How to Determine Conjugation! - UCLA

1 How to Determine conjugation ! Definition: Special stability provided by electron delocalization in three or more adjacent, parallel, overlapping p orbitals. Let's break it down: Special stability=lower electron energy Electron delocalization= electron density is not fixed to one place, it is spread across multiple atoms Three= our magic number/minimum requirement for conjugation Adjacent overlapping p orbitals= p orbitals that are parallel and next to each other. This usually results from (but is not limited to) alternating single and double bonds, which result in partial pi bond behavior (if you're not comfortable with this idea, review resonance structures!) For example: If you see two double bonds separated by a single bond, their orbitals will interact and become conjugated!! o If the double bonds are separated by two sigma bonds, these atoms are not conjugated.

2 O Observe: 1,3 butadiene's double bonds are only separated by one sigma bond whereas 1,4 pentadiene's are separated by two. This means that 1,3 butadiene is conjugated and 1,4 pentadiene is not. o A more accurate representation of 1,3 butadiene is the resonance hybrid, shown on the left below. Notice that all of the carbons are connected by a partial pi bond. That means that all four of these carbons are conjugated! That means that the p orbitals are overlapping, as shown in the image on the bottom right. If you see a nitrogen (or any other atom) with a lone pair/lone pairs that are separated by a sigma bond from a double bond, consider that this nitrogen wants to be conjugated too! o In this case below, the lone pair on nitrogen can be used to create a partial pi bond with carbon. Drawing resonance structures will help you to see which atoms are conjugated.

3 Now, since the oxygen, carbon, and nitrogen (count em, one two three) have partial pi bond behavior, this molecule is conjugated! A quick note: conjugation makes molecules happy. It's like ice cream for little kiddies. Always assume that a molecule wants to be conjugated; however realize that there are cases in which it can't be. (For example if there is too much steric or torsional strain for the molecule to be planar). conjugation is a sub category of resonance. A big difference is that conjugation requires 3 p orbitals while resonance does not. Remember that all conjugated atoms and their immediately attached substituents always lie in the same plane o In the example above, which atoms lie in the same plane? Practice Problems: Thinkbook: (starting page 60) o CFQ's: 1,2,6,9,11 o Owls: 1, 2 o Practice Problems: 1,2,5,11,15 Old midterms: o Fall 2010 #16 o Fall 2011 #17, 18 o Winter 2011 #15, 16, 17 o Spring 2011 Lecture 1 #3 o Spring 2011 Lecture 2 # 1, 12, 15 o Winter 2012 #6,7,8 Works Cited: Dr.

4 Hardinger's Lecture Supplement Google images