Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328

Brief Organic Chemistry

Summer 2004

Lecture Notes: 10 June

© R. Paselk 2004
 
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Stereoisomerism, cont.

Racemic mixtures: 1:1 mixtures of enantiomers. Such mixtures do not rotate polarized light since one cancels the other.

 

Physical Properties of Stereoisomers

Note that the physical properties of enantiomers are identical to one another, as you might expect. However, the physical properties of diastereomers can vary, and the physical properties of a racemic mixture can be quite different from those of the pure enantiomers (racemic mixtures behave as if they were pure compounds with their own set of physical properties).

 Isomer MP (°C) Specific rotation  Density (g/ml) Solubility @ 20°C (g/100 ml water)
(+) 171-4 +12 1.76 139
(-) 171-4 -12 1.76 139
meso 146-8 0 1.67 125
racemic 206 0 1.79 20.6

 

Alkenes & Alkynes

Today we begin our study of unsaturated hydrocarbons - CH compounds with one or more multiple bonds. THere are three classes of unsaturated hydrocarbons:

Alkenes have one or more double bonds between carbons, thus compared to alkanes they have fewer hydrogens - they are unsaturated. Note the formula for single double-bond alkenes will then be CnH2n (this is the same formula as a single ring alkane, but a cycloalkane is saturated - a carbon chain would need to be broken to add hydrogens). With two double bonds the general formula would then be CnH2n-2, etc.

 
IUPAC Nomenclature: Alkenes are named using a similar set of rules to alkanes, but the infix -en-. Alkynes use the infix -yn-. If a compound has both double and triple bonds it is named as an alkyne. Let's look at an example with an alkene:

Aryl groups. We will look at the chemistry and properties of benzene structures, but we need to look at aryl substituents for nomenclature now.

Cycloalkenes. Cycloalkenes are named just as cycloakanes, except the numbering begins with on of the alkene carbons as number one (chose the carbon giving the lowest number for the first substituent encountered.

Common Names.

 

Alkene and Alkyne Structure

We looked at molecular geometry in Lecture 1, including ethene as an example. Reviewing then:

Cis-trans Stereoisomerism: Remember that stereoisomerism refers to systems of atoms connected in same order and same way, but differering in 3-D geometry.

E,Z Designation: need for double bonds with 3 or 4 different substituents to determine whether "cis or trans" - use sequence rules and E,Z system of nomenclature. Use sequence rules to determine priority of substituents, if highest priority substituents are on same side, then Z (German, zusammen = together), if they are on opposite sides and across double bond then E (German, entgegen = opposite). The sequence rules are:

Cycloalkene cis-trans Stereoisomerism: For small rings (< 7 C's) only the cis- isomers exist. If you try buiding a model of these rings you will see why - for a trans isomer one of the substituents would have to point in toward the center of the ring, distorting the other bonds etc. For eight memnbered rings the trans isomer is possible but still not very stable. Larger rings can accomodate both isomers. Note that all of these rings can still have cyclic cis-trans isomerism in addition to isomerism about the double bond.


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Last modified 10 June 2004