Just Chemistry Department

Organic Chemistry 217

Chapter 3

Alkenes And Alkynes

كيمياء عضوية

217ك حسين المغيض. د

Dr. Hussein Al-Mughaid

Part 2

Direct hydration: Addition of H2O

H OH

General reaction:

• Addition of water to the double bond forms an alcohol.

• The addition follows Markovnikov’s rule.

• Uses dilute solutions of H2SO4 or H3PO4 to drive equilibrium toward hydration.

catalyst

(Nu) (Nu)

(Acid-catalyzed hydration)

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Hydration Mechanism

Step 1. Protonation of C=C

Step2 . Nucleophile (H2O) attacks the carbocation

C C

H

+H2O.... C C

OHH +

H

C C + H+ C C

H

+

Additional step: C C

OHH +

H

C C

OH H

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Give the products for the following two reactions?

ether

alcohol

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Alkylboranes

• Stoichiometry : Three moles of alkene can

react with each mole of BH3.

General reaction: Two steps

Indirect Hydration: Hydroboration–Oxidation Gives alcohol with anti-arkovnikov orientation.

unsymmetrical

3

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Overall reaction:

unsymmetrical

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Give the products for the following two reactions?

4

• Hydrogen (H2) can be added across the double bond in a process known as catalytic hydrogenation.

• The reaction only takes place if a catalyst is used.

Example:

Q: What alkene would you start with if you wanted to synthesize methylcyclohexane?

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Addition of Halogens

• Cl2, Br2, and sometimes I2 add to a double bond to form a vicinal dihalide.

5

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Give the product?

6 Oxidation of Alkenes: Two methods

General reaction:

1. Cold, dilute solution of potassium permanganate, KMnO4 in base.

purple

+ MnO2

Dark brown Give the product?

(A chemical test)

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Then add O

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2. Ozonolysis

Then add O

Zn

Zn, H+

• Ozone (O3) will oxidatively cleave (break) the double bond to produce aldehydes and ketones.

• A second step of the ozonolysis is the reduction of the intermediate by zinc or dimethyl sulfide.

2. Zn, H+

1. O3

Ozonolysis–reduction of an unknown alkene gives an equimolar mixture of cyclohexanecarbaldehyde and butan-2-one. Determine the structure of the original alkene.

We can reconstruct the alkene by removing the two oxygen atoms of the carbonyl groups (C=O) and connecting the remaining carbon atoms with a double bond. One uncertainty remains, however: The original alkene might be either of two possible geometric isomers.

Solved Problem

Solution

Unknown alkene 1. O3

2. Zn, H+ +

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Alkynes: Hydrocarbons containing

General Formula: CnH2n-2

(1s + 2p)

CnH2n-2

symmetrical Unsymmetrical

R = R'R = R'

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Alkynes Nomenclature

1. Find the longest continuous chain that contain the triple bond

2. Proceed from the side that gives the double bond the lowest number

1 2 3 4 7 The triple bond determines the numbering

1 2 3 4

5 6

Alkane -a+y

Alkyne

6-bromo-2-methyl-3-heptyne

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Another one

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Give the IUPAC name?

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Try this one

Acidity of terminal alkynes

•Terminal alkynes are more acidic than other hydrocarbons due

to the higher S character of the sp hybridized carbon.

Slightly acidic

Acidity increases Dr. Hussein Al-Mughaid-39

How alkynes react

Alkynes are nucleophiles and react with electrophiles using

electrophilic addition reaction mechanisms!

like Alkenes

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The electrophile adds to the sp carbon of alkyne

Nu

Addition Product

Addition of H-X

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The 2 Cl,s ended on the same C

Can the addition to an alkyne be stopped after the addition of one equivalent of HX?

geminal dihalide

due to the inductive electron withdrawal

of the halogen substituent

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• Mercuric sulfate (MgSO4) in aqueous sulfuric acid adds H—OH to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone.

Mercuric Ion-Catalyzed Hydration of Alkynes

• The hydration is catalyzed by the mercuric ion. • In a typical reaction, a mixture of mercuric sulfate in aqueous sulfuric acid is used. • The addition produces an intermediate vinyl alcohol (enol) that quickly tautomerizes to the more stable ketone

or aldehyde.

(slow)

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rearrange

Carbonyl group

• Enols (vinyl alcohol = OH on sp2 carbon)

are not stable, and they isomerize to the

corresponding aldehyde or ketone

in a process known as

keto–enol tautomerism.

sp2

Keto–Enol Tautomerism

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Draw the structure of the unknown alkyne ?

Give the structures of A and B?

A + B

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methyl ketone

H2O+HgSO4, H2SO4

O O OH OHOH O

A) B) C) D)

3

• Alkynes has 2 p bonds and therefore, add tow equivalents of the halogen to form a tetrahalide

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Addition of Halogens

+

4

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it is difficult to stop this addition reaction after formation of the alkene

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Give the structure of the product?

… but not impossible - “poisoned” catalysts (Lindlar catalyst) are partially

deactivated metals that are more effective at catalyzing the

addition of hydrogen to an alkyne than to an alkene