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

In summary, Benedict's test is not specific to reducing sugars and would give a false positive if acid was present.
  • #1
anisotropic
59
0
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)?
 
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  • #2
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.
 
  • #3
'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).
 
  • #4
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?
 
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  • #5
Borek said:
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.
 
  • #6
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?
 
  • #7
alxm said:
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).
 
  • #8
anisotropic said:
That's precisely what they're asking, and my verdict is no (for the latter two).

Sounds good to me.
 
  • #9
It also sounds to me like you're in 2nd year organic chem @ UWO, and have a lab coming up :)
 
  • #10
newb said:
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).
 

1. How does the Benedict's test work?

The Benedict's test is a chemical test used to detect the presence of reducing sugars. It works by reacting with the aldehyde or ketone functional groups present in reducing sugars, causing a color change from blue to green, yellow, orange, or red depending on the amount of sugar present.

2. Why do acids sometimes give false positives in the Benedict's test?

Acids can give false positives in the Benedict's test because they also contain carbonyl functional groups, which can react with the Benedict's reagent and produce a color change. However, this color change is not due to the presence of reducing sugars, so it is important to confirm the presence of sugars through other tests.

3. How can false positives in the Benedict's test be avoided?

To avoid false positives in the Benedict's test, it is important to first perform a control test without the addition of Benedict's reagent. This will help determine if the color change is due to the presence of reducing sugars or other substances. Additionally, performing other tests such as the Fehling's test can help confirm the presence of reducing sugars.

4. Can other compounds besides reducing sugars give false positives in the Benedict's test?

Yes, other compounds such as certain amino acids and sulfur-containing compounds can also give false positives in the Benedict's test. This is why it is important to confirm the results with other tests and to perform control experiments.

5. What are some limitations of the Benedict's test?

The Benedict's test is limited to detecting only reducing sugars and cannot accurately detect non-reducing sugars. It also relies on visual interpretation, which can be subjective. Additionally, certain substances can interfere with the test and give false results. Therefore, it is important to use caution and confirm the results with other tests when using the Benedict's test.

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