Orgo questions from crack the science

[LTami]

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can anyone explain these two questions to me:

1. HBr reacts with CH2=CH-COOH to produce
a. CH2=CH-COBr
b. CH3-CHBr-COOH
c. BrCH2-CHBr-COOH
d. BrCH2CH2-COOH
e. none of the above

2. which of the following arrangement is correct according to increasing acidity:
a. benzoic acid < formic acid < acetic acid
b. acetic acid < benzoic acid < formic acid
c. acetic acid < formic acid < benzoic acid

the answer for Q1 is d and Q2 is b.
I know e-withdrawing groups increase acidity and e-donating groups decrease acidicity. CH3- is e-donating and decreases acidity and benzene is e-withdrawing and should increase acidity. But the answer to Q2 is b. why?
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[Ryltar]

Acetic acid is less acidic than formic acid because it has an electron donating group, CH3. The phenyl group attached to the carboxylic acid group is a mild electron donor, but not as strong of an electron donor as a methyl group, because benzoic acid is resonance stabilized, and the conjugation between the benzene ring and the carbonyl provide an insulating effect on electron donation by the the phenyl group.

 

[Ryltar]

For the first question, I would think that the strong positive dipole on the carbonyl carbon would repel a carbocation on carbon 2, making it less stable than the carbocation on carbon 3. These questions are really hard btw. I've heard crack science is a lot harder than the real DAT.

 

[LTami]

Thanks. For the 2 questions above, crack the science explains that there are -I effect and -R effect. I have no clue what that is. Your explanation is much better than theirs. Thanks

 

[bannanaslug]

(I effect) is Inductive effect which is when an electron negative atom pulls using sigma bonds. resonance effect (r effect) is the stabilizing with pi bonds or the double bond.

 

[peyman2002]

the first question us probably HBr with light