RE: 2 O chem questions

[sw1sschez]

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Why does a base attack the least subsituted and and why does an acid attack the most substituted end? I cant figure it out.

I am not sure why the Cl from SOCl2 didnt add a Cl to the meta position since the COOH is meta directing on a benzene ring. Instead it adds to the carboxylic acid and displaces the OH instead. I do know that SOCl2 is used to add a Cl though. The answer is B btw.

 

[wantVCUdental]

when attacking epoxides, base attacks less substituted, because it proceeds to something similar to an Sn2 mechanism.
In acid, the O is first protonated and the ring opens up in somehting similar to the Sn1 mechanism. a carbocation forms on the most substituted carbon and the incoming nucleophile adds there (which is the more substituted carbon)

In the second question, SOCl2 is a halide adder, it replaces alcohol groups with halide (Chloride) and the benzoic acid becomes more reactive because now it's an acyl chloride, it reacts quickly with amonia to form the amide (why is B the answer? confused)

 

[sw1sschez]

when attacking epoxides, base attacks less substituted, because it proceeds to something similar to an Sn2 mechanism.
In acid, the O is first protonated and the ring opens up in somehting similar to the Sn1 mechanism. a carbocation forms on the most substituted carbon and the incoming nucleophile adds there (which is the more substituted carbon)

I'm not sure about your second answer but the first one. You started me thinking but it's different from how you arrived though.

In my answer, I have an Acid protonating the O, it turns into a + charge, but then the least substituted end is attacked because it is the least hindered. I dont see how your carbocation comes into play at all. Why is there a carbocation to begin with? I dont see a leaving group. I thought both would go via Sn2 reaction of some sort.

 

[sw1sschez]

(why is B the answer? confused)

It's because the Cl is a good leaving group. And the ammonia is a good nucleophile.

Ok, I think I see your answer for #1. It makes sense when I draw it out.

 

[UCB05]

I'm not sure about your second answer but the first one. You started me thinking but it's different from how you arrived though.

In my answer, I have an Acid protonating the O, it turns into a + charge, but then the least substituted end is attacked because it is the least hindered. I dont see how your carbocation comes into play at all. Why is there a carbocation to begin with? I dont see a leaving group. I thought both would go via Sn2 reaction of some sort.

Imagine the -OH+ leaving and breaking the bond to the more-substituted carbon. The -OH doesn't really leave because it's still attached to the least-substituted carbon. This leaves a HO-C-C+(CH3)2 carbocation intermediate. Next a nucleophile attacks it at the more-substituted carbon. Instead of a concerted nucleophilic attack+ring opening, protonation creates a good leaving group which caues the ring opens first, before nucleophilic attack, similar to Sn1 mechanics.

 

[wantVCUdental]

It's because the Cl is a good leaving group. And the ammonia is a good nucleophile.

Ok, I think I see your answer for #1. It makes sense when I draw it out.

By the way, for the second question, A is the answer. it's from TS right?

 

[c3p]

to answer why this certain reagent doesnt add meta, or to any position on the benzene:

in order for a Cl to add meta you would need AlCl3 and Cl2 (or if Br you need FeBr3 and Br2)...remember that benzene doesnt do nucleophillic rxns, it does electrophillic so if the halide is to be added to the benzne it will be electropositive.....so the whole reason Al, Fe are attached is that they take one Cl or Br with the electrons in the bond so they render the remaining halide (from the original X2) an electrophile....same thing with adding R groups to the benzne (ie alkylation, acylation).

 

[americanpierg]

For question 2, review the mechanisms for carboxylic acid derivatives. acyl halides, anhydrides, amides, esters, etc. Know them by heart. They're all very similar so it shouldn't be too difficult. Learn which can be converted to which, ie almost everything can be converted to amides, but amides have a hard time converting into everything else. acyl halides are most reactive, and amides are least reactive.

 

[sw1sschez]

Thanks for the input everyone. No the answer is still B and yes its from TS.

c3p:

What's the diff between SOCl2 and Al/AlCl3?

to answer why this certain reagent doesnt add meta, or to any position on the benzene:

in order for a Cl to add meta you would need AlCl3 and Cl2 (or if Br you need FeBr3 and Br2)...remember that benzene doesnt do nucleophillic rxns, it does electrophillic so if the halide is to be added to the benzne it will be electropositive.....so the whole reason Al, Fe are attached is that they take one Cl or Br with the electrons in the bond so they render the remaining halide (from the original X2) an electrophile....same thing with adding R groups to the benzne (ie alkylation, acylation).

 

[wantVCUdental]

Thanks for the input everyone. No the answer is still B and yes its from TS.

c3p:

What's the diff between SOCl2 and Al/AlCl3?

AlCl3 is a lewis acid (it can accept electrons) and so it reacts favorably with a benzene ring and activate it so it's more willing to accept the Chlorides coming in.
SOCl2 on the other hand is not a good lewis acid (S already has its quartet, so it's unlikely to accept more electrons) and won't activate the benzene for Cl2 to react.

 

[americanpierg]

Thanks for the input everyone. No the answer is still B and yes its from TS.

c3p:

What's the diff between SOCl2 and Al/AlCl3?

The answer for question 2 isn't B, it's A. I'm 110% sure. If topscores says the answer is B, then topscore is wrong.

 

[c3p]

AlCl3 is a lewis acid (it can accept electrons) and so it reacts favorably with a benzene ring and activate it so it's more willing to accept the Chlorides coming in.
SOCl2 on the other hand is not a good lewis acid (S already has its quartet, so it's unlikely to accept more electrons) and won't activate the benzene for Cl2 to react.

yep exactly....so just think of AlCl3 as making one Cl electropositive (which benzne "wants") and think of SOCl2 as rendering one/the reacting Cl as a chloride ion which is not electropositive)

to summarize: AlCl3 makes one Cl as ClS+ (the delta partial positive symbol) so it will react w/ benzenes

SOCl2 will make one Cl as Cl- (nucleophillic so it reacts with things like ROH, RCOOH and NOT benzene)

same goes for Br and PBr3 and FeBr3

 

[c3p]

The answer for question 2 isn't B, it's A. I'm 110% sure. If topscores says the answer is B, then topscore is wrong.

i second this 110% guarntee, ts is wrong. acyl chlorides are the most reactive of the derivatives and react quite readily with NH3 to form an amide.

 

[sw1sschez]

i second this 110% guarntee, ts is wrong. acyl chlorides are the most reactive of the derivatives and react quite readily with NH3 to form an amide.

Oops. Nope you guys are right. It is A. B was the answer for step 1 before the amide attacked.😀

 

[K Niner]

to answer why this certain reagent doesnt add meta, or to any position on the benzene:

in order for a Cl to add meta you would need AlCl3 and Cl2 (or if Br you need FeBr3 and Br2)...remember that benzene doesnt do nucleophillic rxns, it does electrophillic so if the halide is to be added to the benzne it will be electropositive.....so the whole reason Al, Fe are attached is that they take one Cl or Br with the electrons in the bond so they render the remaining halide (from the original X2) an electrophile....same thing with adding R groups to the benzne (ie alkylation, acylation).

Benzene does do nucleophillic reactions. Its called the SnAr mechanism, and covered in regular ochem classes.

 

[c3p]

Benzene does do nucleophillic reactions. Its called the SnAr mechanism, and covered in regular ochem classes.

yes i know but as far as the dat i highly doubt you will need it. the conditions under which snar occurs is too specific for the dats. if this were an ochem class fourm i would have specified. but yes nucleophilic attack on benzenes can occur, but dont worry about it on the dats.