Understanding chemical reactions using electronegativity and resonance — Master Organic Chemistry

Blog Blog

Online Organic Chemistry Tutoring » Online Organic Chemistry Tutoring » Study Aids » Study Aids » Reaction Guide Reaction Guide Resources Resources Feedback  Feedback 

Master Organic Chemistry Master Organic Chemistry

Getting Started Getting Started Organic 1 Organic 1 Organic 2 Organic 2

Study And Exam Tips Study And Exam Tips Summary Sheets Summary Sheets Thoughts On O-Chem Thoughts On O-Chem Feedback  Feedback 

Ho

How to apply

w to apply electron

electronegativity a

egativity and reson

nd resonance to

ance to

understand reactivity

understand reactivity

iinn Drawing Reaction MechanismsDrawing Reaction Mechanisms,, Organic Chemistry 1Organic Chemistry 1,, Understanding Electron FlowUnderstanding Electron Flow,, Where Electrons AreWhere Electrons Are One thing

One thing has beehas been missing from the discussion of resonance.n missing from the discussion of resonance. What’s the point?What’s the point?

Who care

Who cares if we can writes if we can write out resonance structures? What does it matter if we can figure out the two or threeout resonance structures? What does it matter if we can figure out the two or three most stab

most stable resonle resonance structuance structures? res? So So whwhat?at? Here’s th

Here’s the point: we ce point: we can apply resonance (and electronegativity) to figure out the electron densities of an apply resonance (and electronegativity) to figure out the electron densities of  mole

molecules from first princicules from first princi pl ples,es, and we can apply these electron densities toward understanding how aand we can apply these electron densities toward understanding how a molecule will react.

molecule will react.

Put it ano

Put it another wayther way: if you learn this skill, you will rely: if you learn this skill, you will relyless on memorizationless on memorizationfor understanding reactions,for understanding reactions,

because

because you’ll be abyou’ll be able to figure out the chemical behavior of molecules you’ve never seen before.le to figure out the chemical behavior of molecules you’ve never seen before.

For instan

For instance:ce: if you’re a non-chemistry major I can pretty much guarantee you’ve never seen this reactionif you’re a non-chemistry major I can pretty much guarantee you’ve never seen this reaction  bef

 before. Bore. Buut it if f yyou ou applapply y somsome of e of ththe prie prinnciciplples ies in n ththiis posts post, y, you ou shshououlld be abld be able to me to make somake some he headweadway ay on on iit.t.

Let’s look at these two aspects really quickly. Let’s look at these two aspects really quickly.

1.

1. ApApplplyinying eg e lectronegativitieslectronegativities.. When you have a bond between two atoms with differentWhen you have a bond between two atoms with different

Now av

electronegativities, there will be a dipole (two opposite charges separated in space). That dipole will give you a clue about the electron densities of those two atoms. For example in the molecule below, the oxygen is more electronegative than carbon which means that the C–O bond will be polarized towards oxygen (it will have a higher electron density). This is different than formal charge, which is where we have to assign a charge to an atom for “accounting” purposes.

2. Applying resonance: when you know the most stable two (or three) resonance forms, you’ll have a good idea of what the resonance hybridlooks like. The resonance hybrid also tells you electron densities, sometimes in a way that isn’t immediately apparent from electronegativity (see below).

Here’s some examples of resonance hybrids, along with the electron densities we get from applyingboth

electronegativity and resonance. In the picture, the partial charges (δ) represent electron densities on the hybrid.

 Now for the punch line.

Once you know the partial charges on a molecule, you can then use it to figure outpotential chemical reactivity.How so?

Remember the “one sentence summary of chemistry”:opposite charges attract, like charges repel.

So any region of negative charge on a molecule will have some degree of attraction to aregion of  positive charge on another molecule. In reactions electrons flow from areas of high electron density to low electron density.Another way of putting it: the partial negative charge (i.e. high electron density) will go to a region of partial positive charge (i.e. low electron density).

So in the diagram below I’ve put down some of the resonance hybrids (along with other molecules), and drawn a selection of the interactions between the opposite charges. Although these arrows do not necessarily representactual reactions (although many do!) they at least represent potentially feasible reactions.

The key take-home skill from these examples is to be able so see how the resonance hybrid will determine electron density, and how this can end up leading to hypotheses for feasible reactions.

Let’s go back to the original question:

By applying electronegativity, we can judge that the C–Zn bond will be polarized towards carbon, which makes it electron rich; it should be attracted to the carbon of the second molecule, which both

electronegativity and resonance tell us should bear a partial positive charge. In fact this is a real reaction, althoughwe can’t fully determine how well a reaction will work from first principles.Experimental evidence is the one and only arbiter as to whether a reaction works or not.

Is this technique perfect, without exceptions? No.It’s not perfect. It’s not completely without

exceptions.* But it’s a good mental model for the underlying principles of chemical reactivity. The point here is to give you a glimpse of how to apply the concepts of electronegativity and resonance towards new and unfamiliar situations.

*Two prominent exceptions: electronegativity isn’t the best for figuring out the reactivity of nitrile ion (CN(–) and oxymercuration of alkenes. It doesn’t predict reactivity of Cl-Cl and Br-Br, etc. which are not polarized. **Note that this model doesn’t tell you howreactive different species will be. That will require another set of 

mental models.

PS – a long enough post as it is, but here are some “unproductive” interactions from the diagram above.

0 0 0 Tweet Tweet 2

Related Posts:

7 Factors That Stabilize Positive Charge in Organic Chemistry Evaluating Resonance Forms (4): Positive Charges

Evaluating Resonance Forms (1) – The Rule of Least Charges Introduction to Resonance (2) : Curved Arrows!!

Tagged as: charges, electronegativity, formal charge, opposite charges attract, resonance { 9 comments… read them below or add one }

azmanam January 17, 2012 at 1:22 pm

7 Share

Did you mean to have H’s on the enol and protonated(?) ketone in the middle row of your examples? The methyl group looks… odd. :)

Reply

james January 17, 2012 at 2:07 pm Thanks!… fixed.

Reply

dr klbajaj January 19, 2012 at 1:22 am

FROM THE REACTION BETWEEN DIETHYLZINC AND ENAMINE THERE IS BOND FORMATIONBETWEEN C-C RESULTING INTO AN INTERMEDIATE CONTAING ZINC.IT WOULD HAVE BEEN MUCH BETTER TO SHOW HOW THE BREAKING OF

BOND BETWEEN C AND ZINCTAKES PLACE.THANKS FOR NICE EXPLANATION FOR  USE OF RESONANCE AND ELECTRNEGTIVITY IN REACTION MECHANISM

Reply

james January 19, 2012 at 11:04 pm

True, I didn’t include the specific details of the arrows because they hadn’t been introduced in this series yet and the exact details weren’t important ; just wanted to show that this was a *plausible* reaction. Sorry if this wasn’t clear.

Reply

dr klbajaj January 24, 2012 at 4:42 pm Iagree woth you.thanks

Reply

vu quoc January 19, 2012 at 4:15 pm thanks for this topic. It is good.

Reply

Thanks! Reply

Joseph McClaren January 21, 2012 at 7:25 pm

‘The formal charge of + 1 on oxygen does not represent electron density’ Thank You! I think this is something that confuses a lot of students.

Great article, very nice summary.

I’d love to see more examples of working through the logic of where the electron density is. This is essential for understanding how and when reactions will take place.

Reply

james January 22, 2012 at 2:52 am

Talked about it a bit here, but it didn’t involve resonance.

http://masterorganicchemistry.com/2011/11/15/how_to_use_electronegativity/

I’ve been meaning to write a post called “Formal Charge has its plusses and minuses…” Reply Leave a Comment  Name * E-mail * Website Submit

Sign Up To Get Weekly Sign Up To Get Weekly Organic Chemistry Tips Organic Chemistry Tips

And get the free "How To Succeed And get the free "How To Succeed In Organic Chemistry" E-book In Organic Chemistry" E-book

enter your name enter your email Subscribe!

Reagent Guide

Currently logged out

You are not currently logged in. Username:

Password:

Remember Me

Login

» Register 

» Lost your Password?

Search

To search, type and hit en

Get The Latest Updates

facebook  twitter  rss email

About Master Organic Chemistry

Imagine having a comprehensive online guide to help you solve your own problems in organic chemistry. That's my mission with this site. After earning a Ph.D. at McGill and doing a postdoc at

MIT, I applied to be a professor. That didn't work out. So I decided to teach organic chemistry anyway. Master Organic Chemistry is the site I wish I had when I was learning the subject.

Recent Posts Popular Posts Recent Comments

Hydroboration of Alkenes: The MechanismPosted on: Mar 28, 2013

Hydroboration of AlkenesPosted on: Mar 22, 2013

Alkene Addition Pattern #2: The "Three-Membered Ring" PathwayPosted on: Mar 20, 2013

Bromination of Alkenes: The MechanismPosted on: Mar 15, 2013

Common Mistakes: Drawing Tetrahedral CarbonsPosted on: Mar 12, 2013

Categories

Drawing Reaction Mechanisms Esters Functional Groups General Chemistry Ketones  Nomenclature Off Topic

Organic Chem Study Tips Organic Chemistry 1 Organic Chemistry 2 Organic Reactions Organic Reagents Spectroscopy Stereochemistry Summary Sheets

Synthesis Teaching

Thermodynamics and Kinetics Uncategorized

Understanding Electron Flow Where Electrons Are

Copyright © 2013 MasterOrganicChemistry.com. All rights reserved

“Reagents”: A Pocket Encylopedia of Reagents for Organic Chemistry About James

Confirming your email address… Daily Organic Chemistry Tips Dashboard

Feedback  Getting Started

How To Succeed In Organic Chemistry Mission

 NEW! Reagents App

Online Organic Chemistry Tutoring Org 2 Post Index

Organic 1 Organic 2

Organic Chemistry Study Advice Post Index

Post Index 2 Post Index Draft  post index draft 2

Premium Products

Reagent Guide Reaction Guide

1,4-addition of enolates to enones (“The Michael Reaction”) 1,4-addition of nucleophiles to enones

1,4-addition of organocuprates (Gilman reagents) to enones Acidic cleavage of ethers (SN2)

Addition of aqueous acid to alkenes to give alcohols

Addition of Dichlorocarbene to alkenes to give dichlorocyclopropanes Addition of dichloromethylene carbene to alkenes

Addition of Grignard reagents to aldehydes to give secondary alcohols Addition of Grignard reagents to esters to give tertiary alcohols

Addition of Grignard reagents to formaldehyde to give primary alcohols Addition of Grignard reagents to ketones to give tertiary alcohols

Addition of Grignard reagents to nitriles to give ketones (after hydrolysis) Addition of HBr once to alkynes to give alkenyl bromides

Addition of HBr to Alkenes

Addition of HBr twice to alkynes to give geminal dibromides Addition of HCl once to alkynes to give alkenyl chlorides Addition of HCl to Alkenes to Give Alkyl Chlorides

Addition of HCl to alkynes twice to give geminal dichlorides Addition of HI once to alkynes to give alkenyl iodides

Addition of HI twice to alkynes to give geminal diiodides

Addition of Hydroiodic Acid to Alkenes to Give Alkyl Iodides Addition of LiAlH4 to aldehydes to give primary alcohols Addition of LiAlH4 to ketones to give secondary alcohols Addition of NaBH4 to aldehydes to give primary alcohols Addition of NaBH4 to ketones to give secondary alcohols

Addition of organocuprates (Gilman reagents) to acid chlorides to give ketones Addition to alkenes accompanied by 1,2-alkyl shift

Additions to alkenes accompanied by 1,2-hydride shifts Aldol addition reaction of aldehydes and ketones

Aldol Condensation

Alkylation of enamines with alkyl halides Alkylation of enolates

Allylic bromination of alkanes using NBS Baeyer-Villiger Reaction

Base-promoted formation of enolates from ketones Basic hydrolysis of esters (saponification)

Bromination of alkenes with Br2 to give dibromides Bromination of aromatic alkanes to give alkyl bromides Bromination of Aromatics to give Bromoarenes

Chlorination of alkenes with Cl2 to give vicinal dichlorides Chlorination of Arenes to give Chloroarenes

Claisen Condensation of esters

Cleavage of ethers using acid (SN1 reaction)

Clemmensen Reduction of Ketones/Aldehydes to Alkanes Conversion of acid chlorides to aldehydes using LiAlH(O-tBu)3 Conversion of acid chlorides to esters through addition of an alcohol Conversion of alcohols to alkyl bromides using PBr3

Conversion of alcohols to alkyl chlorides using SOCl2 Conversion of alcohols to alkyl halides using HCl Conversion of Alkyl halides to ethers (SN1)

Conversion of carboxylic acids into acid chlorides with SOCl2 Conversion of carboxylic acids to carboxylates using base

Conversion of carboxylic acids to esters using acid and alcohols (Fischer Esterification) Conversion of tertiary alcohols to alkyl bromides using HBr 

Conversion of tertiary alcohols to alkyl iodides with HI Conversion of thioacetals to alkanes using Raney Nickel Curtius Rearrangement of Acyl Azides to Isocyanates Decarboxylation of beta-keto carboxylic acids

Dehydration of amides to give nitriles

Deprotonation of alcohols to give alkoxides

Deprotonation of alkynes with base to give acetylide ions Diels Alder Reaction of dienes and dienophiles

Dihydroxylation of Alkenes to give 1,2-diols (vicinal diols)

Dihydroxylation of alkenes with cold, dilute KMnO4 to give vicinal diols Elimination (E1) of alkyl halides to form alkenes

Elimination (E1) with hydride shift

Elimination (E2) of alkyl halides to give alkenes Elimination of alcohols to give alkenes using POCl3

Elimination of water from alcohols to form alkenes using acid Formation of Acetals from Aldehydes and Ketones

Formation of alkynes through double elimination of vicinal dibromides Formation of amides from acid chlorides and amines

Formation of Amides Using DCC

Formation of anhydrides from acid halides and carboxylates Formation of Bromohydrins from alkenes using water and Br2 Formation of bromohydrins from alkenes using water and NBS Formation of Carboxylic Acids from Acyl Chlorides

Formation of carboxylic acids from Grignard reagents and CO2 Formation of chlorohydrins from alkenes using water and Cl2 Formation of Cyanohydrins from ketones and aldehydes

Formation of cyclopropanes from alkenes using methylene carbene (:CH2) Formation of Diazonium Salts from Aromatic Amines

Formation of enamines from ketones/aldehydes and secondary amines Formation of epoxides from alkenes using m-CPBA

Formation of epoxides from bromohydrins

Formation of Gilman reagents (organocuprates) from alkyl halides Formation of Grignard Reagents from Alkenyl Halides

Formation of Grignard Reagents from Alkyl Halides Formation of hydrates from aldehydes/ketones and H2O Formation of imines from primary amines and ketones Formation of organolithium reagents from alkyl halides Formation of thioacetals from aldehydes and ketones Formation of tosylates from alcohols

Free Radical Bromination of Alkanes Free Radical Chlorination of Alkanes Friedel Crafts alkylation of arenes

Friedel-Crafts acylation of aromatic groups to give ketones Hell-Vollhard-Zelinsky Reaction

Hofmann elimination of alkylammonium salts to give alkenes Hofmann Rearrangement of Amides to Amines

Hydroboration of Alkenes

Hydroboration of alkynes using BH3 to give aldehydes Hydrogenation of Alkenes to give Alkanes

Hydrogenation of Alkynes to Alkanes using Pd/C Hydrolysis of acetals to give aldehydes and ketones

Hydrolysis of esters to carboxylic acids with aqueous acid Hydrolysis of imines to give ketones (or aldehydes)

Hydrolysis of nitriles with aqueous acid to give carboxylic acids Iodination of alkenes to give vicinal diiodides (1,2-diiodides) Iodination of Aromatics with I2

Keto-enol tautomerism  Nitration of aromatic groups

Opening of epoxides with acid and water to give trans diols Opening of epoxides with nucleophiles under acidic conditions

Oxidation of aldehydes to carboxylic acids using Cr(VI) Oxidation of aldehydes to carboxylic acids with Ag2O

Oxidation of aromatic alkanes with KMnO4 to give carboxylic acids Oxidation of primary alcohols to aldehydes

Oxidation of Primary Alcohols to Aldehydes using PCC Oxidation of primary alcohols to carboxylic acids

Oxidation of secondary alcohols to ketones using PCC Oxidation of thiols to disulfides

Oxidative cleavage of 1,2-diols to give aldehydes/ketones

Oxidative cleavage of alkenes to give ketones/carboxylic acids using ozone (O3) – (“oxidative workup”)

Oxidative cleavage of alkenes to ketones/carboxylic acids using KMnO4 Oxidative Cleavage of Alkynes with KMnO4

Oxidative Cleavage of Alkynes with Ozone (O3)

Oxymercuration of Alkenes to form Ethers using Hg(OAc)2 Oxymercuration of Alkynes

Oxymercuration: Alcohols from alkenes using Hg(OAc)2 and Water  Ozonolysis of alkenes to ketones and aldehydes (reductive workup) Partial reduction of alkynes to trans alkenes using sodium and ammonia Partial reduction of alkynes with Lindlar’s catalyst to give cis alkenes Pinacol Rearrangement

Polymerization of dienes with acid Protection of alcohols as silyl ethers

Protonation of alcohols to give oxonium ions Protonation of Grignard reagents to give alkanes

Reaction of alkyl halides with water to form alcohols (SN1) Reaction of epoxides with nucleophiles under basic conditions Reactions of Diazonium Salts

Reduction of aromatic ketones to alkanes with Pd/C and hydrogen Reduction of aromatic nitro groups to amino groups

Reduction of carboxylic acids to primary alcohols using LiAlH4 Reduction of esters to aldehydes using DIBAL

Reduction of esters to primary alcohols using LiAlH4 Reduction of nitriles to primary amines with LiAlH4 Reductive Amination

SN2 of Cyanide with Alkyl Halides to give Nitriles

SN2 reaction between azide ion and alkyl halides to give alkyl azides SN2 Reaction of Acetylide Ions with Alkyl Halides

SN2 reaction of alkoxide ions with alkyl halides to give ethers (Williamson synthesis) SN2 reaction of alkyl halides with hydroxide ions to give alcohols

SN2 reaction of amines with alkyl chlorides to give ammonium salts SN2 reaction of carboxylate ions with alkyl halides to give esters SN2 reaction of hydrosulfide ion with alkyl halides to give thiols

SN2 reaction of organocuprates (Gilman reagents) with alkyl halides to give alkanes SN2 reaction of thiolates with alkyl halides to give thioethers (sulfides)

SN2 reaction of water with alkyl halides to give alcohols Substitution (SN1) with hydride shift

Substitution with accompanying alkyl shift Sulfonylation of Arenes to give sulfonic acids

The Gabriel synthesis of amines

The haloform reaction: conversion of methyl ketones to carboxylic acids The Malonic Ester Synthesis

The Robinson Annulation

Transesterification promoted by alkoxides

Wittig Reaction – conversion of ketones/aldehydes to alkenes

Wolff Kishner Reaction – conversion of ketones/aldehydes to alkanes Wolff Rearrangement

Resource Guide Resources

Signup for the Reaction Guide Study and Exam Tips

Summary Sheets testtypg

Thanks for Joining!

Thanks! You’re signed up for the newsletter. Thoughts On O-Chem

Videos

A Simple Trick For Determining R/S

Applying E2 Reactions with Newman Projections Bond Rotations: Exercise 1

Bond Rotations: Exercise 2 Bond Rotations: Exercise 3 Bond Rotations: Exercise 4 Bond Rotations: Exercise 5

Bond Rotations: The “Steering Wheel” Analogy Bronsted and Lewis Acidity

Bulky Bases in Elimination Reactions Carbocation Stability

Comparing E1 and E2 Mechanisms Comparing E1 and E2 Stereochemistry Comparing the E1 and SN1

Comparing the SN1 and SN2

Converting a Line Diagram to a Fischer Projection Converting a Line Diagram to a Fischer Projection Converting a Newman Projection to a Line Diagram Curved Arrows

Determining R/S on a Fischer Projection E1 with Rearrangement

E1 With Rearrangement (2)

Elimination Exercise: Zaitsev’s Rule Elimination Reactions in Cyclohexanes Elimination Reactions in Cyclohexanes (2) Evaluating Resonance Forms (1) Charges Evaluating Resonance Forms (2) Octets

Evaluating Resonance Forms (3) Negative Charge Evaluating Resonance Forms (4) Positive Charge Evaluating Resonance Forms (5) Aromaticity Exercise: Condensed Formula (1)

Exercise: Condensed Formula (2)

Factors That Affect Acidity (1) Charge Density Factors That Affect Acidity (2) Electronegativity Factors That Affect Acidity (3) Polarizability

Factors That Affect Acidity (4) Electron Withdrawing Groups Factors That Affect Acidity (4) Resonance

Factors That Affect Acidity (6) – Orbitals Factors that affect acidity – Aromaticity Formal Charge (1) – Atomic Charge

Formal Charge (2) – Introduction to Formal Charge Formal Charge Exercise: Allyl Carbocation

Formal Charge Exercise: CH2N2 Formal Charge Exercise: CH3NO2 Formal Charge Exercise: CN

Formal Charge Exercise: CO3

Formal Charge Exercise: Hidden Hydrogens Formal Charge Exercise: Hidden Lone Pairs Formal Charge Exercise: N3

Formal Charge Exercise: NH4 Formal Charge Exercise: O3

Formal Charge Exercise: Radicals and Carbenes Hidden Hydrogens

How Formal Charge Can Mislead

How Heat Affects Elimination Reactions How to draw an enantiomer 

How To Use A pKa Table In Summary: Resonance Introduction to Elimination Introduction to pKa

Introduction to Rearrangements Introduction to Resonance Introduction to the E2 Reaction

Introduction to the SN1: Experiments Introduction to the SN2: Experiments Key Patterns in Formal Charge

Line Drawings

Making OH Into A Good Leaving Group Rearrangement Reactions: Alkyl Shifts Rearrangement: Hydride Shift

Rearrangements: Carbocation Stability Resonance – Common Mistakes (1) Resonance – Common mistakes (2) SN1 Exercise: The Substrate

SN1 Reaction Energy Diagram SN1 vs. SN2 Overview

SN1 With Alkyl Shift (1) SN1 With Alkyl Shift (2) SN1 With Hydride Shift SN1/SN2/E1/E2 – Substrate

SN1/SN2/E1/E2 Decision – Overview SN1/SN2/E1/E2 Decision – Solvent SN1/SN2/E1/E2 Decision – Temperature

SN1/SN2/E1/E2 Decision – The Nucleophile/Base SN1: Applying the SN1 Reaction

SN2 Exercise: Apply the SN2 SN2 Exercise: Leaving Groups SN2 Exercise: The Substrate

Solvents in SN1 and SN2 Reactions Stereochemistry Exercise 1

Stereochemistry Exercise 2 Stereochemistry Exercise 3 Stereochemistry Exercise 4 Stereochemistry Exercise 5 Strong and Weak Acids

Substitution: What is Substitution?

The 4 Components of Every Acid Base Reaction The E1 Reaction

The Golden Rule of Acid Base Reactions The Single Swap Rule

The SN1 Mechanism The SN2 Mechanism

The SN2 Reaction Energy Diagram Understanding R/S Relationships Unequal Resonance Forms

Using Electronegativity to Find Reactive Sites on a Molecule What Makes A Good Leaving Group?

What Makes A Good Nucleophile? (1) What Makes A Good Nucleophile? (2) What Makes A Good Nucleophile? (3) What’s A Nucleophile?

Zaitsev’s Rule