Chapter6 Alkenes Structure & Reactivity

7/27/2019 Chapter6 Alkenes Structure & Reactivity

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6. Alkenes: Structure

and Reactivity

Based on McMurry‟s Organic Chemistry , 7th edition



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Alkene - Hydrocarbon With

Carbon-Carbon Double Bond

Also called an olefin but alkene is better

Includes many naturally occurring materials

Flavors, fragrances, vitamins



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Why this Chapter?

C-C double bonds are present in most

organic and biological molecules

To examine consequences of alkene

stereoisomerism

To focus on general alkene reaction:electrophilic addition



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6.1 Industrial Preparation and

Use of Alkenes

Ethylene and propylene are the most

important organic chemicals produced



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6.2 Calculating Degree of

Unsaturation Relates molecular formula to possible structures

Degree of unsaturation: number of multiple bonds or rings

Formula for a saturated acyclic compound is CnH2n+2

Each ring or multiple bond replaces 2 H's



Degrees of Unsaturations (DoU)…

Defined as the number of rings and/or

multiple bonds present in a molecule due to

the lost of 2 hydrogen atoms from the

saturated formula, CnH2n+2.

Also known as Index of Hydrogen Deficiency

(IHD)

1 ring 1 DoU

1 double bond 1 DoU

1 triple bond 2 DoU

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DoU….

7

2

HC-)HC(

=DoU

unsn2+n2n

Where,

Huns = #H contain or suppose to contain in an

unsaturated compound



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Example: C6H10

Saturated is

C6H14 Therefore 4 H's are not

present This has two

degrees of

unsaturation Two double bonds?

or triple bond?

or two rings

or ring and double bond



What happen if the compound have other atoms

together with H and C???

How do we calculate the DoU???

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Degree of Unsaturation With Other

Elements….



1) Organohalogen compounds…

contain C, H and X, where X= halogen= F, Cl, Br or I

Halogen acts simply as a replacement for H in an

organic molecule. Huns = #H + #X

Eg: C4

H6

Br 2

, Huns

= #H + #X = 6 + 2 =8

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Degree of Unsaturation With Other

Elements….



Degree of Unsaturation With

Other Elements

2) Organooxygen compounds…

contain C, H and O

Since O forms 2 bonds (-O-), its insertion or substraction

between C-C (C-O-C) and C-H (C-O-H) doesn‟t changethe number of H atoms, therefore doesn‟t affect the DoU.

Huns = #H

Eg: DoU for C5H8O = DoU for C5H8



Organonitrogen compounds

3) Organonitrogen Compounds

Nitrogen has three bonds

So if it connects where H was, it adds a connection point

Subtract one H for equivalent degree of unsaturation in

hydrocarbon



Summary - Degree of

Unsaturation

2

HC-)HC(=DoU

unsn2+n2n

Where,

For pure HC compound: Huns = #H

For organohalogen compound: Huns = #H + #X

For organooxygen compound: Huns = #H (ignore the #O)

For organonitrogen compound: Huns = #H - #N



Naming of alkenes…

Step 1:

Name the parent carbon chain. The parent

carbon chain must be the longest carbon

chain that have at least 1 C=C double bondalong it. Name the compound accordingly

using suffix „ene‟.

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Step 2:

Number the carbon atom in the chain.

Begin at the end nearer to the double bond

If the double bond is equidistant from both

ends, begin numbering at the end nearer to

the first branch point.

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Step 3:

Write the full name of the alkene compound.

List the substituent alphabetically.

Indicate the position of double bond by numbering itbased on the number of the first alkene carbon

beside it. Place that number directly before the

parent name.

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If more than 1 double bond present, indicate

the position of each and use one of the

suffixes –diene, triene, and so on.

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but….

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Naming Cycloalkenes…

Start numbering parent ring from the first carbon with

double bond.

The first substituent has as low a number as possible.

Not necessary to indicate the position of double bond inthe name because it is always between C1 and C2.

BUT, indicate the position of double bond if you have

more than 1 double bond.

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Many Alkenes Are Known by

Common Names

Alk b tit t



Alkene as substituents….

•In some cases, a group containing an alkene

may need to be treated as a substituent.

•In these cases the substituent is named in a similar

fashion to simple alkyl substituents.

•The method is required when the alkene is not the priority group.

•The substituent is named in a similar way to the parent alkene.

•It is named based on the number of carbon atoms in the branch

plus the suffix -yl. i.e. alkenyl

•There are two common names that are widely used:



6.4 Cis-Trans Isomerism in 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 twoatoms forms bond between them

Additive interaction of p orbitals creates a bonding

orbital

Subtractive interaction creates a anti-bonding orbital Occupied orbital prevents rotation about -bond

Rotation prevented by bond - high barrier, about

268 kJ/mole in ethylene



Rotation of Bond Is Prohibitive

This prevents rotation about a carbon-carbondouble bond (unlike a carbon-carbon singlebond).

Creates possible alternative structures



The presence of a carbon-carbon double bond cancreate two possiblestructures

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

t rans isomer - similar groups on oppositesides

Each carbon must havetwo different groups for these isomers to occur



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Cis, Trans Isomers Require That End

Groups Must Differ in Pairs

180°rotation superposes

Bottom pair cannot be superposed without breaking C=C



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6.5 Sequence Rules: The E,Z

Designation

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



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E,Z Stereochemical Nomenclature

Priority rules of Cahn,

Ingold, and Prelog

Compare where higher

priority groups are with

respect to bond anddesignate as prefix

E -entgegen , opposite

sides

Z - zusammen ,together on the same

side



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Ranking Priorities: Cahn-Ingold-

Prelog Rules

RULE 1

Must rank atoms that are connected at comparison point

Higher atomic number gets higher priority

Br > Cl > S > P > O > N > C > H



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RULE 2

If atomic numbers are the same, compare at nextconnection point at same distance

Compare until something has higher atomic number

Do not combine – always compare

Extended Comparison



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RULE 3

Substituent is drawn with connections shown and no

double or triple bonds

Added atoms are valued with 0 ligands themselves

Dealing With Multiple Bonds:



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6.6 Stability of Alkenes

Cis alkenes are less stable than trans alkenes

Compare heat given off on hydrogenation: Ho

Less stable isomer is higher in energy

And gives off more heat

tetrasubstituted > trisubstituted > disubstituted > monosusbtituted hyperconjugation stabilizes



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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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Hyperconjugation

Electrons in neighboring filled orbital stabilize vacant

antibonding orbital – net positive interaction

Alkyl groups are better than H



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6.7 Electrophilic Addition of

Alkenes

General reaction

mechanism:

electrophilic addition

Attack of electrophile

(such as HBr) on bondof alkene

Produces carbocation

and bromide ion

Carbocation is an

electrophile, reacting

with nucleophilic bromide

ion



What is electrophile???

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Two step process

First transition state is high energy point

Electrophilic Addition Energy Path



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Electrophilic Addition for

preparations

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

HBr

HI is generated from KI and phosphoric acid



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6.8 Orientation of Electrophilic

Addition: Markovnikov‟s Rule

In an unsymmetricalalkene, HX reagents canadd in two different ways,but one way may bepreferred over the other

If one orientationpredominates, the reactionis regiospecific

Markovnikov observed inthe 19th century that in theaddition of HX to alkene,the H attaches to the

carbon with the most H‟sand X attaches to the other end (to the one with themost alkyl substituents)

This is Markovnikov’s

rule



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Example of Markovnikov‟s Rule



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Markovnikov‟s Rule (restated)

More highly substituted carbocation forms as intermediate

rather than less highly substituted one

Tertiary cations and associated transition states are more

stable than primary cations



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6.9 Carbocation Structure and

Stability

Carbocations are planar and the tricoordinate carbon issurrounded 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 presenceof alkyl substituents



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I d ti t bili ti f ti



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Inductive stabilization of cation

species



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6.10 The Hammond Postulate

If carbocation intermediate is more stable than another,

why is the reaction through the more stable one faster?

The relative stability of the intermediate is related to an

equilibrium constant (Gº)

The relative stability of the transition state (whichdescribes the size of the rate constant) is the activation

energy (G‡)

The transition state is transient and cannot be

examined



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Transition State Structures

A transition state is the highest energy species in areaction step

By definition, its structure is not stable enough to exist for one vibration

But the structure controls the rate of reaction So we need to be able to guess about its properties in an

informed way

We classify them in general ways and look for trends inreactivity – the conclusions are in the Hammond Postulate

E i ti f th H d



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Examination of the Hammond

Postulate

A transition state

should be similar to

an intermediate

that is close in

energy

Sequential states

on a reaction path

that are close in

energy are likely tobe close in

structure - G. S.

Hammond

C ti R ti d th



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Competing Reactions and the

Hammond Postulate

Normal Expectation: Faster reaction gives more stable

intermediate

Intermediate resembles transition state



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6.11 Mechanism of Electrophilic Addition:

Rearrangements of Carbocations

Carbocations undergo

structural rearrangements

following set patterns

1,2-H and 1,2-alkyl shifts

occur Goes to give more stable

carbocation

Can go through less stable

ions as intermediates

H dride shifts in biological



Hydride shifts in biological

molecules

Chapter6 Alkenes Structure & Reactivity

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