Lecture_II_Day

Lecture_II_Day - Organic Chemistry 308, Lecture II, Chapter...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon
Organic Chemistry 308, Lecture II, Chapter 14 1 ORGANIC CHEMISTRY 308 LECTURE II CHAPTER 14 So far, in approaching the chemistry of functional groups, we have focused directly on the functional group. But in many cases the chemistry of the functional group manifests itself in the way in which the functional group influences nearby parts of the molecule or interacts with other nearby functional groups. Such effects are especially important when the interactions take place through unhybridized p-orbitals on sp 2 or sp hybridized atoms. When three or more such adjacent atoms have unhybridized p-orbitals through which such interactions take place, the system is said to be conjugated and the interactions are π interactions . I. Allylic Systems. The simplest conjugated systems are ones in which there are parallel p-orbitals on three adjacent carbons. Remember for π interactions the p-orbitals must be parallel. These systems arise by forming a reactive intermediate on a carbon next to a double bond. They can be described most easily in one of two ways. In the valence bond approach, we draw two contributing structures in which the reactive site is distributed between the 1,3 carbons.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Organic Chemistry 308, Lecture II, Chapter 14 2 In the MO approach, we construct molecular orbitals by overlap of the 3 parallel unhybridized p orbitals. Three atomic orbitals give three molecular orbitals. We then fill these orbitals (each holds two electrons) in order of increasing energy. (the Aufbau principle). Notice that even in the anion, which has four electrons; it is not necessary to use the antibonding orbital. Let’s look at some of the chemical consequences of the stabilization by a double bond of an adjacent reactive center. A. Halogenation. As we know halogens add electrophilically to alkenes to form vicinal dihalides. But with low concentrations of halogens and elevated temperatures, the reaction mechanism changes and a different product is formed.
Background image of page 2
Organic Chemistry 308, Lecture II, Chapter 14 3 a. There are two reasons for this change. At low concentrations of halogen and high temperatures, the concentration of halogen radicals is relatively high and any vicinal dihalide that forms by electrophilic addition can dehalogenate back to the alkene. b. The mechanism of the free radical halogenation is the same as for alkanes. The X . abstracts the H at the weakest C-H (the one that leads to the most stable free radical. in this case the allylic free radical) In the second propagation step, the allylic radical reacts with another Br 2 continuing the chain. Notice that this reaction is synthetically useful, only when all allylic hydrogens are equivalent (as in propene or cyclohexene) and when the two resonance contributors are exactly the same. c. Normally this reaction is carried out using a reagent
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 04/04/2008 for the course CHEM 308 taught by Professor Boikess during the Spring '08 term at Rutgers.

Page1 / 23

Lecture_II_Day - Organic Chemistry 308, Lecture II, Chapter...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online