superhero1219 Posted August 29, 2009 Posted August 29, 2009 We had a quiz and I got one of the questions wrong and it had to do with the Tollen's test. I missed the lecture on this topic and my book doesn't do a very good job of explaining it. I've posted the question followed by the general equation for the test, which I understand. It converts an aldehyde to a carboxylic acid. The problem I'm having is applying it to something that's in a chair conformation... there is no aldehyde. Can someone show me what the product would be and explain why?
UC Posted August 29, 2009 Posted August 29, 2009 (edited) We had a quiz and I got one of the questions wrong and it had to do with the Tollen's test. I missed the lecture on this topic and my book doesn't do a very good job of explaining it. I've posted the question followed by the general equation for the test, which I understand. It converts an aldehyde to a carboxylic acid. The problem I'm having is applying it to something that's in a chair conformation... there is no aldehyde. Can someone show me what the product would be and explain why? The pyranose (ring form) and chain form interconvert readily in water. Pay close attention to the attached image. I'm sure you can figure out the rest. On a technical note, those arrows should actually all be double-ended equilibrium arrows. The back-and-forth conversion allows for the interconversion of alpha and beta dextrose (which is what that molecule is). Reducing sugars are capable or ring opening to the chain form and have an aldehyde group. Do you see the ether-like portion of the ring and the hydroxide on the next carbon over? This hydroxide becomes the aldehyde group. The configuration is called a hemiacetal. You can look that up for more information. Nonreducing sugars have another ether-like bond formed with that hydroxy group (an acetal), preventing them from opening easily in this manner. As a side note, some ketoses (sugars with a ketone group in the chain form, instead of an aldehyde), which are intuitively not reducing sugars can interconvert into aldoses (ones with the aldehyde) via tautomerism. This is the case for fructose. Edited August 29, 2009 by UC
superhero1219 Posted August 31, 2009 Author Posted August 31, 2009 Thanks for the reply! So in the case of the quiz question the answer would be an open chain with a carboxylic acid on one end?
UC Posted September 1, 2009 Posted September 1, 2009 Thanks for the reply! So in the case of the quiz question the answer would be an open chain with a carboxylic acid on one end? Yes, it's called gluconic acid.
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now