# Organic Chemistry/Questions and Discussions

Name the reagent used to distinguish the following pair Carboxylic and
ethanol Alkyne and alkene Butan-1-yne and Butan-2-yne

## Q1

**What is more acidic, cyclopentadiene or penta-1,4-diene? What is the
application of Huckel\'s rule here?**

### Discussion

In this context acidity is the ability to lose a proton.We will decide
the acidity on the basis of the stability of the resulting carbanion
\[Why did we emphasize the word \"context\" here? How else can a
molecule behave as an acid?\]\[Why is it that a carbanion is formed?\]
\[How did we relate stability of carbanion with acidity here?\]

Also, the number of carbons in both the compounds are same and are of a
similar structure.It is only now that we can compare the stability of
their conjugate bases \[What are they?\] \[carbanions we were talking
about in this case\...\]

So, first we decide what is the most acidic hydrogen in both the
molecules. In the straight chain carbon molecule, the two hydrogens
attached to the central carbon are more acidic than the rest of the
hydrogens. This is for a specific reason.Once any of the hydrogen is
extracted by a base, the central carbon should acquire a negative
charge.Instead,the molecule is represented by a resonance hybrid of two
degenerate \[equivalent\] resonance structures with a negative charge on
the terminal carbons and a third one with the negative charge on the
central carbon. This is a stabilizing factor of the carbanion. The
presence of this stabilizing factor makes the two hydrogens in the
original molecule acidic.

\[What exactly are we talking about here? What is the conjugate base
whose stability we discussed?\]

In the cyclic molecule, the same thing applies. We again have a molecule
which is represented by resonating structures.But the speciality is that
it has six pi electrons and is cyclic.In other words, by Huckel\'s rule
we find that the cyclopentadiene anion is aromatic. Aromaticity is a
better stabilizing factor than simple resonance. Hence the
cyclopentadiene molecule is the more acidic than the straight chain
molecule. Remember that the comparison could be made only because they
are similar molecules.

What is the smallest aromatic species? Is penta-1,4-diene hydrolyzed by
water? Is cyclopentadiene hydrolyzed by water? How do you relate these
observations to what we just discussed above?

### Additional Questions

#### Nucleophillicity and basicity

Proton is a lewis acid.It\'s also an electrophile. Consider hydroxide
ions- OH-(- indicates negative charge) They are nucleophiles. They are
Lewis bases as well.What property makes them nucleophiles and what
property makes them bases? Still another question: Give an example of a
strong base but a weak nucleophile. What gives the molecule these
characteristics?

#### A corrosive phenol

Which phenol has a boiling point of 132 to 134 degree Celsius and is
corrosive in nature? Dark orange in colour?

#### Answers

#### A little variation

What if the linear chain is hex-1,5-diene? Does aromaticity still
dominate?

#### Answers

If it\'s linear it cannot be aromatic, although it can still contain a
delocalised p-system.
