Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 328	Brief Organic Chemistry	Summer 2004
Lecture Notes: 10 June	© R. Paselk 2004
PREVIOUS		NEXT

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).

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 - compounds with one or more double bonds,
• Alkynes - compounds with one or more triple bonds,
• Arenes - compounds containing one or more benzene rings.

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 C_nH_2n (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 C_nH_2n-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:

• First find the longest chain and name as an alkene. Thus a compound with 9 carbons in its longest chain would be a nonene.
• Number the chain starting at the end nearest the double bond. If the double bond is in the center, then start at the end with the lowest number branch point.

 

• Write the full name with substituents etc. as for alkanes. Note with multiple double bonds use special suffixes: -diene, -triene, etc.

Note that the position of each double bond is numbered before the suffix in the same way substituents are.

• Cycloalkenes are named similarly, but the double bond carbons are numbered 1 & 2, since there are no terminii.
• CH₂CHCCH = 1-Buten-3-yne

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

• Aryl groups in general are given the symbol Ar- as opposed to R-
• A benzene ring substutuent is called a phenyl group = Ph- = C₆H₆- =

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.

• For single double bond cycloalkene the number one is assumed in the name, e.g. 3-methylcyclohexene
• For multiple double bonds both bonds are numbered, e.g. 1,4-cycloheptadiene

Common Names.

• A variety of compounds are known almost exclusively by common names, e.g.: ethylene (ethene), propylene (propene), isobutylene 2-methylpropene), acetylene (ethyne).
• Two and three carbon alkenyl groups are also often known by their common names: vinyl = CH₂CH- (ethenyl, e.g. vinyl alcohol), allyl = CH₂CHCH₂- (propenyl, e.g. allylchloride)
• Acetylene is often used as a base for acetylene derivatives, e.g. methylacetylene = propyne

Alkene and Alkyne Structure

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

• Alkenes: sp² overlaps to give sigma bonds at 120°, p-p overlaps to give pi bonds.
  • Thus alkenes have a planar structure across the double bond.
  • Note that the pi bonding lobes are exposed and electron rich!
• Alkynes: sp overlaps to give sigma bonds at 180°, p-p overlaps to give pi bonds.
  • Thus alkenes have a linear bond.
  • Note that the pi bonding lobes are exposed and electron rich!

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

• Like a cyloalkane, an alkene has two sides, so we can again get cis-trans isomerism. These isomers are quite stable, but can be interconverted using acid and a catalyst.
• Under these conditions the trans isomer will come to predominate since it has less steric hindrance.

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:

• First look at atoms directly attached to carbons of double bond - highest atomic number = highest priority. [examples: hydroxide vs. amine vs. methyl]
• If all aubstituents can't be prioritized by atomic number (two or more are the same), then go to the second atom on these groups and prioritize as before. Multiple atoms with same priority count more than single atoms. [examples: ethyl vs. methyl, alcohol vs. ethyl, isopropyl vs. ethyl]
• Multiple bonded atoms are counted with each bond = a single bonded atom. [ethenyl vs. ethyl, cabonyl vs alcohol]

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.

Schedule

C328 Home

Lecture Notes

Last modified 10 June 2004