Organic Chem: Benedict's test and acids - false positives?

[anisotropic]

Since Benedict's test is testing for the presence of a reducing agent (in the form of a carbohydrate), would that not mean it would give a false positive if acid was present (i.e. is acid not considered to be a reducing agent, it being a proton donor)?

 

[alxm]

There's no correlation in general between acid-base and oxidant-reductant. But if anything, acids tend to be oxidants. Stick some metal in acid and see what happens.

 

[anisotropic]

'Oxidants' as in oxidizing agents?

I'm really quite confused because I'm trying to figure out whether or not Benedict's test would be positive or negative for sorbitol and gluconic acid, at unspecified pH levels (assume pH 7? seems like a trick problem, hence my confusion).

 

[Borek]

To be precise it is H⁺ that is an oxidizer.

What is a starting pH of Benedict's reagent? Will it be affected by the acid presence?

 

[anisotropic]

To be precise it is H[up]+[/sup] that is an oxidizer.

What is a starting pH of Benedict's reagent? Will it be affected by the acid presence?

No conditions are specified.

I'm being asked a neutral question; that is, to select which of the following compounds will react positively (i.e. yield a red precipitate) with Benedict's test:

• glucose
• lactose
• sorbitol
• gluconic acid

Again, the question wants a simple "yes/no" answer. I know the answer to the first two, but the latter two have me confused, given the ambiguity of the question.

As far as I have been told, a red precipitate occurs under basic conditions where there is a 2-ketose (which isomerizes to an aldehyde), as well as an aldehyde group under "any" (generic) conditions.

But again, no pH is specified in the question, and I see no 2-ketose or aldehydes in either sorbitol or gluconic acid.

 

[alxm]

Sounds to me like they're asking which of those compounds are reducing sugars.

I'd suggest looking into what makes a reducing sugar reducing.
What's the mechanism?
What does the molecule have to look like?
Do these compounds fit the bill?

 

[anisotropic]

Sounds to me like they're asking which of those compounds are reducing sugars.

I'd suggest looking into what makes a reducing sugar reducing.
What's the mechanism?
What does the molecule have to look like?
Do these compounds fit the bill?

That's precisely what they're asking, and my verdict is no (for the latter two).

 

[chemisttree]

That's precisely what they're asking, and my verdict is no (for the latter two).

Sounds good to me.

 

[newb]

It also sounds to me like you're in 2nd year organic chem @ UWO, and have a lab coming up :)

 

[Suzie Q]

It also sounds to me like you're in 2nd year organic chem @ UWO, and have a lab coming up :)

Except you fail to realize that the question was posted on February of 2009 which is, wait for it, wait for it, LAST YEAR. Newb :P. (I go to UWO and have an organic chemistry lab tomorrow and found this quite hilarious).