4. Alkenes: Structure and

Reactivity

Based on McMurry’s Organic Chemistry, 6th edition

Alkenes

Hydrocarbons With Carbon-Carbon Double Bond

Also called olefins but alkenes is better

Because of their double bond, alkenes have fewer hydrogens than related alkanes and are therefore referred to as unsaturated(general formula CnH2n)

Includes many naturally occurring materials

Flavors, fragrances, vitamins

2

Important industrial products

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Naming Alkenes

Are named according to a series of rules similar to those used for alkanes, with the suffix –ene in place of -ane

Find longest continuous carbon chain for root

Number carbons in chain so that double bond carbons have lowest possible numbers

If more than one double bond is present, indicate the position of each and use one of the suffixes –diene, -triene, and so on

Rings have “cyclo” prefix

4

Naming Alkenes

5

Many Alkenes Are Known by Common Names

Ethylene = ethene

Propylene = propene

Isobutylene = 2-methylpropene

Isoprene = 2-methyl-1,3-

butadiene

6

Electronic Structure of Alkenes

Carbon atoms in a double bond are sp2-hybridized

Three equivalent orbitals at 120º separation in plane

Fourth orbital is atomic p orbital

Combination of electrons in two sp2 orbitals of two atoms forms bond between them

Additive interaction of p orbitals creates a orbital

Occupied orbital prevents rotation about -bond

7

Cis-Trans Isomerism in Alkenes

The presence of a carbon-carbon double bond can create two possible structures

cis isomer - two similar groups on same side of the double bond

trans isomer two similar groups on opposite sides

Each carbon must have two different groups for these isomers to occur

8

Sequence Rules: The E,Z Nomenclature

Neither compound is clearly “cis” or “trans”

Substituents on C1 are different than those on C2

We need to define “similarity” in a precise way to distinguish the two stereoisomers

Cis, trans nomenclature only works for disubstituted double bonds

a system for comparison of Priority of Substituents has been developed

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E,Z Stereochemical Nomenclature

Priority rules of Cahn, Ingold, and

Prelog

Rank atoms that are connected at

comparison point

Higher atomic number gets higher

priority

Br > Cl > O > N > C > H

Compare where higher priority group

is with respect to bond and designate

as prefix

E -entgegen, opposite sides

Z - zusammen, together on the

same side10

E,Z Stereochemical Nomenclature

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Extended Comparison

If atomic numbers are the same, compare at next connection point at same distance

Compare until something has higher atomic number

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Dealing With Multiple Bonds

Substituent is drawn with connections shown and no double or triple bonds

Added atoms are valued with 0 ligands themselves

Multiple bonds

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Comparing Stabilities of Alkenes

Evaluate heat given off when C=C is converted to C-C

More stable alkene gives off less heat

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

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Alkene Stability: electronic and steric effects

Less stable isomer is higher in energy

And gives off more heat

Cis alkenes are less stable than trans alkenes: minor steric

strain

Alkyl substituents (electron donors) increase the electron density

on the double bond and are better separated by a double bond

(120° vs 109°):

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Kinds of Organic Reactions

In general, we look at what occurs and try to learn how it happens

There are four broad types of organic reactions (describe the

changes)

Addition reactions – two molecules combine

Elimination reactions – one molecule splits into two

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Kinds of Organic Reactions

Substitution reactions – parts from two molecules exchange

Rearrangement reactions – a molecule undergoes changes in

the way its atoms are connected

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How Organic Reactions Occur: Mechanisms

In a clock the hands move but the mechanism behind the face is

what causes the movement

In an organic reaction, we see the transformation that has occurred.

The mechanism describes the steps behind the changes that we can

observe

Reactions occur in defined steps that lead from reactant to product

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

Steps can occur in individually or in combination with other steps

When several steps occur at the same time they are said to be

concerted18

Types of Steps in Reaction Mechanisms

Formation of a covalent bond

Homogenic or heterogenic

Breaking of a covalent bond

Homolytic or heterolytic

Oxidation of a functional group

Reduction of a functional group

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Homogenic Formation of a Bond

One electron comes from each fragment

No electronic charges are involved

Not common in organic chemistry

Heterogenic Formation of a Bond

One fragment supplies two electrons

One fragment supplies no electrons

Combination can involve electronic charges

Common in organic chemistry

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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

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Indicating Steps in Mechanisms

Curved arrows indicate breaking and

forming of bonds

Arrowheads with a “half” head (“fish-

hook”) indicate homolytic and homogenic

steps (called ‘radical processes’)

Arrowheads with a complete head

indicate heterolytic and heterogenic steps

(called ‘polar processes’)

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Radical Reactions and How They Occur

Note: Polar reactions are more common

A “free radical” is an “R” group on its own:

CH3 is a “free radical” or simply “radical”

Has a single unpaired electron, shown as: CH3.

Its valence shell is one electron short of being complete

Radicals react to complete electron octet of valence shell

A radical can break a bond in another molecule and abstract a partner with an electron, giving substitution in the original molecule

A radical can add to an alkene to give a new radical, causing an addition reaction

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Steps in Radical Substitution

Three types of steps

Initiation – homolytic formation of two reactive species with

unpaired electrons

Example – formation of Cl atoms form Cl2 and light

Propagation – reaction with molecule to generate radical

Example - reaction of chlorine atom with methane to give

HCl and CH3.

Termination – combination of two radicals to form a stable

product: CH3. + CH3

. CH3CH3

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Polar Reactions and How They Occur

Molecules can contain local unsymmetrical electron distributions

due to differences in electronegativities

This causes a partial negative charge on an atom and a

compensating partial positive charge on an adjacent atom

The more electronegative atom has the greater electron density

Carbon bonded to a more electronegative element has a partial

positive charge (+)

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Polarity of common functional groups

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Generalized Polar Reactions

Polar reactions occur between regions of high electron density and regions of low electron density

An electrophile, an electron-poor species, reacts with a nucleophile, an electron-rich species

An electrophile is a Lewis acid

A nucleophile is a Lewis base

The electron movement is represented with a curved arrow from nucleophile to electrophile

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Alkenes Reactivity

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H+

C

H

CH3

CH3

H3C

H3CC

H

CH3

CH3

H3CH3C

Br

H-Br

H+ + Br -:.... ..

Br -:....

..

The bond of an alkene (the substrate) 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

Let for a moment imagine H-Br dissociated in H+ and Br-

H+ adds to the part of C-C double bond, giving a carbocation

Br- adds to the positive carbon

The results is the addition of H-Br

The reaction is named “electrophilic addition” (electrophilic because the

electrophile first react with the substrate)

Alkenes Reactivity: Electrophilic Addition

General reaction mechanism: electrophilic addition

The alkene bond attacks the electrophile (such as HBr)

Produces carbocation and bromide ion

Carbocation is an electrophile, reacting with nucleophiles (bromide ion)

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Writing Organic Reactions: Not necessarily balanced

Reactants can be before or on arrow

Solvent, temperature, details, on arrow

Electrophilic Addition mechanism

The reaction is successful with HCl and with HI as well as HBr30

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.

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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 alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents)

This is Markovnikov’s rule

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+ Br_

+

+CH3 C

CH3

CH CH3

H

CH3 C

CH3

CH CH3

H

H Br

CH3 C

CH3

CH CH3

X

Orientation of Electrophilic Addition: Markovnikov’s Rule

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Energy of Carbocations and Markovnikov’s Rule

More stable carbocation forms faster

Tertiary carbocations and associated transition states are more

stable than primary carbocations

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Normal Expectation: Faster reaction gives more stable intermediate

Intermediate resembles transition state

Regiospecificity in Addition Reactions

If addition involves a

carbocation intermediate

and there are two

possible ways to add

the route producing the

more alkyl substituted

cationic center is lower

in energy

alkyl groups stabilize

carbocation

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Carbocation Structure and Stability

Carbocations are planar and the tricoordinate carbon is surrounded by only 6 electrons in sp2

orbitals

The fourth orbital on carbon is a vacant p-orbital

The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents, which tend to donate electrons to the positively charged carbon atom

Therefore stability of carbocations: 3º > 2º > 1º > +CH3

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Hyperconjugation

Electrons in neighboring filled orbital stabilize vacant antibonding orbital – net positive interaction

Alkyl groups are better than H

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End of chapter 4