6.7 Nucleophiles and Electrophiles A major focus in this course is on predicting reaction products for ionic reactions and explaining HOW such reactions work Ionic or polar reactions result from the force of attraction between opposite charges Ionic reactions are also guided by the octet rule Consider how methyl chloride and methyl lithium might react Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-1

6.7 Nucleophiles When an atom carries a formal or partial negative charge and an available pair of electrons, it is considered a nucleophile It will love to attack a nucleus. WHY? Explain how the molecules below are nucleophiles What is the difference between a nucleophile and a Lewis Base? NOTHING! Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-2

6.7 Electrophiles When an atom carries a formal or partial positive charge and can accept a pair of electrons, it is considered a electrophile It will love available electrons. WHY? Explain how the molecules above are electrophiles What is the difference between an electrophile and a Lewis Acid? NOTHING! Practice with SkillBuilder 6.2 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-3

6.7 Electrophiles Label all of the nucleophilic and electrophilic sites on the following molecule Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-4

6.8 Mechanisms and Arrow Pushing We use arrows to show how electrons move when bonds break and form It will be a huge benefit in this course to master the skill of arrow pushing There are four main ways that electrons move in ionic reactions 1.Nucleophilic Attack 2.Loss of a Leaving Group 3.Proton Transfers (Acid/Base) 4.Rearrangements Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-5

6.8 Nucleophilic Attack When you identify a nucleophilic site and an electrophilic site, the arrow shows the nucleophile attacking The tail of the arrow starts on the electrons (- charge) The head of the arrow ends on a nucleus (+ charge) The electrons end up being sharing rather than transferred Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-6

6.8 Nucleophilic Attack Nucleophilic attack may appear to occur in two steps The alcohol is the nucleophile in this example. It attacks a carbon with a δ+ charge The second arrow shows the flow of negative charge. WHY is it necessary? The second arrow could be thought of as a resonance arrow. HOW? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-7

6.8 Loss of a Leaving Group Loss of a leaving group occurs when a bond breaks and one atom from the bond takes BOTH electrons For the molecule below, draw the structure that will result after the leaving group is gone Which arrow shows the loss of a leaving group? What is the purpose of the other arrows? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-8

6.8 Proton Transfers Recall from Chapter 3 that a base is protonated when it uses a pair of electrons to take an H + from the acid. The acid retains its electron pair A group can also be deprotonated (sometimes shown by writing –H + over the reaction arrow) or Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-9

6.8 Proton Transfers Multiple arrows may be necessary to show the complete electron flow when a proton is exchanged Such electron flow can also be thought of as a proton transfer combined with resonance Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-10

Carbocations can be stabilized by neighboring groups through slight orbital overlapping called hyperconjugation 6.8 Carbocation Rearrangements Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-11

6.8 Carbocation Rearrangements Hyperconjugation and induction can both be invoked to explain the stability trend below. HOW? If a carbocation can INTRAmolecularly rearrange to become more stable, it will likely do so before reacting with a nucleophile. WHY? Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-12

6.8 Carbocation Rearrangements Two types of carbocation rearrangement are common – Hydride shift – Methyl shift Shifts can only occur from an adjacent carbon. WHY? Do the shifts above make the carbocation more stable? Practice with SkillBuilder 6.3 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-13

6.9 Combining Arrow Pushing Patterns Classify each step in the following mechanism Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-14

6.9 Combining Arrow Pushing Patterns Many times a single step in a mechanism will include more than one arrow pushing pattern Identify the patterns below There are hundreds of mechanisms that involve these key patterns Practice with SkillBuilder 6.4 Copyright © 2015 John Wiley & Sons, Inc. All rights reserved. Klein, Organic Chemistry 2e 6-15