Acidity of lactic acid and propanoic acid

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In summary, lactic acid (CH3CH(OH)COOH) is a stronger acid than propanoic acid (CH3CH2COOH) due to the electronegativity of the oxygen atom in the hydroxyl group. Ketones react with phosphorus pentachloride (PCl5) to form alkyl chlorides, not acid chlorides.
  • #1
Michael_Light
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Homework Statement



i) Is lactic acid,CH3CH(OH)COOH a stronger or weaker acid than propanoic acid (CH3CH2COOH)

ii) Another question is does ketone reacts with phosphorus pentachloride PCl5?

Homework Equations





The Attempt at a Solution



(i) I guess lactic acid is a stronger acid because the oxygen atom on the hydroxyl group is very electronegative thus increases the polarity of the -OH bond on the carboxyl group, so H+ are more easily released. (Please correct me if i am wrong)

(ii) I can't find any relevant information from my textbook and even on google, but I encountered this type of question from my exercise book.. Can anyone guide me?
 
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  • #2
Hi Michael!

For (i) you are correct that lactic acid is stronger. -OH in an alkyl chain acts as electron withdrawing due to -I effect.

For (ii) you probably didn't google it well, as it is quite a standard reaction in organic chemistry. :-p

Ketones react with phosphorus penta-chloride to give acid chlorides.

http://www.ch.ic.ac.uk/local/organic/tutorial/DB_Carbonylnotes1.pdf
 
  • #3
Infinitum said:
...snip

Ketones react with phosphorus penta-chloride to give acid chlorides.

Close, but not quite. The product isn't called an acid chloride.
 
  • #4
chemisttree said:
Close, but not quite. The product isn't called an acid chloride.

Whoops, I meant alkyl chlorides, specifically geminal.
 
  • #5


(i) Your reasoning is correct. The presence of the electronegative oxygen atom in lactic acid makes it a stronger acid than propanoic acid, which only has a hydrogen atom attached to the carboxyl group. This allows for easier release of H+ ions in lactic acid, giving it a lower pKa value (3.86) compared to propanoic acid (4.87).

(ii) Ketones do not typically react with phosphorus pentachloride (PCl5) because they do not contain a reactive hydrogen atom. However, in some cases, ketones can react with PCl5 under harsh conditions (high temperature and pressure) to form chlorinated products. This reaction is not commonly studied or used in organic chemistry.
 

FAQ: Acidity of lactic acid and propanoic acid

What is the difference between lactic acid and propanoic acid?

Lactic acid and propanoic acid are both organic acids, but they have different chemical structures. Lactic acid has a 3-carbon chain, while propanoic acid has a 2-carbon chain. This difference in structure affects their acidity and other chemical properties.

Which acid is stronger, lactic acid or propanoic acid?

Lactic acid is a stronger acid than propanoic acid. This is because lactic acid has a larger difference in electronegativity between its functional groups, making it more polar and therefore more acidic. Additionally, lactic acid has a lower pKa value, indicating a stronger tendency to donate a hydrogen ion.

What is the acidity of lactic acid and propanoic acid affected by?

The acidity of lactic acid and propanoic acid is affected by several factors, including the strength of the acid, the concentration of the acid, and the surrounding environment. For example, a higher concentration of the acid will result in a lower pH and a stronger acidity. The type of solvent or solution the acid is dissolved in can also affect its acidity.

How do lactic acid and propanoic acid affect the human body?

Lactic acid is produced in the body during intense exercise and can cause muscle fatigue and soreness. Propanoic acid is produced by certain bacteria in the gut and can contribute to digestive issues. Both acids play important roles in various metabolic processes in the body.

Can lactic acid and propanoic acid be found in food?

Yes, lactic acid and propanoic acid are both commonly found in food products. Lactic acid is naturally present in fermented foods like yogurt and sauerkraut, while propanoic acid is used as a preservative in some baked goods and dairy products. Both acids contribute to the sour or tangy taste in these foods.

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