Most acidic/basic: carbocation+carboanion

[2thDoc11]

Hey guys. So I'm curious when you add a carbocation vs carboanion? They say which is the most acid/ basic organic compound... Answers are all cyclic but one will be aromatic and others won't but some can become aromatic with either and electronics pair or +...but how do I decide if I'm placing an electronic pair or + because the answer will be different for each.

 

[DAT Destroyer]

If you see a cyclic compound..... and you form a carbocation or a carbanion......be alert for possible aromaticity. If the compound is cyclic, planar, fully conjugated, and contains either 2, 6, 10, 14, 18, or 22 pi electrons.....aromaticity is a likely possibility. For example the cyclopropenium carbocation has aromaticity,,,,,,as would the cyclopentadienyl anion......these would display 2 and 6 pi electrons respectively. Consequently, this will be reflected in much lower pKa values than the usual hydrocarbon. I hope this helps. Dr. Romano

 

[2thDoc11]

If you see a cyclic compound..... and you form a carbocation or a carbanion......be alert for possible aromaticity. If the compound is cyclic, planar, fully conjugated, and contains either 2, 6, 10, 14, 18, or 22 pi electrons.....aromaticity is a likely possibility. For example the cyclopropenium carbocation has aromaticity,,,,,,as would the cyclopentadienyl anion......these would display 2 and 6 pi electrons respectively. Consequently, this will be reflected in much lower pKa values than the usual hydrocarbon. I hope this helps. Dr. Romano

Thank you! I'm actually doing the questions in your book and that's where I'm having trouble. The question asked which was a strongest acid and listed four compounds. One of which was a cyclic propene with the one double bond and another was cyclic ?dipentene? With the two double bonds.... I chose the propene because with the +, it'd be aromatic right? But the answer was the pentene and the solution used - with electron pair.....so how do I differentiate between which one I use??

 

[DAT Destroyer]

For acidity,,,,,,you remove an H.....after you do that....you EXAMINE the anion. The anion of cyclopentadiene has 6 pi electrons. Now ....this means that it is aromatic . The hydrogen " wants " to come off easily to attain this anion stabilization. This is reflected nicely in the literature value of 16 for cyclopentadiene. Hope this helps.

 

[2thDoc11]

For acidity,,,,,,you remove an H.....after you do that....you EXAMINE the anion. The anion of cyclopentadiene has 6 pi electrons. Now ....this means that it is aromatic . The hydrogen " wants " to come off easily to attain this anion stabilization. This is reflected nicely in the literature value of 16 for cyclopentadiene. Hope this helps.

Awesome thank you! And just to make sure, when asked about basicity you remove the hydrogen and inspect the cation? Anion-Acidic, Cation-Basic

 

[DAT Destroyer]

Awesome thank you! And just to make sure, when asked about basicity you remove the hydrogen and inspect the cation? Anion-Acidic, Cation-Basic

That is a bit tricky.....For your purpose focus on an amine. Amines are bases, and aromatic amines are less basic. In the gas phase, the amine that has the most alkyl groups is MOST basic. Why ? Simply add an H.,,,,,and we get 3 alkyl groups and a Hydrogen around the positive alkylammonium cation.....these 3 alkyl groups help to stabilize the POSITIVE charge. Hence, more stability translates into a better base. In water, however, Secondary amines are most basic because hydrogen bonding is also involved in stabilizing the cation. Also....remember....a strong base comes from a weak acid. For example, alkanes are horribly weak acids, with pKa values coming in over 50 !!! Thus, when we remove an H......the basic anion ( a carbanion ) is wickedly strong. I hope this helps. If you can read the chapter in the David Klein book, these comments would be of even greater value to you. Best wishes, Dr. Romano.

 

[Faris_]

Awesome thank you! And just to make sure, when asked about basicity you remove the hydrogen and inspect the cation? Anion-Acidic, Cation-Basic

What seems to be confusing to you is absolute basics of O-chem, no pun intended. You should put the Destroyer down for now and go learn the concepts before testing yourself in a book meant for people already done learning concepts.