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Understanding Conjugated Molecules: Properties, Characteristics, and Reactions, Exams of Stereochemistry

University of Central Florida (UCF)Stereochemistry

The concept of conjugated molecules, their characteristics, and the reactions they undergo. It includes examples of conjugated and non-conjugated molecules, the difference between vinylic and allylic hydrogens, and the mechanism of radical reactions. It also covers the stability of radicals and cations, and the addition reaction of conjugated dienes.

Typology: Exams

2021/2022

Uploaded on 09/27/2022

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Conjugated Molecules
- Conjugated molecules have alternating single and multiple (i.e. double or triple) bonds.
Example 1:
Nomenclature: 2,6-dimethylhepta-2,5-diene
This molecule is not conjugated because it does not have alternating single and multiple
bonds (the arrangement of the bonds starting from C2 is double, single, single and
double). Between the two double bonds, there is a saturated center (C4) and two
intersecting single bonds.
Example 2:
Nomenclature: 2,5-dimethylhexa-2,4-diene
This molecule is conjugated because the arrangement of the bonds starting at C2 is
double, single, and double. They are alternated.
Example 3:
Nomenclature: (2E)-hept-2-en-5-yne
note: (2E) is a stereochemical identifier. The letter “E” indicates the arrangement of the
double bond (where E usually refers to trans and Z refers to cis). The number “2”
indicates the position of the double bond.
This molecule is not conjugated because the single and multiple bonds are not
alternated.
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Conjugated Molecules

  • Conjugated molecules have alternating single and multiple (i.e. double or triple) bonds. Example 1 : Nomenclature: 2,6-dimethylhepta-2,5-diene This molecule is not conjugated because it does not have alternating single and multiple bonds (the arrangement of the bonds starting from C2 is double, single, single and double). Between the two double bonds, there is a saturated center (C4) and two intersecting single bonds. Example 2 : Nomenclature: 2,5-dimethylhexa-2,4-diene This molecule is conjugated because the arrangement of the bonds starting at C2 is double, single, and double. They are alternated. Example 3 : Nomenclature: (2 E )-hept- 2 - en- 5 - yne note : (2 E ) is a stereochemical identifier. The letter “E” indicates the arrangement of the double bond (where E usually refers to trans and Z refers to cis ). The number “ 2 ” indicates the position of the double bond. This molecule is not conjugated because the single and multiple bonds are not alternated.

Example 4 : Nomenclature: (2 Z )-hex- 2 - en- 4 - yne This molecule is conjugated because there is an alternation of multiple and single bonds (double, single, and triple starting from C2). More examples: note : the term conjugation refers to parts of the molecule. If you can find one conjugated system within the molecule, that molecule is said to be conjugated. Example: In this molecule, the double bond A is not conjugated. However, since double bond B is conjugated with double bond C, the molecule is said to be conjugated. Special Nomenclature: The Letter "S" stands for “single” and indicates that we are talking about the conjugated double bonds. The term cis / trans refers to the stereochemistry of the double bonds around the single bond.

The hydrogen directly attached to the double bond carbon is vinylic , whereas the hydrogen attached to the carbon adjacent to the double bond is allylic. The allyl radical Note the condition of reaction is in gas phase. Mechanism of the radical reaction Initiation: Propagation: The chlorine radical formed here can also be used in step 1 of propagation, hence propagate the reaction. Termination: The termination step may produce side product.

In solution, the radical reaction is carried out using N-bromosuccinimide (NBS), which is a source of bromine radical. The radical reaction is carried out only at the allylic position. Why? The allylic radical is resonance stabilized and will be formed more easily than others: The combination of the two resonance form is represented as: The dashed partial bonds represent the bonds that are double in one resonance form and single in the other. The carbon at each end carries ½ of the radical electron density. Orbital description Double bond: a double bond is held together by a bond made up of overlapping sp^2 orbitals and a π bond made from 2 p orbitals overlapping side by side. Methyl radical: a methyl radical is sp 2 hybridized and has one electron occupying the p orbital.

The positive charge is shared equally between the two end carbons. Orbital description As with allylic radical, the allylic cation has all three carbons sp^2 hybridized with overlapping 2 p orbitals. However, there are only 2 electrons delocalized through 3 carbon atoms. Stability of cations: The allylic cation with alkyl groups attached is more stable than allylic cation with only H attached. This again, depends on electron donating ability of R groups. Addition reaction of conjugated dienes Mechanism:

The first step is the protonation of the double bond. There is a choice of an allylic cation which is resonance stabilized, and the un-stabilized primary cation. The allylic cation is formed since it is lower in energy. The resonance description of the allyl cation shows the source of the two products. Question of the day: Why does change in temperature give different yield of the product?