3. Alkenes: The Nature of Organic Reactions

Alkene - Hydrocarbon With Carbon-Carbon Double Bond Also called an olefin but alkene is better The characteristic feature of alkene structure

is the carbon- carbon double bond. The characteristic reactions of alkenes are those that take place at the pi portion of the double bonds

Naming of Alkenes Find longest continuous carbon chain that contains the double bond for

parent name Number carbons in chain so that double bond carbons have lowest

possible numbers Rings have “cyclo” prefix; the two doubly bonded carbons are C1 and

C2; and the numbering directions is such that the first group has the smallest number

Many Alkenes Are Known by Common Names Ethylene = ethene Propylene = propene Isobutylene = 2-

methylpropene

Electronic Structure of Alkenes . Electronic structure of alkenes: Cis - Trans Isomerism

The double bond of an alkene consists of a strong sigma bond and a weaker pi bond. Info leading to this statement was obtained through a study of the energy requirements for bond breakage. Ethene C C bond strength (sigma +pi) 152 kcal/moleEthane C C bond strength 88 kcal/mole

Subtracting these two values give a rough estimate of the amount of energy required to break the pi bond of an alkene (its bond strength).

152 kcal/mole - 88 kcal/mole Difference (pi) 64 kcal/mole

The Presence of the Double Bond in Alkenes has Numerous Consequences

One of these is the phenomenon of "restricted rotation". There is no rotation about the carbon-carbon double bond (CCDB). If we were to force rotation we would have to break the pi bond temporarily. This lack of rotation about the CCDB introduces the possibility of cis - trans isomerism

Cis-Trans Isomerism in Alkenes

The presence of a carbon-carbon double can create two possible structures cis isomer - two similar

groups on same side of the double bond

trans isomer similar groups on opposite sides

Each carbon must have two different groups for cis-trans isomerism to exist

Cis, Trans Isomers Require That The Two Groups Bonded to Each Doubly Bonded Carbon Be Different

Cis-Trans does not pertain to top pair because one of the DB Carbons is bonded to the same two groups. So, if you switched A & B you would get the same molecule.

Cis-Trans does pertain to bottom pair. Switching A with B produces a different molecule.

X

Alkene Stability Cis alkenes are less stable than trans alkenes because of steric

crowding Relative stabilities of various alkenes can be determined by

comparing heats given off on hydrogenation to alkanes The less stable isomer is higher in energy to begin with and therefore

it gives off more heat upon being hydrogenated to the alkane Similar 'heats of hydrogenation' studies for various alkenes have lead

to the following general statement about alkene stability: tetrasubstituted > trisubstituted > disubstituted >

monosusbtituted

Comparing Stabilities of Alkenes

Evaluate heat given off when C=C is converted to C-C The more stable alkene gives off less heat

Trans-2- butene generates 5 kJ less heat than cis-2-butene

H2

H2

Energy

C 3C 3

Sequence Rules: The E,Z Designation Neither compound is

clearly “cis” or “trans” Cis, trans

nomenclature only works for disubstituted double bonds

These are tri-substituted double bonds and a new system of designation is needed. Let’s hope its E,Z.

Develop a System for Comparison of Priority of Substituents

Assume a valuation system If Br has a higher

“value” than Cl If CH3 is higher than H

Then, in A, the higher value groups are on opposite sides

In B, they are on the same side Requires a universally

accepted “valuation”

E,Z Stereochemical Nomenclature

Priority rules of Cahn, Ingold, and Prelog

Compare where higher priority group is with respect to bond and designate as prefix

E -entgegen, opposite sides

Z - zusammen, together on the same side

Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at the same comparison

points Higher atomic number gets higher priority

Br > Cl > O > N > C > H

In this case,The higher priority groups are opposite:(E )-1-bromo-1-chloro-propene

If atomic numbers are the same at the same point of comparison, then compare at next and then if necessary at the next connection point until a difference is determined.

Compare until something has higher atomic number

Extended Comparison

A multiple bond is equivalent to the same number of single bonds

Dealing With Multiple Bonds

Organic Reactions Can Be Categorized On Two Levels

Kinds of Reactions – this is more of a surface level of categorizing org. rxns.; it looks only at the Reactants and the final Products

How the Reactions Occur – this is a deeper level of categorizing org. rxns.; it looks at the individual steps that transform the Reactants into Products, the Reaction Mechanism

Kinds of Organic Reactions

Addition reactions – two molecules combine to form one A + B

Elimination reactions – one molecule splits into two A

Substitution – two molecules exchange partners AB + CD AD + CB

Rearrangement –a molecule undergoes a reorganization of bonds and atoms to form an isomeric product A Z

C

B + C

How Organic Reactions Occur: Mechanisms

In a clock the hands move but the mechanism behind the face is what causes the movement

In the overall organic reaction, we see the reactants and the final products. The mechanism describes the steps behind these changes

Reactions occur in defined steps that lead from reactant to product

Steps in Mechanisms

A step in a reaction mechanism involves either the formation or breaking of a covalent bond

Different Ways of Breaking and Making a Covalent Bond Breaking of a covalent bond

Homolytic or heterolytic

Formation of a covalent bond Homogenic or heterogenic

Heterogenic Formation of a Bond

One fragment supplies two electrons One fragment supplies no electrons Usually involves oppositely charged ions Common in organic chemistry

Homogenic Formation of a Bond

One electron comes from each fragment No electronic charges are involved Not common in organic chemistry

Homolytic Breaking of Covalent Bonds

Each product gets one electron from the bond Not common in organic chemistry

Heterolytic Breaking of Covalent Bonds

Both electrons from the bond that is broken become associated with one resulting fragment

A common pattern in reaction mechanisms

Method of Indicating the Movement of Electrons in the Breaking & Forming of Bonds

Curved arrows indicate breaking and forming of bonds

Arrowheads with a “half” head (“fish-hook”) indicate homolytic and homogenic steps (called ‘radical processes’) The “fish-hook” indicates the movement of only one electron

Arrowheads with a complete head indicate heterolytic and heterogenic steps (called ‘polar processes’) The arrowhead indicates the movement of two electrons

Types of Reaction Mechanisms

Most organic reactions can be categorized in terms of two reaction mechanisms Radical Reactions – the individual steps of

this mechanism involve homolytic bond cleavage and homogenic bond formation

Polar Reactions – this is the most commonly occurring organic reaction. These reactions occur between two polar molecules or between a polar molecule and a charged polyatomic ion. The individual steps of this mechanism involve heterolytic bond cleavage and heterogenic bond formation.

Radical Reactions and How They Occur

Radicals are highly reactive chemical species that have an unpaired electron (odd # of electrons). These reactive species need to stabilize themselves by acquiring an extra electron from somewhere. There are two ways that a radical can do this:

Radical Stabilization

1. A radical can abstract an atom from another molecule, giving substitution in the original molecule and generating another radical in the process

Radical Stabilization

2. One radical can combine directly with another radical

Example Radical Reaction: The Radical Chlorination of Methane The overall reaction is pictured below. What

kind of reaction is this?

Substitution

The Mechanism for the Radical Chlorination of Methane

Initiation:

Propagation:

Termination:

Polar Reactions and How They Occur

These reactions occur between two polar molecules or between a polar molecule and a charged polyatomic ion

Polar organic molecules result from polar bonds within functional groups that are part of the organic molecule

Polar bonds result whenever two covalently bonded atoms have substantially different electornegativity values

Electronegativity of Some Common Elements

The relative electronegativity is indicated Higher numbers indicate greater electronegativity Carbon bonded to a more electronegative element has a partial

positive charge (+)

The Keys to Understanding All Polar Reactions

Opposite charges attract

There should be 4 bonds to every carbon in a neutral molecule

The Fundamental Characteristics of all Polar Reactions are:

An electron rich reagent (nucleophile) donates a pair of e-'s to the electron poor reagent (electrophile) and subsequently forms a new bond.

Simultaneous to the formation of this new bond is the rupture of an old bond. The leaving group departs the molecule with both e-'s of the old bond.

An Example of a Polar Reaction: Addition of HBr to Ethylene

HBr adds to the part of C-C double bond The bond is electron-rich, allowing it to function as a

nucleophile H-Br is electron deficient at the H since Br is much

more electronegative, making HBr an electrophile

HBr

S-

Using Curved Arrows in Polar Reaction Mechanisms

Curved arrows are a way to keep track of changes of bonds in a polar reaction

The arrows track “electron movement”

In polar reactions electrons always move in pairs

Charges change during the reaction

Example Polar Reaction

Mechanism of Addition of HBr to Ethylene HBr electrophile is attacked by electrons of

ethylene (nucleophile) to form a carbocation intermediate and bromide anion

Bromide anion adds to the positive center of the carbocation, which is an electrophile, forming a C-Br bond

The result is that ethylene and HBr combine to form bromoethane

All polar reactions occur by combination of an electron-rich site of a nucleophile and an electron-deficient site of an electrophile

Rules for Using Curved Arrows The arrow goes from the nucleophilic reaction site to the

electrophilic reaction site The nucleophilic site can be neutral or negatively charged The electrophilic site can be neutral or positively charged

Just as the chemistry of alkenes is dominated by addition reactions, the prep of alkenes is dominated by elimination reactions. Additions and eliminations are two sides of the same coin.

7.1 Preparation of Alkenes: A Preview of Elimination Reactions: A B + C

Alkenes are commonly made by elimination of H,X from alkyl halide

(dehydrohalogenation) Uses heat and KOH

elimination of H,OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC)

H

Elimination of H-X from an Alkyl Halide to produce an Alkene

The Mechanism for this reaction involves the base pulling off a proton (H+) from a carbon adjacent to the halogen bearing carbon. When using KOH then the base is OH-, The electron pair that is left behind after the proton leaves, becomes the new alkene pi bond and the halogen departs as a halide anion (:X-) with the two electrons from the C-X bond. 

Transition State Species

OH-OH

-

HO-H

K+

K+

Zaitsev’s Rule for Elimination Reactions Elimination reactions almost always give a mixture of alkene products as the 1st step in the mechanism

involves the removal of a proton from a carbon adjacent to the halogen bearing carbon, and most alkyl halides have more than one carbon adjacent to the halogen bearing carbon. In these cases the more substituted alkene predominates because it is the more stable (Zaitsev’s Rule) 

KOH

KOH

If the H+ is pulled off this carbon

Then this product forms

Dehydration of Alcohols to Yield Alkenes

This reaction involves forming an alkene from an alcohol through loss of OH- from one carbon and H+ from an adjacent carbon (hence dehydration: loss of HOH.

Unsymmetrical alcohols give a mix of products. Zaitsev’s Rule applies.

H+ Cat

Diverse Reactions of Alkenes Alkenes react with many electrophiles to give

useful products by electrophilic addition reactions (often through special reagents)

Alcohols; (add H,OH) across the double bond Alkanes; (add H,H) across the double bond Dihalides; (add X,X) across the double bond Halides; (add H,X) across the double bond Halohydrins; (add HO,X) across the double

bond Diols;(add HO,OH) across the double bond Cyclopropane; (add :CH2) across the double

bond

Reactions of Alkenes

7.1 Preparation of Alkenes: A Preview of Elimination Reactions: A B + C

Alkenes are commonly made by elimination of H,X from alkyl halide

(dehydrohalogenation) Uses heat and KOH

elimination of H,OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC)

Elimination of H-X from an Alkyl Halide to produce an Alkene

The Mechanism for this reaction involves the base pulling off a proton (H+) from a carbon adjacent to the halogen bearing carbon. When using KOH then the base is OH-, The electron pair that is left behind after the proton leaves, becomes the new alkene pi bond and the halogen departs as a halide anion (:X-) with the two electrons from the C-X bond. 

Transition State Species

OH-OH-

HO-H

Zaitsev’s Rule for Elimination Reactions

Elimination reactions almost always give a mixture of alkene products as the 1st step in the mechanism involves the removal of a proton from a carbon adjacent to the halogen bearing carbon, and most alkyl halides have more than one carbon adjacent to the halogen bearing carbon. In these cases the more substituted alkene predominates because it is the more stable (Zaitsev’s Rule) 

KOH

KOH

If the H is pulled off this carbon

Then this product forms

Describing a Reaction: Equilibria, Rates, and Energy Changes

Any chemical reaction may be thought of as an equilibrium process.

The exact position of equilibrium is expressed by the equilibrium constant

If the Keq is large then the product concentratioins [C] [D] are larger than the reactant [A] [B] and the reaction proceeds far to the right.If the Keq value is small then the reaction does not takeplace at all.

Keq = [Products]/[Reactants] = [C]c [D]d / [A]a[B]b

Describing a Reaction: Equilibria, Rates, and Energy Changes

The position of any equilibrium always favors the more stable(lower in energy) species, reactant or product

If a reaction has a large Keq value, then the energy of the products must be lower (more stable) than the energy of the reactants and the reaction must be exothermic(exergonic).

If a reaction has a small Keq value, then the energy of the reactants must be lower (more stable) than the energy of the products and the reaction must be endothermic (endergonic)

Magnitudes of Equilibrium Constants If the value of Keq is greater than 1, this

indicates that at equilibrium most of the material is present as products If Keq is 10, then the concentration of the product

is ten times that of the reactant

A value of Keq less than one indicates that at equilibrium most of the material is present as the reactant If Keq is 0.10, then the concentration of the

reactant is ten times that of the product

Describing a Reaction: Energy Diagrams and Transition States

The highest energy point in a reaction step is called the transition state

The energy needed to go from reactant to transition state is the activation energy (G‡)

First Step in Addition

In the addition of HBr the (conceptual) transition-state structure for the first step

The bond between carbons begins to break The C–H bond begins

to form The H–Br bond

begins to break

Describing a Reaction: Intermediates If a reaction occurs in more than one step, it must

involve species that are neither the reactant nor the final product

These are called reaction intermediates or simply “intermediates”

Each step has its own free energy of activation The complete diagram for the reaction shows the

free energy changes associated with an intermediate

Formation of a Carbocation Intermediate HBr, a Lewis acid, adds to

the bond This produces an

intermediate with a positive charge on carbon - a carbocation

This is ready to react with bromide

Carbocation Intermediate Reactions with Anion Bromide ion adds an

electron pair to the carbocation

An alkyl halide produced

The carbocation is a reactive intermediate

Reaction Diagram for Addition of HBr to Ethylene Two separate steps,

each with a own transition state

Energy minimum between the steps belongs to the carbocation reaction intermediate.

Reactions of Alkenes Please recall that: The carbon-carbon double bond consists of a strong

sigma and a weaker pi bond and that the e-'s of the pi bond are very accessible as they are located above and below the plane of the bond.

Understanding these characteristics of the carbon-carbon double bond, one would correctly assume that the loosely held, highly exposed pi e-'s would be particularly vulnerable to chemical species that are seeking electrons.

Electrophiles and Nucleophiles

Chemical species that seek e-'s are usually electron deficient. These electron seeking reagents are collectively referred to as electrophiles. (Greek for electron loving).

The alkenes themselves are referred to as nucleophiles. (Greek for nucleus loving). Nucleophiles are reagents with electron-rich sites which form a bond by donating a pair of e-'s to an electron-poor reagent (electrophile)

Alkene chemistry should, therefore, be dominated by reaction of the electron rich carbon-carbon double bond (nucleophile) with electron-poor species (electrophiles). This is exactly what we find. *The most important reaction of alkenes is their reaction with electrophiles.

Electrophilic Addition of HX to Alkenes General reaction mechanism: electrophilic addition Attack of electrophile (such as HBr) on bond of alkene Produces carbocation and bromide ion Carbocation is an electrophile, reacting with nucleophilic

bromide ion

Example of Electrophilic Addition

Addition of hydrogen bromide to 2-Methyl-propene

H-Br transfers proton to C=C

Forms carbocation intermediate More stable cation

forms Bromide adds to

carbocation

Energy Diagram for Electrophilic Addition

Writing Organic Reactions

No established convention – shorthand Not necessarily balanced Reactants can be before or on arrow Solvent, temperature, details, on arrow

Two step process First transition state is high energy point

Electrophilic Addition Energy Path

Electrophilic Addition for preparations The reaction is successful with HCl and with HI as well as

HBr Note that HI is generated from KI and phosphoric acid

Orientation of Electrophilic Addition: Markovnikov’s Rule

In an unsymmetrical alkene, HX reagents can add in two different ways, but one way may be preferred over the other

If one orientation predominates, the reaction is regiospecific

Markovnikov observed in the 19th century that in the addition of HX to an alkene, the H always attaches to the carbon with the most H’s to begin with and the X attaches to the other Carbon (to the one with the most alkyl substituents) This is Markovnikov’s rule

Addition of HCl to 2-methylpropene Regiospecific – only one product forms where two

are possible

Example of Markovnikov’s Rule

Energy of Carbocations and Markovnikov’s Rule The more stable carbocation intermediate always forms faster

The stability order for carbocations is as follows:3 2 1 0

R3C+ > R2CH+ > RCH2+ > CH3 +

Markovnikov’s Rule Restated

All electrophillic addition reactions proceed by way of the more stable (more highly substituted) carbocation intermediate