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Which of these is more acidic and why exactly: fluoroacetic acid or chloroacetic acid; p-fluorobenzoic acid or p-chlorobenzoic acid?
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Mike Oc
Which of these is more acidic and why exactly: fluoroacetic acid or chloroacetic acid; p-fluorobenzoic acid or p-chlorobenzoic acid?
The strength of acids is related to the equation below where HA is any acid (lowry-bronsted acid).
HA = H(1+) + A(1-)
The further to the right the equilibrium is, the stronger the acid. So, if A has electron withdrawing substituents attached, these will reduce the charge in A(1-) which resides largely on the functional group that H+ came off of (e.g., -COOH). So, in terms of electronegativity, F is greater than Cl so F will increase acidity more than Cl. This explains the order fluoroacetic acid > chloroacetic acid as well as p-fluorobenzoic acid > p-chlorobenzoic. The acetic acid derivatives are more acidic than the benzoic acid derivatives due to the proximity of the F or Cl. It is closer to the COOH in the acetic acid derivatives than in the benzoic acid derivatives.
There is one caveat, acetic acid is a weaker acid than benzoic acid (pKa 4.75 and 4.2, respectively) which gives the benzoic acids a head start toward being more acidic. So, what are the pKas
The strength of acids is related to the equation below where HA is any acid (lowry-bronsted acid).
HA = H(1+) + A(1-)
The further to the right the equilibrium is, the stronger the acid. So, if A has electron withdrawing substituents attached, these will reduce the charge in A(1-) which resides largely on the functional group that H+ came off of (e.g., -COOH). So, in terms of electronegativity, F is greater than Cl so F will increase acidity more than Cl. This explains the order fluoroacetic acid > chloroacetic acid as well as p-fluorobenzoic acid > p-chlorobenzoic. The acetic acid derivatives are more acidic than the benzoic acid derivatives due to the proximity of the F or Cl. It is closer to the COOH in the acetic acid derivatives than in the benzoic acid derivatives.
There is one caveat, acetic acid is a weaker acid than benzoic acid (pKa 4.75 and 4.2, respectively) which gives the benzoic acids a head start toward being more acidic. So, what are the pKas
My prediction is
fluoroacetic acid > chloroacetic acid > p-fluorobenzoic acid > p-chlorobenzoic acid
The strength of acids is related to the equation below where HA is any acid (lowry-bronsted acid).
HA = H(1+) + A(1-)
The further to the right the equilibrium is, the stronger the acid. So, if A has electron withdrawing substituents attached, these will reduce the charge in A(1-) which resides largely on the functional group that H+ came off of (e.g., -COOH). So, in terms of electronegativity, F is greater than Cl so F will increase acidity more than Cl. This explains the order fluoroacetic acid > chloroacetic acid as well as p-fluorobenzoic acid > p-chlorobenzoic. The acetic acid derivatives are more acidic than the benzoic acid derivatives due to the proximity of the F or Cl. It is closer to the COOH in the acetic acid derivatives than in the benzoic acid derivatives.
There is one caveat, acetic acid is a weaker acid than benzoic acid (pKa 4.75 and 4.2, respectively) which gives the benzoic acids a head start toward being more acidic. So, what are the pKas
fluoroacetic acid pKa: 2.66
chloroacetic acid pKa: 2.87
p-fluorobenzoic acid pKa: 4.14
p-chlorobenzoic acid pKa: 3.98
OK, well 3 out of 4 is not bad. I don’t know why p-chlorobenzoic acid is slight stronger acid than p-fluorobenzoic acid.
My prediction is
fluoroacetic acid > chloroacetic acid > p-fluorobenzoic acid > p-chlorobenzoic acid
The strength of acids is related to the equation below where HA is any acid (lowry-bronsted acid).
HA = H(1+) + A(1-)
The further to the right the equilibrium is, the stronger the acid. So, if A has electron withdrawing substituents attached, these will reduce the charge in A(1-) which resides largely on the functional group that H+ came off of (e.g., -COOH). So, in terms of electronegativity, F is greater than Cl so F will increase acidity more than Cl. This explains the order fluoroacetic acid > chloroacetic acid as well as p-fluorobenzoic acid > p-chlorobenzoic. The acetic acid derivatives are more acidic than the benzoic acid derivatives due to the proximity of the F or Cl. It is closer to the COOH in the acetic acid derivatives than in the benzoic acid derivatives.
There is one caveat, acetic acid is a weaker acid than benzoic acid (pKa 4.75 and 4.2, respectively) which gives the benzoic acids a head start toward being more acidic. So, what are the pKas
fluoroacetic acid pKa: 2.66
chloroacetic acid pKa: 2.87
p-fluorobenzoic acid pKa: 4.14
p-chlorobenzoic acid pKa: 3.98
OK, well 3 out of 4 is not bad. I don’t know why p-chlorobenzoic acid is slight stronger acid than p-fluorobenzoic acid.
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Please check your info properly. I have got pka that supports p-chlorobenzoic acid being stronger acid than benzoic acid.
Please check your info properly. I have got pka that supports p-chlorobenzoic acid being stronger acid than benzoic acid.
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