March 28, 2024, 11:18:45 AM
Forum Rules: Read This Before Posting


Topic: Difference in wave number for the O-H stretch in carboxylic acids and alkanols  (Read 6025 times)

0 Members and 1 Guest are viewing this topic.

Offline ajax0604

  • Regular Member
  • ***
  • Posts: 76
  • Mole Snacks: +3/-1
Could someone explain why the wave number associated with the O-H stretch in carboxylic acids is lower at 2500-3300, while the O-H stretch in alcohols is higher at 3230-3550? 

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Can you think of any relationship between wavelength and bond energy?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline ajax0604

  • Regular Member
  • ***
  • Posts: 76
  • Mole Snacks: +3/-1
The wave number for the OH stretch in carboxylic acid is lower which means that the bond energy isn't as great as it is in an alcohol? Does this have something to do with the C=O group?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
What happens to acids in water? What happens to alcohols in water?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline ajax0604

  • Regular Member
  • ***
  • Posts: 76
  • Mole Snacks: +3/-1
So carboxylic acid will partially ionise whereas alcohol will form hydrogen bonds with water? 

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
Yes, acid will ionize, alcohol will NOT ionize.

Do you see a connection with bond strength?
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline ajax0604

  • Regular Member
  • ***
  • Posts: 76
  • Mole Snacks: +3/-1
Yes I get the connection. Why does the proton come off more readily from the carboxylic acid?

Offline Borek

  • Mr. pH
  • Administrator
  • Deity Member
  • *
  • Posts: 27637
  • Mole Snacks: +1799/-410
  • Gender: Male
  • I am known to be occasionally wrong.
    • Chembuddy
One answer you already know: because its bonding energy is lower.

Second answer (which is generally the same, just makes a handwavy argument about it): -COO- is stabilized by resonance.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info

Offline ajax0604

  • Regular Member
  • ***
  • Posts: 76
  • Mole Snacks: +3/-1
Thanks. It's your second answer I didn't think of until you mentioned it - the -COO- is stabilized by resonance whereas alcohols have no such resonance forms.

Sponsored Links