Upload
melissa-garcia
View
221
Download
0
Embed Size (px)
Citation preview
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
1/17
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
2/17
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
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
3/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
4/17
Stereochemistry in Halogenation of Alkenes
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
5/17
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
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
6/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
8/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
9/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
13/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
CHEM203 R d M h S d G d K M ll
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
14/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
CHEM203 R i d M h i S d G id K i M ll
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
15/17
Hydration of Alkynes
Addition of H2O across the triple bond
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
CHEM203 R ti d M h i St d G id K i t M ll
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
16/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012
CHEM203 R ti d M h i St d G id K i t M ll
8/10/2019 Reactions of Alkenes and Alkynes Study Guide
17/17
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
CHEM203 Reactions and Mechanisms Study Guide Kristen Mascall
Boston University
Nov 2012