chapter 7 alkenes and alkyne

7. Alkenes and Alkyne

CHEM 221 Dr Wong Yau Hsiung

McMurry Organic Chemistry 6th edition Chapter 6 (c) 2003

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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 Important industrial products

These are feedstocks for industrial processes


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7.1 Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is CnH2n+2

Each ring or multiple bond replaces 2 H's


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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

H3CC

CC

CCH3

H H

H H

H H

H H


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Degree of Unsaturation With Other Elements Organohalogens (X: F, Cl, Br, I)

Halogen replaces hydrogen C4H6Br2 and C4H8 have one degree of unsaturation Oxygen atoms - if connected by single bonds

These don't affect the total count of H's


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7.2 Naming of Alkenes Find longest continuous carbon chain for root Number carbons in chain so that double bond carbons

have lowest possible numbers Rings have “cyclo” prefix


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

methylpropene Isoprene = 2-methyl-1,3-

butadiene


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7.3 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 bonding orbital

Occupied orbital prevents rotation about -bond Rotation prevented by bond - high barrier, about

268 kJ/mole in ethylene


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Rotation of Bond Is Prohibitive

This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond).

Creates possible alternative structures


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7.4 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 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

X


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7.5 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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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”


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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


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Ranking Priorities: Cahn-Ingold-Prelog Rules Must rank atoms that are connected at comparison point Higher atomic number gets higher priority

Br > Cl > O > N > C > H

In this case,The higher priority groups are opposite:(E )-2-bromo-2-chloro-propene


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

Compare until something has higher atomic number Do not combine – always compare

Extended Comparison


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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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7.6 Alkene Stability 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


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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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7.7 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


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Two step process First transition state is high energy point

Electrophilic Addition Energy Path


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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


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Energy Diagram for Electrophilic Addition Rate determining

(slowest) step has highest energy transition state Independent of

direction In this case it is the

first step in forward direction


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

HBr


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7.8 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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Addition of HCl to 2-methylpropene Regiospecific – one product forms where two are possible If both ends have similar substitution, then not

regiospecific

Example of Markovnikov’s Rule


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Mechanistic Source of 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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7.9 Carbocation Stability

The stability of the carbocation (measured by energy needed to form it from R-X) is increased by the presence of alkyl substituents

Therefore stability of carbocations: 3º > 2º > 1º > +CH3 More stable carbocation forms faster


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Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful

products by addition (often through special reagents) alcohols (add H-OH) alkanes (add H-H) halohydrins (add HO-X) dihalides (add X-X) halides (add H-X) diols (add HO-OH)


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7.10 Preparation of Alkenes: A Preview of Elimination Reactions Alkenes are commonly made by

elimination of HX from alkyl halide (dehydrohalogenation)

Uses heat and KOH elimination of H-OH from an alcohol (dehydration)

require strong acids (sulfuric acid, 50 ºC)


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7.11 Addition of Halogens to Alkenes

Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process F2 is too reactive and I2 does not add

Cl2 reacts as Cl+ Cl- Br2 is similar


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Addition of Br2 to Cyclopentene

Addition is exclusively trans

+


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7.12 Halohydrin Formation This is formally the addition of HO-X to an alkene (with

+OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrin

The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)


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7.13 Addition of Water to Alkenes

Hydration of an alkene is the addition of H-OH to to give an alcohol

Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol


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7.14 Reduction of Alkenes: Hydrogenation Addition of H-H across C=C Reduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2

Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)


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Hydrogen Addition- Selectivity

Selective for C=C. No reaction with C=O, C=N

Polyunsaturated liquid oils become solids If one side is blocked, hydrogen adds to other


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7.15 Oxidation of Alkenes: Hydroxylation and Cleavage Hydroxylation adds OH to each end of C=C Catalyzed by osmium tetroxide Product is a 1,2-dialcohol or diol (also called a glycol)


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Alkene Cleavage: Ozone Ozonolysis is a widely used method for locating the double

bond Ozone, O3, adds to alkenes to form ketones and/or aldehydes


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Examples of Ozonolysis of Alkenes

Used in determination of structure of an unknown alkene


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Structure Elucidation With Ozone Cleavage products reveal an alkene’s structure


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Permangante Oxidation of Alkenes

Oxidizing reagents other than ozone also cleave alkenes Potassium permanganate (KMnO4) can produce carboxylic

acids and carbon dioxide if H’s are present on C=C


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7.16 Addition of Radicals to Alkenes: Polymers A polymer is a very large molecule consisting of repeating

units of simpler molecules, formed by polymerization Alkenes react with radical catalysts to undergo radical

polymerization Ethylene is polymerized to poyethylene, for example


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Free Radical Polymerization of Alkenes Alkenes combine many times to give polymer

Reactivity induced by formation of free radicals


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Other Polymers Other alkenes give other common polymers


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Alkynes

Hydrocarbons that contain carbon-carbon triple bonds

Acetylene, the simplest alkyne is produced industrially from methane and steam at high temperature

Our study of alkynes provides an introduction to organic synthesis, the preparation of organic molecules from simpler organic molecules


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7.17 Electronic Structure of Alkynes Carbon-carbon triple bond result from sp orbital on

each C forming a sigma bond and unhybridized pX and py orbitals forming a π bond

The remaining sp orbitals form bonds to other atoms at 180º to C-C triple bond.

The bond is shorter and stronger than single or double

Breaking a π bond in acetylene (HCCH) requires 318 kJ/mole (in ethylene it is 268 kJ/mole)


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7.18 Naming Alkynes General hydrocarbon rules apply wuith “-yne”

as a suffix indicating an alkyne Numbering of chain with triple bond is set so

that the smallest number possible include the triple bond


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7.19 Reactions of Alkynes: Addition of HX and X2

Addition reactions of alkynes are similar to those of alkenes

Intermediate alkene reacts further with excess reagent

Regiospecificity according to Markovnikov


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7.20 Reduction of Alkynes Addition of H2 over a metal catalyst (such as palladium

on carbon, Pd/C) converts alkynes to alkanes (complete reduction)

The addition of the first equivalent of H2 produces an alkene, which is more reactive than the alkyne so the alkene is not observed


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Conversion of Alkynes to cis-Alkenes

Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene

The two hydrogens add syn (from the same side of the triple bond)


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Conversion of Alkynes to trans-Alkenes Anhydrous ammonia (NH3) is a liquid below -33 ºC

Alkali metals dissolve in liquid ammonia and function as reducing agents

Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia


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7.21 Oxidative Cleavage of Alkynes Strong oxidizing reagents (O3 or KMnO4) cleave

internal alkynes, producing two carboxylic acids Terminal alkynes are oxidized to a carboxylic acid

and carbon dioxide Neither process is useful in modern synthesis – were

used to elucidate structures because the products indicate the structure of the alkyne precursor

chapter 7 alkenes and alkyne

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