[etherkid21]

I have a question that says

Draw the structures and write the IUPAC names of the two alkenes that are formed when 2-hexanol undergoes a condensation reaction in the prescence of an acid catalyst

correct me if i'm wrong but doesnt an alcohol condensation reaction produce an ether + water?, how can it form two alkenes? is it a series of reactions?..

[sjb]

Whilst you are right in that a condensation reaction between two alcohol molecules will produce an ether, it's not the full story.

What is a condensation - can you write a generic equation for it?

In my opinion there are actually 3 alkenes possible, but this may depend on the level you're asking the question at..

[azmanam]

I'm confused.  If you're forming alkenes, then the reaction would be called a dehydration.  You will get two constitutional isomers.

If you're really doing condensation reaction, you will make ethers.  you will not form alkenes, and the question should not ask you about alkenes.  You will get 1 ether.

Because it is more likely that alkenes was used purposefully, and it is almost excusable to mistake dehydration with condensation, I would say this is supposed to be a dehydration reaction, and you are supposed to form alkenes.  You should look for the two constitutional isomers formed but sjb is right - there are technically 3 alkenes formed.

[sjb]

Maybe this is a language thing? To me, the formation of alkenes from alcohols is just as much a condensation as forming ethers, as in both cases you're losing water in each case - my idiolect would make no distinction between one molecule of water for each alcohol or one molecule of water between two alcohols. Does IUPAC formally define dehydration / condensation?

S

[azmanam]

Perhaps its just the way I organize it in my head?  Condensation to me implies the joining of 2 molecules with concomitant loss of water.  Dehydration to me implies the fragmentation of one molecule into two, with one of them being water.

Wikipedia seems to agree with me on condensation, but not on dehydration.  It calls a condensation reaction a reaction between 2 molecules with loss of a small molecule.  If it loses water, it calls it a dehydration, but loss of HCl, MeOH, or AcOH can also accompany a condensation reaction.  In the article on dehydration reactions, it lists alkene formation from an alcohol like we're talking about here, but also lists ether formation (and anhydride formation) as dehydration reactions.  Reactions I would have classified as condensations but not dehydrations.

IUPAC Gold book says two or more reactants (or remote reactive sites within the same molecule, like the Dieckmann condensation), but does not define dehydration reaction.

At any rate, with the question specifically referring to two alkenes formed, I think the ether product is not the product the question is looking for.  I think we are both in agreement that the question is asking for the two constitutional isomers (plus the geometric isomers that are formed with Zaitsev's rule).

http://en.wikipedia.org/wiki/Dehydration_reaction
http://en.wikipedia.org/wiki/Condensation_reaction
http://goldbook.iupac.org/C01238.html

[Astrokel]

hey azmanam,sjb

(plus the geometric isomers that are formed with Zaitsev's rule).

could you tell me how it looks like beside the two alkenes formed, is it a trans and cis of one of the alkene formed? i have read up on Zaitsev's rule and it only mentions more substitued alkene will be the major product? just being curious

thank you!

[azmanam]

2 hexanol could dehydrate in either of two directions.  It could form 1-hexene or 2-hexene.  2-hexene will be favored as a result of Zaitsev's rule.  2-hexene has 2 geometric isomers (E and Z).  They will be formed in roughly equal mixture (the E might be slightly more favored due to sterics).  Thus there are 2 constitutional isomers formed, and one of the constitutional isomers has 2 geometric isomers.

[Astrokel]

thanks i got it now! how do i determine which follows Zaitsev's rule?

C-C-C-C-C  ---->elimination    C-C-C-C=C +  C-C-C=C-C
          |                               1  2 3 4   5     1 2  3  4  5
          Br

the 2-pentene formed is the major product. is it because it has two substitutes carbon 2 and 5 on the left and right of the double bond respectively? whereas 1-pentene has only carbon 3 as substutes?

[azmanam]

a) number it the other direction.  You named it correctly (2-pentene), but you drew it with the double bond at position 3.  The substituents should be carbons 1 and 3, not 2 and 5.

b) Zaitsev's rule follows from the mechanism.  In the transition state (at least for E2), the base has partially removed the beta proton (the proton on the carbon adjacent to the carbon bearing the leaving group), the electrons from the carbon-hydrogen bond are partially collapsing to form the new pi bond, and the leaving group has partially left.  Double bond character is so highly developed in this transition state that the relative stability of the alkenes determines which alkene is the major product.  (Same is true for E1, but the leaving group has already left...)

Put another way, more substituted alkenes are more stable than less substituted alkenes.  The transition state energy leading to more substituted alkenes is lower.  This means the activation energy leading to the transition state leading to the more substituted alkene is lower.  This means the more substituted alkene is formed preferentially over the less substituted alkene.

[Astrokel]

thanks so much for the detailed info! i have better understanding now