Chapter 7:Structure and Synthesis of

Alkenes

Ú Introduction (7-1)Ú Alkenes are hydrocarbons with C=C in their structure. They are also known as “olefins”.

Ú We have already covered the basics of alkenes nomenclature, but here are a few important groups with common names that you can use as well.

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Orbital Description of the Alkene Double Bond (7-2)Ú We already covered this in the first 2 chapters. The carbon of the alkene group are sp2 hybridized, leaving a p orbital available to form a ππππ bond

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Ú Cis/trans isomerism conversion is not possible

Element of Unsaturation (7-3)

ÚAlkenes are said to be “unsaturated” because they react with hydrogen to give the corresponding alkane (a saturated compound). Alkenes are said to have one element of unsaturation.

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element of unsaturation.

ÚThe element of unsaturation of a compound can be calculated using the following formula:

Index = # C - # H - # halogen + # N +12 2 2

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Practice QuestionsÚQuestion:What is the unsaturation index for:

C15H13ClO

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ÚHow many element of unsaturation are present in podophyllotoxin?

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Nomenclature of cis-trans Isomers (7-5)ÚWe have seen before that cis and trans alkenes were possible. Those were called geometric isomers.

ÚHowever, sometimes this notation cannot

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ÚHowever, sometimes this notation cannot produce an unambiguous name.

Ú In these cases where the substituents are all different, use the E/Z system. This system can be used all the time, replacing the cis/trans notation.

– E: entgagen (opposite)

– Z: zusammen (together)

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– Z: zusammen (together)

This system follows the same principles studied in Chapter 5 for the Cahn-Ingold-Prelog notation of chiral carbons.

ÚProcedure:A) assign the substituent a priority for each carbon of the double bond as: #1 and #2.

– #1 is the highest priority and is determined by the atomic number (or molecular weight) of the atom directly

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molecular weight) of the atom directly attached to the C=C carbonB) if a priority cannot be determined using the atom directly attached to the C of C=C, go to the next atom until a priority can be determined.

ÚLooking at both carbons, determine if the substituents labeled (1) on each carbon of the double bond are on the same side or opposite sides of the double bond– If both (1) are on the same side: notation is Z

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Z– If both (1) are on opposite sides: notation is E

Practice QuestionsÚDraw and label the E and Z isomers for each of the following:

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ÚWhat are the configurations of the double bonds in the following compounds?

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ÚDraw the structure of (Z)-3-isopropyl-2-heptene.

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ÚStability of Alkenes (7-7)The stability of alkenes can be summarized by the Zaitsev’s Rule:

– More highly substituted double bonds are usually more stable

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are usually more stable

– In a reaction: chemical reactions producing alkenes will tend to produce the most substituted C=C

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ÚPractice QuestionClassify the following alkenes in order of increasing stability.

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A B C D

ÚCycloalkenes stability is related to the ring strain involved:C5 or larger are stableC3 and C4 are less stableSmall rings (C4 to C7) are usually “cis” (more stable)

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“cis” (more stable)

ÚFor larger ring (C8 and larger) it is possible to have “trans” geometry

H

H

H

H

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cis

H

trans

Trans cyclodecene is almost as stable as the cis isomer

Physical Properties (7-8)

Bp and Mp: increases with molecular weights. But branching tends to decrease it (similar to alkanes).

– Relatively non-polar

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Synthesis of Alkenes by Eliminations of Alkyl Halides (7-9)

DehydrohalogenationIs the elimination of a hydrogen an halogen from an alkyl halide: Mechanism: E2

Base: abstract a proton from the carbon adjacent to C-X (needs strong base, HO-, CH3O-)

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C-X (needs strong base, HO , CH3O )X-: leaves at the same time

ØMust have an anti-coplanar arrangementin order for the reaction to take place

ØMechanism: concerted (bonds are formed and broken at the same time)

X

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HC C

X

HB

-

Works best with 3o halides and hindered secondary halides.

If other types of halides (unhindered 2o and 1o) are used, it is best to use a very bulky base. This helps

C Br NaOH

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used, it is best to use a very bulky base. This helps in avoiding substitution reactions.

Ú Example: any of the bases from previous slide would give this product

BrEt3N

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Difficulty:ØWith bulky bases and sterically hindered alkyl halides, it is not unusual to get the Hoffmanproduct instead of the Zaitsev’s product.Zaitsev’s product: most substituted C=CHoffman product: least substituted C=C

ÚBecause:Abstraction of the most hindered proton is not possible

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Ú Since the reaction is concerted, and an anticoplanar arrangement of the proton and leaving group is necessary, there are stereochemical consequences to the reaction.

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E

Elimination from Substituted Cyclohexanes

Ú E2 elimination can take place from either the most stable or least stable chair conformation of the molecule. The requirement to follow is an anticoplanar arrangement of H and X in axial positions.

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Ú Therefore, the most substituted alkene may not be produced in these reactions, and the most stable chair conformation may not be the one leading to the product.

ClCl

H

this H cannot participate in theelimination since it is equatorial

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more stable H

BaseE2 conditions

Practice QuestionÚ What it the major organic product of the E2 elimination of trans-1-chloro-2-methylcyclohexane.

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Dehalogenation of vicinal dibromidesUsually performed by reduction with iodide ion or Zn/acetic acid.

Br

Br

I-

acetone

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Br

Br

Br

Zn

+ ZnBr2

Acetic acid is not involved, it is there simply to cleanthe surface of the metal.

ÚNeed anticoplanar arrangement for the 2 bromines…or the molecule must be able to reach this conformation for the reaction to proceed.

ÚWhat is the product of this reaction?

C CBr

PhH Br

PhH

NaIacetone H

Ph H

Ph

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ÚWhat is the product of this reaction?

NaIPhBr

HBr

H Ph

Practice Question

ÚWhat is the product of this reaction?

Br NaI

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Br

Br

NaI

Alkene Synthesis by Dehydration of Alcohols(7-10)Dehydration simply means: removal of waterMechanism is E1 (elimination 1) and will produce a carbocation… therefore, rearrangement to the most stable carbocation is possible.

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Mechanism of Dehydration of Alcohols

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Practice Questions

ÚComplete the following reactions carried out under acidic conditions and heat.

H2SO4

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OH

OH

OH

H3PO4

H2SO4

ÚFor the last reaction from the last question (giving the blue product), show how the reaction takes place by drawing a detailed mechanism.

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Study Problems

7-33Label each structure as Z, E or neither.

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7-44Predict the major product of the acid-catalyzed dehydration of the following alcohols.

2-pentanol

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1-methylcyclopentanol

2,2-dimethyl-1-propanol

7-45Propose a detailed mechanism for the following reaction.

Br CH3OH

heatO

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7-45 Propose a mechanism for the following reaction.

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