Understanding chemical reactions using electronegativity and resonance — Master Organic Chemistry

April 20, 2019 | Author: Benni Wewok | Category: Alkene, Alcohol, Aldehyde, Ester, Amine
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How How to apply electronegativity electronegativity and and resonance resonance to understand reactivity in Drawing Reaction Mechanisms, Mechanisms, Organic Chemistry 1, 1, Understanding Electron Flow, Flow, Where Electrons Are One thing has been been missing from the discussion of resonance. What’s the point? Who cares cares if we can write out resonance structures? What does it matter if we can figure out the two or three most stable stable resonance resonance structures? structures? So what? what? Here’s the the point: we can can apply resonance (and electronegativity) to figure out the electron densities of  molecules molecules from first princi ples,  ples, and we can apply these electron densities toward understanding how a molecule will react. Put it another another way: way: if you learn this skill, you will rely less on memorization for understanding reactions, because you’ll be able able to figure out the chemical behavior of molecules you’ve never seen before. For instance: instance: if you’re a non-chemistry major I can pretty much guarantee you’ve never seen this reaction  before.  before. Bu But if if you apply apply some some of the the prin principl ciples es in in thi this post, post, you you shou shoulld be able able to make make some some headw headway ay on it.

Let’s look at these two aspects really quickly. 1. Appl Applyin ying g e lectronegativities. lectronegativities . When you have a bond between two atoms with different

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 hybrid looks 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 applying both 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 out potential 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 a region 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 represent actual 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, although we 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 how reactive 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.



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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

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


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

james January 19, 2012 at 11:07 pm

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


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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

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Categories Alcohols Aldehydes Alkanes Alkenes Alkyl Halides Alkynes Amines Aromaticity Carboxylic acids Chemical Bonds Conformations 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

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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 1,2-alkyl shift

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 Powered by WishList Member - Membership Site Software

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