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SCH 102 Dr. Solomon Derese 1 Overview of Types of Organic Reactions and Basic Concepts of Organic Reaction Mechanisms
SCH 102
Dr. Solomon Derese 1
Overview of Types of Organic Reactionsand
Basic Concepts of Organic Reaction Mechanisms
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Reactions of organic compounds can be organizedbroadly in two ways by:
I. What types of reactions andII. How these reactions occur.
A chemical reaction is the transformation of onechemical or collection of chemicals into anotherchemical or collection of chemicals.
A chemical reaction involves making new chemicalbonds and breaking old chemical bonds.
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Overview of Types of Organic Reactions
The types of reactions organic compounds undergo isdivided into four categories, these are:
I. Additions reactions
II. Substitutions reactions
III. Elimination reactions
IV. Rearrangement reactions
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I. Addition Reactions
Addition reactions occur when two starting materialsadd together to form only one product with no atomsleft over.
Examples
In an addition all parts of the adding reagent appearin the product; two molecules become one.
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II. Substitution Reactions
Substitution reactions occur when two startingmaterials exchange groups to form two new products.
In a substitution, one group replaces another.
Examples
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III. Elimination Reactions
Elimination reactions are the opposite of additionreactions.
A single reactant is split into two two products.
Elimination reactions give us a method for preparingcompounds with double and triple bonds.
Two s bonds are broken, and a p bond is formedbetween adjacent atoms.
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Addition and elimination reactions are exactlyopposite. A p bond is formed in elimination reactions,whereas a p bond is broken in addition reactions.
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IV. Rearrangement Reactions
Rearrangement reactions occur when one startingmaterial forms one product with a differentarrangement of atoms and bonds.
The product is an isomer of the starting material.
We have so far seen the different types of organicreactions, let’s now see how reactions occur.
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Assignment 22
Classify each of the following reactions as addition,elimination, substitution or rearrangement
a)
b)
c)
d)
e)
f)
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Basic Concepts of Organic Reactions Mechanisms
An overall description of how a reaction occurs iscalled a reaction mechanism.
A mechanism describes in detail exactly what takesplace at each stage of a chemical transformation—which bonds are broken and in what order, whichbonds are formed and in what order.
A reaction mechanism must account for all reactantsused and all products formed.
It is detailed explanation of the electron movement inthe course of chemical reactions.
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A reaction mechanism is a reasonable, stepwiseillustration of how the electrons in the bonds of thereactants of a reaction are reorganized into itsproducts under a given set of reaction conditions.
During a course of a chemical reaction chemicalbonds in the reactants are broken and bonds in theproducts are formed, bonds can be brokensymmetrically (one electron remains with eachproduct fragment) or unsymmetrically (bondingelectrons remain with one product fragment).
The symmetrical cleavage is homolytic cleavage, andthe unsymmetrical cleavage is heterolytic.
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Homolytic bond cleavage
RadicalsSymmetrical bond-breaking: bonding electrons sharedbetween the two atoms.
Heterolytic bond cleavage
Unsymmetrical bond-breaking: One of the elementstaking both bonding electrons.
Ions
Fish hook arrow
Double sided arrow
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Just as there are two ways in which a bond can break,there are two ways in which a covalent two-electronbond can form.
Symmetrical bond-making (Homogenic)
Two bonding electrons are donated by one reactant.
One bonding electron is donated by each reactant.
Unsymmetrical bond-making (Heterogenic)
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• Arrowheads with a “half” head (“fish-hook”) indicate homolytic andhomogenic steps (called ‘radicalprocesses’)—the motion of one electron
• Arrowheads with a complete headindicate heterolytic and heterogenicsteps (called ‘polar processes’)—themotion of an electron pair.
Curved/Curly/Pushing arrows indicate breaking andforming of bonds.
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Reactions that involve symmetrical bond-breakingand bond-making are called radical reactions. Aradical, often called a “free radical,” is a neutralreactive chemical species that contains an oddnumber of electrons and thus has a single, unpairedelectron in one of its orbitals.
Reactions that involve unsymmetrical bond-breakingand bond-making are called polar reactions. Polarreactions occur between negatively charged (anions)and positively charged (cations) ions.
Polar reactions are by far the most common reactiontype in organic compounds.
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Radical species are mostly generated in nonpolarbonds, while ions are generated during reactions ofpolar molecules.Heterolysis is more likely if the two atoms in the bondhave different electronegativities (polar covalentmolecules).
A polar covalent bond between two elements withdiffering electronegativities is polarized in thedirection d+A-Bd-, where B is the more electronegativeelement.
In a heterolysis, the bond will almost always break inthe direction which will leave both bonding electronson the more electronegative atom, B.
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The less electronegative element, A, will have positivecharge while the more electronegative element, B,will have negative charge.
X
Polar reactions proceed by the movement of pairs ofelectrons from areas of high electron density(nucleophiles) to areas of low electron density(electrophiles), or from filled orbitals to emptyorbitals.
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Free Radical Reactions
The term free radical refers to any atom or group ofatoms with an odd number of electrons. As a resultthe electrons in a free radical cannot all be paired.
Even though a free radical does not have a +ve or –vecharge, such a species is highly reactive.
A classical example a reaction that involve a freeradical is halogenation of alkanes in the presence ofultraviolet (UV, hν) light.
As an example of a free radical reaction let’s look atthe mechanism of monochlorination of methane.
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Mechanism of chlorination of methane
For the reaction to occur a C-H and CI-CI bond mustbe broken and a C-CI and H-CI bond must be formed.
The CI-CI bond (bond energy = 58 kcaI/mole) isweaker than C-H bond (bond energy = 104 kcal/mole)and it is the weaker CI-CI bond that will break first toinitiate the reaction.
The above reaction involves three steps, initiation,propagation, and termination.
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Step I: Initiation
The initiation step in the chlorination of methane isthe homolytic cleavage of Cl2 into two chlorine freeradicals.
The energy required for the heterolytic cleavage ofCl2, 58 kcal/mole, is provided by UV light or heat.Halogenation of alkanes does not take place in thedark.
free radical (highly reactive, unstable, initiates a reaction)
58 kcal/mole
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Step II: Propagation
The unstable chlorine free radical forces, the strong C-H to break homolytically in the propagation step(formation of new free radicals).
The reactive chlorine free radical abstract a hydrogen from methane to yield a methyl free radical and HCl.
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The methyl fee radical is also reactive. In the secondpropagation, it abstracts a chlorine from Cl2 to formanother chlorine free radical, one radical generatesanother.
This step yields one of the products of the overallreaction, chloromethane. This step also generates anew chlorine free radical that can abstract a hydrogenfrom atom from another methane molecule andstarts the propagation step all over again.
Thus, once the sequence has been initiated, itbecomes a self-sustaining cycle of repeating steps (I)and (II), making the overall process a chain reaction.
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Step III: Termination
The propagation cycle is broken by terminationreaction. Occasionally, two radicals might collide andcombine to form a stable product. When thathappens, the reaction cycle is broken and the chain isended, terminated. Removing radicals from thereaction mixture without generating any new radicalsstops the reaction.
Coupling of free radicals to form a stable product.
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Assignment 23
a) Write equations and mechanism for the initiation,propagation and termination reactions leading tothe formation of chlorocyclohexane fromcyclohexane and chlorine.
b) When methane is chlorinated, among the productsfound are traces of chloroethane. How is itformed?
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Polar Reactions
Polar reactions occur between oppositely chargedspecies, cations and anions, or between an electrondeficient and an electron rich species.The driving force of the reaction is the Coulombicelectrical force of attraction between positivelypolarized and negatively polarized centers onfunctional groups in molecules. It is based on theprinciple “unlike charges attract and like chargesrepel”.Bond polarity is a consequence of an unsymmetricalelectron distribution in a bond and is due to thedifference in electronegativity of the bonded atoms.
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Polarity Patterns in Some Common Functional Groups
d+
d+
d+ d+d+ d+
d+ d+ d+
d+
d+ d+d+ d+ d+
d- d-
d-
d-
d- d-
d-
d-
d-
d-
d-
d-
d-
d-d-
d- d-
d-
d-
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What does functional-group polarity mean withrespect to chemical reactivity? Because unlike chargesattract, the fundamental characteristic of all polarorganic reactions is that electron-rich sites, d-, reactwith electron-poor sites, d+.
Bonds are made when an electron-rich atom donatesa pair of electrons to an electron poor atom, andbonds are broken when one atom leaves with bothelectrons from the former bond.
In referring to the electron-rich and electron-poorspecies involved in polar reactions, chemists use thewords nucleophile and electrophile, respectively.
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A nucleophile is a substance that has a negativelypolarized, electron-rich, atom and can form a bond bydonating a pair of electrons to a positively polarized,electron poor atom. Nucleophiles can be eitherneutral or negatively charged.
Nucleophile (nucleus loving)
Examples of nucleophilesd-d-d-
The p-bond
acts a
nucleophile
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An electrophile has a positively polarized, electronpoor, atom and can form a bond by accepting a pair ofelectrons from a nucleophile. Electrophiles can beeither neutral or positively charged.
Electrophile (electron loving)
Examples of electrophiles
d+
d+d+
d-
d-d-
H+
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Note that neutral compounds can often react eitheras nucleophiles or as electrophiles, depending on thecircumstances. After all, if a compound is neutral yethas an electron-rich nucleophilic site, it must alsohave a corresponding electron-poor electrophilic site.
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a) Which of the following species is likely to behaveas a nucleophile and which as an electrophile?
Assignment 24
b) Which of the following species are likely to benucleophiles and which electrophiles? Whichmight be both?
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Using Curved Arrows in Polar Reaction Mechanisms
One example polar reaction is the synthesis ofbromoethane by electrophilic addition of hydrogenbromide to ethene.
Let us work out the mechanism of this reaction usingcurved arrows.
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d+ d-
Nucleophile Electrophile
Mechanism of electrophilic addition reaction to alkenes
The C = C the double bond is nucleophilic and thechemistry of alkenes is dominated by reactions withelectrophiles.
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Let us look at another example of polar reaction, areaction between acetic anhydride and methyl amineto yield N-methylacetamide and acetic acid.
d+
d+ d+
d-
d-
d-
Amines are strong nucleophiles
The carbonyl carbon is very electrophilicd-
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Mechanism
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Rules for Using Curved Arrows in Polar Reaction Mechanisms
Rule 1Electrons move from a nucleophilic source (Nu: orNu:-) to an electrophilic sink (E or E+). The nucleophilicsource must have an electron pair available, usuallyeither as a lone pair or in a multiple bond.
Electrons usually flow from one of thesenucleophiles.
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The electrophilic sink must be able to accept anelectron pair, usually because it has either a positivelycharged (+) atom or a positively polarized (d+) atom ina functional group.
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Rule 2The nucleophile can be either negatively charged orneutral. If the nucleophile is negatively charged, theatom that donates an electron pair becomes neutral.
If the nucleophile is neutral, the atom that donatesthe electron pair acquires a positive charge.
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Rule 3The electrophile can be either positively charged orneutral. If the electrophile is positively charged, theatom bearing that charge becomes neutral afteraccepting an electron pair.
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If the electrophile is neutral, the atom that ultimatelyaccepts the electron pair acquires a negative charge.
For this to happen, however, the negative chargemust be stabilized by being on an electronegativeatom such as oxygen, nitrogen, or a halogen. Carbonand hydrogen do not typically stabilize a negativecharge.
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Rule 4The octet rule must be followed. That is, no second-row atom can be left with ten electrons (or four forhydrogen). If an electron pair moves to an atom thatalready has an octet (or two for hydrogen), anotherelectron pair must simultaneously move from thatatom to maintain the octet.
This hydrogen already has two electrons. When anotherelectron pair moves to the hydrogen from the double bond,the electron pair in the H–O bond must leave
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Assignment 25
Identify the nucleophile and electrophile in each ofthe following reaction. Give a step by step mechanismfor the reactions using curved arrows.
