- #1
maverick280857
- 1,789
- 5
Hello
Here are a few questions:
1.
[tex]O_{2}N-CH_{2}-COOH[/tex] pKa = 1.68
[tex]Me_{3}N^{+}-CH_{2}-COOH[/tex] pKa = 1.83
According to my book (Guidebook to Mechanism in Organic Chemistry), the first carboxylic acid is more acidic than the second. I fail to understand the reason for this, since in class we were told that groups which have actual positive charge (such as the N,N,N-trimethyl amino group) have greater -I effect than those with lone pairs or partial positive charges (or polarizable groups). With this line of reasoning, the second acid should be more acidic. Can someone explain why this is not so? Has it got something to do with +I effect/steric effects of three Me groups?
2. Why is o-bromobenzoic acid more acidic than o-chlorobenzoic acid? (pKa of o-bromobenzoic acid = 2.85, pKa of o-chlorobenzoic acid = 2.94)
I understand that both acides will be stronger than benzoic acid on account of Ortho Effect. As ortho effect is active in both acids, we must find some other parameter to compare relative acidity. If -I effect is chosen as a parameter, then Cl is a stronger inductively withdrawing group than Br so the chloro substituted benzoic acid should be stronger contrary to experimental data.
I would be very grateful if someone could help me with these problems.
Thanks and cheers
Vivek
Here are a few questions:
1.
[tex]O_{2}N-CH_{2}-COOH[/tex] pKa = 1.68
[tex]Me_{3}N^{+}-CH_{2}-COOH[/tex] pKa = 1.83
According to my book (Guidebook to Mechanism in Organic Chemistry), the first carboxylic acid is more acidic than the second. I fail to understand the reason for this, since in class we were told that groups which have actual positive charge (such as the N,N,N-trimethyl amino group) have greater -I effect than those with lone pairs or partial positive charges (or polarizable groups). With this line of reasoning, the second acid should be more acidic. Can someone explain why this is not so? Has it got something to do with +I effect/steric effects of three Me groups?
2. Why is o-bromobenzoic acid more acidic than o-chlorobenzoic acid? (pKa of o-bromobenzoic acid = 2.85, pKa of o-chlorobenzoic acid = 2.94)
I understand that both acides will be stronger than benzoic acid on account of Ortho Effect. As ortho effect is active in both acids, we must find some other parameter to compare relative acidity. If -I effect is chosen as a parameter, then Cl is a stronger inductively withdrawing group than Br so the chloro substituted benzoic acid should be stronger contrary to experimental data.
I would be very grateful if someone could help me with these problems.
Thanks and cheers
Vivek