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aromatic rings

im really stuck :frown:

In my textbook it says that the oxygen that is connected to benenze donates its electrons/lone pair into benzenes aromatic pi system ...making the ring electron rich

but i thought only the pi electrons in the ring are the only ones that are involved in aromaticity? :dontknow:

any help??

Reply 1

Prested

You might have been told, that phenol is a weak acid, while ethanol is not. Both have the -OH group, but somehow the O-H bond in phenol may be broken easier and that's why it has acidic character. Why is that? Of course, because the group in phenol is connected to the aromatic ring. The ring influences in the following way: Lone pair of the oxygen is attracted to the ring. This way the ring gains a negative charge near the bond with oxygen, and oxygen gains a positive charge. The negative charge is then 'moving' around to finally get to its initial position. This process is going over and over (see attached photo). This way the bond O-H is weaker, because the electron density is 'pulled' toward the aromatic ring. Notice, that is actually why -OH group activated positions ortho- and para-.
Answering the question: As you can see, the photo shows creating a double bond. This indicates that pi electrons of the oxygen become delocalized just as the electrones of the ring. They thus become a part of the aromaticity
See the photo, blue arrows show electrons' movement while the green arrows show different forms (these are called resonance structures)

(This is the appropriate version)

Just to finish about the acidity: Of course if oxygen loses the electron density from the side of aromatic ring, it pulls the electrons of the -OH bond. Thus, the bond is broken easier.
I'm actually not an undergraduate student. I'm still in high school, so I might have said something wrong. Sorry for the possible inconvenience. :wink:

(edited 10 years ago)

hydroxy-group-bonded-to-benzene-ring.png6.6KB

Reply 2

username913907

Original post by Prested

You might have been told, that phenol is a weak acid, while ethanol is not. Both have the -OH group, but somehow the O-H bond in phenol may be broken easier and that's why it has acidic character. Why is that? Of course, because the group in phenol is connected to the aromatic ring. The ring influences in the following way: Lone pair of the oxygen is attracted to the ring. This way the ring gains a negative charge near the bond with oxygen, and oxygen gains a positive charge. The negative charge is then 'moving' around to finally get to its initial position. This process is going over and over (see attached photo). This way the bond O-H is weaker, because the electron density is 'pulled' toward the aromatic ring. Notice, that is actually why -OH group activated positions ortho- and para-.
Answering the question: The donation is not a part of aromaticity, but it's a result of oxygen being attached to aromatic ring (An oxygen atom alone cannot be attached, as it would need a double bond.)
See the photo, blue arrows show electrons' movement while the green arrows show different forms (these are called resonance structures)

EDIT: As you can see, the photo shows creating a double bond. This indicates that pi electrons of the oxygen become delocalized just as the electrones of the ring.
Just to finish about the acidity: Of course if oxygen loses the electron density from the side of aromatic ring, it pulls the electrons of the -OH bond. Thus, the bond is broken easier.
I'm actually not an undergraduate student. I'm still in high school, so I might have said something wrong. Sorry for the possible inconvenience. :wink:

Your resonance structures are correct but you don't seem to appreciate that these forms never actually exist. The correct picture is a simultaneous combination of these resonance structures leading to a net donation of electron density from the OH into the aromatic ring.

You said something about donation not being part of the aromaticity, odd thing to say..... The O lone pair is part of the delocalised system of Pi electrons

Reply 3

Prested

I do understand, but these structures are well simplifying the whole situation. Sorry not to mention it.

About the delocalisation: I have edited the post a while later, but yes, I did state clearly that they are not a part of aromaticity. My fault, sorry. :wink:

Reply 4

EierVonSatan

Original post by JMaydom

Your resonance structures are correct

Are they?

The main 'take home message' here is that the oxygen pi electrons create an extended pi system with the aromatic ring - this lowers it's energy, making phenol more acidic than simple chain alcohols :yep: