Organic Chemistry/Questions and Discussions

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Q1[edit]

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

Discussion[edit]

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[edit]

Nucleophillicity and basicity[edit]

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[edit]

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

Answers[edit]

A little variation[edit]

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

Answers[edit]

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