiple bonds count as multiple atoms of that atomic number
  4. R,S is then assigned by looking down bond from center atom to lowest prioity atom
    1. Center is R if priority decreases clockwise (right-handed rotation, as in a right-hand screw thread).
    2. Center is S if priority decreases clockwise (left-handed rotation, as in a left-hand screw thread).

 

Optical Isomers and Light

Optical Activity: It turns out that chiral molecules interact with polarized light in such a way as to rotate the plane of polarization. Thus we can use polarized light in a device known as a polarimeter to characterize chiral substances.

Note that different chiral isomers of the same substance rotate plane polarized light by equal amounts but in opposite directions.

Discussion on the nature of polarized light and its interaction with molecules.


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Last modified 8 June 2004