reactions of alkenes and alkynes study guide

8/10/2019 Reactions of Alkenes and Alkynes Study Guide

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Hydrohalogenation of Alkenes

Addition of H-X across the double bond

Notes:

The C=C double bond is degraded to a CC single bond and two new bonds are formed: a

CH single bond and a CX single bond

A carbocationintermediate is formed: the carbocation is sp2-hybridized and trigonal planar

Markovnikov regioselectivity is observed: the more stable carbocation is formed; the

halogen atom ends up on the more substituted carbon

Carbocation rearrangements can occur if a more stable carbocation can be formed

CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall

Boston University

Nov 2012


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Halogenation of Alkenes

Addition of X2across the double bond

Notes:

The C=C double bond is degraded to a CC single bond and two new CX single bonds are

formed

A cyclic halonium cationintermediate is formed: this occurs on the less sterically hindered

face of the double bond

Xattacks from the direction opposite to X+of the cyclic halonium cation: anti-additionis

observed

Xattacks the carbon more able to support a partial positive charge


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Nov 2012


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Stereochemistry in Halogenation of Alkenes


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Stereochemistry in Halogenation of Alkenes

Notes:

Products can be identical, enantiomers or diastereomers

Some double bonds have no facial differentiation: X2can approach from either side; other

double bonds have facial differentiation: X2approaches from the less hindered side

Kristen Mascall

Boston University

Nov 2012

CHEM203 Reactions and Mechanisms Study Guide


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Interrupted Halogenation

Another nucleophile opens the cyclic halonium cation

Notes:

Examples of nucleophiles: H2O (product is a halohydrin), alcohols such as CH3OH (product is a

halo ether), carboxylic acids, amines

Nucleophilic attack can be intramolecular or intermolecular


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Nov 2012


7/17

Hydration of Alkenes

Addition of H2O across the double bond

Notes:

The C=C double bond is degraded to a CC single bond and two new bonds are formed: a

CH single bond and a CO single bond

A carbocation intermediate is formed: Markovnikov regioselectivity is observed, and

carbocation rearrangements can occur

The reaction is catalysed by acid (H+, usually H2SO4): the acid is regenerated at the end of the

reaction


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Nov 2012


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Hydroboration-Oxidation of Alkenes

Addition of H2O across the double bond

Notes:

In the first step, the C=C double bond is degraded to a CC single bond and two new bonds

are formed: a CH single bond and a CB single bond The BH3 molecule approaches the double bond from the less sterically hindered side: cis-

additionis observed

B bonds to the less subsituted carbon and H bonds to the more substituted carbon: anti-

Markovnikovregioselectivityis observed

In the second step, a reaction with H2O2, NaOH, H2O replaces the boron atom with OH: the

final product is an alcohol


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Nov 2012


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Hydrogenation of Alkenes

Addition of H2across the double bond

Notes:

The C=C double bond is degraded to a CC single bond and two new CH single bonds are

formed

The reaction occurs on the metal surface: cis-additionis observed

Hydrogenation occurs on the less sterically hindered face of the alkene


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10/17

Osmylation of Alkenes

Preparation of a vicinal diol

Notes:

The C=C double bond is degraded to a CC single bond and two new CO single bonds are

formed

Osmylation occurs on the less sterically hindered face of the alkene

cis-additionis observed


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Nov 2012


11/17

Epoxidation of Alkenes

Preparation of a cyclic ether (epoxide)

Notes:

The C=C double bond is degraded to a CC single bond and two new CO single bonds are

formed in a three-membered ring

The reaction proceeds through a concerted mechanism

Epoxidation occurs on the less sterically hindered face of the alkene

cis-additionis observed

A carboxylic acid is formed as a side product from the peroxy acid


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Nov 2012


12/17

Ozonolysis of Alkenes

Cleavage of the double bond to form two new carbonyl compounds

Notes:

The C=C double bond is completely broken: two new carbonyl groups are formed

The workup conditions determine the final products: reductive workup (solvolysis) gives

aldehydes and ketones, oxidative workup gives ketones and carboxylic acids

Ozonolysis of a terminal alkene followed

by oxidative workup gives carbonic acid,

which decomposes into CO2and H2O


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Hydrogenation of Alkynes

Addition of H2across the triple bond

Notes:

The CC triple bond is degraded to a CC single bond or a C=C double bond depending on thecatalyst and reaction conditions used

With Lindlar catalyst (poisoned Pd), hydrogenation stops at the alkene and cis-addition is

observed: the product is the Z-alkene


Boston University

Nov 2012

CHEM203 R d M h S d G d K M ll


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Dissolving Metal Reduction of Alkynes

Addition of H2across the triple bond

Notes:

The CC triple bond is degraded to a C=C double bond

The product is the trans-alkene


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Nov 2012

CHEM203 R i d M h i S d G id K i M ll


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Hydration of Alkynes

Addition of H2O across the triple bond


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Nov 2012

CHEM203 R ti d M h i St d G id K i t M ll


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Hydration of Alkynes

Addition of H2O across the triple bond

Notes:

The CC triple bond is degraded to a CC single bond: a new carbonyl group and two newCH single bonds are formed

The initial product is an enol, which further reacts to form the final ketone

Terminal alkynes form methyl ketones

Symmetrical internal alkynes produce a single ketone

Unsymmetrical internal alkynes give a mixture of constitutional isomers


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Nov 2012

CHEM203 R ti d M h i St d G id K i t M ll


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Deprotonation and Alkylation of Alkynes

Removal of the proton on a terminal alkyne and introduction of a carbon

chain

Notes:

A terminal alkyne is transformed into an internal alkyne

Only somebases can be used to deprotonate an alkyne: bases whose conjugate acid have pKa

values > 25

Examples of suitable bases: NaH (H), NaNH2(NH2); unsuitable base: NaOH (OH)

The alkyl halide R

X must be 0

(methyl) or 1


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Nov 2012

reactions of alkenes and alkynes study guide

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