Formation of glycols with syn addition
1 / 39

Formation of glycols with Syn Addition - PowerPoint PPT Presentation

  • Updated On :

Formation of glycols with Syn Addition. Osmium tetroxide. Syn addition. also. KMnO 4. Anti glycols. Using a peracid, RCO 3 H, to form an epoxide which is opened by aq. acid. epoxide. The protonated epoxide is analagous to the cyclic bromonium ion. Peracid: for example, perbenzoic acid.

Related searches for Formation of glycols with Syn Addition

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Formation of glycols with Syn Addition' - soo

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Formation of glycols with syn addition l.jpg
Formation of glycols with SynAddition

Osmium tetroxide

Syn addition



Anti glycols l.jpg
Anti glycols

Using a peracid, RCO3H, to form an epoxide which is opened by aq. acid.


The protonated epoxide is analagous to the cyclic bromonium ion.

Peracid: for example, perbenzoic acid

An example l.jpg
An example

Are these unique?

Diastereomers, separable (in theory) by distillation, each optically active

Ozonolysis l.jpg

Reaction can be used to break larger molecule down into smaller parts for easy identification.

Ozonolysis example l.jpg
Ozonolysis Example

For example, suppose an unknown compound had the formula C8H12 and upon ozonolysis yielded only 3-oxobutanal. What is the structure of the unknown?

The hydrogen deficiency is 18-12 = 6. 6/2 = 3 pi bonds or rings.

The original compound has 8 carbons and the ozonolysis product has only 4

Conclude: Unknown  two 3-oxobutanal.




Simply remove the new oxygens and join to make double bonds.

But there is a second possibility.

Another example l.jpg
Another Example


Hydrogen Deficiency = 8. Four pi bonds/rings.



Unknown has no oxygens. Ozonolysis product has four. Each double bond produces two carbonyl groups. Expect unknown to have 2 pi bonds and two rings.


To construct unknown cross out the oxygens and then connect. But there are many ways the connections can be made.

Look for a structure that obeys the isoprene rule.

Mechanism l.jpg

Consider the resonance structures of ozone.

Electrophile capability.

Nucleophile capability.

These two, charged at each end, are the useful ones to think about.

Hydrogenation l.jpg

No regioselectivity

Syn addition

Heats of hydrogenation l.jpg
Heats of Hydrogenation

Consider the cis vs trans heats of hydrogenation in more detail…

Heats of hydrogenation 2 l.jpg
Heats of Hydrogenation - 2

The trans alkene has a lower heat of hydrogenation.

  • Conclusion:

  • Trans alkenes with lower heats of hydrogenation are more stable than cis.

  • We saw same kind of reasoning when we talked about heats of combustion of isomeric alkanes to give CO2 and H2O

Heats of hydrogenation14 l.jpg
Heats of Hydrogenation

Increasing substitution

Reduced heat of Hydrogenation

By same reasoning higher degree of substitution provide lower heat of hydrogenation and are, therefore, more stable.

Slide15 l.jpg

Acid Catalyzed Polymerization

Principle: Reactive pi electrons (Lewis base) can react with Lewis acid. Recall

Which now reacts with a Lewis base, such as halide ion to complete addition of HX yielding 2-halopropane

Variation: there are other Lewis bases available. THE ALKENE.

The new carbocation now reacts with a Lewis base such as halide ion to yield halide ion to yield 2-halo-4-methyl pentane (dimerization) but could react with another propene to yield higher polymers.

Examples of synthetic planning l.jpg
Examples of Synthetic Planning

Give a synthesis of 2-hexanol from any alkene.


Alkene is a hydrocarbon, thus we have to introduce the OH group

How is OH group introduced (into an alkene): hydration

  • What are hydration reactions and what are their characteristics:

  • Mercuration/Reduction: Markovnikov

  • Hydroboration/Oxidation: Anti-Markovnikov and syn addition

Slide17 l.jpg

What alkene to use? Must involve C2 in double bond.

Which reaction to use with which alkene?

Markovnikov rule can be applied here. CH vs CH2.

Want Markovnikov!

Use Mercuration/Reduction!!!

Markovnkov Rule cannot be used here. Both are CH.

Do not have control over regioselectivity.

Do not use this alkene.

For yourself : how would you make 1 hexanol, and 3-hexanol?

Slide18 l.jpg

Another synthetic example…

How would you prepare meso 2,3 dibromobutane from an alkene?


Alkene must be 2-butene. But wait that could be either cis or trans!

We want meso. Have to worry about stereochemistry

Know bromine addition to an alkene is anti addition (cyclic bromonium ion)

Slide19 l.jpg

This did not work, gave us the wrong stereochemistry!

This worked! How about starting with the cis?

Slide20 l.jpg

Addition Reaction General Rule…

Characterize Reactant as cis or trans, C or T

Characterize Reaction as syn or anti, S or A

Characterize Product as meso or racemic mixture, M or R


Characteristics can be changed in pairs and C A R will remain true.

Want meso instead?? Have to use trans. Two changed!!

Structure l.jpg

sp hybridization

Acidity of terminal alkynes l.jpg
Acidity of Terminal Alkynes

Stronger base

Weaker base

Other strong bases that will ionize the terminal alkyne:


Important synthetic method dehydrohalogenation l.jpg
Important Synthetic Method: Dehydrohalogenation

1. Dehydrohalogenation…

An alkyl halide can eliminate a hydrogen halide molecule, HX, to produce a pi bond.

Recall that HX can be added to a double bond to make an alkyl halide.

HX can also be removed by strong base, called dehydrohalogenation.

Preparation of alkene

Strong base


Or rewriting



Also, if we start with a vinyl halide and a very strong base (vinyl halides are not very reactive).



Synthetic planning retrosynthesis l.jpg
Synthetic planning (Retrosynthesis)

Work Backwards…..

Trace the reactions sequence from the desired product back to ultimate reactants.

Starting reactant

Target molecule.

Overall Sequence converts alkene alkyne

But typical of synthetic problems side reaction occurs to some extent and must be taken into account.


More sythesis nucleophilic substitution l.jpg
More Sythesis: Nucleophilic Substitution

Use the acidity of a terminal alkyne to create a nucleophile which then initiates a substitution reaction.

Note that we still have an acidic hydrogen and, thus, can react with another alkyl group in this way to make RCCR’

Alkyl halides can be obtained from alcohols

Reactions alkyne with halogen l.jpg
Reactions: alkyne with halogen


No regioselectivity with Br2.

Stereoselective for trans addition.

Reactions addition of hx l.jpg
Reactions: Addition of HX

The expected reaction sequence occurs, formation of the more stable carbocation.

Markovnikov orientation for both additions.

Now for the mechanism….

Mechanism31 l.jpg

The expected reaction sequence occurs, formation of the more stable carbocation.

Reactions acid catalyzed hydration markovnikov l.jpg
Reactions: Acid catalyzed Hydration (Markovnikov).

Markovnikov addition, followed by tautomerism to yield, usually, a carbonyl compound.

Reactions anti markovnikov hydration of alkynes regioselectivity l.jpg
Reactions: Anti Markovnikov Hydration of Alkynes, Regioselectivity

Step 2

Step 1

Similar to formation of an anti-Markovnikov alcohol from an alkene

Step 1, Internal Alkyne: addition to the alkyne with little or no regioselectivity issue.

Alternatively Asymmetric, terminal, alkyne if you want to have strong regioselectivity then use a borane with stronger selectivity for more open site of attack.

Less exposed site.

Aldehyde not ketone.

More exposed site.


Tautomerism enol carbonyl l.jpg
Tautomerism, enol Regioselectivity carbonyl

Step 2, Reaction of the alkenyl borane with H2O2, NaOH would yield an enol. Enols are unstable and rearrange (tautomerize) to yield either an aldehyde or ketone.


internal alkyne   ketone (possibly a mixture, next slide)

Terminal alkyne   aldehyde

Examples l.jpg
Examples Regioselectivity

Used to insure regioselectivity.

As before, for a terminal alkyne.

But for a non-terminal alkyne frequently will get two different ketones

Get mixture of alkenyl boranes due to low regioselectivity.

Reduction alkyne alkene l.jpg
Reduction, Alkyne Regioselectivity Alkene

1. Catalytic Hydrogenation

If you use catalysts which are also effective for alkene hydrogenation you will get alkane.

You can use a reduced activity catalyst (Lindlar), Pd and Pb, which stops at the alkene. You obtain a cisalkene.

Syn addition

Reduction 2 l.jpg
Reduction - 2 Regioselectivity

2. Treatment of alkenyl borane with a carboxylic acid to yield cis alkene.

Instead of H2O2 / NaOH

Alkenyl borane

3. Reduction by sodium or lithium in liquid ammonia to yield the trans alkene.

Plan a synthetic sequence retrosynthesis l.jpg
Plan a Synthetic Sequence RegioselectivityRetrosynthesis


Synthesize butan-1-ol from ethyne. Work backward from the target molecule.

A big alkyne can be formed via nucleophilic substitution. This is the chance to make the C-C bond we need.

Is read as “comes from”.

Major problem: make big from small. Be alert for when the “disconnect” can be done.

Catalytic Lindlar reduction

  • BH3

  • H2O2, NaOH

Convert ethyne to anion and react with EtBr.

Do a “disconnect” here.

Target molecule

Catalytic reduction Lindlar

Addition of HBr.

Now, fill in the “forward reaction” details

Can we get an alkyne from smaller molecules?

Not yet! So how can we get it?

How about joining molecules to get an alkene? Not yet!! So how can we get an alkene?

Ask yourself! Do we know how to join any two molecules together to yield an alcohol?